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Full text of "Maryland weather service. [Reports. New ser.] v. 1-3"

I I 1 1 




Mai\yland 
Weather Service 




MARYLAND WEATHER SERVICE 



VOLUME TWO 



MARYLAND 



WEATHER SERVICE 




VOLUME TWO 



BALTIMORE 

THE JOHNS HOPKINS PRESS 

1907 




Z^i Boxi (§aitimovt (prcee 

BALTIMORE, MD., V. S. A. 



BOARD OF CONTROL 

W.A[. BULLOCK CLAKK, Director. 

REPRESEM'INC; THE JOII>"S HOPKIXS UMVKKSITY. 

W. T. L. TALIAFERRO, . . Secretary axd Treasurer. 

REPRESENTING THE MARYLAND AGRICULTURAL COLLEGE. 

OLREK LAXARt) FA.SSIG,, .... Meteohologist. 

REPRESENTING THE U. S. WEATHER BUREAU. 



TJie Maryland Weather Service is conducted under the joint auspices 
of the institutions above mentioned, the Central OflBce being located at 
the Johns Hopkins University. The meteorological work is under the 
immediate supervision of the Meteorologist who is detailed by the Chief 
of the U. S. Weather Bureau. Other lines of investigation are carried 
on in co-operation with various State and National organizations. 



LETTER OF TRANSMITTAL 

To His Excellenc}', Edwin Warfield, 
Governor of Maryland, 
Sir: — I have the honor to present herewith the second volume of the 
new series of reports of the Maryland Weather Service. The first volume 
contained a general account of the physiography and meteorology of the 
State while the present volume is chiefly devoted to a special study of the 
climatic features of Baltimore and vicinity. I am, 

Very respectfully, 

Wm. Bullock Clark, 

Director. 

Johns Hopkins Univebsitt, 
December 1, 1907. 



CONTENTS 



^ PAGE 

PREFACE 17 

INTRODUCTION, OPERATIONS OF THE SERVICE. By Wm. Bullock 

Clark 21 

Physiography and Climate of the State 21 

Climate and Weather of Baltimore 22 

Climate of the Counties 22 

Distribution of Plant Life in the State 23 

SL'B\^y of the Swa:mp Lands of the State 23 

Other Lines of Work 25 

THE CLIMATE AND WEATHER OF BALTIMORE. By Oliver L. 

Fassig 27 

THE CLIMATE OF BALTIMORE 29 

Introduction 29 

The Geographic Horizon of Baltimore 30 

Atmospheric Pressure 31 

The Diurnal Variations of the Barometer 34 

The Normal Diurnal Variation at Baltimore 34 

Phases of Diurnal Oscillation 35 

Diurnal Variations of Pressure on Clear and Cloudy Days 40 

The Diurnal Barometric Wave 41 

Corrections for Reduction to true Mean Pressure 43 

The Annual March of Atmospheric Pressure 44 

Average Monthly and Annual Pressure 47 

Annual and Secular Variations of Pressure 50 

The Average Variability of Pressure 53 

Extremes of Pressure Sfi 

Temperature of the Atmospheke 56 

Introduction o6 

Average Temperatures 57 

The Normal Hourly Temperature 59 

Phases of the Diurnal Variation 63 

Diurnal Variation as Affected by Clouds and Rain 66 

Effect of a Snow Covering 69 

The Effect of Wind Velocity on Temperature 70 



CONTENTS 

PAGE 

Range of Temperature on Calm and Windy Days 72 

Reduction to the True Mean Temperature 72 

The Hourly Rate of Change 73 

Mean Daily Temperature 76 

Average Inter-diurnal Changes of Temperature 79 

Average Daily Range 83 

Diurnal Variability of Temperature 83 

The Probable Error of the Mean Daily Temperatures 90 

Mean Monthly, Seasonal, and Annual Temperatures 91 

The Normal Temperature 95 

The Variability of the Monthly and Annual Mean 96 

Warm Months and Seasons 97 

Frequency of Stated Departures from the Monthly Seasonal and 

Annual Mean Temperatures 99 

The Probable Error of the Monthly and Annual Means 101 

Succession of the Seasons 103 

Daily Extremes of Temperature 104 

The Greatest Daily Range of Temperature 109 

Monthly and Annual Extremes 112 

The Greatest Monthly Range 114 

Frequency of Days with Frost 115 

The Frequency of Cold Waves 126 

Killing Frdsts 129 

The First and Last Occurrence of a Minimum of 32° 131 

Light Frosts 135 

The Period of Effective Temperatures for Plant Growth 136 

The Frequency of Warm Days in Summer 137 

Time of Occurrence of Annual Minimum and Maximum Temperatures 145 

Temperature of the Water in the Harbor 147 

Humidity 148 

Introduction 148 

Hourly Variation in Humidity 152 

Phases of the Diurnal March of Relative Humidity 154 

Mean Monthly and Annual Relative Humidity 156 

Absolute Humidity 158 

Mean Vapor Pressure 159 

Pkecipitation 159 

Introduction 159 

The Causes of Precipitation 161 

The Geographical Distribution of Rainfall 161 

The Influence of Wind Direction 162 

The Influence of Topography 162 

The Influence of Atmospheric Pressure 163 

The Seasonal Distribution of Rainfall 164 

Hourly Amount of Rainfall 165 



MARYLAXD WEATHER SEKYICE 7 

PAGE 

Hourly Rainfall Frequency 167 

Duration of Precipitation 170 

Frequency of Precipitation of Stated Amounts 174 

Average Daily Rainfall 178 

Daily Rainfall Frequency 182 

The Probability of Rain 183 

The Monthly Precipitation 185 

The Seasonal and Annual Precipitation 190 

Monthly and Annual Departures 195 

Excessive Rains 197 

Greatest Rainfall in 24 Hours 199 

Excessive Rates of Precipitation 205 

Dry Spells 214 

Wet Spells 219 

The Distribution of Precipitation in Normal, Dry, and Wet Years. . . 223 

Snowfall 227 

Dates of First and Last Snow 231 

The Frequency of Days with Snowfall 232 

Heavy Snowfalls 235 

Duration of Snowfall 236 

Fogs 237 

Sunshine and Cloudiness 239 

Sunshine 239 

Average Daily Sunshine 243 

Sunshine Phases 244 

Cloudiness 245 

Clear, Partly Cloudy, and Cloudy Days 246 

Frequency of Clear Days 248 

Frequency of Partly Cloudy Days 250 

Cloudy Days 251 

The Winds 251 

Introduction 251 

Average Hourly Wind Movement 252 

Average Daily and Total Monthly Wind Movement 255 

Maximum Wind Velocities 258 

Frequency and Duration of Stated Wind Velocities 261 

Average Duration of Storm Winds 262 

Gales 263 

Prevailing Hourly Wind Directions 265 

Prevailing Monthly and Annual Directions 268 

Monthly Frequency of Stated Directions 273 

The Direction of Upper and Lower Clouds 274 

Electrical Phenomena 276 

Thunderstorms 276 

Thunderstorm Probability 280 



8 CONTENTS 

PAGE 

Consecutive Days with Thunderstorms 280 

Direction of Thunderstorms 281 

Pressure Changes during Thunderstorms 282 

Hail 284 

Auroras 288 

Sunspots and Weather 288 

General Character of the Seasons 295 

Observations and Instrumental Equipment 296 

Historical Notes 296 

Observers and Observations 301 

Instrumental Equipment 301 

Hours of Observation 302 

Changes in the Location of the Station and Officials in Charge 304 

Summary of Average and Extreme Values 306 

THE WEATHER OF BALTIMORE 311 

Introduction 311 

The Synoptic Weather Chart 312 

Cyclones and Anti-cyclones 313 

Areas of Unsettled Weather (Cyclones) 316 

Pressure and Winds 318 

Temperature and Wind Direction 319 

Distribution of Clouds and Precipitation 320 

Areas of Fair Weather (Anti-cyclones) 321 

Isobars and Winds 321 

The Winds and Distribution of Temperature 323 

Distribution of Clouds 324 

The Eastward Drift of Cyclones and Anti-cyclones 324 

Weather Charts of the Northern Hemisphere 327 

Weather of the Principal Climatic Zones 328 

The Tropical Zone 328 

The Temperate Zones 329 

The Polar Zones ; 330 

The Seasons 331 

AVinter Weather 333 

Winter Cyclones 334 

The Lake Storm 335 

The Storm of December 24-26, 1902 335 

The Storm of January 7-8, 1903 341 

The Storm of February 27-March 1, 1903 345 

The Southwest Storm 350 

The Storm of February 3-5, 1903 350 

The Storm of December 26-28, 1904 354 

The Storm of December 12-13, 1903 359 



MARTLAXD W EATHEll SEilVICE 9 

PAGE 

The Gulf Storm 363 

The Storm of February 1-3, 1902 364 

The Storm of January 5-7, 1905 368 

The Storm of February 20-22, 1902 373 

The Blizzard 378 

The Blizzard of March 11-13, 1888 378 

The Blizzard of February 12-14, 1899 382 

Areas of Fair Weather (Anti-cyclones) 389 

Cold Waves 391 

The Cold Wave of December 13-15, 1901 ^ 392 

The Cold Wave of February 10-13, 1899 395 

The Origin of Cold Waves 396 

The Cold Winter of 1903-04 397 

The Warm \\'inter of 1889-90 399 

The Distribution of Atmospheric Pressure during the Cold Winter 

of 1903-04 and the Warm Winter of 1889-90 400 

The Variability of Winter Weather 401 

The Weather of Christmas Day 404 

The AVeather of Washington's Birthday 409 

Si'Ki.Nc; Wkathek 410 

March Winds and Storms 412 

Ice Storms 413 

The Squall of March 1 , 1907 413 

Equinoctial Storms 415 

Hail Storms 417 

The Storm of May 19, 1904 418 

The Storm of April 27, 1890 418 

Spring Frosts 421 

Ice without Frost 423 

Periods of Unsettled Weather 424 

The Rainy Period of April 19-25, 1901 425 

The Rainy Period of May 16-26, 1894 426 

The Variability of Weather in Spring 428 

The Weather of March 4 429 

The Weather of May 1 432 

The Weather of Easter Sundays 432 

Sim .mi;k Wkathki! 436 

Summer Storms 437 

The Thunderstorm of July 20, 1902 438 

Ttic 'j'hunderslorm of July 3, 1902 444 

The Thunderstorm of July 12, 1904 446 

The Tornado of July 12, 1903 447 

* Waterspouts 452 

Summer Hot Spells 453 

The Summer of 1900 454 

General Weather Conditions 459 



10 CONTENTS 

PAGE 

The Summer of 1901 462 

The Hot Periods of August, 1900, and July, 1901, Compared 463 

Days with a Maximum Temperature of 90° or above 466 

The Cold Summer of 1816 467 

Distribution of Pressure during the Cool June of 1903 469 

Distribution of Pressure during the Normal June of 1902 470 

The Variability of Summer Temperatures 470 

The Weather of July 4 472 

West Indtan Hurricanes 475 

Frequency of Hurricanes 476 

The Hurricane of October 13, 1893 476 

Autumn Weather 480 

Indian Summer 482 

The Variability of Autumn Temperatures 486 

The Weather of September 12 486 

The Weather of October 1 489 

The Weather of Thanksgiving Day 489 

The Heavy Rains of September 24-26, 1902 492 

FOEETEtLING THE WEATHEB (HISTORICAL) 493 

Introduction 493 

Natural Signs 494 

Astro-Meteorology 496 

Symbolic Days 498 

Early Books on Weather Proverbs 498 

Forecasts Based on Average and Extreme Values 499 

Temperature Variability 500 

Rainfall Probability 500 

Special Days 501 

Recurring Periods 502 

The Method of the Synoptic Weather Chart 504 

The Indian Seasonal Forecasts 507 

Index 511 



ILLUSTRATIONS 



PLATE FACIXG PAGE 

I. The Diurnal Barometric Wave 42 

II. Typical Barograms 44 

III. Daily March of Temperature and Pressure 80 

IV. Daily March of Temperature 82 

V. Typical Thermograms 86 

VI. Departures of Mean Monthly Temperature from Normal for 87 

Years 87-88 

VII. Departures of Mean Monthly, Seasonal, and Annual Tempera- 
ture from Normal for 87 Years 92 

VIII. Selected Relative Humidity Curves 158 

IX. Precipitation Probability 184 

X. Monthly, Seasonal, and Annual Departures from the Normal 

Precipitation (1817-1904) 194 

XI, Average Hourly Wind Direction 266 

XII. Sunspots, Solar Prominences, and Weather Conditions 294 

XIII. General Character of the Seasons. — Winter 296 

XIV. General Character of the Seasons. — Spring 296 

XV. General Character of the Seasons. — Summer 296 

XVI. General Character of the Seasons. — Autumn 296 

XVII. General Character of the Year 296 

XVIII. Office of the U. S. Weather Bureau and Maryland State Weather 

Service 300 

XIX. Storm Warning Display Station 304 

XX. Hourly Observations at Baltimore during the Blizzard and Cold 

Wave of Feb. 9-14, 1899 388 

XXI. Frost Figures 311 

XXII. Effects of Ice Storm of March, 1906 413 

XXIII. Depanures in Temperature during the Hot Spell of 1900 462 

XXIV. Distribution of Pressure, Winds, and Temperature during Normal, 

Cold, and Warm Seasons in the United States 470 

FIGURE ■ PAGE 

1. Hourly Variations of the Barometer 33 

2. Isopleths of Hourly Pressure 36 

3. Principal Phases of Diurnal Oscillation of Pressure 38 

4. Diurnal Variations of Pressure on Clear and on Cloudy Days 40 

5. The Diurnal Barometric Wave 42 



12 ILLCSTRATIOXS 

PAGE 

6. Mean Monthly Atmospheric Pressure 44 

7. Variations in the Mean Monthly Pressure 46 

8. Annual Variations of Pressure Expressed as Departures from the 

Normal Value 50 

9. Monthly Means and Extremes of Pressure 54 

10. Mean Hourly Temperature 60 

11. Isopleths of Hourly Temperature 62 

12. Principal Phases of Diurnal Variation of Temperature 64 

13. Effect of Cloudiness and Rain on the Hourly Variations of Tempera- 

ture 67 

14. Effect of Snow-covering on the Hourly Variations of Temperature.. 68 

15. Effect of Wind Velocity on the Hourly Variations of Temperature.. 71 

16. Hourly Rate of Change of Temperature 74 

17. Curves Representing the Average Hourly Pressure and the Hourly 

Rate of Change in Temperature for the Year 76 

IS. Inter-diurnal Temperature Changes 80 

19. Total Seasonal and Annual Frequency of Stated Diurnal Changes of 

Temperature 84 

20. Diurnal Changes of Temperature of less than 6°, +6°, +8°, and 

-1-10° each month 85 

21. Diurnal Changes of Temperature of —6°, -^6°, +8°, -M0°, -|-20°... 88 

22. Frequency of Stated Departures from the Monthly Normal Tem- 

perature 101 

23. Frequency of Stated Departures from the Normal Seasonal and An- 

nual Temperatures 102 

24. Greatest Daily Range of Temperature 109 

25. Extreme, Average, and Mean Maximum and Minimum Temperatures. Ill 

26. Absolute Maximum and Minimum Temperatures 114 

27. Greatest Monthly Range of Temperature 115 

28. Longest Period of Consecutive Days with a Minimum Temperature 

of 32° or Below 119 

29. Number of Days with Mean Temperature Below 14° and 32° 120 

30. Annual Frequency of Days with a Maximum Temperature Below 32° 121 

31. Annual Frequency of Cold Days 122 

32. Monthly Frequency of Cold Days 122 

33. Interval Between Last and First Occurrence of a Minimum Tem- 

perature of 32° 132 

34. Interval Between Last and First Occurrence of Minimum Tempera- 

ture of 40° 134 

■ 35. Annual Number of Days with Mean Temperature Above 42° 136 

36. Annual Number of Days with Maximum Temperature of 90° and 

Over 137 

37. Time of Occurrence of the Lowest and Highest Temperature of 

the Year 144 

38. Air and Water Temperatures in Baltimore Harbor 144 

39. Mean Hourly Relative Humidity 151 



:martlaxd weather service 13 

PAGE 

40. Mean Hourly Relative Humidity 153 

41. Phases of the Diurnal Variations in Relative Humidity 155 

42. The Mean Monthly Relative Humidity 156 

43. Variations in the Mean Annual Relative Humidity 156 

44. Average Hourly Precipitation 165 

45. Average Hourly Amounts of Precipitation in January and July 167 

46. Average Hourly Frequency of Precipitation in January and July. . . . 169 

47. Average Hourly Frequency of Precipitation 170 

48. The Average Duration of Precipitation 172 

49. Variations in the Annual Frequency of Days with Appreciable Pre- 

cipitation 177 

50. Monthly Frequency of Precipitation 182 

51. The Monthly Amount of Precipitation 185 

52. Mean Monthly Precipitation 188 

53. Variations in the Annual Amount of Precipitation for 1817 to 1904. . 191 

54a. Departures from Mean Monthly Precipitation (1817-1859) 193 

54b. Departures from Mean Monthly Precipitation (1860-1904) 194 

55. The Heaviest Precipitation in any 24 Consecutive Hours 201 

56. Rainfalls Equalling or Exceeding 2.50 Inches in a Day 202 

57. Rainfalls Equalling or Exceeding One Inch per Hour 203 

58a. Excessive Rates of Rainfall 210 

58b. Excessive Rates of Rainfall 211 

59. Dry Periods 215 

60. Dry Periods 218 

61. Wet Periods 220 

62. Total Monthly Precipitation During a Dry Year, a Normal Year, and 

a Wet Year 224 

63. Daily Precipitation During a Dry Year, a Normal Year, and a Wet 

Year 225 

64a. Annual Frequency of Days with a Snowfall to the Amount of One- 
tenth of an Inch 228 

64b. Annual Depth of Snowfall in Inches 229 

65. Monthly Frequency and Amount of Snowfall 233 

66. Mean Hourly Sunshine 241 

67. Mean Hourly Sunshine for the Year 242 

68. Average Hourly Cloudiness 245 

69. Relative Frequency of Clear. Partly Cloudy, and Cloudy Days 247 

70. Hourly and Annual Variations of Wind Velocity 253 

71. Average Hourly Variations in Wind Velocity 254 

72. The Frequency of Storm Winds 258 

73. Average Hourly Wind Direction facing page 266 

74. Prevailing Morning and Afternoon Wind Directions in January. . . . 267 

75. Relative Frequency of Prevailing Wind Directions 269 

76. Prevailing Monthly Directions of the Wind in Warm, in Normal, 

and in Cold Seasons and Years 270 

77. The Frequency and Distribution of Thunderstorms 277 

78. The Average Monthly P'requency of Occurrence of Thunderstorms ., 277 



14 ILLUSTRATIONS 

PAGE 

79. The Annual Frequency of Occurrence of Thunderstorms from 1871 

to 1904 278 

80. The Direction of Movement of Thunderstorms 281 

81. Some Typical Barograms during Thunderstorms and Squalls 283 

82. The Frequency of Occurrence and the Hourly and Seasonal Distribu- 

tion of Hailstorms 286 

83. Barograms during Hailstorms 286 

84. Sunspots, Solar Prominences, and Weather Conditions 294 

85. Typical Cyclone of Dec. 27, 1904 (Pressure and Winds) 317 

86. Typical Cyclone of Dec. 27, 1904 (Complete Chart) 317 

87. Typical Anticyclone of April 4, 1904 (Pressure and Winds) 322 

88. T3T)ical Anticyclone of April 4, 1904 (Complete Chart) 322 

89. Typical Cyclone and Anticyclone of March 3, 1904 325 

90. Pressure Distribution over the Northern Hemisphere, Dec. 4, 1886 . . 327 

91. The Lake Storm of Dec. 24, 1902 336 

92. The Lake Storm of Dec. 25, 1902 336 

93. The Lake Storm of Dec. 26, 1902 337 

94. The Lake Storm of Dec. 24-26, 1902 (Diagr.) 339 

95. The Lake Storm of Jan. 7, 1903 342 

96. The Lake Storm of Jan. 8, 1903 342 

97. The Lake Storm of Jan. 6-8, 1903 (Diagr.) 343 

98. The Lake Storm of Feb. 27, 1903 346 

99. The Lake Storm of Feb. 28, 1903 346 

100. The Lake Storm of March 1, 1903 347 

101. The Lake Storm of Feb. 27-March 1, 1903 (Diagr.) 348 

102. The Southwest Storm of Feb. 3, 1903 351 

103. The Southwest Storm of Feb. 4, 1903 352 

104. The Southwest Storm of Feb. 5, 1903 352 

105. The Southwest Storm of Feb. 3-6, 1903 (Diagr.) 353 

106. The Southwest Storm of Dec. 26, 1904 356 

107. The Southwest Storm of Dec. 27, 1904 356 

108. The Southwest Storm of Dec. 28, 1904 357 

109. The Southwest Storm of Dec. 26-28, 1904 (Diagr.) 358 

110. The Southwest Storm of Dec. 12, 1903 360 

111. The Southwest Storm of Dec. 13, 1903 360 

112. The Southwest Storm of Dec. 12-14. 1903 (Diagr.) 361 

113. Paths and Rain Areas of Southwest Storms of Jan., 1898 362 

114. The Gulf Storm of Feb. 1, 1902 365 

115. The Gulf Storm of Feb. 2, 1902 365 

116. The Gulf Storm of Feb. 3, 1902 366 

117. The Gulf Storm of Feb. 1-3, 1902 (Diagr.) 367 

118. The Gulf Storm of Jan. 5, 1905 369 

119. The Gulf Storm of Jan. 6, 1905 369 

120. The Gulf Storm of Jan. 7, 1905 370 

121. The Gulf Storm of Jan. 5-7, 1905 (Diagr.) 371 

122. The Gulf Storm of Feb. 20, 1902 373 

123. The Gulf Storm of Feb. 21, 1902 374 



MARYLAXD WEATHER SERVICE 15 

PAGE 

124. The Gulf Storm of Feb. 22, 1902 374 

125. The Gulf Storm of Feb. 20-22, 1902 (Diagr.) 375 

126. Paths of the Gulf Storms of February, 1902 376 

127. Diagram of Rainj^ Sundays of the Winter of 1801-2 (Diagr.) 377 

128. The Blizzard of March 11, 18SS 379 

129. The Blizzard of March 12, 1888 379 

130. The Blizzard of March 13, 1888 380 

131. The Blizzard of March 11-13, 1SS8 (Diagr.) 381 

132. The Blizzard of Feb. 9, 1899 384 

133. The Blizzard of Feb. 10, 1899 384 

134. The Blizzard of Feb. 11, 1899 385 

135. The Blizzard of Feb. 12. 1899 385 

136. The Blizzard of Feb. 13, 1S99 386 

137. The Blizzard of Feb. 14, 1899 386 

138. Snow on the Ground after the Blizzard of February, 1899 387 

139. Cold Wave of Dec. 13, 1901 393 

140. Cold Wave of Dec. 14, 1901 393 

141. Cold AVave of Dec. 15, 1901 394 

142. Cold February 11, 1899 402 

143. Warm February 11, 1887 402 

144. The Weather of Christmas Day (December 25) (Diagr.) 406 

145. The W^eather of Washington's Birthday (February 22) (Diagr.).. 408 

146. The Squall of :\Iarch 1, 1907 414 

147. The Hail Storm of May 19, 1904 419 

148. The Hail Storm of May 19, 1904 ( Diagr. ) 419 

149. The Frost of May 9, 1906 422 

150. Ice without Frost, April 17, 1905 424 

151. The Weather of March 4 (Diagr.) 430 

152. The Weather of May 1 (Diagr.) 433 

153. The Thunderstorm of July 20, 1902 440 

154. The Thunderstorm of July 20, 1902 (Diagr.) 441 

155. The Movements of the Thunderstorm of July 20, 1902 (Diagr. "i 442 

156. The Thunderstorm of July 3, 1902 444 

157. The Thunderstorm of July 3, 1902 (Diagr.) 445 

158. The Thunderstorm of July 12, 1904 447 

159. The Tornado of July 12, 1903 (8 a. m.) 448 

160. The Tornado of July 12, 1903 (8 p. m.) 448 

161. Chart of August 6, 1900 (during Hot Spell) 460 

162. Temperature during Hot Spells of 1900 and 1901 (Diagr.) 464 

163. The Cold July 1, 1885 471 

164. The Warm July 1, 1901 471 

165. The Weather of July 4 (Diagr.) 474 

166. The Hurricane of Oct. 13, 1893 (8 a. m.) 478 

167. The Hurricane of Oct. 13, 1893 (8 p. m.) 478 

168. The Hurricane of Oct. 14, 1893 (8 a. m.) 479 

169. The Weather of Oct. 29, 1903 (Indian Summer) 484 

170. The Weather on September 12 (Defenders' Day) (Diagr.) 488 



PREFACE 

The present volume is the second of a series of reports dealing with 
the climatic features of Maryland. The first volume was general in 
character and presented all that was then known regarding the physi- 
ography and meteorolog}- of the State. The present and succeeding 
volumes will be devoted to more special studies within the province of 
climatological research. 

The Introduction to the present volume, prepared by Wm. Bullock 
Clark, is devoted chiefly to an account of the operations of the Service 
together with the plans for future work. An account is given of the 
Swamp Lands of the State whicli are atti-aeting wide attention. The 
writer refers to the Botanical Survey of the State, which has been made 
under the auspices of the State Weather Service, the results of which 
will be shortly printed in Volume III of the present series. 

The Report on the Climate and WeatJier of Baltimore and Vicinity, 
discussed by Oliver L. Fassig, constitutes the chief portion of the vohune 
and represents the result of many years of exhaustive study of the Balti- 
more region. All of the available records both public and private have 
been employed in this work and the result may be regarded as remarkably 
complete. It is doubtful if any district luis received as thorough study 
as Dr. Fassig has given to that of Baltimore and vicinity. The report 
is divided into two parts. The first deals with the average and extreme 
values of the meteorological elements recorded in the city of Baltimore. 
The discussion is based upon careful ol)Servatious extending over a period 
of nearly a century. The second part deals with types of weather experi- 
enced in Baltimore and vicinity — hence with the actual physical condi- 
tion of the atmosphere at stated times, during the prevalence of storms, 
cold and warm waves, etc. 



18 PREFACE 

The Maryland Weather Service desires especially to extend its thanks 
to Professor Willis L. Moore, Chief of the U. S. Weather Bureau, who 
has generously aided the conduct of the investigations discussed in the 
present volume. Dr. Fassig has had access to the complete records of the 
U. S. Weather Bureau as well as to those of other official organizations. 

Mr. E. W. Berry, of the State Geological Survey, has materially aided 
in editing the manuscripts for the volume. 



INTRODUCTION 



OPERATIONS OF THE SERVICE 



BY 

WM. BULLOCK CLARK 



INTRODUCTION 
OPERATIONS OF THE SERVICE 

BY 

WM. BULLOCK CLARK 



The Maryland Weather Service has been engaged for many years in a 
study of the climatic features of Maryland. These investigations have 
resulted in the accumulation of a vast amount of information relating to 
the meteorology, the physiography, the agricultural soils, and the distri- 
bution of plant life. Much aid has been rendered the State Weather 
Service in this work by both the National and State bureaus. 

Physiography and Climate of the State. 

The results of the physiographic and meteorological studies of the 
entire state down to 1899 were presented in Volume I of this series of 
reports. These investigations were based on all the then existing obser- 
vations and records, both official and private. The physiographic studies 
had been conducted largely under the auspices of the State Geological 
Survey, but as the results were so fundamental to an interpretation of 
the climatic features of the State, their publication in the very first 
volume of the new series of reports seems desirable. 

The meteorological data relating to Maryland climate had been ac- 
cumulated for over a century, but little attempt had been made hitherto 
to draw conclusions from them or to seek an explanation for the many 
variations that are found in the different sections of the state and in the 



T-i INTRODUCTION 

same regions at different seasons of the year. These studies may be 
regarded as preliminary to the more exhaustive investigations which have 
followed, as well as to those which still await completion. 

Climate and Weather of Baltimore. 

The investigations of the climate and weather of Baltimore and vicin- 
ity, discussed in the pages of the present volume, may be regarded as 
fully meeting the requirements of such a detailed study. The author has 
treated exhaustively the elements entering into the interpretation of the 
conditions found to prevail in the Baltimore region. It is probably the 
most complete study that has ever been given to the climate and weather 
of a single city and its environs, and will afford a most important store- 
house of information for those who may be seeking for an accurate knowl- 
edge of the exact conditions that prevail in Baltimore and its immediate 
surroundings. The aid rendered by the Chief of the U. S. Weather 
Bureau has alone made it possible to secure the results here recorded. 

Climate of the Counties. 

Special reports on the climate of Allegany, Garrett, Cecil, Calvert, and 
St. Mary's counties have been prepared by the Maryland Weather Service 
and issued under the auspices of the Maryland Geological Survey in its 
series of county reports. It is the intention of the State Weather Service 
ultimately to bring together and publish these chapters when complete 
for all the counties in a single volume of the State Weather Service 
s'eries. When brought out this report on the climate and weather of the 
Mar3dand counties will present for each political district of the State an 
exhaustive discussion of its special features that will be of great benefit 
to the inhabitants and to those seeking information regarding the special 
climatic conditions of any particular county. This study will take sev- 
eral years for its consummation, but with the co-operation so generously 
furnished by the Chief of the U. S. Weather Bureau it will be finally 
completed in a form that will be recognized as thoroughly authoritative. 



MARYLAND WEATHER SERVICE 23 

The Meteorologist in charge of the State Weather Service, who has 
always been the representative of the U. S. Weather Bureau in Balti- 
more, has been hitherto designated by the Chief of the National Service 
to prepare these reports and has had access not only to the United States, 
but to the State records in this worlv. He lias been able to employ the 
services of a trained body of men who would be otherwise entirely beyond 
the reach of the State for such an investioation. 



Distribution of Plant Life in the State. 

The distribution of animal and plant life, and more especially of the 
latter, is so intimately associated with the physiographic and climatic 
conditions that prevail that the Maryland Weather Service has under- 
taken a Botanical Survey of the State as a part of its climatic studies. 
For the past three years several trained botanists under the direction of 
Dr. Forrest Shreve have been engaged in the different sections of the 
state in making a detailed investigation of the botanical conditions. Not 
only has the distribution of plant life been found to be dependent on the 
climate and physiography of the state, but upon the agricultural soils 
which in turn find their ultimate interpretation in the underlying rocks 
from which they have been derived, thus bringing the work of the State 
Geological Survey and State Weather Service into close association. 

The botanical survey is now completed and a report is at the present 
time being prepared, which will be issued as Volume III of the State 
Weather Service. 



Survey of the Swamp Lands of the State. 

A- survey of the swamp lands of Maryland has been made in connection 
with the topographic survey of the state, in which the State Weather 
Service has participated with the State Geological Survey in its co-opera- 
tion with the Topographic Branch of the TJ. S. Geological Survey. This 
survey has shown the following swamp areas. 



24 



IXTRODUCTION 



Area of Swamp Lands in the Various Counties Computed from the 
Maryland Geological Survey Maps. 

County. Fresh. Salt. Total. 

Sq. Mi. Acres. Sq. >li. Acres. Sq. Mi. Acres. 

Baltimore 1.7 1,088 5.4 3,456 7.1 4,544 

Anne Arundel 3.3 2,112 1.9 1,216 5.2 3,328 

Prince George's ... 8.6 5,504 0.2 128 8.8 5,632 

Charles 11.9 7,616 22.1 14,144 34.0 21,760 

Calvert 3.2 2,048 1.2 768 4.4 2,816 

St. Mary's 0.3 192 1.3 832 1.6 1,024 

Harford 0.4 256 11.3 7,232 11.7 7,488 

Cecil 0.2 128 6.5 4,160 6.7 4,288 

Kent 0.4 256 7.9 5,056 8.3 5,312 

Queen Anne's 9.7 6,208 4.5 2,880 14.2 9,088 

Talbot 0.3 192 5.3 3,392 5.6 3,584 

Caroline 9.7 6,208 2.6 1,664 12.3 7,872 

Dorchester 78.3 50,112 123.2 78.848 201.5 128.960 

Wicomico 17.1 10,944 22.1 14.144 39.2 25,088 

Somerset 7.7 4,928 68.5 43,480 76.2 48,768 

Worcester 33.0 21,120 35.4 22,656 68.4 43,776 

Garrett 4.5 2,880 4.5 2,880 

Other counties 4.0 2,460 4.5 2,560 

Total 194.3 124,352 319.4 204,416 513.7 328,768 

it will thus be seen that the State of Maryland has 328,768 acres of 
swamp lands, of which 124,352 acres are fresh-water swamps and 204,416 
acres salt-water marshes. The eastern and southern counties of the state 
bordering the Chesapeake Bay and the Atlantic Ocean have 323,326 
acres, of which 118,912 acres are fresh and 204,416 acres are salt. The 
central and western counties have 5440 acres, all of which are fresh. 

The agi'icultural soil survey of Maryland, which is being carried on in 
co-operation with the TJ. S. Bureau of Soils, shows a considerably larger 
acreage of swamp lands in those counties surveyed than the estimates 
above given, but in the soil survey the small tracts on individual farms 
were computed, while the topographic maps show only the larger areas, 
which would alone be considered in any plan of government reclamation. 
Counting these small tracts the total area would probably reach 500,000 
acres. 

A fuller study of these swamp lands is now in progress and as their 
present condition is intimately connected with the climatic conditions of 



MARYLAND WEATHER SERVICE 25 

the State their study, in part at least, falls within the province of the 
State AVeather Service. 

Other Lixes oe Work. 

The far reaching influence of climate on the economic and social 
development of the state suggests other lines of investigation that require 
the attention of the State Weather Service. 

The character of the agricultural soils, although fundamentally deter- 
mined by the underlying rocks, is also to no small degree dependent on 
the physiography and climatic features of the State. These factors must 
be considered in any comprehensive study of the agricultural soils. 

The health of any community is also to no inconsiderable extent de- 
pendent on the climate, and this is recognized in the field of investigation 
known as medical climatology. In Volume I the present writer said in 
discussing this subject in his " Plan of Operation of the Service "' that 
" the healthfulness of Maryland as a place of residence is a question of no 
small importance to those who may be considering the advisability of 
seeking homes in our midst, and actual facts should be presented in such 
a manner as to command their attention. The various sections of the 
state, their marked differences in temperature and rainfall, may be shown 
to be adapted to the physical requirements of different people, and it is 
highly important that these facts should be made known. 

" It is also probable, as the meteorological records over considerable 
periods are carefully studied, that some districts will be found highly 
beneficial to people suffering from certain ailments. It is the purpose of 
the Maryland AYeatlier Service to have some expert upon medical clima- 
tology carefully study its records and prepare a report upon this subject, 
and already arrangements to this end have been perfected." 

The general and special studies earlier enumerated naturally afford the 
basis for a considei-ation of tlie crop condition? of tlie State and this 
subject should be taken up in a comprehensive way as the data collected 
become adequate to the discussion of so great a subject. The agricul- 
tural products of the State far surpass those in every other line, and tlie 
State Weatlier Service should give whatever assistance it can in the study 
of the important problems involved. 



26 INTfiODUCTIOJSr 

It is also a well recognized fact that the character and distribution of 
forest growth is in no small degree determined by the climatological 
features which have already been described. Since forestry studies were 
organized by the State Geological Survey a few years ago a State Board 
of Forestry has been organized and the investigation of our forests is 
now well under way. The State Weather Service can aid in various ways 
in this work. 

The climate in its various relations touches human life in so many 
points that the investigations already undertaken and proposed will 
prove not only of great interest, but of greater value to the people of 
the state. Eesults of real worth can rarely be obtained quickly, but the 
investigations now being conducted by the State Weather Service are of 
a fundamental character, and when completed will cover as fully as 
possible the jfield of climatology in its various relations to the economic 
interests of the State. 



REPORT ON THE 
CLIMATE AND WEATHER 

OF 

BALTIMORE AND VICINITY 

(Based on the Observations of the U. S. Weather Bureau; Supplemented by Obser- 
vations of the Maryland State Weather Service, and the U. S. 
Army Medical Department.) 

PREPARED BY DIRECTION OF 
WILLIS L. MOORE 

V 

Chief oh U. S. Wkather Bureau 



BY 

OLIVER LANARD FASSIG 



THE CLIMATE OF BALTIMORE 



I^siTEODUCTIOX 



For more than thirty years the United States Weather Bureau has 
maintained a station of the first order in Baltimore City. During all 
these years the weather conditions have been carefully and accurately 
noted and recorded at several stated hours of the day by trained observers. 
In 1893 the instrumental equipment of the station was greatly increased 
and the value of the records enhanced by the acquisition of additional 
self-recording instruments by means of which a continuous record has 
been obtained of all the principal elements of the weather. The records 
of the Baltimore station now show the local state of the atmosphere dur- 
ing every hour of the day and night since 1893, barring an occasional 
brief break in the record due to accidental causes. The factors thus 
continuously noted are the temperature of the atmosphere, the pressure, 
rainfall, sunshine, wind velocity, wind direotion and, since 1902, the 
humidity. This mass of exceedingly valuable raw material for the study 
of problems in local climatology, supplemented by an almost unbroken 
series of local observations made since 1817 under the auspices of the 
United States Army Medical Department and the Smithsonian Insti- 
tution, has never before been subjected, as a whole, to a critical analysis 
and reduction. It is evident that such observations, secured at enormous 
expense of time and money, should yield benefits beyond their immediate 
uses at the time of recording, however valuable these may be. 

The weather conditions at Baltimore are typical of conditions within 
a wide area. Allowing for small differences in amplitude of variation 
due to local surface conditions, an analysis of the Baltimore observations 
may with safety be applied to much of that portion of the Middle Atlantic 
States lying east of the Appalachian Mountains. This area lies about 
midway between the rigorous north and the mild south, the equable ocean 
region anrl the region of great variability in the interior of the continent. 



30 THE CLIMATE OF BALTIMORE 

Eainfall is abundant and quite uniformly distributed throughout the 
3^ear. Storms of destructive violence are of rare occurrence; tornadoes 
are almost unknown. The season of safe plant growth is long, and sun- 
shine is abundant. 

In the following report the analysis of the observations is divided into 
two distinct parts. The first part deals with the average conditions of 
the atmosphere, derived from many years of statistical data relating 
to temperature, pressure, humidity, rainfall, clouds and sunshine, winds, 
etc., and to departures from their normal values. In brief, it deals with 
the climate of the region about Baltimore. The second part is devoted 
to the iveather, or actual conditions of the atmosphere at any given time 
as regards temperature, humidity, rainfall, clouds, wind — the sum 
total of the atmospheric conditions. Hence weather is a passing phase of 
climate. Attention will be directed largely to storms, cold waves, hot 
waves, etc., as well as to the gentler phases of the atmosphere Avhich con- 
stitute the daily routine of weather. This division into climate and 
weather is necessarily more or less arbitrary, and the lines of demarcation 
employed by different writers will seldom be foimd in exactly the same 
places, nor will the strict definition be consistently adhered to in the 
practical treatment of the subjects by the same writer. But the division 
is, in the main, logical and- a convenient one for all practical purposes. 

Without entering unduly into details regarding the plan of Part I, 
attiention may be directed to the order of discussion of the climatic factors. 
As far as possible each element has been considered with reference, (a) to 
its diurnal period, (b) its annual period, (c) its variability, or non- 
periodic aspects of short and long duration. Tables and diagrams have 
been freely employed, the statistical tables permitting of greater accuracy 
of statement, the graphic method affording a readier means of presenting 
at a glance the salient features of the variability of the climatic elements 
from hour to hour, or from season to season. 

The Geographical Horizox of Baltimore. 

The State of Maryland is situated within three distinct physiographic 
provinces. The low, flat Coastal Plain, averaging about 60 feet above 
mean tide and cut up by tidal estuaries, extends from the Atlantic 
seaboard westward to a line joining Philadelphia. Baltimore and Wash- 



MARYLAND WEATHER SERVICE 31 

ington. where it is separated sharply from the Piedmont Plateau, or 
middle province. The Piedmont Plateau is an undulating area with 
elevations rising to 700 feet or 800 feet, and extending westward to the 
mountainous and high plateau region of the Appalachian Province. The 
mountains of this latter province form a system of parallel ranges extend- 
ing from northeast to southwest across the state, rising to heights of 
3000 feet. The city of Baltimore is partly on the Piedmont Plateau 
and partly on the Coastal Plain. The country to the north and west is 
gently undulating; to the east and south it is level and but a few feet 
above the adjacent estuaries of Chesapeake Bay. 

ATMOSPHEEIC PEESSUEE. 

As a direct climatic factor the pressure of the atmosphere, and varia- 
tions in this pressure, are of comparatively minor importance. The 
effect of changes in the height of the barometer upon the human system 
does not begin to be recognized until the rise or fall is very marked. A 
diminished pressure causes in most persons a feeling of lassitude with 
increased difficulty of breathing. But this physiological effect is not ex- 
perienced, excepting by extremely sensitive persons, until the barometer 
shows a fall of three or four inches, equivalent to an ascent of three or 
four thousand feet above sea-level. The extreme variations of pressure 
at any one place do not often exceed an inch within the period of a few 
days. At Baltimore the extreme range has been but slightly over two 
inches in the past thirty-three years. The change in the pressure ex- 
perienced during the passage of the severest type of cyclonic disturb- 
ance is less than the permanent diU'crcncc in pressure between the east- 
ern and the higher western portions of the State of Maryland; and 
hence less than is experienced by travelers daily in passing from Balti- 
more to Pittsburg, over the Alleghany ^Mountains. As an indirect fac- 
tor, however, in the climates of the world, and as a direct agency in 
causing movements of the atmosphere, the pressure changes are of the 
highest importance and take rank witli those of temperature and rain- 
fall. 

In anotlier part of this report, the more general relations of pressure 
will be discussed in connection with the consideration of storm move- 
ments. Tlie following pages are devoted mostly to the local conditions 



32 



THE CLIMATE OF BALTIMORE 



and variations of pressure at Baltimore, based upon observations made 
since 1871 under the auspices of the United States Weather Bureau. 
Observations were begun in Baltimore on January 1, 1871, and have 
been maintained in an unbroken series to the present time. Standard 



29.000 inches. 



TABLE I.-MEAN HOURLY BAROMETRIC PRESSURE. 

[In inches and thousandths.] 

Local time is 6 minutes slow. 



75th mer. time. Jan. Feb. Mar. Apr. [ May June July jAug. Sept. Oct. Nov. Dec. Year 



1 A.M. 

2 

3 



.938 
.940 



.937 
.947 
.953 



8 .965 

9 j .978 

10 I .981 

11 I .974 

Noon .953 

1 931 

.920 
.918 
.921 
.926 
.932 
.940 
.943 
.945 



10 945 

11 943 

Midnight 939 



Average. 



.885 
.883 
.880 

.881 
.886 
.894 
.906 
.912 
.911 
.906 
.891 
.869 
.855 

.a5i 

.851 

.857 
.866 
.875 
.878 
.883 
.885 
.883 
.881 



.943 



.881 



.892 
.888 
.883 
.883 
.89] 
.900 
.910 
.916 
.920 
.917 
.908 
.896 
.877 
.862 
.854 
.849 
.853 
.860 
.871 
.880 
.888 



.893 



.874 
.869 
.867 
.868 
.875 
.886 
.897 
.901 
.901 
.900 
.892 
.879 
.866 
.852 
.840 
.835 
.835 
.838 
.848 
.863 
.873 
.877 
.880 
.881 



.821 
.817 
.816 
.818 
.826 
.SSI 
.845 
.851 
.852 
.850 
.843 
.832 
.819 
.806 
.794 
.787 
.784 
.786 
.795 
.806 
.817 
.820 
.832 
.823 



.820 



.840 
.836 
.835 
.840 
.849 
.858 
.866 
.871 
.871 
.868 
.863 
.854 
.841 
.830 
.819 
.810 
.807 
.810 
.817 
.836 
.836 
.843 
.844 
.843 



.841 



.835 
.831 
.830 
.835 
.844 
.853 
.861 
.866 
.867 
.865 
.859 
.850 



.813 
.805 
.802 
.804 
.810 
.819 
.830 
.835 
.836 
.835 



.850 
.847 
.846 
.848 
.855 
.864 
.874 
.880 
.883 
.883 
.878 
.868 
.856 
.842 
.831 
.835 
.823 
.823 
.831 
.841 
.851 
.855 
.857 
.857 



.923 
.920 
.930 
.932 
.929 
.938 
.948 
.954 
.959 
.958 
.949 
.938 
.933 
.908 
.897 
.891 
.891 
.894 
.903 
.914 
.933 
.926 
.938 
.936 



.959 
.955 
.952 
.954 
.960 



.993 
.994 
.9fl3 
.985 
.969 
.951 
.937 
.931 
.929 
.933 
.939 
.946 
.954 
.957 
.961 
.960 
.958 



.959 
.9.59 
.958 
.959 
.964 
.970 
.981 
.991 
.995 
.993 
.981 
.963 
.944 
.934 
.933 
.9.36 
.941 
.9.50 
.957 
.964 
.965 
.966 
.965 
.963 



.966 
.967 
.967 
.964 
.963 
.968 
.978 
.990 
.996 
1.003 
.990 
.971 
.950 
.940 
.939 
.944 
.950 
.956 
.966 
.971 
.973 
.973 
.973 
.970, 



0.895 
0.893 
0.891 
0.893 
0.898 
0.906 
0.916 
0.934 
0.927 
0.937 
0.919 
0.905 
0.889 
0.876 
0.868 
0.865 
0.867 
0.873 
0.880 
0.888 
0.895 
0.898 
0.899 
0.897 



.853 



.924 i .959 ' .962 I .968 0.895 



Table I contains the average hourly values of the station pressure at 
Baltimore for the period of ten years from 1893 to the close of 1902. The 
values are derived from the continuous record of a Richard barograph, cor- 
rected to agree with personal observations of a mercurial barometer made 
daily at 8 a. m. and 8 p. m. Each mean hourly value for the year is based on 
over 3600 observations; hence these values may be regarded as very close 
approximations to normal averages for each hour of the day for the year. 
The station elevation has been 123 feet above mean tide since August 1, 1896. 
The average hourly pressures are also shown graphically in Figs. 1 and 2. 



mercurial barometers were read at stated hours from two to five times 
daily. Since 1893 a self-recording barograph has furnished a continu- 
ous record of the pressure conditions and changes, affording excellent 
material for an analysis of the diurnal fluctuations of the barometer. 
The rich and abundant material accumulated by the Weather Bureau 
during the past thirty-three years has been reduced and di.scussed with 



ilARYLAND WEATHER SERVICE 



33 



a view to disclosing the nature of the diurnal and annual periodic 
changes of pressure, as well as the irregular and secular changes. 



Mdt. 3 6 9 Noon 3 6 9 Mot Mot. 3 6 9 Noon 3 6 9 Mot. 

29.00+ In- 
.98 92 



i 1 






' ■ 


1 ! 










' 
















' L/^ 








/ i N 






' I 1 




' 


1 ■ ; 1 




<C-*-Li 


\-. j._^-_^. 


— _f-^^**»N^- 






\~ , 


y^ , , . "^ 


1,1 




\ Lt^ 




' M 


1 * j ; 


! >.^^ ' 


■ , , 




1 ' 1 ' 


, j 1 ■ 


' 






M 1 1 










! ; , 


: ' 








' 






' 









-90 .84 









-n — '^ 








r Arfl. 




1 
















1 ' 1 




' ' 1 


' 


_.^-*-i_ 


' j 






: 1 










' , 




: i 






"\ "^"*^" 


V"^*"" 






i i ' 


rv 1 i 


i' ' I ' 




1 1 i 1 


1 iNi 1 


X : 1 1 






I T^T' 


1 1 , 






Mi 








1 1 






' 







Inches 
• 94 



2 9 90 



1 , 


n 


"^ 






"■ 






1 1 ' 










-^n 


















1 ' 1 ' 1 


^ 




] 1 


1 1 M 






Year. - 


' 










1 




i i ' 




1 ' 






1 ' 


1 ' 








1 1 


: 








1 


1 i ' 




. , 1 I 








' 


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1 


! 1 


■ ' 






n ; ; 






' ■ > 1 




TT- 


..,__. 


t M ' ' 


; 1 ' ' 


' ' ' ■ 


1 1 i ' ' 










' , 1 








MM 


1 ' 1 






— -^ 1 ; ] 










^1.1 


1 i 


1 










! ! 




1 




■ .^! 1 


1 










M ' 


i 












: 1 1 : 


1 1 ; 


' 




■ ' 'X' 












MM 




' ' 










1 , 






Zl ' ' 


i M \ 


1 








1 


' ' M 












, 


^ ' ' ' 


. ^ 






MM 




; 


' 




i 




./ 


1 




*■ 1 i 1 


Ml. 




1 


' 


' 1 


1 




' 




\ ' ' 








^-^-^TH 


1 ^^^T- 


-T""'^ 


■ r^ 




\ 




f i l'^ 


C ' I'M 




, ' 1 


! ! j I 


I'll 


\' 


Mil' 


1 1 j/f 










dix 1 






! 




\' 


1 , I 










J ' 1 




1 1 ' ' 




; 






' ■ ;\. ' 














r~ 


■ • 1 


1 1 1 


, 1 i ' 


i 


1 








i > 








; 














1 ■ 




1 


, ^ 










ia-. J-^ 




[ 


1 














1 1 


1 ■ i j 




' : 












1 


T* 






1 
















' 








1 ' 


j 








i 






















-n 


, ' ■ , ! 


: ' : 1 1 




1 1 


; 1 


j 






j 1 


t 


1 




1 








i 1 1 1 






pi: - ^ - 




' ' 


: 








1 


t 












• ' M 
















1 



Inches 
.94- 



■86 



Mot. 3 6 9 Noon 3 6 9 Mdt. mdt. 3 6 9 Noon 3 6 9 Mot. 

29.00r In- 
•88 30.0 




j ; 1 j , 






1 1 --.„ 1 


MI 




! 1 ' 








1 ' ■ ' 






! 












1 






! I 




■ 












I 


1 y^ 


' X 




. ■ ' 








i-*''***^**- 








/^ 


' 1 . 




V ' ' ' / 


'^'V^T 




. ^ 


\^ y 








^NtO^' 


' * 








' 






' 


' 






, 


! 




1 


t. 1 . . : 


' 1 ■ ! 



Fig. 1. — Hourly Variatious of the Barometer. 

Fig. 1. The (trcinue height of the barometer at each hour of the day for the ten years 
from 1892 to 190.S is shown in the alwve diagrams for the months of January, April, July, 
Oct., and for the entire year. The height of the column of mercury which the pressure of 
the atmosphere sustained is expressed in inches and hundredths of an inch. See also Fig. 2, 
and Table I. 



34 the climate of baltimore 

The Diurxal Variations of the Barometer. 

Within the tropics, wliere C3'clonic storms are of infrequent occur- 
rence, the diurnal variation of the barometer is the most marked fea- 
ture of the barometric curve. So regular in form and distinct in out- 
line are these changes that it is possible by inspection of the curve to tell 
approximately the time of day. The amplitude of oscillation near the 
equator is about one-eighth of an inch. This amplitude decreases with 
distance from the equator but is still recognizable in the latitude of 70 
degrees. Along the parallel of Baltimore the amplitude is quite marked 
in a curve representing average hourly changes for the period of a month 
or more, but is detected only by the experienced eye in the daily curve, 
owing to the relatively large irregular changes due to the passage of the 
cyclonic storms of the middle latitudes. At times, especially in the 
summer months when tropical conditions prevail for a considerable 
period in our latitudes, the diurnal variation is very distinct for days 
at a time. (See the curve for August 7-13, 1900, on Plate II.) 

THE normal DIURXAL VARIATION AT BALTIMORE. 

The mean hourly values of barometric pressure for Baltimore are pre- 
sented in Table I for each month and for the year. The results for each 
season and for the entire year are also shown graphically in Fig. 1 on 
page 33 and Fig. 2 on page 36. In Table II the same values are ex- 
pressed in terms of departures from the average value for the entire day. 
These tables and diagrams reveal for Baltimore the characteristic double 
barometric curve so well known to the meteorologist from the results of 
analyses of observations in all parts of the world, with perhaps minor 
peculiarities due to local conditions. The fluctuations are well marked 
in all months of the year, the amplitude varying from 0.060 inch in 
August to 0.071 inch in March. In Fig. 2 the distribution of pressure 
is presented by a method not frequently employed but one which shows 
clearly and in compact form the successive changes from hour to hour 
throughout the year. Upon a system of coordinates representing the 
hours of the day and the months of the year, the curved lines of equal 
pressure are projected in such manner as to enable one to find the exact 
pressure at any hour of any month. These curved lines are sometimes 
called " isopleths.'^ For example, to find the average pressure at noon^ 



MARYLAND WEATHER SERVICE 



35 



in Ajiril, you run clown the vertical line marked noon, until the horizontal 
line marked April is intercei^ted, and find the isopleth of 29.875. This 
method enables ns also to see at a glance the chief characteristics of the 
seasonal distribution, further emphasized by differences in shading, the 
lighter shades indicating the lower pressures of the warm months and the 
darker shades the higher pressures of the colder months. 

TABLE II.-HOUKLY DEPARTCKES FROM MEAN DAILY PRESSURE. 
,In thousandths of an inchj 

Local time is 6 minutes slow. 



75th mer. time. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year 



1 A. M —.004 

2 -.005 

3 -.003- 

4 — .OOT- 

5 —.006 

.004 
.010 
.022 
.035 
.038 
.031 
.009 
-.012- 

— .023 - 

— .025- 

— .022- 

5 —.017- 

6 -.011 



10.... 
11.... 
Noon 

1.... 

2 

3.... 

4.... 



7... 

8. 



—.003- 
.000 
.002 
.003 
.000 



Midnight —.004 



.004 

.002 

-.001- 

-.002- 

.000 

.005 

.013 

.025 

.031 

.030 

.025 

.010 

-.012- 

-.026 - 

-.030- 

-.030- 

-.034- 

-.015- 

-.006- 

■.<m- 

.004! 
.002! 
.000, 



.006 

.002 

-.003 

-.003 

.005 

.014 

.024 

.030 

.034 

.031: 

.022i 

.010: 

-.009 

-.024 

-.032 

-.037i 

■.ft33 

-.026 

-.015 

-.OOfJ 

.CK)2 

.006 

.006 

.007 



.003 

.002- 

.004 

.003- 

.004; 

.015 

.026 

.030' 

.0:30 

.029 

.021; 

.OOS 

.005- 

.019- 

.031- 

.036- 

xm- 

.033- 
.023 - 
.008- 
.001 - 
.0061 
.009| 
.010; 



.001 
.003 
.004 
.003 
.006 
.017 
.025 
.031 
.032 
.030 
.023 
.012 
.001 
.014 
.026 
.033 
.036 
.034 
.025 
.014 
.003 
.000 
.002 
.002 



.001 
-.005 
.006 
.001 
.008 
.01 
.025 
.030 
.030 
.02 
.023 
.013 
.000 
.011 
.022 
.031 
.034 

.o;ii 

.(V24 
.015' 

.005!- 

.001 

.003 
.002 



.000- 


-.OftS 


-.001' 


-.004- 


.(K)6 


-.004 


-.005- 


.(K)7 


-.004 


.000- 


.005 


-.003- 


.009 


.(K»3 


.005 


.017; 


.011 


.014 


.026 


.021 


.034' 


.0311 


.(t'7 


.OiO. 


.032, 


.030 


.oa5: 


.o;30, 


.03(1 


.034 


.024! 


.035 


.035 


.0151 


.015 


.OI4I 


.003! 


.(K)3 


— .003 - 


.010- 


.011 


— .016 - 


.023- 


.023 


-.037 - 


.030- 


(138 


-.0:33- 


.033- 


0:^0 


-.0:33- 


.031 - 


.030 


— .0:30'- 


.025- 


.(t'3 


-.031:- 


.016- 


.(tl3 


-.010- 


.005 — 


.003 


-.001 - 


.000 


Am 


.003 


.(Kin 


.004 


.004 


.OOOj 


.004 


.003- 



.000 -.003 
-.004— .003 
-.007 -.004 
-.005— .003' 

.001 .002 



.008 
.020 
.034 
.0:35 
.033 
.026 
.010 
-.008- 



.008 
.019 
.029 
.033 
.031 
.019 
.001 
-.018 



-.033— .038 ■ 
-.028— .030 ■ 
-.030— .036 
-.037— . 031- 
-.030— .012 - 
-.013— .(K)5- 



.(K15 


.002 


.(H)2 


.C03 


.(K)2 


.004 


.001 


.003 


.001 


.001 



-.00-. 
-.001 
-.001 
-.(H)4 
-.005 
.000 
.010 
.032 
.028 

.0:35 

.022 
.003 
-.018 
-.038 
-.029 
-.024 
-.018 
-.012 

-.m 

.003 
.004 
.005 
.005 
.002 



.000 

.003 

-.004 

.003 

.003 

.011 

.021 

.029 

.032 

.033 

.034 

.010 

-.006 

-.019 

-.037 

.030 

-.028 

-.033 

015 

-.007 

.000 

.003 

.004 

.003 



Table II contains the average hourly departures from the normal monthly 
status pressures recorded in Table I, the values being expressed in thou- 
sandths of an inch of pressure. Figures preceded by the minus sign represent 
values below the normal for the day; those without sign represent values 
above the normal. For example, examining the column headed " year " we 
observe that: the atmospheric pressure is at or below the average for the day 
from 1 a. m. to 4 a. m., above the average from 5 a. m. to noon, falls below 
the mean again at 1 p. m., remaining below until 8 p. m., then again exceed- 
ing the mean to midnight; that the average time of the primary maximum 
for the day occurs between 9 a. m. and 10 a. m., and the primary minimum at 
4 p. m.; that a secondary maximum occurs at 11 p. m., and a secondary 
minimum occurs at 3 a. m. The maximum and minimum phases vary from 
month to month as shown in Table III and in Fig 3. 



PHASES OF THE DIURNAL OSCILLATION. 

The four principal phases are distinctly revealed in all of the normal 
curves (see Figs. 1, 2 and -1). The primary, or morning, ma.ximum 



36 



THE CLIMATE OF BALTIMORE 



occurs about 9.15 a. m., local time, on the average during the year. The 
time varies with the season. During the winter months the crest of the 
maximum wave occurs about half an hour later and during the summer 
months half an hour earlier, making a difference of about an hour be- 
tween the earliest and latest average appearance. The most variable in 
time of appearance is the primary, or afternoon, minimum. From 
January, when this phase occurs about 3 p. m., there is a steady retard- 
ation in the time of occurrence of the minimum to about 5 p. m. in 
May, where it remains until August, when the time again moves for- 
ward to the winter minimum at 3 p. m. The average occurrence of the 



9 '0 11 NOON 12345678910 




opleths of Hourly Pressure. 



Fig. 2 shows the average distribution of atmospheric pressure throughout the day and 
year, based on observations of ten years of hourly readings of the barograph. The upper 
marginal figures indicate the hour of the day, the marginal letters indicate the months of the 
year. The line enclosing the area of lightest shading defines the time of day and month 
"when the barometer is normally lowest ; increase in the intensity of shaded areas shows an 
increase In the height of the barometer. The diagram shows that the barometer is lowest 
at about 5 p. m. in the month of May ; that it is highest at 10 a. m. in the month of December. 
The curved lines show the hours of the day and the months of the year when the barometer 
readings are, on the average, equal; these lines are called chrono isobars, or isopleths of 
pressure. The dotted lines marked S.R. and S.S. show the time of sunrise and sunset. See 
also Table I. 



minimum for the year is about 4.15 p. m., local time. The secondary, 
or night, maximum occurs about 11 p. m., the time of occurrence being 
fairly constant but varying from 10 p. m. in December to 11.30 p. m. 
in the summer months. The secondary, or night, minimum occurs 
quite uniformly at 3 a. m. from May to December, with extreme limits 



MARYLAXD WEATHER SERVICE 



of 3.30 a. m. in January and February and 2.30 a. m. in April. The 
most marked feature of the variations in time of occurrence of the dif- 
ferent phases is the gradual increase in the time interval between the 



TABLE III.— PRESSURE PHASES. 
(75th meridian time.— Local time is 6 minutes slow. 



January 1 

February I 

March ] 

April 

May 

June 

July 

Aug'uet 

Sejjtember 

October 1 

November I 2 

December 



Morning- 
maximum. 



8 9 10 



6..i 

6 2 
4 5 

7 3 
6 4 



Sums . 



Annual average 



9:30 
9:30 
9:00 
9:00 
8:30 
8:30 
9:00 
9:30 
9:00 
9:00 
9:00 
10:00 




210 3 9:0 



Afternoon 
minimum. 



p.m. 



2 3 



533 



4 5 



4 
8 
9 
4 

3^ 

7 1 



.. 3:00 

.. 3:00 

.. 4:00 

. . 4:30 

2 5:00 
.. 5:00 

3 5:00 
5 5:30 
1 4:30 

.. 4:00 

.. 3:00 

.. 3:00 



354011 



4 6 1 4:08 



Night 
maximum. 



8 9 101112 



I I 



■iViV 



4 8294327 17 



> ft 
< 



11:00 
10:30 
IhOO 
11:30 
11:30 
11:00 
11:30 
IIMO 
11:0U 
11:00 
10:00 
10:00 



10:55 



Night 
minimum. 



Mid- 
n't. 



12 12 3 4 



6| 2. 
1. 
3. 

5 2. 

1..I 



1 1129582915 



1 9 3! 1 



3:30 
3:30 
3:00 
2:30 
3:00 
3:00 
3:00 
3:00 
3:00 
3:00 
3:00 
3:00 



3:05 



Table III shows the average hour of occurrence of the daily maximum and 
minimum station pressure for each month and for the year during a period 
of ten years, and also the extent to which the times of occurrence of these 
phases have varied from the average time. For example, examining the 
afternoon minimum phase, the most variable of all, we find that in 10 years 
it occurred in January 7 times at 3 p. m., 4 times at 4 p. m., and once at 5 
p. m.: and that the average time of occurrence for the year is about 3 p. m.; 
that in August it occurred at 5 p. m. 6 times and at 6 p. m. 5 times; and as 
an average time we have about 5.30 p. m., etc. The average values are also 
shown graphically in Fig. 3. 



primary maximum and primary minimum with the approach of sum- 
mer, from five hours and a half in the winter months to eight hours 
and a half in May and June. This increasing interval is due mostly to 
variations in the time of occurrence of the primary minimum. These 
phases are shown in detail in the following table and in Fig. 3. 



38 



THE CLIMATE OF BALTIMORE 



INTERVALS BETWEEN' PRINCIPAL PHASES OK PRESSURE. 

(Expressed in Hours and Minutes.) 



Intervals 
between. 


Jan. 


Feb. 


Mar. 


Apr. 


May June 


i 
July Aug. [Sept. 


Oct. 


Nov. 


Dec. 


Year 


A. Primary 
max. and min. 

B. Secondary 
max. and min. 


5:30 
4:30 


5:30 
5:00 


7:00 
4:00 


7:30 
3:00 


8:30 
3:30 


8:30 
4:00 


8:00 
3:30 


8:00 
3:30 


7:30 
4:00 


7:00 
4:00 


6:00 
5:00 


5:00 
5:00 


7:00 
4:05 



FMAMJJASOND 





■ 


'~ 










ill: ' ' , 1 




-- 


■^ 














: 1 ' 1 Mi , 1 




V 










1 














s 




> 


































1 1 ' ; 


































\ 






















































































































































































/ 










s. 


-* 


y 








^ 








;a 


' [ 




^^ 


_ 


> 






























S 




1 








































v, 


k-j 


















































































































































































































































































































































































































^ 


■" 


V 


























l^ 
















'V 






rf 











_. 




^ 






























, 








p 






























S 






3PM 


1 ^ 


/ 


































s 


1^ 










































r"~ 
























































































































1 




















































































h-j ' ' ' 


















































































• 










































































1 1 1 






































A 


9 A. M. - 


i; 






L 




_ 


. 


.. 








t^'j 






. 




^-1 














> 


SJ 






,<" 


-? 










"n 
















































































































































































6A M.- 




















































































































































































































































fr/ 


1 




3AM- 


" 


N 






































! 






V 


r' 


































































i 1 




^ i 














' i 1 




r : I 



3 A. M. 



9P.M. 



3 P. M. 



- 9A.M. 



6 A. M. 



Fig. 3. — Principal Phases of Diurnal Oscillation of Pressure. 

Fig. 3 indicates the time of occurrence of the maximum and minimum points reached by 
the barometer in the diurnal oscillation. The upper marginal letters represent the months 
of the year ; the figures indicate the hours of the day. The curved lines represent i-espec- 
tively (a) the time of occurrence of the secondary maximum; {h) the primary minimum ; 
(c) the primary maximum ; (d) the secondary minimum. See also Table III. 



MARYLAND WEATHER SERVICE 



39 



AMPLITUDE OF OSCILLATIOX. 

(In Thousandths of an Inch.) 



Jan. Feb. . Mar. I Apr. , May June 



A. Diurnal 

Amplitude. 63 61 71 66 

B. Nocturnal 

Amplitude. 9 6 10 14 



ft* 64 
6 9 



July, Aug. Sept.' Oct. | Nov. Dec. 



60 68 65 63 64 

1 

11 8 1 9 8 10 



Tear 



63 



TABLE IV.-HOL'RLY VARIATIONS OF PRESSURE ON CLEAR AND ON 
CLOUDY DAYS. 

(Expressed In thousandths of an inch as departures from the daily average.) 





Clear days in 


To'al 

(90 days 

Departure. 


Cloudy days in 


Total 

(90 days) 

Departure. 


75th Meridian 
Time. 


Jan. and 

Feb. 

(60 days) 

Departure. 


July 

(.30 davs) 

Departure. 


Winter 

(60 days) 

Departure. 


Summer 

(30 days) 

Departure. 


Midnight . ... 

1 

2 

3 

4 

5 

6 


-.053 
-.037 
-.029 
-.021 
-.018 
-.005 
-.004 
+ .022 
+ .041 
-.056 
+ .063 
-r.058 
-r.040 
+ .019 
+ .003 
-.008 
-.013 

— .014 
-.014 
-.012 
-.015 

— .019 
-.026 
-.036 
-.051 


-.006 
-.006 
-.007 
-.008 • 
-.002 
+ .009 
+ .018 
+ .028 
+ .034 
+ .0.34 
+ .a32 
+ .028 
+ .023 
+ .009 
-.004 
-.017 
-.026 
-.0.38 
-.035 

-.0:31 

-.025 
-.016 
-.013 
-.008 
-.006 


-.028 

— .022 
-.018 
-.014 
-.010 
+ .002 
+ .011 
+ .025 
+ .038 
+ .045 
+ .048 
+ .043 
+ .032 
+ .014 

.000 

— .013 

— .020 
-.024 

— .024 
-.022 

— .020 
-.018 
-.020 
-.022 
-.028 


-.003 
+ .010 
+ .013 
+ .012 
+ .006 
-^.006 
-.008 
+ .015 


-.001 
-.004 
-.011 
-.014 
-.011 
-.004 
+ .011 
+ .023 


.000 
+ .003 
+ .001 
-.001 
—.002 
+ .001 
+ .010 




+ .019 


8 


-.024 +.029 


4-. 026 


9 

10 

11 

Noon 

1 

2 

3 

4 

5 

6 


+ .032 i +.034 1 +.033 
-+-.034 +.030 +.033 
+ .024 +.027 -J-.026 
+ .U04 +.020 , -.012 
-.016 +.005 -.006 
-.025 -.003 -.014 
—.027 —.011 -.019 
—.027 -.017 -.022 
-.023 -.020 -.022 
-.025 -.017 -.021 
—.011 —.013 —.013 


8 


— .006 —.004 —.005 


9 


— .003 +.001 —.001 


10 

11 

Midnight 


-.003 -^.003 .000 
+ .003 +.001 +.002 
+ .001 +.001 .000 



Table IV shows the amount of the diurnal variation of pressure on clear 
days as compared with cloudy days in order to detect any difference due 
to cloudiness. For this purpose CO clear days in January and February and 
?,0 clear days in July were chosen, to be compared with CO cloudy days in 
January and Feliruary and HO cloudy days in July. The effect of irregular 
\ariations of the barometer due to the passage of storms was first eliminated 
from the actual means. The results are also graphically shown in Fig. 4. 

In individual months the time of occurrence of these phases varies 
considerably from the average times for the entire ten years, as may be 
seen in Table III, which shows the frequency of occurrence of the differ- 
ent phases for each month for the ten -year period. 



40 



THE CLIMATE OF BALTIMORE 



DIURNAL VARIATIONS OF PRESSURE ON CLEAR AND CLOUDY DAYS. 

In Tables I and II and Figs. 1 and 2 the average distribution of 
pressure is shown for all conditions of the weather during a period of 
ten years. In order to determine the effect, if any, of cloudiness upon 
the oscillation of pressure, selection was made of 60 clear days in Janu- 
ary and February and 30 in July to be compared with a like number of 



MDT 
+ .05 



9 Noon 



-.0 5 
••-.05 



-.05 
♦.05 





y"" ^^ 


7 \ 




j/ .'' ■* X, 


•' — ~ - . ^'^ ^ S 


j' ■ _i,_.j_^_:^Jl2_. -,,_.,.-,__,.- — .».— -^«»-. --!.--. .ii. J." *. ir ,^ f'> 


'- " ^^^ "^ ""-^ XL ^■^j^r'^X- 




xS ^"^i 


/ s 








i I 




""" ' : .^sf--^ :" "'-^-^ 




Tfy' ; ..^>s^ ,„ . v.--^-^-v^ 




~'--+' "*?.-'—•,_ "'^ ^ — -"''^ 1 


>^ ^jfyf"^ 


"*-._ --"'' 










' *" " ~'^5» 1 


>;:, — H^ ^> 


^^P 1 "'nj^ V 




-.. . .>^ > S < ^. ..-.., 


1 '•i-jfj;^ ■' "■" -■ ■ ■ - TT-- '-S^ ' -1- \ i ,-r-- 


\ \ ^ 1 ^"Hn ' •!, "«.._ 1 '•'f\ -t— — 1 1 


-j-^ r "t--sii-+«-— T "*■-«, 


1 . 1 




i i ' 1 i 





-.05 
ud5 



/) 



-.05 
+.05 



Fig. 4. — Diurnal Variations of Pressure on Clear and on Cloudy Days. 
Clear. Cloudy. 

Fig. 4 shows the hourly chanKes of the barometer on selected da3'S approximately similar 
in all respects excepting as to tlie amount of cloudiness. The cootinuous lines show the 
movements of the barometer on clear days and the dotted lines on cloudy days, for (a) win- 
ter months, Q>) summer months, and (c) for the year. The hourly heights of the barometer 
are expressed in hundredths of an inch, as departures from the average height for the entire 
day. See also Table IV. 

cloudy days in the same months, as far as possible. The computed 
variations are shown in tabular form in Table IV, and graphically in 
Fig. 4, on page 40, after first eliminating the effect of irregular fluc- 
tuations of the barometer due to passing cyclonic disturbances. The 
primary maximum and minimum phases differ but little from those of 



MARYLAND WEATHER SERVICE 41 

the normal curve, although the amplitude on clear days is somewhat 
exaggerated, especially so during the winter months. The curves for 
the normal and for the cloudy days coincide very closely for the sum- 
mer months and for the year, but diverge in the early morning hours 
of the winter months. The most striking feature of the curve for 
totally clear days is the wide divergence from the normal curve in win- 
ter during the night and early morning hours. 

THE DIURXAL BAROMETOIC WAVE. 

The diurnal variations of the barometer described in the preceding 
paragraphs are not simply of local occurrence but are part of a general 
phenomenon extending over the greater portion of the earth's surface. 
The maximum and minimum phases pointed out occur in all localities 
at approximately the same hours of local time. As stated above, this 
pressure wave, as it may be called, has its greatest development in or 
near the equatorial belt, and diminishes in amplitude with distance 
north and south of the equator. It has some resemblance to a double 
atmospheric wave passing completely round the earth from east to west 
every twenty-four hours, having a velocity at the equator of about one 
thousand miles per hour. By plotting upon a map of tlic world the 
departures from the normal daily pressure for successive hours of the 
day at a large number of stations uniformly distributed over the north- 
ern and southern hemispheres, and joining such stations as have equal 
departures of pressure for the same hour, we have presented to us four 
systems of pressure-distribution, consisting of two areas of low pressure 
and two areas of high pressure. These systems completely encircle the 
globe and closely resemble in form the cyclonic and anticyclonic systems 
of the middle latitudes, but differ from them, among other things, in 
covering an area vastly greater, and in moving in the opposite direction. 
The diurnal fluctuations of the barometer are the local evidence of this 
vast double atmospheric wave passing round the globe daily. The west- 
ward propagation of these waves near the equator is represented in Fig. 5 
on page 42 ; the curve shows the time of occurrence of the different 
phases of the double wave, its amplitude, and the direction of propaga- 
tion along the path of greatest development. The character of these 
waves is further indicated in fhe diagrams of Plato T, in which the succes- 



42 



THE CLIMATE OF BALTIMORE 



sive areas of high and low pressure are exhibited at intervals of two 
hours in passing from east to west across the North and South American 
continents/ 

This double atmospheric wave, or tide, is so intimately associated 
with the apparent diurnal movements of the sun that the conclusion is 
almost irresistible that the pressure changes are due primarily to changes 
of temperature. This relationship has not yet been satisfactorily demon- 
strated to be that of direct cause and effect, but there seems to be a gen- 
eral consensus of opinion that the primary maximum and the primary 
minimum phases of pressure are direct effects of the sun's heat. The 







N. 




t 


om 3am M^dn 9 


pm 


, 6pr, 


3om Noon 9am t 


am 


























*040 
t020 






















fO-IO 
tO?0 












/ 




\ 














/ 




\ 


















/ 




\ 












W.o 

-020 
-040 
















/ 




\ 


oE 

-0?0 

-040 


\ ^ 










/ 












V 


/ 


/ 












V 


y 












s. 


"""1 



Fig. 5. — The Diurnal Barometric Wave. 

Fi^. 5 shows the direction of movement of the diurnal barometric wave, from east to west 
around the globe ; also the local time at which the crests and the hollows of the wave pass 
over any locality along the path of the greatest development of the wave, near the equator. 
The extent of the diurnal rise and fall of the barometer is shown by the figures to the right 
and left of the diagram, which express the departures above and below the normal height 
for the day, in thousandths of an inch of mercury. See also Plate I. 

theory advanced many years ago to account for the chief maximum and 
minimum phases seems plausible. At the time of day, between 9 a. m. 
and 10 a. m., when the atmosphere is being warmed most rapidly and 
the tendency of the air to rise in consequence is greatest, the upper and 
colder layers impede this upward movment, resulting in a temporarily 
increased tension at the surface of the earth. When this tension is re- 
lieved the barometer begins to fall, reaching its lowest point about the 



'Fassig, O. L. The Daily Barometric Wave. BulL No. 31, U. S. Weather 
Bureau. 8°. Washington. D. C, 1902, pp. 62-65, 12 pis. 




rHK DHUNAI. IIAKIIMETKIC W A\ !•:. 
(75T]1 MERIDIAN TIMK.) 

?:in pressure of tin- day arc expressed in Ihonsaiidtlis of an inch of mercury.) 



MARYLAXD WEATHER SERVICE 43 

middle of the afternoon when the upward movement of the warm air 
may be assumed to be least impeded. In this connection, Fig. 17, on 
page 76, is significant, showing the average hourly rate of change of 
temperature for the year, compared with the curve representing the 
average hourly variation of pressure for the year. 

As has already been stated above, the pressure-wave attains its greatest 
amplitude in the equatorial belt where the diurnal temperature changes 
are greatest, and over the continental masses north and south of the 
equator where the diurnal range of temperature is most marked. (See 
Plate I.) 

According to Dr. Hann,^ in seeking an explanation of the diurnal 
variations of the barometer: "We had better deal with the action of 
the sun on the upper strata of the atmosphere and treat this as the 
principal cause. The actinometrical observations show us that these 
upper strata absorb a considerable amount of heat. The diurnal heat- 
ing action of the sun on the upper strata would harmonize far better 
with the general uniformity of the daily barometric oscillation along the 
different parallels of latitude as well as with its general independence 
of weather. We need not quite exclude local influences, but these seem 
to be more of a secondary character." This view is also held by Lord 
Kelvin, who seems to have been the first to suggest this explanation. 

CORRECTIONS FOR REDUCTION TO TRUE MEAN PRESSURE. 

The determination of the daily mean barometric pressure based on 
24-hourly observations for ten years enables us to apply the necessary 
corrections to any given combination of daily observations in order to 
obtain a true mean. The following table contains the corrections for 
each month and for the year which must be applied to the series of 
observations made according to any of the five systems most frequently 
employed in barometric observations in this country. The average of 
the three observations made at 7 a. m., 2 p. m., and 9 p. m. approaches 
most nearly the true 24-hourly mean for the day, when the 9 p. m. ob- 
servation is given double weight. 

'Hann, J. The Theory of the Daily Barometric Oscillation. Quart. Journ. 
Roy. Met. Soc, London, 1899, p. 40. 
4 



44 



THE CLIMATE OF BALTIMORE 



CORRECTIONS FOR DIURNAL VARIATIONS OF THE BAROMETER. 

(In Thousandths of an Inch.) 



Hours of 
observation. 



7A.+2P.+9P. 



7A.+2P.-*-2(9P.) 



7A.+3P.+11P. 



10 A.+IO P. 



8A.+8P. 



Jan. 


Eeb. 


Mar. 


Apr. 


May 


June 


+4 


+4 


-1 


-3 


-3 


-3 


+2 


+2 


—1 


—2 


-1 


-1 


+5 


+5 


+1 


-1 





-2 


-20 


-17 


-18 


-18 


-15 


-14 


-11 


-11 


-12 


-11 


-8 


-8 



July 


Aug. 


Sept. 


Oct, 


Nov. 


Dec. 


Year 


-4 


-3 


-2 


+1 


+2 


+5 


-1 


2 


—2 


-2 


+2 


+1 


+2 





-2 


-1 





+2 


+3 


+5 


+1 


—15 


-16 


-18 


-18 


-18 


-20 


-18 


—1 


-8 


—10 


-14 


-16 


-12 


-11 



The Annual March of Atmospheric Pressure. 
In order to determine the changes of pressure from day to day during 
the course of the year, the daily averages of the Baltimore observations 
covering a period of 30 years were reduced to what may be called normal 
values for each day of the year. In obtaining these normals the sea-level 
values were employed, but corrections for diurnal variation were not 
applied. The results are shown in Table Y on page 45 and graphically 
by means of curve (d) of Plate III. This curve shows a fall in pres- 
sure from month to month from January to May. During May, June 
and July the pressure remains fairly uniform, followed by a compara- 
tively rapid rise in August and September. During October there is a 
slight fall followed by a rise to January. The curve of daily changes 
does not, however, show a steady rise and fall from season to season. 



J 






F 






[V 






A 






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29.84 



Fig. 6. — Mean Monthly Atmospheric Pressure. (See Table VI). 



MARYLAND WEATHER SERVICE. 



VOLUME 2, PLATE II. 




MARYLAXD WEATHER SERVICE 



45 



The progression is marked by successive waves varying in period from 
two to eight or ten days' duration, which persist even in the average 
dailv values for 30 vears. The variation from dav to dav is smallest 



TABLE V.-MEAN DAILY BAROMETRIC PRESSURE, 

Reduced to sea level. 

[In inches and hundredths.] 



29.00 Inches. 



Date. 



9.. 
10.. 
11.. 
12. 
13.. 
14.. 
15.. 
16.. 
17.. 
18 
19.. 
20 . 
21.. 
22. . 

h'.'. 

24.. 
25.. 
26.. 
27. . 
2S'.'. 
29.. 
30.. 
31.. 



Average 



Amplitude. 



Jan. Feb. 



1.14 



.19 



1.14 
1.21 
1.06 
1.10 
1.22 
1.17 
1.14 
1.10 
1.11 
1.17 
1.13 
1.08 
1.04 
1.14 
1.16 
1.12 
1.12 
1.06 
1.08 
1.08 
1.03 
1.04 
1.08 
1.16 
1.05 
1.08 
1.15 
1.14 
.96 



1.11 



.18 



Mar. : Apr. 



1.07 
1.03 
1.04 
1.06 
1.16 
1.13 
1.09 
1.11 
1.02 
1.05 
1.08 
1.01 
1.03 
1.10 
1.09 
1.04 
1.08 
1.05 
.96 
.95 
1.00 
1.02 
1.03 
1.09 
1.09 
.99 
.97 
.97 
1.02 
1.04 
1.02 



1.04 



1.03 
1.00 
1.00 
.99 
1.02 
1.04 
1.04 
1.05 
l.OI 
1.00 
1.03 
1.08 
1.03 
.98 
.96 
1.03 
1.07 
1.04 
1.02 
1.03 
1.08 
1.08 
1.03 
1.03 
1.03 
1.03 
1.03 
.96 
.96 
1.00 



1.02 



.12 



May 



1.00 

.98 

1.02 

1.01 

.99 

.99 

1.05 

1.03 

1.01 

1.01 

1.01 

1.00 

.99 

1.01 

.99 

1.00 

1.04 

1.03 

.98 

.99 

1.00 

1.01 

1.03 

1.01 

.97 

.97 

.94 

.96 

1.01 

.99 



1.00 



June 



1.01 

1.03 

1.01 

.99 

.99 

l.dO 

1.00 

1.00 

.98 

1.00 

1.01 

1.01 

1.01 

1.03 

1.03 

1.03 

.97 

.97 

.98 

.99 

.96 

.95 

1.00 

1.00 

.98 

.98 

.99 

.95 

.97 

1.02 



0.99 



.08 



July Aug. Sept. Oct. Nov. Dec. 



1.04 

1.03 

1.00 

.99 

.99 

1.03 

1.03 

.98 

.96 

.99 

1.00 

1.00 

.96 

.98 

.96 

.96 

.99 

.99 

1.00 

1.01 

1.01 

1.01 

1.02 

1.03 

1.00 

.97 

.98 

1.00 

.98 

.98 

1.00 



0.99 



1.00 
1.00 

.99 
1.03 
1.04 
1.03 
1.01 
1.01 
1.01 

.99 
1.00 
1.00 
1.00 

.99 
1.02 
1.03 
1.03 
1.00 
1.00 
1.02 

.99 
1.01 
1.03 
1.03 
1.01 
1.05 
1.06 
1.07 
1.04 
1.02 
1.05 



1.03 



.08 



1.09 
1.10 
1.08 
1.08 
1.09 
1.08 
1.08 
1.10 
1.12 
1.13 
1.11 
1.08 
1.07 
1.12 
1.10 
1.04 
1.06 
1.08 
1.02 
1.07 
1.14 
l.!3 
1.09 
1.13 
1.12 
1.07 
1.09 
1.12 
1.09 
1.10 



1.09 



1.11 
1.13 
1.11 

1.04 
1.05 
1.05 
1.09 
1.09 
1.12 
1.11 
1.11 
1.11 
1.08 
1.07 
1.13 
1.11 
1.11 
1.11 
1.11 
1.11 
1.13 
1.13 
1.07 
1.12 
1.14 
1.08 
1.04 
1.07 
1.05 
1.09 
1.06 



1.09 



1.11 
1.10 
1.13 
1.17 
1.15 
1.17 
1.15 
1.08 
1.06 
1.05 
1.09 
1.13 
1.13 
1.12 
1.10 
1.20 
1.18 
1.13 
1.04 
1.09 
1.13 
1.14 
1.06 
1.12 
1.12 
1.11 
1.11 
1.13 
1.12 
1.17 



1.12 



.10 



1.16 
1.15 
1.13 
1.11 
1.08 
1.10 
1.10 
1 14 
1.16 
1.13 
1.10 
1.13 
1.09 
1.10 
1.13 
1.16 
I.IT 
1.15 
1 22 
1.22 
1.17 
1.15 
1.15 
1.16 
1.15 
1.05 
1.06 
1.17 
1.11 
1.17 
1.17 



1.14 



.17 



Average for the year 30.063 inches. 



Average amplitude 0.05 inches. 



Table V shows the average sea-level barometric pressure for each day of the 
year. The period of observation covers the 30 years from 1871-1900. The 
daily mean is based on three readings of the mercurial barometer at about 

7 a. m., 3 p. m. and 11 p. m., from 1871 to June 1888, and on two readings at 

8 a. m. and 8 p. ra., from July 1888 to 1900. The correction for diurnal var- 
iation has not been applied, but this is extremely small for the observations 
made at 7 a. m., 3 p. m., and 11 p. m., and about -f .01 inch for the series of 
readings at 8 a. m. and 8 p. m. "The number 29.00 should be added to each 
of the figures in the body of the table. The monthly range of the mean daily 
pressure is indicated in the last line of the table. The figures of this table 
are also represented graphically in curve D of Plate 3. 



46 



THE CLIMATE OF BALTIMORE 



M 1875 




Fig. 7. — "Variations in the Mean Monthly Pressure (Expressed as Departures from 
the Normal Values for the Month, ia Thousandths of an Inch of Mercury). See 
Table YIL 



MARYLAND WEATHER SERVICE 47 

during the summer months when it is generally less than 0.05 inch, 
and greatest in the winter months, when it rises to 0.10 inch, and, occa- 
sionally, to 0.15 inch. To what extent these irregular interdiurnal 
variations would be eliminated in a longer series of observations is a 
matter of conjecture. To a marked extent at least they are probably 
persistent and due to a periodic recurrence of certain types of weather 
at certain seasons of the year. 

Of special interest is the comparatively rapid rise in pressure from 
August to September, and the arrested upward movement in October, 
more clearly shown in Fig. 6, constructed from monthly averages, than 
in the serrated curve of daily means. 

The barometric waves of short period vary greatly in length and are 
not generally sharply defined, but in most instances they extend over 
a period of three and a half to four days, and are accompanied by in- 
verse variations of temperature as is clearly shown in the temperature 
and pressure curves of Plate III. The individual features of these 
waves are shown in Plate II, in which actual tracings of the baro- 
graph are reproduced as representative types for the different seasons 
of the 3^ear. The great variability of barometric conditions in the 
winter months and the comparatively uniform conditions in the sum- 
mer months are here shown in strong contrast. The curve representing 
the conditions for the week ending August 13th, 1900, is almost entirely 
free from irregular or non-periodic fluctuations, permitting the diurnal 
variations to be plainly recognized. 

AVERAGE MONTHLY AND ANNUAL PRESSURE. 

In an elaborate report on barometry,' Professor Bigelow has discussed 
in detail the reduction of barometric observations at Weather Bureau 
stations in the United States. In this report all observations from 1873 
to 1899 have been reduced to the epoch of January 1, 1900. During 
this long period several different methods of reduction had been em- 
ployed, resulting in series of observations not strictly comparable. In 
order to obtain comparable values all reduction^ were recomputed and 

' Bigelow, F. H. The Reduction of Barometric Pressure Observations at 
Stations of the United States Weather Bureau. Vol. II of the Report of the 
Chief of the Weather Bureau for 1900. 



48 



THE CLIMATE OF BALTIMORE 



TABLE VI.-MEAN MONTHLY STATION PRESSURE REDUCED TO THE WEATHER 
BUREAU SrSTEM FOR THE EPOCH JANUARY 1, 1900. 

[Inches and thousandths.] 

Lat. 39° 18' N., Long. 76° 3~'=5 hrs., 6 m. W. of Gr. Elevation above mean sea level 123.3 

feet. Gravity corr. — .01.5. 
29.000 inches. 



A^ear. j Jan. Feb. Mar. Apr. 



May June 



1873 ! 0.971 0.886! 0.8841 0.808' 0.866 0.859 

1874 ! 1.053 1.0:il .879 .8951 .796 

1875 1.083 .986 .947 .833 .835; .874 



1876. 
1877. 
1878. 
1879. 
1880. 

1881. 

1882. 
1883. 
1884. 
1885. 

1886. 

1887. 

1888 

1889. 

1890. 

1891. 
1892 . 
1893. 
1894. 
1895. 

1896. 
1897. 
1898. 
1899. 
1900. 

1901 . 

1902 . 
1903. 



Mean (1873-99). 



Corr. for sea- { 
level ) 



1.044 

1.042 

.963 

.963 

LOGO 

1.023 
1.048 
1.075 
1.025 
1.006 

.928 
.934 

1.070 
.912 

1.080 

.921 
.917 
.847 
1.023 
.909 

1.034 
1.009 

.895 
1.032 

.951 



.995 



.127 



1.014 
.972 
.831 
.974 
.993 

1.063 
1.025 
1.155 

.958 



.957 
1.080 

.958 
1.026 

.956 

.954 
.993 
.996 



.764 
.935 
.969 

.918 
.896 



.762 
.947 



.968 



.137 



.918 
.974 
.841 
.980 
.943 

.642 
.983 
.873 
.909 
.922 

.819 
.852 
.942 

.783 
.908 

.949 

.876 
.920 
.963 

.874 

.864] 

.938 

1.073 

.8161 



.855 



.851 



.809 
.891 



.907 
.889 
.756 
.906 



.870 

.983 
.819 

.965 

.874 
.943 
.891 

.895 



.9811 
.960 

.828, 
.939 

.884 

.8011 
.803 
.761 



.938 .868 
.884! .865 



.796: 

.939 
.929 

.907 
.879 
.839 
.824 
.814 

.78l! 
.884 
.843 
.816 
.819 

.9011 
.838 
.759 
.793 
.933 
I 
.877 
.831 
.793 
.913 
.840 

.726 
.894 
.994 



.853 



.773 
.783 
.841 
.935 

.877 

.839 
.864 
.801 



.920 

.839 
.823 
.863 
.903 
.831 

.858 

.783 
.792 



.899, .877 .852 .851 



.136 .133 .133 .139 



Mean, sea-level 1.122 1.105 1.025 1.010 .985 .990 



July Aug. Sept.' Oct. Nov. 



Dec. 



0.888 0.916' 0.959! 0.934' 0.865 1.039 
.871 .884 .940' .981 1.056 1.U45 
.838 .873, .934' .900 .993 .925 



.877i 
.830 
.811 
.848 
.853 

I 
.817 
.894 
.856 
.744 
.833 

I 
.800 
.830 
.879 
.838 
.880 



.934 
.834 
.870 
.840 



.814 
.899 
.839 
.856 

.818 
.871' 



.8341 
.761 
.831 

.918: 

.894! 
.9001 
.905} 
.9161 

.833, 

.8411 
.847! 
.859 1 
.930, 
.878 

.858 

.865 1 
.834 1 

.881 
.838; 

.901 
.843 

.873 
.831 

.895 



.850 
.953! 

.9771 

1.013 

.918, 



.906 
.935 



.857 



1.043 

.984' 



l.OlO 
1.1171 



.928 
1.077 

.911 
1.0.50 

.928 



.941' 1.013 1.065! 1.037 

.938, .964' 1.053; 1.015 

.9.34 1.036 1.0.53 1.004 

.968 1.016 .964| 1.047 

.936 .8711 .830 .911 



.984! 1.028! 

.976 .9101 

.918 .848 

.881 .886: 

.971! .771! 



.898 1.004 

.953, .996 

.992! .941 

.932 1.022 

.936: .928 



1.015' 

1.0241 

.910 

.949 

.9211 

.895! 
1.006 
.9421 
.9211 
.941! 



.955, 1.036 

.895' .938 

.959! .980' 

.862' .976! 

.914 1.033; 



.910 
.990 



1.053 
1.029, 



1.063' 
.974 
.953 
.926 
.918 



.883 .931 1.033 .918 
.833 .904 .9411 .951 
.855 1.009 .9171 .964 



1.054 

.940 

1.004 

1.000 

.983 

1.053 
.947 
.928 
.962 
.966 

.950 
.971 



Year 



.850 .869 .945 .942 .976, .988 29.917 

1 : 1 I 



.130 .136 .129 .138 .134 .13" 



.980' 1.005 1.075' 1.080, 1.110 1.125 30.050 



.133 



Table VI presents the average monthly and annual station pressures for 
each month and year from 1873 to 1899, as recorded in Professor Bigelow's 
Report on Barometry, wjth the addition of values for 1900 to 1903. All ob- 
servations used in this table were reduced to the same plane (123 feet above 
mean tide), to the true mean of 24 hourly observations 'and corrected for the 
force of gravity at the Station. The values in the footings of the table 
are Professor Bigelow's " normals " for the period 1873 to 1899. 



MARYLAND WEATHER SERVICE 



49 



uniform corrections for gravity and for diurnal variation were applied 
by Professor Bigelow. The corrected monthly and annual means for 
Baltimore as given by Professor Bigelow are reproduced in Table VI 
on page -iS, with the additional values for the years 1900 to 1903, simi- 
larly reduced. For the methods employed in the reduction of observa- 

TABLE VII.-DEPARTURES FROM AVERAGE STATION PRESSURE. 
[In thousandths of an inch.] 



Jan. 



Feb. Mar. 



Apr. 



May 



1873 !— .034!— .082— .015— .069 +.014 

1874 ! + .058 +.053 -.030+. 018 -.05B 

1875 | + .088| + .018| + .048-.044-.017 

1876 ' + .049 +.046 +.019 +.011 +.076 

1877 +.047 +.004 +.075 -.036 +.032 

1878 -.033 —.137 -.058 —.188 —.0,56 

1879 -.032 +.006 +.081 -.068 +.087 

1880 I + .065 +.025 +.044, + .014 +.077 

1881 ' + .028+.095— .2.571— .091 +.055 

1882 +.053 +.057 +.084 +.030 +.027 

1883 I + .080+.187— .026+.012— .013 

1884 I + .030— .010 + .010|— .121 —.028 

1885 + .0U|— .093: + .023 + .029 —.038 



1886. 
1887. 



1889.. 
1890.. 



1891. 
1892. 
1893. 
1894. 
1895. 

1896. 
1897. 
1898. 
1899. 
1900. 

1901. 
1902. 
1903. 



.— .067 -.011 -.080 +.072— .071 
. I— .071 +.112— .047— .007 +.032 
. +.0751— .010 +.043 +.106— .010 
. — .083' + . 058— .116]— .0.58— .03« 
. +.085— .0131 + .009 +.088— .033 



-.074 -.014 +.050 
-.078| + .024-.023 
-.148 +.028, + .021 
+ .028 +.018 +.064 
.^.086,— .089 -.026 



-.003' + .049 
+ .065— .014 
+ .014 -.093 
+ .018— .060 
—.008+. 071 



June 



+ .008 
—.023 
+ .023 

+ .017 
+ .014 

— .063 
+ .002 
+ .014 

-.078 

— .068 

— .010 
+ .084 
+ .026 

—.022 
+ .013 
-.050 
+ .031 
-.006 

-.024 
-.004 
+ .015 
+ .011 
+ .069 



+ .038 
+ .021 
-.012 



+ .027 +.067 — .0P6[— .036 

— .020— .045 +.006| — .007 

.039! -.108 +.031 —.040 

.002— .038 +.066!+. 101 

+ .002 +.049 —.028, + .042 



Means. 



Dept. 



'-.0391— .204 —.035 +.104 +.025— .013 
+ .014 — .0:33 +.039 J-.OKi -.031 -.028 
— .100 +.001 +.174 -.049 — .059 +.01S 
+ .0;j7i-.0.50— .OKS +.062 +.0<;0l + .O51 
-.044 -.072 —.011 +.0371 -.012 -.020 



.107 
.020 
.129 



-.140'-. 113 

— .206— .044 

— .021 +.169 



.078 



.076— .126 
.074' + .042 
.116 +.143 



.852 



+ .00' 
—.069 
-.059 



.851 



051 —.018 —.040 -.065 -.066 -.067 



July 



Aug. Sept 



+ .047 
+ .015 
+ .004 



Oct. 



+ .013 -.008 
-.006; + . 039 
-.023 -.042 



.033' + .025 
+ .044 +.031 

+ .006 +.036 

— .106; + .047 
-.027-. 037 

-.050— .028 

— .020— .022 
+ .0291 — .010 
-.022 +.061 
+ .030 +.009 



+ .038 
+ .074 
-.016 
+ .020 



-.011 
-.004 
— .035 
+ .012 



-.010,-. 041 

+ .039 +.032 
-.036! -.026 
+ .049 +.004 
-.011 —.038 
+ .006 +.026 



-.032 
+ .021 
-.032 



.850 



+ .014 
-.03' 
-.014 



-.005 +.071 
-.018 +.022 
-.012! + . 094 
+ .022: + .074 
-.010-. 071 

+ .a38 +.086 
+ .030— .03; 
-.028! — .094 
— .065!— .056 
+ .025 -.171 



Nov. Dec. 



-.111 +.051 

+ .080 +.057 
+ .017|— .063 

-.119!— .060 
+ .020! + .089 
-.109— .077 
+ .064 +.062 
+ .141J-.060 

+ .089! + .049 
+ .076 +.027 
+ .076 +.016 

— .012, + .059 
-.146-.07 

—.078' + . 016 

— .023! + .008 
+ .016— .047 
-.044 +.034 
—.040— .060 



+ .069' + .013+.C60! + .066 
+ .078— .047 -.038— .048 
-.036 +.017 +.004 +.016 
+ .ro3-.080 .000+. 012 
-.025!— .038+.057|-.005 

-.0511-. 033 +.087, + .065 
+ .060 +.048!-. 003 -.041 
-.004 +.030 —.023 —.060 
-.025 +.111 -.050 —.026 
-.005 +.087— .058 -.033 



-.015 
-.043 
+ .063 



+ .091 
-.001 
-.025 



946 .943 



.048 .029 



.G25 



-.058-.038 
-.025 -.017 
—.012 —.061 



.059 



.988 



.071 



Year 



-.011 

.020 

+ .001 

+ .001 

+ .008 

.073 

+ .028 
+ .033 

.004 
+ .031 
+ .038 

.005 
-.033 

015 
-.001 
+ .002 

024 
-.006 

+ .019 
-.001 
-.017 
+ .005 
-.009 

+ .005 
+ .005 
-.001 
+ .004 
—.009 

-.048 
— .038 
-.007 



19.917 



Table VII presents the average monthly and annual pressures expressed in 
terms of departures, in thousandths of an inch, from the normal values for 
the period 1873-1899. The normal for each month and for the year is shown 
in the first line of lootings, and the departures of the monthly normals from 
the annual normal are shown in the last line. These departures are also 
graphically shown in Fig. 7 and Fig. 8. 



50 



THE CLIMATE OF BALTIMORE 



tions and for further particulars in reference to the Baltimore pres- 
sure data the report of Professor Bigelow should be consulted, espe- 
cially pages 176, 646 and 798. 

In Table VII the mean monthly and annual pressures from 1873 to 
1903 are given in terms of departures from the monthly and annual 
normal values. The normal monthly and annual pressures derived 
from the mean of the daily averages (see Table V) differ somewhat 
from those derived from Professor Bigelow's monthly means (see Table 
VI) after reducing the latter to sea-level. This discrepancy is due to 
the fact that the daily means were taken directly from the original 
record of observations of the Baltimore Office of the Weather Bureau 
to which the correction for diurnal variation had not been applied. 



IBTr 1875 




Fig. 8. — Annual Variations of Pressure Expressed as Departures from the Normal 
Value. (See Table VII.) 



ANNUAL AND SECULAR VARIATIONS OF PRESSURE. 

The average atmospheric pressure of a year is by no means a constant 
quantity. The fluctuations in value from year to year are sometimes 
considerable. This is most readily recognized when the variations are 
graphically presented as in Fig. 8 on page 50. Here the Baltimore ob- 
servations are plotted in terms of annual departures from the normal 
value for the entire period from 1871 to 1903. The amplitude of 
fluctuation is expressed in thousandths of an inch. The resulting curve 
presents a series of waves or surges varying in amplitude from a few 
thousandths of an inch to nearly one-tenth of an inch. The period of 
oscillation also varies considerably, yet there is a remarkable uniformity 
in the length of these periods. Measuring from crest to crest and from 
hollow to hollow of these waves Ave have the following figures repre- 
senting the periods in years and fractions: 





MAKYLAXD "WEATHER SERVICE 








51 


Number of Years. 






Mean. 


From crest to crest 

(from 1871). 


4 


4 


4 


4 


5.5 


4.5 


4 


3.5 


3.5 


5.5 


4.2 


From hollow to hollow 

(from 1873). 


3.5 


3.5 


4 


4 


5 


5 


5 


3 


4 


4 


4.1 



Since 1871, the beginning of the series of observations at Baltimore, 
no crest of a wave has been heloiv the normal value for the entire period 
and no hollow has been al)ove the normal level. In order to fall into 
harmony with this series the year 1903 should form the crest of a 
wave and be followed by approximately equal pressure in 1904 and 
lower pressure in 1905; but we must not overlook the fact that it is the 
unexpected which is most likely to follow a long-range forecast. Tlie 
period from 1871 to 1903 includes ten waves with an average length 
from crest to crest, or from hollow to hollow, of slightly over 4 years, 
the limits of variability being three years and five years and a half. 

A conspicuous feature of the annual variation of pressure at Balti- 
more is the abnormally low pressure of 1878. The departure in this 
year was nearly five times the average annual variability. Upon first 
examination it appears suspiciously large. It is, however, substantiated 
by similar departures, though not so marked, at stations in all parts of 
the United States. A few of the larger departures occurring in this 
year are here given : 



DEPARTURES FROM NORMAL PRESSURE IX 1878. 



Baltimore — .072 

Washington — .057 

New York — .052 

Cincinnati — .080 

St. Louis —.049 



New Orleans . 

St. Paul 

San Francisco 
Key West . . . . 
Boston 



—.068 
—.052 
—.058 
—.059 
—.056 



The abnormally low pressure evidently extended over a very large 
territory in 1878. At Baltimore the pressure was decidedly below the 
average during every month of the year, excepting September, when it 
was but slightly above. Usually there is considerable fluctuation above 
and below the annual average during the course of the year. In 1901 
the average pressure was also abnormally low, but not as low as in 1878. 

Another marked feature of the curve is the steadv diminution in the 



52 



THE CLIMATE OF BALTIMORE 



amplitude of fluctuation from 18T8 to 1900, diminishing with consider- 
able uniformity from nearly one-tenth of an inch to about one-hundredth 
of an inch. Since 1900 the amplitude has again increased. The curve 



30 000 inches. 



TABLE VIII.-MAXIMUM STATION PRESSUKES. 
[In inches and hundredths.] 





Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


Year 


1875 


.68 
.52 
.51 
.61 
.44 

.61 


.57 
.85 
.50 
.26 
.70 

.58 


.37 
.43 
.48 
.40 

.65 

.40 


.32 
.43 
.26 

.20 
.37 

.32 


.24 
.32 
.30 
.14 
.42 

.31 


.13 

.12 
.19 
.11 
.22 

.22 


.16 
.18 
.16 
.06 
.23 

.12 


.20 
.16 
.16 
.10 
.13 

.34 


.35 
.13 
.24 
.40 
.36 

.31 


.48 
.36 
.41 
.29 
.75 

.41 


.61 
.30 
.51 
.46 
.50 

.68 


•61 
.59 
.51 
.49 
.56 

.47 


.68 


1876 


.85 


1877 


.51 


1878 


.61 


1879 




1880 


.68 


1881 


.61 


.73 


.29 


.30 


.41 


.12 


.16 


.22 


.24 


.52 


.66 


.57 


.73 


1882 


.83 


.68 


.65 


.43 


..38 


.18 


.23 


.26 


2'i 


.29 


.51 


.48 


.83 


1883 


..58 


.73 


.48 


.37 


.28 


.40 


.15 


.20 


.38 


.56 


.60 


.57 


.73 


1884 


.79 


.68 


.40 


.16 


.24 


.41 


.00 


.24 


.41 


.53 


.38 


.59 


.79 


1885 


.75 


.41 


..38 


..50 


.10 


.21 


.09 


.15 


.27 


.24 


.29 


.70 


.75 


1886 


.77 


.49 


.36 


.43 


.21 


.18 


.13 


.25 


.34 


.41 


.38 


.46 


.77 


1887 


.57 


.81 


.65 


.55 


.23 


.26 


.11 


.19 


.35 


.39 


.74 


.83 


.83 


1888 


.73 


.57 


.44 


.51 


.19 


.16 


.15 


.19 


.44 


.31 


.63 


.53 


.73 


1889 


.51 


.78 


.45 


.39 


.28 


.35 


.16 


.18 


.23 


.28 


.59 


.73 


.78 


1890 


.62 


.47 


.49 


.54 


.21 


.28 


.19 


.15 


.27 


.24 


.28 


.51 


. .62 


1891 


.40 


.56 


.55 


.34 


.26 


.10 


.18 


.08 


.27 


.38 


.67 


.51 


.67 


1892 


.49 


.57 


.37 


.35 


.17 


.11 


.36 


.04 


.27 


.31 


.31 


.46 


.01 


1893 


.27 


.64 


.46 


.31 


.23 


.16 


.10 


.07 


.19 


.37 


..53 


.71 


.71 


1894 


.48 
.34 


.66 
.40 


.45 

.,38 


.28 

.48 


.30 
.17 


.15 
.32 


.06 
.10 


.05 
.12 


.38 
.23 


.23 
.40 


.59 
.51 


.60 


.66 


1895 


.60 


1896 


.38 


.39 


.49 


.38 


.24 


.12 


.22 


.20 


.22 


.36 


.62 


.78 


.78 


1897 


..59 


.51 


.58 


.50 


.28 


.13 


.17 


.07 


.27 


.48 


.46 


.42 


.59 


1898 


.40 


.50 


.52 


.12 


.16 


.15 


.20 


.12 


.29 


.33 


.42 


.46 


.52 


1899 


.84 


.56 


.30 


.27 


.20 


.11 


.12 


.11 


.29 


.41 


.40 


.49 


.84 


1900 


.44 


.58 


.47 


.30 


.21 


.04 


.08 


.18 


.20 


.30 


.52 


.36 


.58 


1901 


.61 


.23 


.22 


.33 


.03 


.08 


.04 


.06 


.33 


.47 


.34 


.38 


.61 


1902 


.79 


.24 


.32 


.18 


.26 


.35 


.11 


.11 


.32 


.43 


.38 


.56 


.79 


1903 


.42 


.49 


.43 


.31 


.34 


.18 


.35 


.27 


.30 


.21 


.57 


.46 


.57 


Extremes 


.84 


.85 


.65 


.55 


.42 


.41 


.36 


.34 


.44 


.75 


.74 


.83 


.85 



Table VIII presents the highest station pressure observed at any of the 
regular hours of observation for each month and for the entire year, from 
1875 to 1903, together with the absolute extremes for each month and for the 
entire period of observation. The absolute extremes are also indicated in 
Fig. 9 curve '(a). The number 30.00 should be added to each of the figures 
in the table. 



also shows some suggestions of a wave of greater period. From 1885 to 

1899 there seems to have been a gradual rise in pressure. The Baltimore 

series of observations is, however, too short to place much reliance 
upon the evidence of a long period of variation. 



IMARYLAXD "WEATHER SERVICE 



53 



THE AVERAGE VARIABILITY OF PRESSURE. 

Some interesting facts regarding the variability of pressure conditions 
are revealed in Talkie Til showing the departures of monthly average 
pressures from 18T3 to 1903. The month of greatest variability is March 

TABLE IX.-MIXIMUM STATION PRESSURES. 

[In inches and hundredths.] 
29.000 inches. 





Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


1 
Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


Tear 


1875 


..55 


.36 


.36 


.52 


.35 


.66 


.61 


.64 


.59 


.49 


.52 


.37 


.35 


1876 


.49 
.31 
.39 
.29 

.48 


.20 
.38 
.28 
.43 

.27 


.21 
.02 
.18 
.34 

.26 


.48 
.36 
.22 

.52 


.58 
.46 
.55 
.64 

.73 


.64 
.52 1 
.40 
.44 

.59 


.72 
.64 
.55 
.51 

.59 


.72 
.63 
.56 
.59 

.66 


.18 
.66 
.49 
.65 

.59 


.49 

.29 

8.74 

.54 

.42 


.48 

.38 

8.90 

.45 

.41 


.01 

.29 

8.65 

.57 

..59 


.01 


1877 


.02 


1878 


28.65 


1879 


.29 


1880 


.26 


1881 


.26 


.32 


.no 


.46 


.56 


.54 


.60 


.60 


.77 


.50 


.64 


.34 


.00 


1882 


.29 


.26 


.48 


.31 


.46 


.47 


.57 


.56 


.50 


.74 


.69 


.60 


.26 


1883 


.54 


.65 


.29 


.56 


.29 


.60 


.64 


.61 


.50 


.44 


.69 


.48 


.29 


1884 


.17 


.25 


.45 


•1*. 


.57 


.63 


.55 


.64 


.66 


.66 


.42 


.45 


.14 


1885 


.40 


.19 


..53 


.46 


.44 


.43 i 


.61 


..52 


.42 


.02 


..53 


.25 


.02 


1886 


8.91 


.24 


.20 


.29 


.47 


.47 


.63 


.59 


.&i 


.73 


.41 


.44 


28.91 


1887 


.37 


.30 


.35 


.22 


.52 


.55 


.62 


.64 


.54 


.48 


.39 


.32 


•to 


1888 


.62 


.44 


.34 


.48 


.59 


.«3 


.58 


.35 


.60 


.42 


.38 


.85 


.34 


1889 


.13 


.47 


.35 


.22 


.63 


.59 


.60 


.66 


.47 


.44 


.40 


.54 


.13 


1890 


.63 


.54 


.38 


.42 


.54 


.67 


.64 


.64 


.82 


.35 


.60 


.36 


..33 


1891 


.12 


.37 


.37 


.38 


.66 


.53 


.61 


.56 


.76 


.00 


.28 


.48 


.12 


1892 


.14 


.17 


.16 


.52 


.44 


.50 


.63 


.59 


.59 


.47 


.47 


.41 


.14 


1893 


.07 


.12 


.27 


.35 


.26 


.48 


..56 


.39 


.54 


.01 


.60 


.43 


.01 


1894 


.22 


.39 


.50 


.34 


.40 


.52 


.58 


.66 


.55 


.20 


.54 


.23 


.20 


1895 


.17 


.22 


.35 


.22 


.48 


.64 


.56 


.61 


.60 


.52 


.35 


.31 


.17 


1896 


.45 


8.81 


8.99 


.57 


.55 


.43 


.59 


.70 


.54 


.00 


..50 


.61 


28.81 


1897 


.54 


.43 


.25 


.47 


.40 


.57 


..50 


.00 


.70 


..54 


.28 


.36 


.25 


1898 


.29 


.26 


.69 


.33 


.36 


.52 


.61 


.68 


..59 


.52 


.29 


.11 


.11 


1899 


.18 

.22 
.14 
.32 


.39 

.14 
.44 
.14 


.04 

.22 
.21 
.19 


.48 

.54 

8.97 

.12 


.56 

.30 
.42 
.58 


.66 

.57 
.63 
.34 


.68 

.46 
.66 
.64 


.60 

.75 
.72 
..59 


.57 

.55 
.54 
.64 


.35 

.72 
..53 
.43 


.31 

.19 
.25 

.30 


.13 

.30 
.38 
.20 


.03 


1900 


.14 


1901 


28.97 


1902 


.12 


1903 


.21 


.17 


.46 


.21 


.76 


.50 


.49 


.61 


.62 


.48 


.52 


.26 


.17 


Extremes 


8.91 


8.81 


8.99 


8.97 


.26 


.34 


.46 


.35 


.18 


8.74 


8.90 


8.65 


28.65 



Table IX presents the lowest station pressure observed at any of the regular 
hours of observation for each month and for the entire year, from 1875 to 
1903, together with the absolute extremes for each month and for the entire 
period of observation. The absolute extremes are also indicated in Fig. 9 
curve (e). The number 29.00 should be added to all fractional numbers in 
the table. 

with an extreme limit of 0.431 inch and an average variability of 0.058 
inch. The month of most uniform pressure is June with an extreme 
amplitude of 0.162 inch and an average variability of 0.029 inch. The 
montli of December sbows a remarkable freedom from extreme fluctua- 



54 



THE CLIMATE OF BALTIMORE 



tions, being next to June in this respect, while at the same time exhibit- 
ing a fairly large average variability. In the following table the varia- 
bility of the average monthly and annual pressure at Baltimore is shown 
by means of the average plus or minus departures from the normal 
monthly and annual values, the greatest plus and minus departures and 
the extreme variations of the monthly and annual values. 





J 












VI 






\ 




M 






J 






J 






K 






5 




o 






M 






3 




J 


Inches 




















































1 
























































































, 


































































(a) 








s 


























































/ 




-^ 


^ 










s 






















































X 






















V 










































/ 


' 






' 
























\ 










































^ 


































\ 






































> 






















(b) 






K, 












s 


































/ 




















" 










s 












s. 






























/ 














^ 


-" 






























\ 




























/ 










^ 














.50 














^ 


































/ 










^ 
























t 








N 
















"• 


^ 


^ 








/ 








/ 








































































ff 


























































































































■>«. 




















/ 




























































^ 
















































































■ 














































































































(<=) 








































































- 








>. 






















































.- 


— 


■* 


30.00 


















■4 






























0^ 




' 




M 


































K 


... 




r 


















^ 
















































































































































































































































































































































































_ 




—i 






























































.' 
















s 


















































r* 
























N 












































/ 












, 


L 
















s, 




















.50 




















/ 












/ 


' 




S. 
















^ 








S 




























f 












/ 










































(dl 
















. 












. 
















\ 


























»« 


^ 








■" 




— 














,>• 


r^ 


















\ 




















































/ 
























\ 


















































/ 


























\ 
















































r 


























^ 














































J 






























V 










































































\ 






















(ej 












/ 






























































29.00 












f 
































































\, 






/ 
















































J 












/ 








k. 


^ 














































\ 






' 




\ 








/ 
























































\ 




/ 






^ 












































































, 




y 






































































y 


/j 


f 












































































































































































































































































































.50 






_ 
































_ 


































_ 



Inches 
31.00 

(^) 
fb) 



30,00 



w 



2900 
Ce) 



.50 



Fig. 9. — Monthly Means and Extremes of Pressure. (See Tables VIII, IX 
and X.) 

VARIABILITY OF PRESSURE CONDITIONS AT BALTIMORE. 

(Expressed as departures from the normal values in thousandths of an inch.) 





Jan. 


Feb. 


Mar. 
174 


Apr. 


May 


June 


July 


Aug-. 


Sept. 


Oct. 


Nov. 


Dec. 


Year 


Greatest plus 
departure (+). 


88 


187 


106 


142 


84 


74 


67 


78 


Ill 


141 


89 


38 


Greatest minus 
departure (— ). 


148 


206 


357 


188 


126 


78 


106 


108 


96 


171 


146 


77 


72 


Extreme am- 
plitude. 


236 


393 


431 


294 


268 


162 


18 


175 


174 


282 


287 


166 


110 


Average de- 
parture /' + \ 


59 


.5.5 


58 


54 


45 


29 


30 


33 


32 


.55 


59 


46 


15 



MARYLAXD WEATHER SERVICE 



55 



The negative departures are far more marked than the positive. Only 
in May, June and November have the plus departures exceeded the 
minus. This contrast is particularly strong in the figures representing 

TABLE X.-SUMMARY OF PRESSURE CONDITIONS. 

iln inches and thousandths.] 









Monthly Means. 


Mean Monthly and Annual 
Extremes. 


Absolute Extremes. 




Highest and Low- i . 


1875 


-1903. 






1875-1903 






Means. 


est Means as De- 1 ^ 
















partures from ! i -^ 


















1873 

to 

1899 


Normal. . .:: 
1873-11)03. ii, , .3 


Average of 
Extremes. 


Departures 
from Nor- 
mal. 


o 
til 

a 

2 


Max. 


& 


Min. 


a , n 

21 Zt 








^ |P3 ^ 




. 




tx 


t>l 1 ^i 






CO 


. 1 ■ - ' s 


"^ "S 


m ^-» 


d 












1 


r" 1-5 




» s 
5 c 


2 § 

if £ 




30.000+ 


38. 000 4- 






29.000+ 


1 1 




^.COn-l- 29.000+ 




1 


1 


Jan 


.995 


-h . 088 1 875 - . 1 48 1 893 . 236 . 0.59 


.572 ..307 


J+.577— .688 


1.265 


.843 


1899^ .908 


1886 1.935 


Feb. .. 


.968 


. 1 87 188:3 - . 206 Vm. 393 . 055 


.557 .298 


.589 -.670 


1.259 


.849 


1876; .809 


1896 2.040 


Mar. .. 


.899 


.174 1S98-. 2.57 1881 .431 .0.58 


.443 .291 


.544 -.608 


1.152 


.6.50 


1887] .993 


1.S96 1.6.58 


Apr... 


.877 


.1061,S88 — .1><81S78 .294 .054 


.353 .363 


.476 -.514 


.99C 


.550 


1887! .969 


1901 1.581 


May... 


.852 


.142 I903-.126 1901 .268 .045 


.245 ..50 J 


.393-.a5C 


.743 


.424 


1879 J. 257 


1893 1.167 


Juno .. 


.851 


.084 1884 - .078 1881 . 162 .029 


.191 ..=^42 


' .340 — .309 


.64S 


.414 


1884 1.345 


1902 1.069 


July.. 


.850 


. 074 1 892 — . 1 06 1 884 . 1 80 . 030 


.151 1 .594 


: .301 -.256 


.557 


.3,59 


1892 1.4.57 


1900 .903 


Aug. .. 


.869 


.0671876 -.108 1878 .175 .OSJ 


.159 .608 


.290 -.261 


..551 


.342 


1880 1.3,50 


il8S8 .993 


Sept... 


.946 


. 078 1 892 - . 0!'6 1 876 . 1 74 . 032 


.293 1 .581 


.347-. 365 


.712 


.440 


1888 1.181 


1876 1.259 


Oct.... 


.943 


.11 11899 -.1711.'-90.'J82. 0.55 


.381 .435 


' .439 — .507 


.946 


.753 


1879 .739 


il878 2.013 


Nov. .. 


.976 


.141 1880 — .1461885 .287 .059 


.498 .420 


.522 -.5.56 


1.078 


.740 


1887 .902 


1878 1.8:« 


Dec. .. 


.9^8 


.089 1877 -.077 1878 


.166.046 


.540 


.353 


.552 -.636 


1.188 


.830 


1887 


.648 


1878 2.183 


Year . . 


.917 


.038 1883 -.072 1878 


.110 .015 


.691 


.120 


.777-.797 


1.574 


.849 


1876 


.648 


.878 3.301 



Table X presents a summary of pressure conditions, derived mostly from 
the preceding tables. It shows the mean monthly values; the highest and 
lowest mean values, with year of occurrence and range; the mean variability 
of the monthly means; the mean monthly and annual extremes, with their 
respective departures from the normal value, and their ranges; the 
amount and year of occurrence of the absolute extreme values, and the ab- 
solute range of pressure for each month and year. Much of the data con- 
tained in this table is also shewn graphically In Fig. 9. 



the annual departures. The extreme amplitudes, or the differences be- 
tween the highest and lowest average monthly values are about six times 
larger than the average plus or minus departures. This ratio is remark- 
ably constant throughout the year, excepting the months of December 
and Januarv when the ratio falls to 4. 



56 THE CLIMATE OF BALTIMORE 

EXTREMES OF PRESSURE. 

The extreme range of the barometer at Baltimore from 1871 to 1903, 
according to the official records of the United States Weather Bureau, 
is 2.20 inches. The highest ohserred reading, namely 30.85 inches, oc- 
curred on February 5, 1876, and the lowest, 28.65 inches, on December 
10, 1878. The barometer seldom falls below 29.00 inches in the Middle 
Atlantic states; since 1875 a lower reading has been observed at Balti- 
more but once in each of the months of January, February, March, April, 
October, November, and December, and never in the months of May to 
September. The very low pressures occur only in connection with a 
severe cyclonic storm of the winter type, or in connection with tornadoes. 
In the center of the extremely limited area of a tornado the barometer 
has fallen to 27.00 inches or less for a fcAV minutes, but Baltimore has 
fortunately been visited but two or three times in the past 30 years or 
more by these fierce and destructive storms, and then only by a compara- 
tively mild type. Of the seven occasions referred to above on which the 
barometer fell below 29.00, three occurred in the year 1878, a year 
remarkable for low pressures, two in 1896, one in 1886, and one in 1901. 

The abnormally high pressures likewise occur in the winter months 
only, the most marked of them in connection Avith the intenser types of 
cold waves. The highest observed reading of the barometer occurred 
during the cold wave of February, 1876, when the pressure rose to 30.85 
inches. A detailed record of the highest and lowest observed pressures 
for each month and for the year is given in Tables Till and IX on pages 
52 and 53. For a summary of averages and extreme conditions of 
pressure reference may be made to Table X. In Figure 9 some of the 
chief features of this table are graphically shown. 



TEMPEEATUEE OF THE ATMOSPHEEE. 

Introduction. — There are certain factors which, in the long run, de- 
termine the average temperature of every locality. Of these the latitude, 
the position of the place with reference to large land and water areas, the 
height above sea-level, the nature of the soil, and other factors of minor 
importance are constant and tend to give to a place a fixed mean temper- 



M AH Y LAND WEATHER SERVICE 57 

ature. Other factors, as wind direction, amount of cloudiness, etc., vary 
greatly from day to day and from season to season, and tend to produce a 
variable mean temperature. In some localities, within the tropics for 
example, these variable factors become fairly constant, and enable us to 
determine the average temperature by means of a comparatively short 
period of observations. In others the variable factors are large, as in the 
temperate zones, especially in the usual paths of cyclonic disturbances. 
In such regions a long series of observations is often necessary to de- 
termine the average temperature conditions to within 1° or less. In the 
smaller islands of the tropics five or six years of carefully made temper- 
ature records will yield an annual mean value with a probable error not 
greater than one-tenth of a degree. In the temperate regions an equally 
accurate annual mean niay require observations covering a period of 50 
to 100 years. In the long run the effect of the variable climatic factors is 
eliminated and a given locality secures a position upon the normal tem- 
perature chart, due to its geographical and topographical position and the 
nature of the soil. Baltimore occupies a middle position on the climatic 
chart with average annual and summer temperatures 3° or 4° below tlie 
average for the entire globe, and a winter temperature about 10° below. 
The city lies between a region of equable temperatures, the ocean, and one 
of great variability, the nortli continental area. The factor to which 
is due most of the changeable character of the weather of Baltimore, 
causing a variability greater than is its due on account of latitude, is its 
proximity to the great transcontinental storm paths. Baltimore is within 
the influence of the barometric depressions which continually pass from 
the northwest, across the Lake region and the New England states, and 
which are accompanied by rapid changes in wind direction from the warm 
southerly to the colder west and northwest winds. 

Average Temperatures. 

For purposes of comparison it is essential to have a standard of refer- 
ence. In discussing temperature conditions the standard of value is 
usually assumed to be the average daily temperature. This daily average 
is derived from observations made hourly throughout the day and niglit. 
Approximate averages are obtained from two or more observations made at 



58 THE CLIMATE OF BALTIMORE 

such hours of the day as to give a value more or less closely agi'eeing with 
that derived from hourly observations. Experience has shown that fairly 
accurate daily averages may be obtained by noting the temperature at 
7 a. m., 3 p. m., and 9 p. m., or 7 a. m., 3 p. m., and 11 p. m., or 10 a. m., 
and 10 p. m., or 8 a. m., and 8 p. m., or from the highest and lowest tem- 
peratures recorded during the day. In later years automatically record- 
ing instruments have largely displaced direct observations permitting us 
to obtain a daily mean temperature to any desired degree of -accuracy with 
comparatively little personal attention. Monthly, seasonal, and annual 
means are in turn derived from the daily means. 

In the discussion of temperatures in succeeding pages we must not lose 
sight of the nature of average values. They are not real values in the 
sense of occurring in nature. When we say that the average temperature 
on the 4th of July in Baltimore is 79°, we mean that by adding together 
the hourly temperatures on the 4th of July for a great many years and 
dividing by the total number of hours we obtain the value 79°. This 
may never have been the real average value for the day on any 4th of July. 
It is simply an arithmetical mean ; the real temperatures of the day may 
have had any value from 60° to 100° or more. 

Average values are sometimes very misleading if sole reliance be placed 
upon them to characterize the temperature conditions of a day or a 
season. Two seasons may have the same mean temperature and yet be 
totally different in character. The summer of 1898 left the impression of 
an unusually warm season. The official records show a temperature very 
near the average of a period of thirty years (76°). The average may be 
obtained from any one of a large series of combinations, and our general 
impression of the character of the season will be determined by the 
particular combination of weather experienced. The temperature may 
lemain uniformly near the average throughout the season; there may be 
excessively high temperatures of short duration combined with longer 
periods of moderately low temperatures; or excessively low temperatures 
combined with longer periods of moderately high; or there may be very 
high combined with very low temperatures, etc. All of these combina- 
tions may produce a " normal " average, but the personal effect will be 
different in each instance, and give rise to a variety of opinions as to the 



MARYLAXD WEATHER SERVICE 59 

character of the season. Disregard of such considerations frequently 
leads to unfavorable criticism of official records. Hence the figure rep- 
resenting the average temperature of a period is not of itself a safe cri- 
terion of the temperature conditions ; the variability of temperature is an 
essential factor in revealing the character of the period. 

The XoRiiAL Hourly Temperature. 

The most familiar, and at the same time most regular, feature of 
changes in the weather is the rise and fall of temperature between sunrise 
and sunset. Like the pressure change it is most regular in the tropical 
regions, and diminishes in amplitude with distance from the equator until 
it disappears by merging into the annual change within the Arctic Circle. 
As the amplitude of variation depends very largely upon the character of 
the surface upon whch the rays of the sun fall, there are marked de- 
partures from the general law of decrease in amplitude with increased 
latitude. Over a water surface the daily changes are small: over the 
interior of the continental areas, especially over sandy soils and in a 
dry atmosphere, they are enormously increased. The difference between 
the highest and lowest temperature recorded during an average day a 
few feet above the surface of mid-ocean is not ordinarily more than 1° or 
2°, owing to the relatively large absorbing power of water, and to the 
large quantity of heat employed in the conversion of water into vapor — 
the latent heat of evaporation. The surface of the soil, especially when 
unprotected by vegetation, is rapidly warmed by the sun's rays and attains 
a high temperature, owing to its comparatively low specific heat. The at- 
mosphere above such surfaces is in turn heated by contact and bv con- 
vection currents. In consequence the difference between midday and 
night temperatures over land surfaces is many times larger than over 
water surfaces. For any given locality the diurnal variation also varies 
with the season of the year, following the changes in the altitude of the 
sun, and hence is greatest in the summer months and least in the winter 
months. 

The Baltimore hourly observations of temperature extend over a 

period of ten years, affording ample data for determining all phases of the 

<liurnal variation. 'J'he tracings of the Eichard thermogra))li were cor- 
5 



60 



THE CLIMATE OF BALTIMORE 



rected at four points each day by means of direct observation of a mer- 
curial thermometer at 8 a. m., and 8 p. m., and by readings of the maxi- 
mum and minimum points reached by a mercurial maximum and an 
alcohol minimum thermometer. The average values for the ten years 



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Fig. 10. — Mean Hourly Temperature. (See Table XI.) 



for each hour of the day, for each month, and for the year are given in 
Table XI, and in Fig. 10 and Fig. 11. 

The details of changes in temperature from hour to hour are best 
shown in tabular form from which the exact value for each hour may be 
readily taken. The graphic form, however, presents advantages in afford- 



TABLE XI. 


-MEAN HOURLY TEMPEBATUEE. 






Hours. 


Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. Dec. 


1 A. M 




31.4 
31.0 
30.6 
30.2 
29.8 
29.6 
29.4 
29.7 
30.6 
32.0 
33.7 


29.9 
29.4 

28.9 
28.5 
38.1 
27.9 
27.8 
28.4 
29.7 
31.3 
33.1 
34.6 

:i5.8 

36.9 
37.2 
37.2 
36.5 
35.3 
34.4 
33.6 
32.8 
32.2 
31.6 
31.1 


40.1 
39.4 
38.8 
38.3 
37.9 
37.6 
37.8 
38.8 
40.5 
42.2 
44.0 
45.7 
47.0 
48.3 
48.7 
48.8 
48.0 
47.0 
45.6 
44.5 
43.4 
42.5 
41.7 
40.9 


49.1 
48.3 
47.6 
47.1 
46.5 
46.4 
47.4 
49.5 
51.7 
53.7 
55.6 
57.1 
58.2 
59.0 
59.6 
59.6 
59.0 
57.9 
.56.2 
.54.9 
53.6 
52.5 
51.6 
50.5 


59.6 
58.8 
58.1 
57.5 
57.0 
57.4 
59.0 
61.3 
63.4 
65.2 
67.1 
68.4 
69.5 
70.5 
70.7 
70.7 
70.2 
69.0 
67.0 
65.2 
63.7 
62.6 
61.5 
60.9 


68.0 
67.2 
66.4 
65.8 
65.2 
66.1 
68.2 
70.5 
72.8 
74.6 
76.3 
77.5 
78.7 
79.6 
79.9 
79.8 
78.9 
77.6 
75.7 
74.1 
72.6 
71.3 
70.2 
69.2 


72.9 

72.2 
7l'.4 
70.8 
70.3 
70.6 
72.4 
74.7 
77.2 
79^1 
80.9 
82.4 
83.4 
84.0 
84.2 
84.1 
83.1 
81.8 
80.1 
78.3 
76.6 
75.6 
74.5 
73.7 


71.3 

70.7 
69.9 
69.3 
68.7 
68.8 
70.4 
73.0 
75.4 
77.7 
79.8 
81.0 
82.1 
82.8 
82.9 
82.6 
81.6 
80.5 
78.8 
77.0 
75.6 
74.4 
73.2 
72.2 


65.2 
64.6 
63.8 
63.1 
62.5 
62.2 
63.1 
65.7 
68.0 
70.5 
72.6 
74.2 
75.3 
76.1 
76.4 
76.3 
75.1 
73.4 
71.6 
70.2 
68.6 
67.5 
66.8 
65.7 


53.9 
53.3 
52.7 
52.1 
51.7 
51.3 
51.6 
53.6 
56.0 
58.4 
60.6 
62.3 
63.4 
64.2 
64.4 
64.0 
62.8 
61.2 
.59.5 
58.1 
56.9 
55.9 
55.0 
54.2 


44.1 35.0 
43.7 34.5 


3 


43.2 ' 34.2 


i 


43.7 . 33.8 




42.3 as. 4 


6 


42.0 33.0 




41.9 32.9 


8 

9 


42.8 33.3 
44.8 1 34.5 


10 

11 


46.7 36.0 
48.6 37.9 


Noon 

1 

3.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'. 

4 

5 

6 

8 

9 


35.0 
36.0 
36.8 
37.2 
.37.0 
.36.2 
a5.4 
34.5 
34.0 
33.0 
32.8 
32.1 
31.8 


50.2 39.4 
51.4 40.7 

52.0 41.6 
53.2 42.0 
51.6 41.5 
50.4 40.5 

49.2 39.5 

48.1 : 38.6 

47.3 37.9 

46.4 37.1 


10 

11 


45.7 36.5 
44.9 35.9 
44.2 35.2 








32.9 


32.2 


42.8 


53.0 


63.9 


72.8 


77.3 


75.8 


69.1 


57.4 


46.5 36.9 









MEAN HOURLY TEMPERATURE. 








Spring. 


Summer. | Autumn. 


"Winter. 


Year. 


1 A. M 


49.6 
48.8 
48.2 
47.6 
47.1 
47.1 
48.1 
49.9 
51.9 
53.7 
.55.6 
57.1 
.58.2 
59.2 
59.7 
.59.7 
.59.1 
58.0 
.56.3 
.54.9 
53.6 
52.5 
51.6 
50.8 


70.7 
70.0 
69.2 
68.6 
68.1 
68.5 
70.3 
72.7 
75!l 
77.1 
79.0 
80.3 
81.4 
82.1 
82.3 
83.2 
81.2 
80.0 
78.2 
76.1 
74.9 
73.8 
72.6 
71.7 


54.4 
53.9 
53.2 
52.6 
52.2 
51.8 
53.2 
54.0 
56.3 
58.5 
60.6 
62.2 
63.4 
(U.l 
64.3 
64.0 
62.8 
61.3 
59.7 
58.5 
57.3 
56.4 
55.6 
54.7 


32.1 
31.6 
31.2 
30.8 
30.4 
30.3 

ai.o 

30.5 
31.6 
33.1 
34.9 
36.3 
37.5 
38.4 
38.8 
38.6 
37.7 
36.7 

a5.8 

35.2 
34.3 
33.8 
dii.-Z 
33.7 


51.7 


o 


51.1 


3 

4 


50.5 
49.9 




49.4 




49.4 


- 


.50.2 


8 


51.8 


9 


53.7 


10 


55.6 


11 






.59.0 


1 


60.1 


*> 


61.0 


3 


61.3 


4 


61.1 
60.2 


6 


59.0 




57.5 


S 


.56.3 


9 

10 

11 


.55.0 
54.1 
53.2 




52.5 








53.2 


75.3 


.57.7 


34.0 


55.0 







Table XI shows the mean temperature for each hour of the day, based on 
the continuous record of a Richard thermograph for the ten-year period 
ending December 31, 1902. The thermograph record was corrected daily by 
direct observations of a mercurial thermometer at 8 a. m. and 8 p. m., and by 
the readings of a maximum and a minimum self-registering thermometer. 
The annual mean (55.0°) is the average value of over 87,000 hourly observa- 
tions, and may be regarded as a true normal value for the period covered by 
the observations. The same results are graphically shown in Fig. 10, for 
January, April, July, October, and the year, and in Fig. 11, for all the months 
of the year. 



62 



THE CLIMATE OF BALTIMORE 



ing a readier means of observing the relative changes from hour to hour 
and the distribution of temperature within the day, the month, and the 
year. The method of presentation employed in Fig. 11 is particularly 
well adapted to a rapid survey of the hourly and seasonal distribution. 
In construction it resembles the maps prepared for showing the varying 
topography of an area. In place of the meridians of longitude and par- 
allels of latitude to arrive at the geographical position of a given locality, 
we have vertical lines to represent the hours of the day and horizontal 
lines for the months of the year, the intersection of which gives us the 




Fig. 11. — Isopleths of Hourly Temperature. 

Fig. 11 shows the average distribution of temperature throughout the day and j ear, based 
on observations of ten years of hourly readings of the thermograph. The hours of the day 
are indicated by the upper line of figures, Avhile the marginal letters indicate the months 
of the year. The line enclosing the area of lightest shading defines the time of occurrence 
of the lowest temperature of the year and day ; rise in temperature is indicated by increase 
in the intensity of .shading. The diagram indicates that the lowest temperatures of tlie year 
occur in the early morning hours of January and Februarj', and that the highest occur in 
the eai'ly afternoon hours of Julj% based on the average of a long series of years. The 
curved lines show the liours of the day and the months of the year when the average read- 
ings of the thermometer are equal : these lines are called chrono-isotherms, or. isopleths of 
temperature. The dotted lines marlicd S.R. and S.S. show the time of sunrise and sunset. 
See also Table Xi. 



time sought. In place of the contour lines, or lines of equal elevation 
of the topographic map, we have lines of equal temperature (or isopleths 
of temperature as they are called when used in this manner) projected 
upon the plane of the time lines. The rapid detection of the diurnal 
and annual distribution of temperature is further facilitated by means of 
a system of shaded areas, increase in the intensity of the shade signifying 



:MARYLAXD "WEATHER SERVICE 63 

increase in temperature. Consulting Fig. 11 we find that for Baltimore 
a temperature of 8-i° is limited, under average conditions, to the hours 
from 2 p. m. to 4 p. m., during the month of July; that the temperature 
of 75° occurs, on the average, from June, between the hours of 10 a. m. and 
7 p. m., to September, between 1 p. m. and 5 p. m., etc. In the winter 
months the line of 32°, or freezing weather for example, is limited, on 
the average, to the months of January and February from midnight to 
10 a. m. We see that the average summer temperature of 76° extends from 
June to September during the middle hours of the day, while the average 
winter temperature of 36° is confined to the night and morning hours of 
December, January and February, and to a few of the early morning 
hours of March. The lowest temperature of the day occurs, on the aver- 
age, just before sunrise and hence varies with the advance and retreat of 
the sun. The time of occurrence of the highest temperature varies less 
with the season, occurring throughout the year between 3 p. m. and 4 
p. m., excepting the month of November when the highest temperature 
of the day occurs at about 2.30 p. m. 

The diurnal variation of temperature is represented by a simple curve 
which rises steadily from a minimum point just before sunrise, attains a 
maximum in the early afternoon hours, and then descends without inter- 
ruption to the early morning minimum. In this respect it differs from 
the curve representing the diurnal variation of the barometer which, 
as we have seen, has a double period, Ts-ith primary and secondary maxi- 
mum and minimum points. 

Phases of the Diurnal Variation. 

The principal phases of the diurnal variation of temperature are pre- 
sented in Table XII, containing a summary of the average time of occur- 
rence of the minimum, the maximum, and the mean temperature for the 
day, and the varying interval between the occurrence of the minimum 
and maximum points. In the months of May and June the lowest tem- 
perature of the day occurs at 5.05 a. m., 75th meridian time, which is 
six minutes faster than Baltimore local time; the time advances steadily 
to 6.50 a. m. in January, returning again to 5.05 a. m. in May. Tlie 
maximum of tlio day sliows less variation in time of occurrence. From 



64 



THE CLIMATE OF BALTIMORE 



January to May it remains at about 3.25 p. m., then occurs successively 
earlier in the day until November, when a maximum is attained at 2.25 
p. m. It seems rather remarkable that the maximum temperature of 
the day should appear earliest in the month of November. The average 
temperature occurs first at about 9 a. m. in the summer months and at 
10.30 a. m. in the winter months, and again between 8.30 p. m. and 9.00 
p. m. in summer, and about 10.00 p. m. in the winter months, excepting 































































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— " 














































































































Max 


.. 










































































































































1. Mean 













































, 


■ 


■— ^ 
































MiN. 


























■ 














_,^ 


-^^ 





















_ 

























































































































































Fig. 12. — Principal Phases of Diurnal Variation of Temperature. 

Fig. 12 shows the time of occurrence of the highest and lowest points indicated by the 
thermometer on an average day for each month ; also the morning and afternoon hours 
when the mean temperature of the day is most likely to occur. See also Table XII. 

December, when it occurs as early as 9.20 p. m. The amplitude of varia- 
tion, or the difference between the daily maximum and daily minimum 
temperature, is greatest in the month of June (14°. 7) and is smallest 
in the month of Januarv^ (7°.0) . (See Fig. 12.) 

The temperatures thus far discussed are average values for a period 
of ten years. When we examine into the time of occurrence of the prin- 
cipal phases of the diurnal march of temperature more closely we find 



MARYLAND WEATHER SERVICE 



65 



a wide divergence from the average time of occurrence as recorded in 
preceding paragraphs. The limits of variability in the average time 
for a single month are shown in the following tabular statement contain- 
ing the hour and the frequency of occurrence of each phase in each 
month of the ten-year period. 



TABLE XII.— TEMPERATURE PHASES. 



Minimum. 



Fre- 
quency. 



5' 6 7 8 



January 

February . . 

March 

April 

May 

June 

July 

August 

September ] 2 

October 

November I 1 

December 



Tear. 



4 9 



4 4 



6:50 
6:30 
6:15 
5:«) 
5:05 
5:05 
5:10 
5:25 
5:50 
6:05 
6:25 
6:40 



5:50 



1st Mean. 



Fre- 
quency. 



9 10 11 



5 
5 

6 

3 7 
7 3 
9 1 

9 1 

9| 1 
4| 6 

4 6 
l| 8 

6 



5 5 



9:40 



Maximum. 



Fre- 
quency. 



p. m. 



1 2 3i 4 5 



7 5 

5 4 

41 

6! 5 

7 4 

21 5 



9 2 



II 4. 



3:10 



2nd Mean. 



Fre- 
quency. 



p. m. 



8! 91011 



1 9'.. 

2 8., 

3 7., 



ih-m. 



8 4 



1 9:50 
3 10:00 
9:40 
9:30 
8:50 
8:50 
8:35 
8:50 
8:40 
8:35 
8:55 
9:20 



8-30 
9-00 
9-10 
9-50 
10-20 
10-05 
9-55 
9-20 
9-00 
8-35 
8-00 
8-20 



9:00 9-20 



Table XII indicates the average time of occurrence of the lowest and 
highest temperature of the day for each month and for the year; the morning 
and the afternoon hours when the mean temperature of the day is most 
likely to occur; the frequency of occurrence of these phases at given hours; 
and the average number of hours between the occurrence of the highest and 
lowest temperatures of the day. The values are based on hourly observations 
for the ten-year period from 1893 to 1902. See also Fig. 12. 



The January minimum may occur from 5 a. m. to 8 a. m., the normal 
time being 6.50 a. m. In May, June, and July the minimum occurs 
with great regularity at about 5 a. m., and in September and October at 
6 a. m. There is more uniformity in the time of occurrence of the max- 
imum temperature of the day; this does not vary greatly from the hour 
of 3 p. m. at any season of the year. The earliest occurrence of the 
maximum is in the month of November. The time interval between the 



66 THE CLIMATE OF BALTIMORE 

minimum and maximum of the da}' increases steadily from the winter 
months to the summer months. Beginning with eight hours and thirty 
minutes in January, the interval reaches a maximum in May when it 
amounts to ten hours and twenty minutes, then decreases regularly to 
eight hours in November. This difference in time is due mostly to 
variations in the time of occurrence of the minimum temperature. 

DiuRXAL Variation as Affected by Clouds and Eain. 

In considering the diurnal variation of temperature in the preceding 
paragraphs the character of the day does not enter into the problem. 
The average values given include all days for a period of ten years. The 
amplitude of variation of temperature is manifestly largely dependent 
upon the presence or absence of clouds. On a cloudy day the sun's rays 
are largely absorbed by the cloudmass and comparatively little of the 
sun's heat-rays reach the earth's surface directly. To discover to what 
extent the normal daily variation is affected by the character of the day 
the diurnal variation of temperature has been determined for selected 
days in each season. For this purpose the days were grouped as clear, 
cloud}'', and rainy. Days were regarded as clear during which the per- 
centage of sunshine exceeded 90 per cent of the possible amount for the 
day. They were considered cloudy when the sky was overcast the entire 
day. A day was considered rainy when rain fell for more than four 
hours, not necessarily consecutive. Each group included approximately 
100 days, selected from all seasons of the year. A further restriction 
was imposed by excluding days with a moderate or a high wind, as it 
was desired to eliminate the effect of wind velocity upon the diurnal 
variation in this problem. 

The results of the above classification are shown in Fig. 13, in which 
some interesting and instructive relations are revealed. The ampli- 
tude, or difference between the lowest and highest temperature of the 
day, is manifestly greatest on clear days, with a maximum in the spring 
months. Cloudiness reduces the daily range of temperature to less than 
one-half of that on a clear day. On a rainy day the difference between 
the maximum and minimum is reduced to 2° or 3°, equivalent to about 
one-fourth the range on a clear dav in winter and to about one-sixth 



MARYLAXD WEATHER SERVICE 



67 



the range in spring, summer, and autumn. The principal phases of the 
diurnal march of temperature do not materially change in the summer 
and autumn months. The minimum occurs approximately at sunrise 
and the maximum of the day in the early afternoon hours. There is a 
marked deviation, however, from the normal conditions on rainy days 
in autumn and winter. After attaining the maximum for the dav it is 



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Fig. 13. — Effect of Cloudiness and Rain on the Hourly Variations of Temperature. 



maintained until nearly midnight. One of the most interesting facts 
revealed in the diagrams is the relative position of the curves for clear, 
cloudy, and rainy days in the difTerent seasons. In winter the clear day 
has a temperature decidedly below the normal for the season, while the 
cloudy and raiiiv (Uiys are well al)ovo tb.e normal. In spring the clear 
day lias altout the normal temperature: the cloudy day is far above tbe 
normal : the raiin- dav is decidedlv below tlie normal. In summer the clear 



68 



THE CLIMATE OF BALTIMORE 



day is decidedly warmer than the average for the season ; the cloudy day 
is about normal; the rainy day is much below the normal. In autumn 
the clear day is somewhat below the average temperature, the cloudy day 
is about normal, and the rainy day is well above the normal. These dif- 
ferences in temperature depending upon the extent of cloudiness and pre- 
cipitation are in some cases very large. In spring the early morning 
temperatures may be 10° to 15° lower with a clear sky than with an 
overcast sky. In autumn there is quite as marked a difference between 
a clear and a rainy day in the early morning hours. In the summer 
months the midday temperatures may be reduced 10° to 13° by an over- 



s' 






30* 



Fig. 14. — Effect of Snow-Covering on the Hourly Variations of Temperature, 
(a) A normal winter day. 
{b) Average of days with snow on the ground. 

cast sky, and 15° to 20° during a rain. During the autumn an overcast 
sky will maintain the average temperature of the day 6° to 8° above 
that of a clear day. 

MEAN HOURLY TEMPERATURE ON CLEAR, ON CLOUDY AND ON RAINY DAYS. 



Winter. 



Normal Temperature 34.0° 

Clear days (Departures) —5.2° 

Cloudy days " i +0.6° 

Rainy days " +0.5° 



Spring. 



53.2° 
1 .70 

+3.5° 
-6.2° 



Summer. 



75.8° 

+3.4° 
4 2° 

-:!o° 



Autumn. 



57.7': 



-0.4° 
+0.9° 



ilARYLAXD WEATHER SERVICE 



69 



Effect of a Sxow Coverixg. 

To determine the effect of a snow covering upon the diurnal variation 
of temperature the average hourly temperature was calculated for all 
da3's within the period of ten years from 1893 to 1902 upon which the 
ground was covered with snow to a depth of half an inch or more. The 
values for the entire season are shown in the accompanying table and in 
Fig. 14 in comparison with the normal temperatures for the winter 
season. The two curves are identical in form and run parallel through- 
out their extent, but the days with snow on the ground were uniformly 
about 10° below the normal temperature for the winter months. 



HOURLY TEMPERATURES ON DAYS WITH SNOW ON THE GROUND. 



Hours: A. M. 


1 


2 


3 


4 


5 


6 


7 


8 


9 10 


11 


Noon. 




Winter normal.. 
With snow on 

ground 

Departure below 

normal 


32.1 
21.8 
10.3 


31.6 
21.2 
10.4 


31.2 
20.8 
10.4 


30.8 
20.4 
10.4 


30.4 
20.0 
10.4 


30.2 
19.5 
10.7 


30.0 
19.3 
10.8 


30.5 
19.5 
11.0 


31.6 33.1 

20.7 32.6 
10.9 10.5 


34.9 
24.6 
10.3 


36.3 
26.0 
10.3 




Hours: P. M. 


1 


2 


3 


4 


5 


6 


7 


8 


9 


10 


11 


Mid- 
night. 


Means. 


Winter normal.. 
With snow on 

ground 

Departure below 

normal 


37.5 
27.3 
10.2 


38.4 
28.3 
10.1 


38.8 

28.9 

9.9 


38.6 

28.9 

9.7 


37.7 

27.8 

9.9 


36.7 

26.8 

9.9 


35.8 
25.7 
10.1 


35.2 
24.9 
10.3 


34.3 
24.0 

10.3 


33.8 
23.4 

10.4 


33.2 
22.5 
10.7 


32.7 
23.0 
10.7 


34.0 
23.6 
10.4 



The temperature is lowered during the night by the intenser radiation 
from a snow surface ; it is prevented from rising during the day because 
much of the heat of the sun which would otherwise go to warm tlie 
atmosphere is spent in melting and vaporizing the snow. The air tem- 
perature is likewise reduced by the snow preventing the communication 
of heat from the ground by convection. As observations show that tlie 
difference between the normal hourly winter temperature and the hourly 
temperature over a snow-covered ground is practically constant through- 
out tlie day and night, the daily range is neither increased nor decreased 
by the presence of snow. The low average temperature of the winter of 
l!Hi;;-l!J01 was doul)tlos.s largely due to the exceptional d\iration of a 
snow cover. The depth of snow was not great, in the vicinity of Balti- 



70 THE CLIMATE OF BALTIMORE 

more, but a moderate snow covering persisted during a period of time 
nearly double the usual length. There is some compensation in the 
beneficial protection afforded by snow to winter wheat and to vegetation 
in general by preventing the penetration of frost into the ground. 

The Effect of Wixd Velocity ox Temperature. 

Another factor which largely affects the diurnal range of the ther- 
mometer is the movement of the atmosphere. It is well known that in 
a quiet atmosphere there may be a great difference in temperature at the 
earth's surface and a small distance above. In the night and early morn- 
ing hours of winter the thermometer may register 5° or 10° lower near 
the ground than on the house tops ; on a hot summer's day the difference 
at midday may be quite as large but reversed. In either case the lower 
layers of the quiet atmosphere tend to take on the temperature of the 
ground. Such differences are particularly common in the lower-lying 
portions of any locality. The}'' do not occur in an active atmosphere; a 
breeze will quickly level any marked differences in the temperature of 
any neighboring strata of air by intermingling of the lower and higher 
layers resulting in an approximately uniform temperature. The effect 
of wind movement on the diurnal range of temperature may be clearly 
shown by classifying a large number of days according to total daily 
wind movement, days which in other respects have approximate!}^ similar 
conditions. The results of such a classification are graphically shown in 
Fig. 15. An equal number of clear or approximately clear days was 
selected in each of the months of January, March, July, and October. 
Those having a total daily wind movement of less than 100 miles 
per day were placed in one group; another group contained days with a 
total daily wind movement between 200 miles and 300 miles; still an- 
other group comprised winter and summer days with a wind movement 
exceeding 400 miles per day. The average hourly temperature was then 
determined for each group separately and a comparison made between the 
resulting temperatures. In each case the diurnal range of temperature is 
seen to be markedly lower with increase in wind movement. In the 
following tabular statement tlie total daily range for each condition 
mentioned above is given, while in the .succeeding table the hourly 



MARYLAND WEATHER SERVICE 



71 



3 6 9 Noon 3 6 9 Mot. 



3 6 9 Noon 3 6 9 Mot 



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Fig. 15. — Effect of Wiud Velocity on tlie Hourly Variations of Temperature. 
(a) On days with a light wind. 
ib) On days with a moderate wind. 
(c) On days with a high wind. 



72 



THE CLIMATE OF BALTIMORE 



changes are shown for the year, expressed in terms of departures from 
the normal temperatures for the year. 

Eange of Temperature on Calm axd AYixdt Days. 

Total daily wind movement. Jan. March. July. Oct. Year. 

Less than 100 miles 14.6° 15.4° 19.5° 18.9° 16.8° 

From 200 to 300 miles.... 5.9° 12.0° 11.0° 8.2° 9.0° 

Over 400 miles Winter 5.2° Summer 5.7° 5.4° 



hourly temperature on calm and on windy days. 

(Expressed in terms of departures from the normal temperature.) 



Hours : A. M. 1 

J 


2 


3 4 5 


6 


7 


8 


9 


10 


11 


Noon. 




Normal Temper- 


51.7° 
- 2.6 
-3.6 
-11.1 


51.1° 
- 2.7 
-3.5 
-11.2 


50.5° 
-2.6 
-3.4 
-11.2 


49.9° 
- 2.6 
-3.3 
-11.2 


49.4° 
-2.6 
- 3.2 
-11.3 


49.4° 
- 2.6 
-3.5 
—11.9 


50.2° 

- 2.6 

- 4.6 
-13.2 


51.8° 
- 1.9 
-5.0 
-14.5 


53.7° 

- 1.0 

- 5.4 
-15.7 


55.6° 
+ 0.1 
- 5.7 
-16.6 


57.5° 
+ 1.0 
— 6.4 
-17.7 


59.0° 
+ 1.7 
- 6.2 
-18.1 




50-100 Miles (De- 
partures) 

200-300 Miles (De- 
partures) 

Over 400 Miles 
(Departures).. 




Hours : P. M. 


1 


2 


3 


4 


5 


6 


7 


8 


9 


10 


11 


Mid- 
night. 


Means. 


Normal Temper- 
ature 

50-100 Miles (De- 
partures) 

200-300 Miles (De- 
partures) 

Over 400 Miles 
(Departures).. 


60.1° 
+ 2.1 
— 6.4 
-18.5 


61.0° 

+ 2.1 

- 6.4 

-18.7 


61.3° 
+ 2.3 
-6.5 

-18.9 


61.1° 
+ 2.2 
-6.4 
-19.1 


60.2° 
+ 2.1 
- 6.4 
-19.2 


59.0° 
+ 1.6 
-6.3 
-18.6 


57.5° 
+ 1.7 
- 6.3 

-18.1 


56.3° 
+ 1.2 
— 6.2 
-17.5 


55.0° 
+ 1-1 
- 6.0 

-17.0 


54.1° 
+ 1.1 
-6.0 
-16.7 


53.2° 
+ 1.0 
-6.0 
—16.2 


52.5° 
+ 1.1 
-6.1 
-16.1 


55.0° 
+ 0.1 
— 5.3 
—15.8 



EeDUCTION TO THE TrUE MeAN TEMPERATURE. 

As it is often inconvenient or impossible to make daily observations 
of the temperature at the hours best suited to the purpose of securing an 
accurate average value, it is desirable to know the corrections to be 
applied to any selected combination of hours in order to arrive at a true 
average value for the day for a given locality. This can readily be done 
whenever hourly observations, or continuous records, have been main- 
tained somewhere within a hundred miles or so of the locality, pro- 
vided the physiographical conditions of the two localities do not differ 
widely from one another. In the following table the necessary correc- 
tions have been computed for the horizon of Baltimore for some of the 



ilARYLAXD WEATHER SERVICE 



combinations of hours of observation employed at different times within 
the State of Maryland. 

CORRECTIONS TO REDUCE OBSERVED TEMPERATURES TO THE TRUE 

DAILY MEAX. 



Hours of Observation : — (75th 
Meridian Time.) 



J (7:37 a. + 4:37 p. + 11:37 p.). . .. 
4(7:00 a. + 2:00 p. + 9:00 p.).... 
i (7:00 a. + 2:00 p. + 3(9:00 p.) . . . . 
i (7:00 a. + 3:00 p. + 11:00 p.) . . . . 

J 7:00 a. + 3:00 p. + 10:00 p.) 

A (10:00 a. + 10:00 p.) 

J (Maximum + Minimum) 

H8:00a. + 8:00 p.) 

i(Ta. + lla.+3p. + 7p. + llp.) 



+0.2+0.1+0.1+0.1 
—0.3—0.3—0.3—0.3—0.5 
—0.3—0.4—0.4—0.4—0.3 



0+0.1+0.1 



-0.3-0.2 



+0.5 

0.4 

+1.1 

-0.5 



-0.2 
+0.4 
-0.4 
+1.3 



+0.4 

-0.3 

+1 

-0.6;-0. 81—1.1 



-0.3 
-0.1 



+0.8 



+0 
-0.3 

+0.1 
+0.7 
-1.2 




0.7 
0.4 

0.3 
-0.2 
+0.2 
+0.5 
-1.3 



+0.21+0.3 



-0.5 
-0.3 
+0.3 
-0.1 
-0.2 



0.4 
-0.1 
+0.3 

0.1 
-0.1 

+0.1 0-0.3 
+0.8+1.3+1.1 
-1.1-1.2—1.0—0 



+0.3 
-0.2 

+0.3 
+0.1 
+0.1 



+0.5 
-0.2 

+0.4 
+0.1 
+0.2 
0.4 
+1.6 



+0.5 
-0.3 
0.2 
+0.3 
-0.1 
+0.3 
-0.5 
+1.5 



+0.3 
-0.3 
-0.3 

-0.2 
+0.7 
-0.5 
+1.3 



-0.6-0.6 



0.3 

0.4 
-0.3 
+0.1 

0.2 
+0.3 

0.4 
+ 1.0 
-0.9 



In a system of three hours of observation the combination 7 a.m., 3 p. m. 
and 11 p. m., gives a mean value very close to the 24-hour ly mean, the 
annual average differing from the latter by only 0.1°, while the maximum 
departure is but + 0.4° during the month of widest divergence. During 
four months of the year, namely, January, February, June and December, 
no corrections need be applied. One of the best combinations of two hours 
is that of 10 a. m. and 10 p. m., which yields an average but 0.2° above 
the true annual mean. The maximum and minimum readings of self- 
registering thermometers require a correction of — 0.4° to the annual 
average. Considering the great convenience of one observation a day over 
two or more and the further advantage of showing the highest and lowest 
temperatures, this is the most desirable system to adopt. The United 
States Weather Bureau maintains an organization of about 3500 co- 
operating voluntary observers, all reporting daily maximum and minimum 
temperatures. 

The Hourly Eate of Change. 

While the temperature increases steadily from sunrise to about 3 p. m. 
and then steadily decreases to sunrise, the rate of warming and cooling 



74 



THE CLI^[ATE OF BALTIMORE 



has its own period which differs from that of tlie temperature itself. 
The temperature rises most rapidly from 8 a. m. to 10 a. m., depending 
upon the season of the year, and falls most rapidly from 6 p. m. to 7 p m. 
The hours of least change coincide with those in whieh the maximum 
and minimum temperatures of the day occur. In selecting a combination 
of hours for observation it is important to bear in mind this varying rate 
of change, and to avoid as far as practicable the hours of maximum rate. 
Consideration of this point is of no consequence when maximum and 




Fig. 16. — Hourly Rate of Change of Temperature. 

Fiif. 16 shows the extent of change in the temperature from hour to hour throug-hout the 
day and year. Tlie values are based on hourlj' records during a period of ten years. The 
houi s of the day are indicated liy the upper horizontal line of figures, and the months of the 
year by the marginal letters. The areas without shading show the time of day when the 
change in temperature is least, the heavj- black line within this area marking the time of 
change from falling to rising, or rising to falling temperature. The areas with darkest 
shading show the time of most rapid change. A falling temperature is designated by a 
minus sign, a rising by absence of sign, before the figure representing the amovnit of change 
in degrees and tenths. The dotted lines marked S.R. and S.S. show the time of sunrise and 
sunset. The time of most rapid rise in the temperature is between 8 a. m. and 9 a. m., the 
time of most rapid fall is between 7 p. m. and 8 p. m. See Table XIII and Fig. IT. 



minimum thermometers are employed, or when a continuous record of the 
temperature is maintained. The approximate time at wdiich the rate of 
change is greatest and least for each month and for the year is shown 
below in connection with the hours of maximum and minimum temper- 
ature of the dav. 



MARYLAXD WEATHER SERVICE 



VO 



TIME OF DIURXAL MAXIMUM AND MIXIMUM RATE OF WARMIXG AND 

COOLIXG. 





c 


i 


o 


3:30 


>> 

S 

3:.30 


05 

a 


3 

3:00 


< 
3:00 


P. 
® 
t/J 




o 


6 


a 

3 

a 
c 
< 


Time of Max. temp. (p. m.). 


3:00 


3:30 


4:00 


3:00 


3:00 


3:00 


3:00 


3:00 


3:00 


" Min. rate (p. m.).. 


3:00 


3:00 


3:00 


3:30 


3:30 


3:(K) 


3:00 


3:(K) 


3:(M) 


3.00 


3:00 


3:1 


3:00 


" " Min. temp. (a. m.). 


T:(KI 


T:00 


6:00 


6:00 


5:(J0 


5:(10 


5:00 


5:00 


6:(K) 


6:00 


7:00 


7:00 


6:00 


" " Min. rate (a. m.).. 


7:(H) 


6;()0 


6:00 


5:30 


5:U0 


4:30 


5:00 


5:30 


6:(M) 


6:00 


6:30 


6;.30 


5:30 


" Max. rate (a. m.).. 


1000 


10:00 


10:00 


9:01) 


9:(H) 


H-.m 


S;30 


f<:(H) 


9:U0 


9:00 


9:00 


10:30 


9:30 


" *' Max. rate (p. m.).. 


6:30 


5:30 


6:30 


7:00 


7:00 


7:00 


7;30 


7:30 


6:30 6:30 


6:30 


6:00 


6:30 



TABLE XIII.-MEAX HOURLY CHANGE OF TEMPERATURE. 
(Expressed in degrees and tenths of a degree.) 



Midn't tol a. m.. 

1- 3 

2- 3 

3- 4 

4- 5 

5- 6 

6-7 

7-8 

8- 9 

9-10 

10-. 1 

11-Xoon... 
X^oon-1 p. m.. 

1- 3 

2- 3 

3- 4...'.... 

4- 5 

5- 6. 

6- 7 

7- « 

8- 9 

9-10 

10-n 

11-Midn't.. 



— .4 — .O 

— .4 - .4 

— .4 — .4 



2 •> 



.,T — . t — 



.5 

.4 
.3 
.3 

i!o 

1.7 
1.7 
1.8 
1.7 
1.3 
1.2 
.5 
.1 



1.4 - 

8, 

i 

.6 

.1 
1.0 
2.1 



1.3 

.8 



3.0 
1.9 
1.5 
1.1 



— 1. 



—1.2 -1-0 - 



— .81 —1.1; — : 



1.0 
1.4 

1.1; 

1.1; 
9, 



— .3 — .5 — .8 —1.1 — .6 — 



.0 
.6 
1.1 
1.7 
1.3 
1.3 
1.1 
.9, 
1.1 



- .5 
.4 
1.6 
2.3 
2.1 
1.8i 
1.9 
1.3 
1.1 
1.0 



-1.3 
-3.0 
-1.8i 
-1.5i 
-1.1 
-1.1 
.6 



- .8 

- .8 

- .6 

- .6 
.9 

2.1 
2.3 
2.3 
1.8 
1.7 
1.3 
1.3 
.9 
.3 

- .1 

- .9 
-1.3 
-1.9 
-1.6 
-1.5 
-1.3 
-1.1 
-1.0 



.3 
1.8 
2.3 
3.5j 
1.9; 
1.8 
1.5 
1.0 

.6 

- .1 
-1.0 
-1.3 
-1.7 



- .9 

- .6 

- .8 

- .6 

- .6 
.1 

1.6 
3.6 
3.4 
2.3 
3.1 
1.2 
1.1 

!i 

- .3 
-1.0 

-1.1 

-1.7 
-1.8 
-1.4 
-1.3 
-!.& 
-1.0 



- .5 

- .6 

- .8 

- ie 

- .3 
.9 

2.6 
2.3 
2.5 
2.1 
1.6 
l.ll 
.8 
.3 

- .1 
-1.3 
-1.7 
-1.8 
-1.4 
-1.6| 
-1.1 



- .3 

- .6 

- .6 

- .6 

- .4 

- .4 
.3 

3.0 
2.4 
2.4 
2.2 
1.7i 
l.l' 
.8 
.2 

- .4 
-1.3 
-1.6 
-1.7 
-1.4 
-1.3 
-1.0 

•9: 



- .5 

- .3 

- .4 

- .4 

- .4 

- .1 
.4 

1.3 
1.5 
1.9 
1.5 
1.3 
.9 
.4 
— .5 
-1.0 
-1.3 -1.0 
-1.1 — .9 



- .4 

- .3 

- .1 
.9 

3.0| 
1.9' 
1.9i 
1.6 
1.31 
.6 

- .6 

- .8 



— .« -1.0 -1.1 - .8 — .1 



— .6 
.8, - .6 



- .6 

- .5 
.0 
.8 

1.6 
1.9 
1.9 
1.9 
1.5 
.9 
.9 



-1.3 
-1.5 
-1.2 
-1.3 

- .9 

- .9 

- .7 



Table XIII shows the average amount of change in temperature from hour 
to hour in each month and in the year. The minus sign preceding a number 
indicates a fall in temperature; numbers without a sign show a rise in 
temperature. For example, from midnight to one a. m., the temperature 
falls, on the average, four-tenths of a degree in the month of January, one 
and four-tenths in April, and eight-tenths, on the average, for the entire 
year. The results are also graphically shown for each month in the year in 
Fig. 10, and for the year, in Fig. 17. The values are based on hourly observa- 
tions for a period of ten years. 



In Table XIII, the average amount by wliich the temperature changes 
from liour to hour througliout the day is rocordeil for t-acli mouth and 
6 



76 



THE CLIMATE OF BALTIMORE 



for the year. These values are derived from ten years of hourly observa- 
tions from 1893 to the close of 1902, In Fig. 16, the values are graph- 
ically shown for each month of the year in terms of departures above and 
below a line separating the rising from the falling temperatures. In 
Fig. 17 the curve representing the average hourly rate of change in 
temperature for the year is drawn in connection with the average hourly 
pressure curve. As noted above in the paragraph on the diurnal variation 
of the barometer there is a close resemblance between these two curves, 
suggesting some causal connection between the diurnal warming and cool- 
ing of the atmosphere and pressure changes. 



-to 



\ 1 — ^ 




/'''T^S-s i 


\/A I 1 1 ^J ^ ^^_^ 


T^f-/-| rV-V ' — H h+- 


U- jU- -^ ^i,^±^^^ i^-^SU- 


__ _ .^V _ 2^ _±+ _ _ it - 


:iiii::ii-±^gii:iiii:^ii#iiiii:+iiiHg 


:^::==4^:^==="=====+:^+^fci::±^";^itifc 


- - z^.^^^ - -^t;^ "TV-f-^ /-^t+t 


^.^^ :::^_4__^_^_^ _^ ^^s-h- v - ^' 


-^+ ^J^ J U- -U-.^^ h^^^ J7^*^- 


== =^=F =i+=T= =" ==d==4=r--^-'f =^ 4+ 


"=1^=4="=n^4r"'4'4l?r' " 


. 1_^ i 1 1 M i M ' 1 — ' — 


— ___ 1 . . ^ ' - . . . ..... 1 . — . — , — _ 



+015 



Fig. 17. — Curves Representing the Average Hourly Pressure (a), and the Hourly 
Rate of Change in Temperature (b), for the Year. 



Mean Daily Temperature. 

Just as the daily change in altitude of the sun causes a daily rise and 
fall in temperature, so the annual variations in altitude give. rise to an 
annual rise and fall in temperature. In the. diurnal period, the highest 
temperature is attained about three hours after the sun reaches the mer- 
idian; in the annual period the maximum temperature is reached from 
three to four weeks after the sun attains the greatest elevation. While 
the lowest temperature of the day occurs about sunrise, the minimum for 
the vear lags four to five weeks behind the time of lowest seasonal altitude 



ilAKYLAXD WEATHER SERVICE 



77 



of the Sim. The steady advance and retreat of the sun in his annual 
course would probably cause a uniform increase in temperature from day 
to day from winter to summer, and a corresponding decrease to the winter 
months, if the character of the earth's surface were uniform. The distri- 
bution of land and water surfaces is doubtless responsible for the irreg- 
ular character of the curve representing the annual changes of temperature 
when constructed from mean daily temperatures. 



TABLE XIV.— MEAN DAILY TEMPERATURE. 
(Corrected to hourly mean.) 



1. 

S. 
i. 
5. 
6. 

S. 

9. 
10. 
11. 
12. 
13. 
14. 
15. 
16. 
17. 
18. 
19. 
20. 
21. 
22. 
23. 
24. 
2.5. 
26. 
27. 
28. 
29. 
30. 
31. 



Jan. 


Feb. 


33.5 


33.9 


34.3 


31.5 


32.2 


33.9 


33.2 


3:3.8 


33.2 


30.9 


33.0 


32.7 


34.4 


34.3 


&3.7 


35.3 


33.0 


34.7 


32.2 


34.5 


31. li 


.36.1 


.34.0 


36.7 


33.2 


36.1 


34.8 


a5.3 


33.1 


35.9 


34.6 


35.7 


33.8 


36.5 


33.2 


37.5 


33.9 


.37.3 


34.2 


36.3 


36.0 


37.5 


35.5 


38.1 


35.1 


37.1 


.32.2 


a5.3 


32.4 


37.4 


32.5 


37.7 


.33.8 


36.0 


.34.8 


35.9 


33.6 


36.6 


33.0 




34.3 





38.2 
38.0 
38.7 
37.8 
36.8 
39.0 
40.2 
39.5 
40.3 
43.1 
41.4 
42.8 
41.8 
40.6 
39.0 
39.9 
39.2 
39.8 
43.2 
41.4 
41.2 
42.2 
41.2 
41.0 
42.0 
42.5 
44.0 
45.0 
43.4 
44.5 
46.2 



47.0 
49.2 
48.8 
48.4 
48.7 
49.8 
49.8 
50.6 
.50.2 
50.2 
50.6 
51.2 
52.4 
.55.2 
53.2 
52.8 
52.7 
.53.4 
55.2 
55.4 
.55.9 
55.6 
57.2 
57.6 
.56.4 



.58.0 
57.4 
59.7 



59.3 
60.5 
58.9 
59.5 
61.2 
60.9 
60.7 
61.7 
64.9 
66.2 
65.8 
64.5 
62.7 
61.9 
63.8 
64.0 
63.4 
63.9 
&5.5 
66.5 
66.4 
66.1 
65.2 
65.7 
67.1 
67.5 
67.3 
68.6 
67.3 
68.8 
70.3 



rune 


July 


Aug. 


Sept. 


Oct. 


Nov. 


70.2 


75.5 


76.6 


72.4 


62.5 


51.5 ' 


69.0 


75.7 


76.4 


71.8 


61.6 


53.4 


71.2 


78.1 


76.6 


7'' 7 


62.2 


50.1 


71.4 


78.7 


76.6 


72.0 


63.0 


47.7 


71.2 


77.5 


76.7 


72.4 


60.9 


48.5 


71.6 


77.. 5 


77.0 


73.6 


60.7 


49.0 , 


70.4 


77.7 


76.8 


71.8 


59.0 


48.8 ' 


72.1 


77 7 


77.0 


71.4 


.59.4 


49.6 


72.6 


77.9 


77.4 


70.6 


59.0 


49.9 ; 


72.3 


77.9 


77.5 


70.3 


59.0 


49.3 


72.6 


77.7 


76.8 


68.6 


58.7 


47.7 


72.4 


77.6 


76.8 


69.7 


.57.4 


47.1 


72.4 


78.5 


76.2 


69.6 


58.0 


46.3 


73.3 


78.2 


75.6 


67.6 


58.1 


44.9 


73.8 


78.9 


75.6 


67.1 


56.9 


45.3 


72.8 


79.6 


75.4 


69.0 


57.8 


46.2 


73.9 


78.9 


75.0 


68.8 


.57.4 


45.7 


73.8 


79.1 


75.6 


67.7 


57.2 


46.3 


74.6 


77.3 


75.4 


69.3 


56.2 


44.3 


75.6 


77.3 


75.4 


66.8 


54.7 


42.0 


75.6 


77.5 


75.8 


64.6 


54.6 


41.3 


75.2 


76.7 


75.2 


64.8 


54.3 


42.8 


75.1 


77.6 


74.3 


65.4 


55.0 


43.7 ' 


76.1 


76.8 


74.6 


65.2 


54.1 


41.7 


75.9 


7';. 3 


74.4 


64.8 


.53.1 


41.5 


77.fi 


78.7 


74.0 


65.4 


53.4 


41.6 


76.2 


78.2 


72.8 


64.0 


53.8 


41.9 


76.6 


77.1 


72.1 


64.1 


o«.o 


40.8 


76.2 


77.4 


72.6 


64.0 


.52.6 


38.1 


75.6 


77.5 


73.4 


62.9 


52.0 


36.2 




76.9 


73.2 




50.6 





36.3 
38.3 
38.5 
39.0 
37.9 
38.3 
39.3 
38.6 
38.7 
37.8 
40.0 
39.9 
39.7 
38.3 
36.9 
36.2 
36.3 
36.3 
35.9 
33.9 
a5.7 
37.3 
37.8 
36.5 
35.5 
34.3 
34.0 
33.9 
33.7 
34.1 
33.8 



Table XIV shows the mean temperature for each day of the year as derived 
from the daily maximum and minimum temperatures for 30 years, from 1871 
to 1900. To the average daily values derived from these observations, cor- 
rections have been applied to reduce them to the true mean based on 24 hourly 
observations. The altitude of the thermometers varied from 40 to 60 feet 
above the ground. 



In Table XIV the average temperature for each day of the year is 
shown, based upon the daily maximum and minimum temperature for a 
period of thirty years. The corrections wliieli were found necessary in 



78 THE CLIMATE OF BALTIMORE 

the preceding paragraphs, in order to reduce these values to the true 
daily mean based on 2 4-hour ly observations, have been applied in this 
table. In Plates III and IV the daily mean temperatures for the same 
period (1871-1900) of thirty years, are shown graphically in curves B. 
The irregular serrated appearance of these curves is very marked. The 
advance and retreat of the seasons is accomplished by a succession of 
waves of rising and falling temperature, of unequal period, but averaging 
about three to four days. These changes accompany the areas of high 
and low atmospheric pressure which pass in continual succession from 
west to east within the temperate zones of the northern and southern 
hemispheres, and which have become familiar to us in the daily weather 
charts now isssued by nearly all national governments. 

A study of the curves representing the daily temperatures for the year, 
shows a greater variability in the winter months than in the summer 
months. This is more readily recognized in the curves of extreme tem- 
peratures (Plate IV, curves A and C), than in those representing the 
average temperature for a long period (curve B). In the past thirty 
years the temperature has been lowest, on the average in Baltimore, on 
the 5th of February (30.9°). The day having the highest average tem- 
perature of the year is the 16th of July (79.6°). Hence the temperature 
rises during 161 days and falls during a period of 204 days. From 
April 20th to October 23rd the temperature remains above the average 
for the year; from October 23rd to April 20th it is below. The temper- 
ature rises most rapidly in March and falls most rapidly in November. 

As stated above, the temperature of the air at the earth's surface lags 
behind the temperature of direct solar radiation nearly a month, the latter 
attaining a maximum value on June 22nd, the former about July 17th, 
at Baltimore. This lagging effect is particularly noticeable in the tem- 
perature of late summer and the autumn. On the 22nd of March and of 
September the direct rays of the sun which fall upon Baltimore are 
presumably of approximately equal intensity, as the sun is at these times 
directly over the equator. The temperature of the air, however, screened 
from the direct rays of the sun, is 65° on September 22nd, wliile it is 
only 42° on the 22nd of March. This marked difference between the 



MARYLAXD "WEATHER SERVICE 79 

temperatures of corresponding days of the ascending and descending 
branches of the annual curve holds good throughout the year. 

TABLE XV.-MEAX DAILY CHANGE OF TEMPERATURE. 
(Expressed in degrees and tenths of a degree.) 





Jan. 
—0.4 


Feb. 


Mar. 


Apr. May June 


July 


Aug. 


Sept. 


Oct. Nov. 


Dec. 


0- 1 


—0.5 


1.7 


0.4 —0.5 -0.2 


0.0 


-0.2 


-0.6 


-0.2 1.0 


0.1 


1-3 


0.8 


-2.4 


-0.2 


2.2 1.2 -0.2 


0.3 


—0.2 


-0.6 


-0.9' 0.9 


2.0 


2- 3 


-3.1 


2.4 


0.7 


—0.4 —1.6 2.2 


2.4 


0.2 


0.9 


0.6 -2.3 


0.2 


^4 


1.0 

0.0 

-0.2 


-0.1 
—2.9 

1.8 


-0.9 

—1.0 

2.2 


-0.4 0.6 0.2 
0.3 1.7 -0.2 
1.1 -0.3 0.4 


0.6 
-1.2 

0.0 


0.0 
0.1 
0.3 


-0.7 
0.4 
1.2 


0.8 -2.4 

—2.1 0.8 

0.2 0.5 


0.5 


4-5 


—1.1 


5-6 


0.4 


6-7 


1.4 
-0.7 
-0.7 
-0.8 


1.6 

1.0 

-0.6 

—0.3 


1.2 

-0.7 

0.8 

2.8 


0.0 —0.2 -1.2 

0.8 1.0 1.7 

-0.4 3.2 0.5 

0.0 1.3 -0.3 


0.2 
0.0 
0.2 
0.0 


-0.2 
0.2 
0.4 
0.1 


-1.8 
-0.4 
-0.8 
-0.3 


—1.7 -0.2 
0.4 0.8 

—0.4 0.3 
0.0 -0.6 


1.0 


7 8 


— O.T 


8- 9 


0.1 


9-10 


-0.9 


10-11 


-0.6 
2.4 


1.6 
0.6 


-1.7 
1.4 


0.4 -0.4 0.3 
0.6 -1.3 -0.2 


-0.2 
-0.1 


-0.7 
0.0 


-1.7 
1.1 


-0.3 —1.6 
-1.3 -0.6 


•> 


11-12 


-0.1 


12-13 


-0.8 


-0.6 


-1.0 


1.2 -1.8 0.0 


0.9 


-0.6 


-0.1 


0.6 -0.8 


-0.2 


13-14 


1.6 


-0.8 


—1.2 


3.8 -0.8 0.9 


—0.3 


-0.6 


—2.0 


0.1 -1.4 


—1.4 


14-15 


-1.7 


0.6 


-1.6 


—2.0 1.9 0.5 


0.7 


0.0 


-0.5 


-1.2 0.4 


-1.4 


15-16 


1.5 


-0.2 


0.9 


-0.4 0.2 —1.0 


0.7 


-0.3 


1.8 


0.9 0.9 


-0.7 


16-17 


-0.8 


0.8 


—0.7 


—0.1 0.6 1.1 


-0.7 


-0.4 


-<l.2 


-0.4 -0.5 


0.1 


17-18 


-0.6 


0.1 


0.6 


0.7 0.5 -0.1 


0.2 


0.6 


-1.1 


-0.2 0.6 


0.0 


18-19 


0.7 


-0.2 


3.4 


1.8 1.6 0.8 


-1.8 


-0.2; 


1.6 


—1.0 -2.0 


-0.4 


19-20 


0.3 


-0.1 


—1.8 


0.2 1.0 1.0 


0.0 


0.0 


—2.5 


—1.5 -2.3 


-2.0 


20-21 


1.8 


1.2 


-0.2 


0.5 —0.1 0.0 


0.2 


0.4 


—3.2 


-0.1 -0.7 


1.8 


21-22 


—0.5 


0.6 


1.0 


-0.3 -0.3 -0.4 


-0.8 


-0.6 


0.2 


-0.3 1.5 


1.6 


22-23 


-0.4 
—2.9 


-1.0 
-1.8 


-1.0 
-0.2 


1.6 —0.9 -0.1 
0.4 0.5 1.0 


0.9 

-0.8 


—0.9 
0.3 


0.6 
-0.3 


0.7 0.9 
—0.9 —2.0 


0.5 


23-24 


-1.3 


24-25 


0.2 
0.1 


2.1 
0.3 


1.0 
0.5 


—1.2 1.4 -0.2 
1.1 0.4 1.7 


0.5 
1.4 


-0.2 
-0.4 


-0.4 
0.6 


-1.0 -0.2 
0.3 0.1 


-1.0 


2.5-26 


-1.2 


26-27 


1.3 


-1.7 


1.5 


0.3 -0.2 —1.4 


-0.5 


-1.2 


-1.4 


0.4 0.3 


-0.3 


27-28 


1.0 


-0.1 


1.0 


0.2 1.3 0.4 


—1.1 


-0.7; 


0.1 


-1.5 -1.1 


-1.1 


28-29 


-1.3 


0.7 


-1.6 


—0.6 —1.3 —0.4 


0.3 


0.5 


-0.1 


0.3 -2.7 


0.8 


29-30 


-0.6 




1.1 


2.3 1.5 -0.6 


0.1 


0.8 


-1.1 


-0.6 -1.9 


0.4 


30-31 


1.3 
1.0 


1.0 


1.7 
1.2 


1.5 


-0.6 


-0.2 




-1.4 


-0.3 




0.8 1.0 0.6 


0.6 


0.4 


0.9 


0.7 1.0 


0.8 







Table XV shows the change in the mean daily temperature from day to 
day throughout the year. The minus sign indicates a fall in temperature 
while absence of the sign indicates a rise. For example: The 1st of January 
is 0.4° colder, on the average, than the day preceding; the 9th of May is 3.2" 
warmer than the 8th of May, etc. The same results are shown in curves B 
of Plates III and IV. These values are based on daily average temperatures 
for a period of thirty years. 



Average Inter-Diurnal Changes of Temperature. 

The changes in the average temperature from day to day are indicated 
with greater accuracy in Table XY than in the curves on Plates III and 
IV. The amount of rise or fall in temperature from day to day through- 
out the vear is given to tenths of a degree. The average variability is 



80 



THE CLIilATE OF BALTi:kIORE 



approximately 0.8°, and varies from 1.0° or 1.2° in the winter months 
to 0.5° or 0.6° in the summer months when the changes are considered 
without reference to sign. The month of greatest variability is March, 
while July is the month of least variability. The annual march of tem- 
perature shows some interesting periods of marked rise and fall, per- 
iods of three or more days during which there is a conspicuous departure 
from the path representing a steady progressive change. Such periods 




YiG. 18. — Inter-diurnal Temperature Changes. 

Fig. 18 sliows tlie average monthly frequency of changes of stated amounts in the mean 
temperature of the day from day to day. The marginal figures indicate the degree of 
change, and the heavy curved lines the frequency of stated changes. For example, a change 
of 2° in the mean daily temperature occurs on the average 3.2 times in .January, 2.6 times in 
February, 4.7 times in June, and 5.-5 times in August, etc. Increase in the intensity of the 
shading represents increase in the frequency of occurrence. See also Table XVII, 

have received a great deal of attention from European climatologists and 
there is an abundant literature of a popular as well as scientific character 
grouped about these special days. Throughout central and southern Eu- 
rope there is a popular impression that iiijuriou.s frosts are likely to occur 



MARYLAND WEATHER SERVICE. 



VOLUME 2, PLATE IN. 



5 IOI520 2530 5 l OlsaoaS 5 10 l-j gO SS-^U , ..i^^^ 30 ^ iOi.,,.,,.3 ::.:=m«mnim^^ | " '% , 7„!^ 




A. Average daily maximum temperature. B. Daily mean temperature. *'^^^ ''''''^ "unimuni temperature D D 



aily mean barometric pressure. 



MARYLAXD WEATHER SERVICE 81 

in the early part of May, and their coming is awaited with anxiety by 
the agricultural classes, especially in France and northern Germany. 
May 11, 12 and 13, are the days of most probable occurrence and these 
days have been variously designated as the " Three Ice Saints," the " Three 
Ice Men," etc., by the husbandmen. Similar regressions in temperature 
have been noted at other seasons of the year and carefully investigated 
but the spring drop occurring at a critical period in crop growth has 
received by far the largest share of attention. 

In a study of the tendency to the formation of frosts from the 10th to 
the 13th of May in Europe, Dr. Assmann has shown that the fall in 
temperature is first shown in Scandinavia, spreads in a southerly and 
then in a southwest direction over Central Europe. The maximum de- 
parture is attained on the 10th. Eeceding eastward, at first slowly and 
then more rapidly, it reaches the Russian provinces of the Baltic on the 
13th. Daily weather charts constructed by van Bebber show a pro- 
gressive departure of pressure which readily accounts for a period of 
northerly winds and clear skies and abnormally low temperature, first 
over northern Europe, then southward over central Europe and France. 

Careful study of the daily temperature at Baltimore does not dis- 
close a tendency toward a decided fall in temperature on these days. On 
the contrary, there is a distinct rise from the 9th to the 12th in place of 
the European fall (see curves A. B. and C, Plate III). A similar plus 
departure in temperatures at this time is disclosed in the daily temper- 
ature curves of other localities, namely, Washington, Norfolk, Nashville, 
Columbus, Ohio. The geographical extent of this marked departure has 
not yet been carefully investigated, but it is not confined to the localities 
named. A probable explanation of this phenomenon may be found in a 
periodic recurrence at this time of an area of high barometric pressure 
over the South Atlantic states, or an extension westward of the permanent 
area of high pressure over the North Atlantic in latitude of about 30° 
in conjunction with the development of a barometric depression in the 
Mississippi Valley. Such a pressure distribution invariably causes a rise 
in temperature above the average for the time of year in the central 
states and the Middle Atlantic states. 



82 



THE CLIMATE OF BALTIMOUE 



A similar period of marked rise occurs at Baltimore about March 7 to 
13 and again April 12 and 13. About June 20, and again about July 20, 
there is apparently a tendency for the temperature to fall below the 
normal for two or three days. These marked departures from the steady 
and uniform seasonal advance or retreat of temperature conditions do 
not always occur upon the same days, year after year, but the fact that 



TABLE XVI.— MEAN DAILY RANGE OF TEMPERATURE. 





Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


1 


12.0 


11.3 


14.4 


14.7 


18.0 


19.0 


15.9 


16.0 


15.9 


17.8 


15.9 


13.3 


2 


15.0 


13.9 


13.3 


17.1 


18.4 


17.1 


16.1 


15.3 


16.9 


16.7 


17.8 


14.1 


3 


13. 5 


14.5 


14.4 


17.2 


17.5 


16.6 


18.9 


15.9 


18.6 


18.2 


14.8 


13.7 


4 


13.1 


13.2 


14.5 


15.4 


16.9 


16.1 


17.9 


16.3 


17.1 


15.9 


15.3 


12.6 


5 


13.9 


12.5 


14.9 


17.4 


18.0 


17.4 


15.7 


16.6 


17.7 


16.4 


15.5 


13.4 


6 


11.7 


14.4 


15.9 


16.5 


19.1 


16.9 


16.4 


16.9 


16.9 


16.0 


15.2 


13.8 




12.0 


13.9 


16.5 


16.7 


16.3 


16.0 


17.5 


16.2 


16.5 


12.9 


16.0 


15.0 


8 


13.6 


13.6 


13.6 


16.5 


16.3 


17.4 


17.5 


17.3 


14.7 


16.5 


14.6 


13.3 


9 


13.8 


12.7 


14.4 


16.1 


21.4 


17.9 


10.1 


17.3 


16.1 


17.6 


14.7 


14.1 


10 


14.3 


14.5 


14.8 


15.2 


18.8 


16.7 


17.3 


15.x 


16.6 


19.1 


13.6 


14.2 


11 


12.7 


13.3 


15.8 


16.3 


18.0 


18.1 


17.5 


17.7 


13.2 


18.4 


12.7 


13.4 


12 


13.0 


13.5 


15.5 


17.5 


18.3 


17.7 


17.6 


16.3 


14.6 


14.3 


15.1 


13.3 


13 


12.4 


12.5 


15.6 


17.9 


18.3 


17.6 


17.3 


15.9 


14.6 


14.9 


16.3 


14.9 


14 


14.1 


13.1 


13.6 


18.2 


15.5 


18.3 


17.4 


15.5 


13.7 


14.6 


14.0 


14.4 


15 


12.4 


15.fi 


14.2 


15.2 


17.8 


17.2 


17.5 


15.6 


14.8 


18.1 


13.9 


14.2 


16 


12.6 


15.6 


13.8 


16.1 


17.4 


16.7 


18.0 


16.2 


16.8 


19.1 


14.8 


14.0 


17 


13.7 


15.9 


14.1 


15.8 


17.8 


16.2 


16.9 


15.7 


17.7 


18.5 


14.4 


13.9 


18 


12.9 


15.4 


15.7 


16.0 


17.0 


17.3 


16.7 


16.2 


16.6 


17.2 


14.9 


14.4 


19 


11.2 
13.4 


14.1 
14.6 


14.9 
13.6 


17.9 
16.3 


17.3 
16.2 


18.7 
19.2 


15.5 
16.4 


16.6 
17.3 


16.6 
15.4 


17.3 
16.6 


13.5 
11.8 


13.0 


20 


12.3 


21 


12.7 


15.1 


15.7 


18.2 


17.2 


17.3 


15.8 


17.3 


15.9 


16.7 


14.3 


13.6 


22 


14.4 


13.7 


15.3 


17.5 


16.9 


17.3 


16.6 


16.1 


17.2 


16.2 


13.4 


13.9 


23 


15.2 


14.9 


19.9 


18.5 


15.8 


17.8 


16.2 


14.8 


16.1 


15.1 


13.5 


14.6 


24 


12.3 


15.1 


17.3 


17.5 


16.9 


17.8 


16.0 


15.3 


15.2 


14.6 


12.8 


13.1 


25 


13.5 


15.3 


15.5 


15. 9 


16.5 


17.4 


16.7 


16.7 


16.3 


14.6 


12.4 


12.0 


26 


14.8 


14.2 


14.6 


17.4 


15.9 


17.0 


16.7 


16.2 


16.6 


15.9 


13.6 


11.6 


27 


14.4 
12.2 


14.8 
14.7 


14.1 
13.5 


17.3 
17.5 


16.3 

18.8 


15.3 
17.1 


15.0 
15.7 


14.5 
15.4 


16.1 
16.8 


14.2 
13.0 


15.1 
12.2 


12.6 


28 


12.1 


29 


14.5 


9.6 


14.5 


15.7 


17.9 


16.7 


16.2 


15.9 


15.5 


14.3 


13.3 


13.9 


30 


13.5 




16.8 


20.2 


18.4 


10.3 


15.4 


15.9 


15.6 


13.7 


11.4 


12.4 


31 


11.8 




15.9 




16.7 




15.5 


16.3 




14.7 




13.6 



Table XVI shows the average difference between the daily maximum and 
daily minimum temperatures for each day of the year, based on observations 
covering a period of 30 years. 



they persist in a curve representing average conditions for a long series 
of years (over thirty years in the Baltimore series) would seem to point 
to a decided tendency toward the formation of a given system of pressure 
distribution upon these days. This subject of the periodic recurrence of 
similar weather types is a matter worthy of more attention than has yet 
been given to it in this country. 



MARYLAND WEATHER SERVICE, 



VOLUME 2, PLATE I 




5 10 I5?02530 5 10 15 2025 i lO 15 202530 5 10 I5202S30 5 I 



BASED UPONDAiaO-^-^^'^NSPOU 30,,,^^^ 



A. Daily 



B n-, , C Daily minin'""?"^''^^- q. Ev 

temperature. B. Daily mean temperature. >-. ^^ l.\ 



treme range of temperature. E. Average daily range of temperature. 



MARYLAND WEATHER SERVICE 



83 



Average Daily Kaxge. 

The average maximum and minimum temperatures for each day of the 
3'ear for a period of thirty years are shown graphically in curves A and C 
on Plate III. These curves show the characteristics already described in 
considering the mea7i daily temperatures, which was to be expected as the 
latter were derived from the daily maximum and minimum. The average 
daily range of temperature, or the average difference between the highest 
and lowest readings for each day, is shown in Table XVI. The daily 
range is also shown on Plate III by the difference in value of correspond- 
ing points in curves A and C and directly in curve E on Plate IV. Dur- 
ing the winter months the range is least; it increases in the spring 
months, reaching a maximum in May and June, then decreases steadily 
to a minimum in January. As the daily range is largely dependent upon 
the amount of cloudiness and atmospheric movement, as shown in pre- 
ceding paragraphs, a considerable variation in the range from year to 
year in the same month may be expected. In January, for instance, with 
a range of 13.3° as an average for thirty-two years, it has varied from 
11.2° in 1891 to 17.0° in 1876. The March range has varied from 11.8° 
in 1891 to 22.0° in 1873. The smallest average range for any month 
occurred in Xovember 1884, namely 10° ; the greatest average was that of 
March, 1873, with 22.0°. When the daily range is averaged up for an 
entire year the variability is reduced to comparatively narrow limits. 
The annual average was smallest in the year 1882 (14.1°) and greatest 
in 1900 (16.9°). The ten-year averages have varied only between the 
limits 15.2° and 15.9°. 



AVERAGE DAILY RANGE OF TEMPERATURE. 



Period. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year 



1871-1880 13.8 14.2 15.9 

1881-1890 13.5 14.4 14.6 

18!tl-HK)0 13.5 13.8 15.0 



1871-1902 13.2 



14.1 



15.3 



16.1 
17.1 
17.2 

16.7 



17.6 
16.6 
18.0 

17.4 



16.7 
17.0 
18.0 

17.4 



16.3 14.8 15.7 

16.2 15.7 ; 15.1 

17.3 17.3 I 17.3 



16.7 



16.0 16.1 



16.8 
16.0 
16.7 



14.0 13.6 

13.5 13.3 

14.6 13.4 



16.3 14.0 



13.4 



15.5 
15.2 
15.9 



Diurnal Variability of Temperature. 
A climatic factor of the highest importance, especially to those who are 
not in the best of healtli, is the variability of tomporature conditions from 



84 



THE CLIMATE OF BALTIMORE 



day to day. The magnitude of the diurnal change may be represented in 
various ways : either by comparing the extremes of temperature of one day 
with those of the following, or the average daily temperatures, or readings 





o' 




)° 




2° 




3° 




\° 


5' 


6' 




7" 




B" 


9° 


10 






1 2 






14^ 






16 






18° 






20" 






2 


2 


" 




24 


• 




2 










' 






; 








1 










I 






, 


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-1 



Fig. 19.— Total Seasonal and Annual Frequencj' of Stated Diurnal Chang-es of 
Temperature. 

(a) Total Annual frequency. [d) Total Spring frequency. 

(b) " Summer " (e) " Winter " 

(c) " Autumn " 

rig. 19 shows the total number of stated changes In the mean daily temperature during 
each season and during the year. The upper horizontal line of figures indicates the degree 
of change, and the marginal figures to the right of the diagram show the frequency of 
stated changes. See also Table XVII. 



made at the same hour of the day. The frequencj of changes of a given 
amount will depend somewhat upon the method chosen for determining 
the daily change. "We have seen above that the normal change in temper- 



MARYLAND WEATHER SERVICE 



85 



ature from day to day varies from 0.5° or 0.6° in the summer months to 
1.0° or 1.2° in the winter months. But this average change for a long 
series of years is of less significance than the frequency of changes of a 
given amount. Large, and especially sudden, changes in temperature 
within short periods have never been considered particularly desirable 
from any point of view. Such changes may have advantages, but the un- 
comfortable, if not actually harmful effects, of rapid changes are sure to 
outweigh these. As a general rule proximity to the ocean will insure an 
equable temperature, free from sudden and large changes. Especially 



Jan. 


Feb. 


MCH. Apr. May 




UNE 






July 




Aug. 




Sept 


Oct 






Mo 


/. 




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Fig. 20.— Diurnal Changes of Temperature of less than 6°, 6° + , S°+ and 10° + 
each month. 

Fitr. 30 shows the f reaucncy of changes of 0° to 5°, of 6° and above, 8° and above, and of 10° 
and above, in the mean temperature of the day for each month of the year. The degree of 
change is indicated by the curved lines marked —6°, 6°+, 8°+, and 10°+ respectively, 
while the frequency of the stated changes is shown by the marginal figures to the right of 
the diagram. For example, a rise or fall of .5° or less in the mean temperature of the day 
occurs on the average 1" times in January, 21 times in May, and 25 times in July, etc. ; a rise 
or fall of l(J°or more in the mean daily tempemture occurs 6 times in January, 2 times in 
May, etc. See Table XVII. 

is this true on small islands, or along the western coasts of the continents 
where ocean winds prevail. The diurnal variability increases rapidly as 
the interior portions of the continental areas are approached. 

The changes in the daily average temperature at Baltimore, covering 
a period of thirty years, have been computed and arranged according to 
fro<liioncy and degree of change. These diurnal changes have also been 



86 



THE CLIMATE OF BALTIMORE 



grouped according to months, seasons, and the jesn, in the table which 
follows and presented graphically in Figs. 18, 19, 20 and 21. In the 
summer months changes of one, two, and three degrees largely predomi- 
nate, from which there is a rapid decrease in frequency of larger changes. 
In the winter months changes of one, two, and three degrees are still 
dominant, but the decrease in frequency as the changes increase is much 
more gradual. 

TABLE XVII.-FREQUEXCY OF STATED DIURNAL CHANGES OF TEMPERATURE. 



£ 




0) 




< 


>> 


o 

3 


3 
1-5 


3 
< 


*3 

C 
® 

CO 


O 


> 

o 
2; 


c5 


ii 
a 


Summer. 
Autumn. 


1 




0° 


2.0 


1.9 


1.7 


3.5 


2.5 


2.9 


3.5 


3.7 


3.6 


1.7 


1.9 


l.S 


6.710.2 8.3 


5.7 


.30.8 


1 


3.0 


3.4 


3.1 


3.2 


3.9 


4.4 


5.5 


5.4 


4.4 


3.6 


3.1 


3.1 


10.215.311.1 


8.545.0 


2 


3.2 


3.6 


3.6 


3.4 


4.3 


4.7 


5.6 


5.5 


4.3 


3.8 


3.6 


3.4 


11.115.711.7 


9.347.8 


3 


3.3 


3.7 


3.3 


3.4 


3.9 


4.2 


6.0 


4.9 


4.2 


4.2 


3.9 


3.5 


10. 6,14.012.3 


9.446.3 


4 


3.7 


3.7 


2.6 


3.3 


3.3 


3.3 


3.3 


3.3 


2.9 


3.5 


3.6 


3.0 


9.1 9.610.0 


8.437.1 


5 


3.6 


3.7 


2.6 


3.3 


3.0 


3.7 


3.6 


2.5 


3.6 


3.2 


3.3 


3.0 


8.9' 7.8 9.0 


8..3'.34.0 


6 


3.3 


3.3 


2.2 


3.6 


3.3 


3.0 


1.8 


1.5 


1.9 


3.3 


3.4 


2.5 


7.0 5.3 6.5 


6.925.7 


7 


2.1 


2.0 


2.0 


2 2 


1.9 


1.7 


1.3 


1.1 


1.8 


2.1 


3.0 


2.1 


6.1 4.2 5.8 


6.223.2 


8 


1.8 


1.7 


1.6 


lA 


1.4 


1.2 


0.7 


0.7 


1.2 


1.5 


1.4 


1.5 


4.5 2.6 4.2 


5.016.3 


9 


1.7 


1.5 


1.8 


1.3 


1.3 


0.9 


0.5 


0.5 


1.0 


1.3 


1.3 


1.3 


4.1 1.91 3.5 


4.614.1 


10 


1.3 


1.1 


1.6 


0.8 


0.9 


0.5 


0.3 


0.3 


0.6 


0.7 


0.9 


1.0 


3.3 1.1 2.5 


3.510.2 


11 


1.1 


0.9 


1.3 


0.7 


0.7 


0.3 


0.3 


0.1 


0.6 


0.6 


0.8 


1.0 


2.6 0.7 2.0 


3.0 8 3 


13 


1.0 


0.6 


0.7 


0.5 


0.4 


0.2 


0.1 


0.1 


0.3 


0.4 


0.6 


0.7 


1.7 0.4 1.3 


2.3; 5.6 


13 


0.8 


0.6 


0.6 


0.5 


0.3 


0.1 


0.1 


0.1 


0.3 


0.3 


0.4 


0.6 


1.3 0.3 1.0 


2.01 4.7 


14 


0.6 


0.4 


0.5 


0.3 


0.1 






0.1 


0.1 


0.3 


0.3 


0.5 


l.Oj 0.1; 0.5 


1.5j 3.0 


15 


0.3 


0.5 


0.4 


0.2 


0.1 


0.1 




0.1 


0.1 


0.2 


0.3 


0.4 


0.7 0.2' 0.5 


1.2 2.6 


16 


0.3 


0.3 


0.3 


0.1 




0.1 








0.1 


0.1 


0.3 


0.4 0.1 0.3 


0.8 1.6 


17 


0.1 


0.3 


0.2 


0.1 












0.1 


0.1 


0.2 


0.3 .. 0.3 


0.6 1.3 


18 


0.1 


0.2 


0.1 


0.1 












0.1 


0.1 


0.2 


0.2; .. 


0.2 


0.5i 0.8 


19 


0.1 


0.1 


0.1 


0.1 












0.1 


0.1 


0.1 


0.2 




0.3 


0.4 0.8 


20 


0.1 


0.1 




0.1 












0.1 


0.1 


0.1 


0.1 




0.1 


0.3 0.6 


21 


0.1 


0.1 




















0.1 








0.2 0.3 


29 




0.1 




























0.1 0.2 


^ 




0.1 




























0.1 0.2 


24 




0.1 






















oil 






0.1 0.2 


25 




0.1 




























0.1 0.1 


26 
































0.1 0.1 



The first column indicates the degree of change from day to day in the 
average daily temperature. The figures in the remaining columns show 
the average monthly, seasonal and annual frequency of occurrence of indi- 
cated changes, based upon observations during 30 years, from 1871 to 1900. 



The larger daily changes decrease in frequency on the approach of the 
summer month.*. A cliange of 10° in the average temperature of two 
consecutive days has occurred about 3.5 times in 30 years during each 
winter month and 9 times in each of the months of Julv and August. 



MARYLAND WEATHER SERVICE 



VOLUME 2, PLATE I 




A. Daily maximum temperature. 



B. Daily mean temperature. 



BASED UPON D.«lV0'^^^'ONSPo„3„^.^ 

C. Daily ."ini"'""*"""^' D. £ 



reme range of temperature. E. Average daily range of temperature. 



MARYLAND WEATHER SERVICE 



87 



The details of these changes are shown in the table above and in Figs. 
18, 19, 20 and 21. 

These figures and the diagrams reveal the interesting fact that the 
average departure and the most frequent departure are not identical, or 
that the arithmetical mean of all departures for any given month is not 
the most probable value. In the winter months the average departure is 
about 1°, in the summer months it is about 0.6°. The most probable 
departure is in all months larger than the average departure, as is clearly 
brought out in the following comparison of the average change from day to 
day and the most probable change. 

DIURNAL VARIABILITY. 





Jan. 


Feb. 


Mar. 


Apr. 


May June 


Julj' 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


Year 


Average change. .± 
Most probable 
change ± 


1.0° 
3° 


1.0° 
4° 


1.2° 
2° 


0.8" 
3^ 


1.0° 
2° 


0.6° 

2° 


0.6° 

2° 


0.4° 


0.9° 
1° 


0.7° 
3° 


1.0° 
3° 


0.8° 
3° 


0.8° 
2° 



The true measure of diurnal variability is the change in the average 
temperature from day to day. It is more convenient, however, to express 
this cliange by means of the difference between the highest or the lowest 
temperature of successive days, or between the temperature at any given 
hour, as 8 a. m. of one day to 8 a. m. of the following day, or from 8 p. m. 
to 8 p. m. The results will differ somewhat according to the method 
adopted. Assuming as correct the variability as measured by means of 
the daily average temperature, the variability based on differences of the 
minimum temperature from day to day will give too many changes under 
()°, while those based on the 8 a. m., 8 p. ni. and the maximum tempera- 
tures yield too few small changes, the departure from the true frequency 
being in the order named. On the other hand when we consider the larger 
changes of G°, 8°, or 10° and above, the minimum temperature yields too 
many. Take as an illustration the frequency of changes of 10° and over. 
During the course of a year of average temperature conditions there are -11 
diurnal changes of 10° or more, basing the count on the daily minimum, 
59 on the 8 a. m., 58 on tlie 8 p. m., and 83 on the maximum temper- 
atures. That is to say, the variability computed from differences in the 
uiaxiimiiii temperature from day to day may show more than double tlie 



MARYLAND WEATHER SERVICE. 



jtN Apr. July Oct Jan. Apb, Jul 



1817 1818 

Oct. Jan Apr. July Oct Jan Afr. Jl 





: . [ , [ ^ -| 


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VOLUME 2, PLATE VI. 



1819 
Oct. Jam Apr. Juiv Oct .lAr^j 



DEPARTURES OF MEAN MONTHLY TEMPERATURE FROM NORMAL 

FOR 87 YEARS. 



MARYLAND WEATHER SERVICE. VOLUME 2, PLATE VI. 

I860 1861 1862 1863 1864 

JAN. Apb. jul^ Oct. jam. Apr July Oct. j^n Apb July Oct. Jan. Apr. July Oct. Jan. Apr. July Oct. ja 



1865 




88 



THE CLIMATE OF BALTIMORE 



true frequency of the larger changes of 10° and above, while the smaller 
changes are below the true frequency; hence changes in the daily maxi- 
mum temperature are not a safe guide to the diurnal variability in the 
geographical horizon of Baltimore. In all cases the changes based upon 
observation of the maximum temjjerature from day to day differ most 
widely from those based on changes of the average daily temperature. In 
Fig. 21 and in the following table these results are shown graphically for 
changes under 6°, for 6°+, 8°+, 10°+ and 20°+, when the diurnal 





\ 


\ 




\ 










/ 




\ 


\ 

\ 


\ 


\ 


\ 








/ 






\ 


^ 




\ 






/ 




I 




\ 




\ 


\ 


\ 


> 


/ 





20°+ 10^ bV eV -0 

Fig. 21.— Diurnal Changes of Temperature of —6°, 6° + , 8° + , 10° + , 20° + . 

Fig. 21 shows the frequency of stated changes in the temperature from daj- to day when 
based on the minimum temperature of two successive days, on the mean temperature, on 
the 8 a. m., on the 8 p. m., and on the maximum temperatui-e. respectively. See also Fig. 20, 
and Table XVII. The frequency is indicated bj' the line of figures above the diagram, the 
degree of change, Vjy the line of figures below the diagram. 

variability is based on observations of the maximum, the minimum, the 
8 a. m. and 8 p. m. readings and on the true daily mean. Diurnal changes 
were computed for a period of 30 years from the daily average tempera- 
ture, and for a period of 10 years from the maximum, minimum, the 
8 a. m., and the 8 p. m. observations. 

FREQCENCV OF DIURXAL TEMPERATURE CHAXCiES OF STATED AMOUNTS. 
(Expressed in terms of departures from the frequencj- based on changes in the daily 

mean temperature.) 



Temperature changes. Minimum. 



Departure. 

Below 6° +11 

6°+ - 5 

8'+ i - 10 

10'+ - 9 

20°+ ' -0.4 



Mean. 



241 

119 

71 

41 

1.7 



Departure. 



— 16 
+ 19 
+ 21 

+ 18 

+2.7 



• p. m. 



Maximum. 



Departure. 



— 21 
+ 21 
+ 20 
+ 18 
+2.3 



Departure. 



+ 51 
+ 48 
+ 42 
+6.6 



MARYLAND WEATHEE .SERVICE 



89 



The smaller changes, under G°, increase in frequency very rapidly from 
February to July, then decrease at a similar rate to February. Changes 
of 6° and over occur most frequently in the months of December,. Jan- 
uary, February and March ; the decrease is then uniform until a minimum 
frequence is reached in August : then there is a more rapid increase to 

TABLE XVIir.-FIVE-DAY MEANS OF TEMPERATURE. 
(For five-day periods ending on given daj-s.) 



January. 


February. 


March. 


April. 


May. 


June. 


5th 33.3 


4th 33.5 


1st .37.0 


5th 48.4 


5th 59.9 


4th 70.4 


10 33.3 


9 as. 6 


6 38.1 


10 50.1 


10 63.9 


9 71.6 


15 as.s 


14 35.7 


11 40.9 


15 52.5 


15 6:^.7 


14 72.6 


20 33.9 


19 36.6 


16 40.8 


30 53.9 


20 64.7 


19 73.8 


25 34.3 


24 36.9 


21 41.0 


25 56.5 


25 66.1 


24 75.5 


30 33.5 




26 41.8 
31 44.6 


30 58.1 


30 67.9 


29 76.5 


Jul}-. 


August. 


September. 


October. 


November. 


December. 


4th 76.7 


3rd 76.8 


2nd 72.7 


2nd 63.0 


1st .51.8 


1st 3S.7 


9 77.7 


8 76.8 


7 72.5 


7 61.3 


6 49.5 


6 38.4 


14 78.0 


13 76.9 


12 70.1 


12 58.7 


11 49.1 


11 3S.9 


19 78.8 


18 75.4 


17 68.4 


17 .57.6 


16 46.0 


16 38.3 


24 77.2 


23 75.2 


23 66.6 


22 55.4 


21 43.9 


21 35.6 


29 77.7 


28 73.6 


27 65.0 


27 53.9 


26 42.3 


26 38.3 
31 33.7 



TEN-DAY MEANS OF TEMPERATURE. 

(For ten-da J- periods ending on given days. Derived from above table of five-day means.) 



January. 


February. 


March. 


April. 


May. 


June. 


10th 33.3 
30 33.6 
30 33.9 


9th 33.5 
19 36.2 


1st 37.0 
11 39.5 
21 40.9 
31 43.2 


10th 49.3 
20 53.2 
30 57.3 


10th 61.4 
20 64.3 
30 67.0 


9th 71.0 
19 73.2 
29 76.0 


July. 


August, 


September. 


October. 


November. 


December. 


9th 77.2 

19 78.4 
29 77.5 

I 


8th 76.8 
18 76.2 
28 74.4 


7th 72.6 
17 69.3 
27 65.8 


7th 62.1 
17 58.2 
37 54.6 


6th 50.7 
16 47.5 
26 43.1 


6th 33. 5 

16 38.5 

26 36.0 

Jan. 5 33.5 



Table XVIII shows the mean temperature for each successive period of 
five (lays beginning with January 1st, and also for each successive period 
of ten days. The 5-day and 10-day means were computed from the normal 
daily temperatures for the 30-year period 1871-1900, after reducing the latter 
to the true daily temperature based on hourly observations. 



December. A change of 20° in the average temperature of two successive 
days has occurred at Baltimore about 50 times in the 30 years from 1871 
to 1900. Of these occurrences 15 were recorded in February, 10 in 
January, 8 in December, 5 in ^larch, 5 in November, 2 in each of the 

7 



90 THE CLIMATE OF BALTIMORE 

months of Aprils, May, and October, none in the months of June, July, 
August and September. The most frequent change and hence the most 
probable, is a change of 2° in the spring and summer and 3° in the 
autumn and winter months. 

The Probable Error of the Meax Daily Temperatures. 

No law has yet been discovered governing the departures from the 
normal temperature of a year, month, or day. Departures above and below 
the normal for a long series of observations agree very closely in their 
distribution with chance occurrences. Hence the formula applicable to the 
latter has been employed in the determination of the probable error of 
average temperatures for a given period. 

The equation used for finding the probable error of the daily, monthly, 
and annual means of temperature for Baltimore is a form suggested by 
Fechner * and is as follows : 

J. ^ J. 1.1955 



^271 — 1 

in which E is the probable 

error, v the average departure from the normal temperature (in degrees 

Centigrade) not regarding the sign of the departure, and n the number 

of occurrences, in this case the number of years of observation. The value 

1.1955 
of the factor .'. ^ is as follows for the stated periods of observation : 
\/27l — 1 ^ 

20 30 40 50 60 70 80 90 100 yrs. 

0.191 0.156 0.134 0.120 0.110 0.102 0.095 0.089 0.085 
In order to determine the probable error of the daily mean temperature 
at Baltimore for the 30-year period, from 1871-1900, the above formula 
was applied to a representative day in each season, namely, for the 15th 
day of January, April, July and October. In winter (represented by 
January 15), the average departure v of the mean daily temperature from 
the normal is 7°, in spring (April 15), 5°, in summer (July 15), 4°, in 
the autumn (October 15), 6°. In individual cases these departures vary 
greatly. On January 15, 1871, the mean daily temperature was 62°, 

*See: Hann's Lehrbuch der Meteorologie. Leipzig, 1901, p. 107. 



MARYLAND WEATHER SERVICE 



91 



or 28° above the normal value; on January 15, 1893, the mean tempera- 
ture of the day was 22° below the normal. Thus the loth day of Jan- 
uary shows a range in the average temperature of the day of 50°. The 
extremes on April 15th were 17° above and 11° below, a range of 28° ; on 
July 15th 6° above and 12° below the average, a range of 18° ; on October 
15th the extreme departures were plus 15° and — 19°, a range of 34". 
These figures strikingly illustrate the variability of temperature conditions 
within short periods at Baltimore. 

THE FREQUENCY AND AVERAGE VALUE OF DEPARTURES FROM THE 
NORMAL DAILY TEMPERATURE. 



January 15th. 


April 15th. 


July 15th. 


October 15th. 


Departures. 


Departures. 


Departures. 


Departures. 


Fre- 
quency. 

+18 
-15 


Sums. 

+ 116.6° 
-116.5° 


Aver- 
age. 

+6.5° 

-7.8° 


Fre- 
quency. 

+13 
-20 


Sums. 

+84.9° 
—84.0° 


Aver- 
age. 

+6.5° 
-4.2° 


Fre- 
quency. 

+20 
-13 


Sums. 

+61.0° 
—60.2° 


Aver- 
age. 

+3.0° 
-4.6° 


Fre- 
quency. 

+17 
—16 


Sums. 

+106.6° 
—105.2° 


Aver- 
age. 

+6.3° 
-6.6° 


33 


233.1° 


7.1° 


33 


168.9° 


5.1° 


33 


121.2° 


3.7° 


33 


211.8° 


6.4° 



Entering the values of the average departure v in the formula we 
obtain as the probable error of the mean temperature of a typical winter, 
spring, summer, and autumn day the following values: 

January April July October Average Seasonal 

1.1° 0.8° 0.6° 0.9° 0.8° 

These figures represent the probable error of a daily mean temperature 
in the respective seasons for a series of observations at or near Baltimore 
covering a period of 30 years. The daily mean temperature will not be 
increased or decreased by an amount greater than these values by extend- 
ing the series of observations. 



Mean Monthly, Seasonal, and Annual Temperatures. 
There is an excellent series of local temperature observations extending,, 
with very few interruptions, from 1817 to date. For the series from 1817 
to 1824 we are indebted to Captain Lewis Brantz, who kept a careful 



93 THE CLIMATE OF BALTIMORE 

record of the weather, in what was in his time West Baltimore, and pre- 
sented his published results to the Mar5dand Academy of Sciences, of 
which he was a member. His observations were made at five stated 
hours of the day, at sunrise, 8 a. m., 2 p. m., sunset, and 10 p. m., and com- 
prised the elements of pressure, temperature, wind-direction and force, 
clouds, and rainfall. In 1831 systematic observations of the principal 
climatic elements were made at 7 a. m., 3 p. m., and 9 p. m. at Fort 
McHenry, under the auspices of the U. S. Army. This series was main- 
tained to the year 1892 with the exception of two or three years just pre- 
ceding and during the Civil War. From 1871 to the present time a first 
order station of the U. S. Weather Bureau has been maintained at 
Baltimore. 

To complete the record since 1817 it has been necessary to interpolate 
observations made at neighboring localities, applying, however, the proper 
corrections. This could readily be done as the different records over- 
lapped. The break in the record from 1825 to 1830 was filled in by re- 
ducing Washington, D. C, observations to the Fort McHenry series; for 
the years 1859 to 1863 the excellent record maintained for 20 years at 
Shellman's Hills, about 20 miles due west from Baltimore, was utilized. 
A year's record by Captain Brantz in 1836 afforded a means of reducing 
his earlier observations to the Fort McHenry series. From 1871 to 1892 
the Fort McHenry and the U. S. Weather Bureau records overlapped. 
All of these observations were ultimately reduced to the U. S. Weather 
Bureau series by applying the necessary corrections and were thus con- 
verted into a comparable and continuous record of great interest and value 
in the discussion of the climatic conditions of Baltimore City. 

The monthly, seasonal, and annual means are presented in Table 
XIX. The departures from the normal values are shown in graphic 
form in Plates VI and VII. The table and diagrams afford excellent 
material for the study of the changes in temperature conditions exper- 
ienced by Baltimoreans during the preceding century and incidentally 
the results throw light upon the assertion of the " oldest inhabitant " that 
our winters are growing milder, an assertion which has been persistently 
repeated since the earliest settlers arrived on our shores. 



MARYLAND WEATHER SERVICE 



VOLUME 2, PLATE VIL 
880 1885 leSO .'8?? 1900 <aa= 




1820 182B 1830 1B35 1840 1845 18&0 185& 1860 

DEPARTURES OF MEAN MONTHLY, SEASONAL AND ANN 



865 1870 1875 1880 1886 1890 1895 1900 1905 

UAL TEMPERATLIRE EROM NORMAL FOR 87 YEARS. 



:marylaxd weather service 



93 



TABLE XIX.— MEAN TEMPERATURES AT BALTIMORE FOR 88 YEARS, 18I7-190i. 



Years. 



1817. 

1818. 
1819. 
1830. 

1821. 
1822. 
1823. 
1824. 

1825. 

1826. 
1827. 
1828. 
1829. 
1830. 

1831. 
1832. 
1833. 
1834. 
1835. 

1836. 
18:^7. 
18;}8. 

18:». 

1840. 

1841. 
1842. 
1843. 
1844. 
1845. 

1846. 
1847. 
1848. 
1849. 
18.50. 

1851. 
18.T.2. 
1853. 
1854. 
1855. 

18.56. 
1857. 
1858. 
1S59. 
I8f». 

1W!1. 
lsfi2. 
18(^!. 
1864. 
1865. 

18(i6. 
1867. 
iHtW. 
1869. 
1870. 






33.230.143.461.962.072.2 
, .35. 6:«. 942. 9 50. 861. 7 74. 5 
,40.739.0.39.754.5,63.475.9 

:«). 4 42.944.656.41.59.172.4 



,540, 
,6.36. 
,731. 
437. 
6,39. 

941. 
4 42. 
H4."). 
729. 
,033. 



249.062.877.0 
259.570.775.6 
,4.59.166.4:2.(1 
9.55.56:^.772.7 
a.56. 763. 876.1 



4.53.772. 
.■<(;0.3f!5, 

t;50.3i;2. 

1.56.763. 
8i56.664. 



.31.232.648.4 57.765.075.9 
, 34 . 3 .39 .345. 6 .53 . 9 63 . 5 72 . 9 
. 39.639.241.957.87n.sV3.s 
. 32.246.3 4H.3.56.5H1.,M73.1 
.34.330.942.1.50.264.772.3 

.36.327.933.9.52.764.167.9 
. 31. 335. 941. 950. 363. n;i. (I 
. 39.8 2S.6 43.7 49.660.n:.">.t; 
.34.936.544.41.57.666.971.1 
.26. 740.546. 4L55.4I62.2'72. 3 

..32.9-33.6 11.5 48.6.56.4 70.7 
.38.939.94'.t.].".5.4r,(i.:i:o.l 
. 40. 929. 931. 251. .-.M. 77:!.: 
. 31. 7;i3. 943. 057. 167. 270. 4 
.39.335.945.2.55.861.372.9 

! ' i 

..34.831.4 43.1.54.2 65.569.2 
..3:5.234.339.1.56.961.971.] 
. 40. 0:!S. 041., s5-<.l(l,s. 1176.0 
. .34 . 4 :J2 . 7 45 . 5 53 . 2 61 . 9 76 . 3 
.40. 741. 6143. 9.51. 861. 975. 8 

. .39.841.348.1 55.965.572.5 
30.537.744.149.263.971.2 
, 34.8 :)8. 5 43. 9. 54. 4 65.0 75.1! 
, 36 . 1 :58 .345. 5 .50 . 2 65 . 7:5 . 1 
. 39.1,;50.740.6.58.965.572.3 



26 . 3 28 . 4 35 . 3 56 . 2 63 . 3 76 . 6 
25.94:i.l tl.5 17.7<!:i.(i:2.5 
43. 9:!:!. 54:!. 455. 461. ;i77. 7 
37 . 3 :58 . 7 49 . 5 .54 . 2 64 . 9 70 . 7 
33. 7 ;K. 7 46. 2.52. 164. 9 70. 7 
( I I , 

32.7:59.144.4.54.6.58.573.5 
:54.2:U.4:f9. 951. 561. 969.1 
:!6.2:i2.4:i.S.(l49.964.670.9 
:i7 . 9 40 . 4 4:; . 4 52 . 5 68 . S 74 . 7 
;5:,' . 6 -m . 1 50 . 3 .59 . 1 68 . 2 77 . 3 

:S:i.4:!S.O 11.9.57.161.1 7:!. 3 
2«.5:i0.6:!9.,^.5,'^.:i(il.il7:!.7 
:52.9:iO.()4:i.l51.262.'<71.1 
, 40. M41. 0,42. 5.55. 962. 3 74. 5 
42.937. 741. 156.365.978.8 
I I I I I 



^ P 



77.9 74.8 67.353.8.50.2,38.2 
79..- 76. 865.6,53. 3 50. 6:33. 8 
77.779.8 71.0.51.550.2:^7.9 
77.7177.269.0.51.642.5:36.7 



75.6 
,^1.0 
7S.0 
79.1 



9' 

SO.;!' 

77.9 
74.9' 

81. of 



071 



.067. 
.8j68. 

.273. 

.471. 
.169. 
.066. 
.0170. 



3 55, 
7. "9, 
s.M, 
1.58. 
4|60. 

2.58, 

1 .-.s. 
2.54. 
1.54. 

2|.58. 



6 16. 

244! 
148 
3,46 

8 47 
5 46 

7 .50 
145 
8.54 



38. 

m. 
2 41. 
044. 

537. 

7:57. 
S43. 
441. 
,545. 
,4;38. 



76.676.967.4.59.414.727.0 
67(1.771.959.4 17.S40.6 
275.1 69. 6.-,4.:{45.:540.1 
8l.578.;5!67.]:.52.6|45.6 3-<.l 
i6.6j73.5l62.1|,58.0|49.434.9 

r5.9'71.06S.S47.S42.ri34.2 
:5.,s;i.,^<14.S.V).947.737.9 
~1. 7 :,-^. 467. 9.50. 941. 7:52. ■; 
i8.273.967.4|.59.7i41.2,;i5.9 
r4. 9175.5163.8155. 3144. 7i31. 5 

r7. 575. l.0. 948. 943.1:56. 7 
:ii.574.46,s.4.-i4.o:!;i.<t:i4.:> 
,'6.,s77.471.5.'i4.04:i.o:i6.,s 
8.5 75.1 66.9.52.542.1:54.8 
'7. 2!76.4i67. 1.55. 9)46. 430.1 



5.4 



,270. :!.-,:;. n 1^ 



:!r.i 



;'6.477.2ii.-,.,-,,V. .2i:;.2 15..-, 
1'7 . 3 76 . 5 6,s. :.' :Vi . 7 54 . 6 :5,s . ', 
1-9.7175.0168.21.57.81.52.4142.0 

r9 . 4 74 . 6 119 . 7 58 .517.6 :S3 . 9 
r5. 9:7. 066. 2.57. 944. 042.1 
1 7. 176. 2 70.1. 5:!. 7 49.7:58.2 
r9. 4 76. 4 71. 6.57. 951. 6:55. 4 
r9. 176. 9170.2)63. 9149. 6'39. 4 



sn. 



').0(1,> 

■,.411,- 



5 17.2:55.9 
916. t !:;.-< 
;;i.l ■.:,.si\:.s:,s.', c;.'.) 11. ii 
76.2 75.168.0 52.4 48.4:52.8 
76. 7;75.8|65. 11.56.2145.6,33. 8 

74.872.769.1.59.144.737.8 
74.675.96'.l.:!.",s.l 12.,'<:!6.7 
77.1 77. H64.:!.".2. 7 46.2:51.:! 
78. ;!i79. 569. 557. 248. 5:57. 7 
79.476.174.1.53.147.8:59.8 

79.1 71. 670. .K.5S. 151.5:55.6 
77.9",6.:iii!t.6.5-<.(i49.'. :!4.4 
,s: i . 2 ; 9 . (1 71 1 . 55 . ,>< 4.>< . 4 34 . 5 
78.176.6,70.15:3.143.240.5 
83. .5j80. 671. 1159. 4148.4137. 5 



<^ 



55. 4| .55.8 

.54.6.34.951.8 

56. 7,-37. 81.52. 5 
.55.037.153.4 



g.0|-57. 11817 
6.91.56. 511817-8 
77. 8i57. 611818-9 
5. 8i54. 411819-20 



.55.. 1.35. 



437, 
.340. 
.6:40, 

I 

.538, 
.4:57, 
.24:!, 
.2:^5. 
.6|.38, 



].51.077.9.57.91.<«20-1 
4."9.]78.7 61.61S21-3 ' 

156.6 76.0 55.71822-3 
7. 54. 4i75. 6.57.71823-4 

4.56.6[77..5[58.4jl824-5_ 

9.58.4 77.1.59.9182.5-6' 
6 .■.7.7 77. 9. 5,-^.^1826-7 
:>.":).,^7.s.4.5.s.Jl,S27-8 
1 . 54. 2|75. 1.55. 21828-9 
41.55. 8177. 9161. 11829-30 



.55.1:54.2.57.0176.5.57.21830-1 
56 . 9 :5:! . 5 .54 . ;5 75.7 59 . 7 1831-2 

57 . 1 39 . 8 56 . S 75 . 7 .56 . 4 1832-3 
.56.7:59.5.55.577.6.55.118.3.3-4 
.54. 0,34. 4|.52. 3174. Ij56., 511834-5 

.51. 8:53.0 50.2 71. 6. 5:5.1 183.5-6 
54.l:!:i.s5].77:i.!i5(;.l ls:56-7 
.^4.i:>5.t5].i;s.6."M.5i,s:s7-8 

.55.6:34.7.56.;5i74.4 56.]il8;-58-9 
53 .9 34 . 4 54 . 7j74 . 21.54 . 61839-40 

.52.9.32.748.8 74.4 54.31840-1 
55.l):is..-,,54.9^:!.'; .54. 11841-2 
.'3. 9 :!5. (I 44.,-.! 76. 0.56. 2 1842-3 
54 . 4 ::54 . 1 1 .55 . 8: 74 . 7 .53 . 81843-4 
55. 2i.36 . 7154 . 1 175 . ,5.56 . 511844-5 

i j ! i i 

.■4.7:52.1.54.373.357.5184.5-6 

55. 6 :i4 . 6 .53. 6 75. 4 -''S- 1 1846-7 

57 . 2 :!9 . 2 .56 . 2 ■; 6 . 5 .55 . :! 1847-8 
.56. 2::57. 5.53. 5 76. 7.59. 8 1848-9 
.57. ,5[40. 3152. 5i76. 81.59. 5 1849-50 

.57.2 41.0.56.575.5.58.618.50-1 
,'4 . 9 :!1 . ( 1 .52 . 4 74 . 7 56. (1 18.51-3 
.51 i . 4 : ;,-< . 5 ,",4 . 4 ■; 6 . :! .57 . M852-3 
.56. 6:57. 5. 5:5. 676. ;i60. 4 18.53-4 
.57.ip5.1|55.0j76. 1161.21854-5 

,'4.131.4.51.6 77.5.57.5185.5-6 
,55.ii:i5.ll.50.7';i.,'^,57. 1 18.56-7 
.56.,s4(l.4 .5:>.4 77.5.56..S ly,-)7-8 
.55.6:59.3 ,56.2 74.U.56.0 18.58-9 
54 . 41.33 . 11.54 . 4|74 . 41.55 . 6 1859-60 

i I ! I i 

55 . 35. 2 .52 . 5 73 . 7 57 . 6 1,860-1 
.•3.'; :i4.."51.1 7:5.256.71861-3 
.':!.6:!5.1 .■rt)..s75.:).54.4 1862-3 
.57.3:57.554.9 77.5.58.4186.3-4 
.58.235-5159.277.6160.01864-5 
i i ■ I 

56 . 4 .37 . 1 55 . 4 74 . 7 60 . 1 1.86,V6 
.55. 7:51. 6.5:!. 2711. 059.:! 186(3-7 
.55.3:12. 1 .52.4 7,'<..s.5.s.] 1S67-8 
.56. fi:5,S.,s.>5. 676. 4.55. .51868-9 
.58. £ 40 . 4 54 . 4 81 . .59 .611869-70 



94 



THE CLIMATE OF BALTIMORE 



Table xix Con't.— MEAN TEMPERATURES AT BALTIMORE FOR 8S YEARS, 1817-1904. 



Tears. 



1873 




1873 




1874 




1875 




1876 




1877 




1878 




1879 




1880 




1881 




1882 




1883 




1884 




1885 




1886 




1887 




1888 




1889 




1890 




1891 




1892 




1893 




1894 




1895 




1896 




1897 




1898 




1899 




1900 




1901 

1902 




1903 




1904 





•O ' _. 



fe S h^ f s 



. . .35.■3'.39.847.758.96.^.474.0 
..!34. 436. 0.37. 055. 367. 275. 3 
.. 34. 2!.35. 640. 4 52.062.3 73.9 
..39.3,37.443.,S47.063.175.9 
. .130.329. 439. 649. 564. 173. 7 

..i41.7i37.9j39.9,.52.264.2i75.9 
..132. 340.614]. 553. 762. 773. 
.'34.9 40.4 49.2 58.6^3.570.2 
. . .31.5.'J2.243.S52.S65.7'73.3 
.42.9 41.2 42.855.170.875.3 

..130.334.641.8.51.767.870.8 
..,34.741.345.052.1.59.674.0 
..|32. 339. 1.39. 552. 264. 274. 6 
. 132.042.244.1.52.465.073.2 
..|34. 0:28. 535. 4. 54. 363. 272. 6 

..;29. 131. 341. 7.54. 7162. 169. 9 
..132.4.38.237.9.51.367.672.3 
.. 129.1.35.. 537. 4.52. 7 62. 8 73. 2 
..138.730. 644. 1.54. 8165. 771. 2 
..43. 7:43. 040. 9.53. 963. 775.1 
I , 

..37.341.639.0.56.0162.1171.7 
..31.936.237.3.51.963.175.5 
..24.334.040.4.52.561.672.5 

. . 37.434.447.S.''i2.364.W73.1 
..31.4 26 . 2 40 . 2 52 . 8 62 . 5 74 . 3 

..133.335.237. 8.56., 568. 7 71. 3 

..130.8.36. 6 45. 2. 52. 66;^. 4 69. 9 

}6.7;34.7;48.6.51.46:3.573.7 

!2.7as.041.6r)3.664.875.3 

35.333.138.6.54.864.9 72.9 

34. 428. 742. 951. 062. 0'73. 5 
31 . .529 . 8 46 . 2:.53 . 2,63. 9!71 . 9 
.33. 536. 9149.7 54. 8165. 5167.5 
27.538.641.350.5165.871.6 



Ten-Tear Mean. 

1821-1830 

1831-1840 

1841-18,50 

18.51-1860 

1861-1870 

1871-1880 

1881-1890 

1891-1900 

Gen'l Averages. 

1817-1870 

1871-1903 

1817-1903 



35.737. 043. 653.5j64. 9 
33.6136.440.8.53.064.2 
33.134.041.653.463.9 



.36.337.746.155.765.575.0 
34.135.843.7.54.264.272.6 
36 . 7 35 . 1 42 . 3 .'i 4 . 3 62 . 72 . ( i 
34. 7 36. 3 43. S 53. 4 64. 2 73. 3 
35.2|35.742. 754.663.9 74.0 



35.4 



34.9 



36.1 



34.035.5 



35.8 



43.6 
42.1 
43.1 



54.6 
53.3 
54.1 



63.8 
64.3 
64.0 



74.1 

73.7 
73.0 



73.5 
73.1 
73.3 



76.0177.563.758.745.233.8 
81.7|79.869.5.56.943.7|32.4 
■9. 676.668. 3.55. 341. ,5;40 
■7.573.1 70. 21.57. 145. 8j39. 3 

■8.273. 766. 2:,56. 043. 438. 6 

80 . 6 76 . 2 65 . 9'52 . 9 47 . 6 29 . 
8.977.968.260.24S.443.,s 

80.876.069.71.58.847.4 35.6 
9.075.365.263.046.741.9 
■7.7 74.7|68.3|,56.O42.031.3 

■8.977.2'77.2:a3.,549.3;43.8 
6.9 74.269.361.944.5:^.7 
'7. 173. 265. 41.57. 94S. 430.0 
5. 475. 572. 4,60. ti Hi. r,;;;. t 

■9.974.9,67.2:55.8 t:,.,s;i;.ti 

4.773.869.91.59.046.6131.3 
0.673.665.01,56.345.3137.2 
4.676.565.11.53.048.337.2 
6.573.766.3.54.147.945.1 
6.374.167.8.57.047.534.9 

71. 6(74. .5'70.5|54. 4143. 9143.0 
■6. 8j76. 4:66. 7, 56. 044. 0133. 7 
■6.875.665.9157.1 43. 9'38.] 
■7.473.769.9.57.443.337.3 
3.1 77.271.71.53.4 47. 03S 

7.7 77.068.0,54.3.50.936.2 
7.074.968.9.58..5'46.238. 
8.677.771.2.59.044.6.36. 
77.1 76.066. 5,59. 046. 736. 8 
9. 680. 373. 562. 0,49. ,5,36. 6 



9.876.9168 
77. 274.1167 
77.373.767.7 
75.7. 



.56.1141.8134.7 



58.6 
58.7 



51.735.3 
43.333.9 



78.678.069.8.57.348.3140.8 
7.775.467.1,55.345.135.3 

7.475.96,s.(i.54.ii4(i.;):!7.5 
,s.0T;-).>fi,s.(l.-,7.14;.437.7 
8. 676. 669. 8157. Oi47. 136. 9 



79.0 
77.1 
76.6 



78.1 
77.6 
77.9 



76.1 
74.7 
76 



369 



76.4 
75.6 
76.1 



67.5 
68.6 
3 



57.5145.236.6 
.57.947.038.0 



57.146.0 



56.0 

57.5 



.37.5 



37.7 

37.1 

56.6146.637.4 



47.0 
46.0 



,56.237.5.57.375.8.55.91870-1 



34.7,53.3 
•55.034.1,51.6 
■55.8,39.1.51.3 
•53.533.0,51.1 

■55.3.39.4,52.1 
.56.8.34.0-52.6 
•57.] 39.7.57.1 
•55. 8a3. 1.-4.1 
•56.442.0.56.2 

•57.232.1 .'S. 8 
•55.8.39.9.52.2 
.55.236.0.52.0 
56.337.7 r3.« 
.54.0:33.3.51.0 

.^^3.6132. 7! .52. 8' 
,':4.7,34.0,52.3 
,'■4.133.951.0 
55.635.5.-4.9 
.56.4 43.9,52.8 

•55.437.9.52.4 
."4.037.0.50.8 
.'3.4,30.7.51.5 
.55.6.36.6.55.0 
•54.031.6.51.8 

•55.. 5 35. 7 •■4. 3 
.55.234.5.53.7 
.56.2.36.6.-4.5 
•■4.832.3.-3.3 
.56.635.1.52.8, 

.54.1*3.2.53.0 
■55. 033. 0.54. 4' 
.54.935.2)56. 



■8.9.56.7:1871- 
6.7 .55^5 1872-3 
■5.5.57.71873-4 
■5.2.55.21874-5 

'7.6.55.5187.5-6 
■6.8.58.91876-7 
■5.7.58.61877-8 
5.9.58.31878-9 
■5.9.55.41879-80 

■5.863.31,880-1 
■5.0,58.61881-3 
5.0,57.31883-3 
4.7-59.91883-4 
5.8.56.31884-5 

73.8|.58.5il88.5-6 
■5. 5155. 51886-7 
■4.8.55.51887-8 
3.8.56.11888-9 
5.3 57.41889-90 

■2.61.56.31890-1 
•6.2.55.61891-2 
'5.0-55.6 l,s92-3 
■4.7.56.91h9,3-4 
■4.9.57.41894-5 

■5.3.57.7.1895-6 
3. 9 .57. 9: 1896-7 
■6.71.58.31897-8 
■6.1.57.41f*9,S-9 
■7.6:61.71899-00 

■6.7 55.51900-1 
4.4.59.31901-2 
2.5(56.61902-3 



29.7i52.5l....l 1903-4 

Ten- rear Means. 

57. 4|38. 3.55. 8177. 2'58. 5 1821-1830 
.55.0!35.] .^4.0|75.2.55.8 1S31-1840 
.55.;!:i6.4.52.9 75.3 56.51,«41-18.50 
.55.,s;iti.:.'.-3.,S:75.7.57.7).S,5l-1860 
56.0,35.9,53.7 76.4.58.01861-1870 



.55.836.4,53.7 
55.336.0.53.7 
.55. 134.9 i3.0 



55.936.4 
.55.335.5 
55.636.0 



54.0 
.^3.2 
53.7 



76.456.7:1871-1880 

74.8157. 81881-1890 
75.3.57. 5J1891-1900 



76.057.2 1817-1870 

75.457.31871-1903 

5. 8 57.31817-1903 



Table XIX shows the mean monthly, seasonal and annual temperature for 
Baltimore for 88 years from 1817 to 1904. The table contains three distinct 
records: (a) observations from 1817 to 1824, by Capt. Lewis Brantz, in what 
•was then west Baltimore; (b) observations at Fort McHenry, along the 
Baltimore harbor, from 1831 to 1870; (c) observations under the auspices of 
the United States Weather Bureau from 1871 to 1904. 



MARYLAXD WEATHER SERVICE 95 

The Lewis Brantz observations were reduced to the Fort McHenry series 
by applying corrections derived from an overlapping period in 1836 and 
1837. The monthly means for the period from 1825 to 1830 were derived from 
Washington, D. C, observations, and reduced to the Fort McHenry series by 
adding the departures from the Washington, D. C, normal temperatures to 
the Fort McHenry normal. The means for the years 1859 to 1863 were re- 
duced to the Fort McHenry series in a similar manner by means of a 20-year 
record of overlapping observations made at Shellman's Hills, about 20 miles 
west of Baltimore. The record from 1817 to 1870 was then made conformable 
to the Weather Bureau record from 1871-1903 by means of departures derived 
from overlapping records covering a period of about 20 years. Thus the 
entire record from 1817 to 1^03 may be regarded as an approximately uni- 
form series of Baltimore City temperatures. 

THE XORMAL TEMPERATURE. 

The variations of the mean monthly, seasonal and annual temperature 
during the greater portion of the preceding century are discussed in 
succeeding pages, while the mean for each month, season and year since 
1817 is published in Table XIX, together with the monthly, seasonal, and 
annual averages for each ten-year period, and for the entire 87 years. 
The variations in value of the ten-year averages are observed to be small, 
even in the case of the month of greatest variability. The maximum 
variability (5,3°), occurs in the month of March with a mean tempera- 
ture, for a ten-year period, as low as 40.8° and as high as 46.1°. 

The annual average for ten years has varied between the limits 55.0® 
and o7.-4° a range of 2.4°. Xo progressive increase or decrease is indi- 
cated for the entire period eitlior in tlic monthly, seasonal, or the annual 
means. From the third decade (1831-1840), there was a steady rise in 
temperature to the sixth (1861-1870), and since then a steady fall to 
the present time. The series of observations is not sufficiently long, 
however, to draw the conclusion that there is a periodic change of this 
length. In the absence of changes of long period in the fluctuations of 
tlie annual mean temperature the normal temperature derived from the 
87 years will remain fixed at 55.0° for Baltimore. The probable error 
of this value is not greater than one-tenth of one degree Fahrenheit. 
Hence a longer series of observation will not cliange the result by an 
amount greater than onc-tentli of one degree. 



96 the climate of baltimore 

The Vakiability of the Monthly and Annual Mean. 

In tabulating the following lists of exceptional months and seasons the 
sole basis of selection has been an average monthly temperature decidedly 
above or below the normal for the entire period of 87 years. Such lines 
of division must necessarily be arbitrary as there are no fixed standards of 
cold and warm. The degree of departure from the normal fixed upon 
for classification as cold or warm varied with the variability of the month 
and season. In the comparatively constant summer months a departure 
of 2° may be regarded as exceptional. In the variable winter months a 
departure of 6° may be assumed to be necessary to make the month an 
exceptionally cold or warm one. The seasons and the year being less 
fluctuating, the departures selected were smaller, varying from 2° for the 
year and the summer to 4° for the winter season. 

A close examination of this list of exceptional departures from the 
normal will doubtless cause surprise by the absence of periods which left 
an impression of great heat or cold. Attention has already been called 
to the fact that an average temperature for the period of a month or 
season is sometimes an inadequate measure of the temperature conditions 
of the period. A month with 10 consecutive days of excessively hot 
weather for example, will long remain in memory as a hot month, no 
matter what the average temperature of the entire month may be. Yet a 
moderately cool spell preceding and following the hot days will result in 
an average value for the month little if any above the normal and hence 
would not be found in a list of warm months. 

The month of Jul)', 1898, may be cited as a case in point. The highest 
temperature recorded in the official records of the Baltimore station oc- 
curred on July 3, 1898, namely 104°. The month contained 10 days with 
a temperature of 90° and over. Yet this month is not listed as a warm 
month because the average temperature for the entire month was less than 
2° above the normal. The proper place to look for such excessively hot 
spells is in the list of warm days rather than warm months. As a rule, 
however, the average temperature is a safe guide for expressing the gen- 
eral temperature conditions of a given period. 



MARYLAND WEATHER SERVICE 



97 



Warm Months and Seasons. 

The following list includes the months and seasons since 1817 during 
which the average temperature rose decidedly above the normal in the 
vicinity of Baltimore. The degree of departure required for each month 
and season in order to find a place in the list is shown in the column 
headed " Departure." 

WARM MONTHS AND SEASONS. 



De- 
parture. 



January 6°+ 1824,1828,1843, 

February.... 6°+ 1820,1827,1828, 

March 6°+ 1825,1826,1812, 

April 5°+ 1817,1822,1823. 

May 4°+ 1822, 1820, 1833, 

June 4° + 1828, 185S, ISti;'), 

July 3°+ 1822, 18.30. 1S31, 

August 3°+ 1819, 1821, ls:.'U'. 

September.... 4°+ 1822, 182t;, istio. 

October 4°+ 1855, 187'J, issi, 

November.... 4°+ 1818,1822,1830, 

December 5°+ 1824, 1827, 1829, 

Winter 4° + 

Spring 3° + 

Summer 2° + 

Autumn 3° + 

Year 2° + 



1870, 1876, 1880, 1890. 
1834, 1857, 1884, 1890. 
1859, 1865, 1878, 1903. 
1827. 1865. 

1848, 1864, 1865, 1880, 1896. 
L';70. 

1.S.38, 1868, 1870, 1872. 
1S27, 1S28, 1870, 1872, 1900. 
ISSl, ]!)00. 
lss:.>, 1SS4, 1900. 

1849, 1850, 1866, 1896, 1902. 
1848, 1857, 1877, 1881, 1889, 1891. 



1823-4, 1824-5. 1827-8, 1849-50, 1850-1, 1857-8. 1869-70, 1879-80, 1889-90. 

1822, 1826. 1827, ls;il, 1833. 1865, 1871, 1878, 1903. 

1819, 1821, 1822, lS-7, 182.S, 1830, 1838, 1868, 1870, 1872. 

1822, 18:30, 1854, 1.S55, 1881, 1900. 

1822, 1825, 1826, 1827, 1828, 1830, 1865, 1870. 



The years 1822, 1827, 1828, 1857 and 1870, are conspicuous in the list 
for sustained warmth during several months of the year. During 1822 
there were five months of the year with an excessive departure above the 
normal. During one month only, namely January, was the temperature 
below the normal. The year attained the highest mean annual temper- 
ature on record, namely 3.2° above the normal. 

COLD MONTHS AND SEASONS. 



De- 
parture. 



January 6° + 

February . ... 6*-t- 

March I>° + 

At»ril 5°-i- 

May 4° + 

June 4°-l- 

July 3° + 

Au(fU8t 3° + 

September 4° + 

OctobiT 4° + 

November 4'+ 

December 5° + 

Winter 4° + 

Spriniif 3"+ 

Summer 2° + 

Autumn 3° + 

Year 2"+ 



1821, 
1829, 
IKlf,, 
1821, 

]K2(l, 

iKii;, 
lS2'.t. 
183t;, 
]8:t.^., 

1819, 
1820, 
1831. 



1840, 
1836, 
1843, 
18(1, 
1S3S, 
IS-Ki, 
l.SKI, 
ISlll, 
1S4(), 
1820, 
18:16, 
1840, 



1856, 
1838, 
18.56, 
1857, 
1841, 
lst;2, 

IStil. 

lS6li, 

isti:i, 
lKi4, 
1S38, 
1845, 



1857, 
18.56, 
1872, 
1S74. 
1843, 
190:t. 
1S62, 
l'.t03. 
1S71. 
IKje. 
1839, 
1872, 



1858, 1867, 1893, 1904. 

1875, 1885, 1895, 1899, 1901, 1902, 1904. 

1885. 

1861, 1882. 

1886, 1888, 1891, 1895. 



18,38, 1841, 1844, 18.59. 
1842, 1844, 1873, 1880, 1901. 
1876, 1880, 1886. 



185.5-6, 1866-7, 1892-3, 1894-5, 1901-2, 1903-4. 

1836, 1841, 184:3, 1857. 

]83t!, 1K42, 1H46, 1861. 1862, 1886, 1889, 1891, 1903. 

18:36, 1838, 1841, 1842, 1844. 

1836, 1841, 1863, 1875, 1886, 1893. 



98 



THE CLIMATE OF BALTIMORE 



The year 1836 occurs most frequently in the list of cold months and 
seasons. The average temperature for the entire year was the lowest in 
the record of 87 years. It also contained lowest average August and Oc- 
tober temperatures^ and the lowest summer and autumn averages. Jan- 
uary alone was above the normal, and this but l.-l° above. The tempera- 
ature was decidedly below the normal during nine months of the year. 
The summers of 1903 and 1886 follow close behind the memorable sum- 
mer of 1836. The recent winter of 1903-04: with an average temperature 
of 29.7° was the coldest experienced in Baltimore. jSTo excessively low 
temperatures were recorded, but the entire season was characterized by 
an almost unbroken period of moderately cold weather. There was an 
almost total absence of the usual and sometimes frequent " thaws " of 
previous years. The ice crop was the heaviest in many years. Navigation 
on the Bay was impeded to an unprecedented extent. The Bay was frozen 
from shore to shore to a distance of over 80 miles south of Baltimore. 



WARMEST AND COLDEST MONTHS. 
(Expressed in terms of departures from the normaL) 



Normal — 
Warmest + 

Year 

Coldest — . . 

Year 

Range 



Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec 



34.9' 

+9.0 
1858 
-10.6 
1893 
19.6 



35.8° 

+10.5 

1834 

—9.6 
1895 
20.1 



43.1 

+ 7.' 
1865 
-11.9 
1843 
19.1 



54.1° 

+7.8 
1817 

-7.1 
1874 
14.9 



64.0° 

+8.3 

1826! 

-4.9 

1843: 

20.5 



73.3° 
+5.5 
1870 
—5.8 
1903 
11.3 



77.9° 
+5.61 
1870 
— 6.3| 
1891 1 
11.9! 



76.1° 
+4.9 
1831 
-5.1 
1836 
10.0 



+8.6 
1881 

-6.5 
1835 
1.5.1 



56.6° 46.6 

+7.3 +8.0 
1855 1849 
—6.7 



1836 
16.1 



1842 
14.7 



37.4* 

+8.2 
1829 
-10.4 
1831 
18.6 



WARMEST AND COLDEST SEASONS AND YEARS. 





Winter. 


Spring. 


Summer. 


Autumn. 


Year. 




36.0° 

+7.9 

1889-90 

-6.3 

19a3-4 

14.2 


53.7° 
+5.5 
1865 
-8.9 
1843 
14.4 


75.8° 
+5.2 

1870 
-4.3 

1836 
9.4 


57.3° 

+6.0 
1881 

-4.2 
1836 
10.2 


55 6° 




+3.2 
18'^2 


Year 


Coldest— 

Year 


-3.8 
1836 


Range 


7 







Winter and spring have varied most from the normal, the difference be- 
tween the warmest winter (namely 43.9° in 1889-90) and the coldest 
winter (29.7° in 1903-04) is 14.2°. The spring limits are -f 5.5° (1865) 
and —8.9° (1843), a range of 14.4°. The summer limits are + 5.2° 



MARYLAXD WEATHER SERVICE 99 

(1870) and —4.2° (1836), a range of 9.4°. The autumn limits are 
+ 6.0° (1881) and — 4.2° (1836), a range of 10.2°. 

A warm February may have the average temperature of a normal 
March, or approach that of a cold April, or cold October. A warm Sep- 
tember may have the average temperature of a normal July or August. 
October has been nearly as cold as a warm February. May has been as 
warm as a cold July. A month may have the same mean temperature as 
the second preceding or following month. Hence a season may be one 
month later or earlier than the average time, or, there may be two months 
difference for example, between a very late spring and a very early spring. 

Frequency of Stated Departures fro:m the Monthly Seasonal 
AND AxxuAL Mean Temperatures. 

During a period of 87 years the mean annual temperature was below the 
arithmetical average in 45 per cent of the total number of years, above the 
normal in 49 per cent, and exactly normal (within one-tenth of a degree 
Fahrenheit) in 6 per cent. 

the distribution of seasonal depart cres. 

Above Normal. Below Normal. Normal. 

_ <ro % % 

Winter 41 59 

Spring 48 51 1 

Summer 45 52 3 

Autumn id 54 

Average 45 54 ] 

These figures indicate that the average seasonal temperatures are most 
likely to be below the normal value ; hence the average plus departure must 
be larger than the minus departure. This discrepancy between the depar- 
tures of opposite sign is most conspicuous in the January temperatures 
with departures above the normal in 41 per cent of the past 87 years, and 
below in 59 per cent. A further interesting feature of the tabulation above 
is that the exact average value seldom occurs. Computing the averages to 
tenths of a degree Fahrenheit, the normal value has never been experienced 
in 87 years in winter and autumn, but once in spring, and three times in 
summer. The annual normal has occurred five times. Hence the arith- 
metical average seasonal temperatures are not the most probable or of the 



100 



THE CLIMATE OF BALTIMORE 



most frequent occairrence. The most probable departure differs with the 
month and the season. 

The following table shows the frequency of departures of stated 
values from the normal monthly and annual temperature during the 
87 A-ears from 1817 to 1903. The total number of values included is over 
1000. For example, taking the month of January, the monthly mean 
temperature has been colder or warmer than the normal by 1° or less 
17 times in the 87 years past; it was 3° warmer or colder 16 times; 6° 
7 times; 11° once, etc. The frequency of stated monthly departures 
is also shown in Fig. 22, and the seasonal and annual departures in Fig. 
23, as percentages of total number of occurrences. 



FREQUENCY OF DEPARTURES FROM NOR.MAL MONTHLY TEMPERATURES. 



Departures. 


t-5 




o 


< 

18 
23 
20 
10 
10 
3 
2 
3 


eS 

36 
23 
10 
6 
6 
1 
3 
1 
1 


o 

a 
a 

36 
13 
34 

7 
4 
3 


I-: 

33 

30 
11 
9 

1 
3 

1 


bi 
3 
< 

36 
25 
14 

7 
4 

1 


P. 
® 

30 

31 

17 

10 

6 

1 

1 


O 

O 

31 
18 
34 
13 
3 
4 
3 
2 
1 


5 
•^ 

20 
23 

13 
17 
8 
5 

1 
2 


o 

<B 
P 

34 
16 
11 
13 

1 

5 
3 
3 


a 
< 

45 

32 

8 

2 




±1 


17 
13 
16 
9 
12 

4 

3 

6 

.... 


11 
11 
11 
13 
11 
14 
4 
9 


31 

17 
10 
11 

10 
6 
3 


304 


3 

4 

5 

6 


180 
123 
80 
.•)5 




30 


8 . 


U 


9 








1 


11 


10 


3 

1 


1 
1 










3 


11 


















1 




4 


13 





















+13 






1 

40 
46 

1 


















1 




35 

50 

2 


47 
40 



44 

41 

3 


44 
43 




43 
43 

1 


39 

44 
4 


44 

42 

1 


44 
43 



45 

42 




43 
42 

2 


43 
43 

1 


42 
40 
5 


511 


Minus De|>artures ( — '' 


519 


No Departures (0) 


14 



In most months the departure is likely to be about 1° above or below the 
normal; in April the most probable departure is 2°, in October 3°, and in 
February above 4°. 

Fifty-two per cent of the mean annual temperatures have fallen within 
1° of the normal value in the past 87 years, and in 37 per cent of the 
remaining years the mean was within 2° of the normal. No annual 
mean has risen to 4° above the normal and none fallen 4° below. Hence 
the annual mean temperature has a comparatively small range of depart- 
ure from the normal. The extreme departures occurred in 1822 (3.2° 
above normal) and in 1836 (3.8° below), an extreme range of 7° between 



MARYLAXD WEATHER SERVICE 



101 



the coldest and warmest years on record at Baltimore. The frequency of 
departures of stated values for the seasons is shown in the following table, 
in percentages of the total occurrences in 87 years: 




Fig. 22. — Frequeucy of Stated Departures from the Monthly Normal Temperature. 

Fig. 22 shows the frequency of stated departures from the normal value of the monthly 
temperatures, based on records covering- 87 j-ears. The upper line of figures represents the 
degree of departure above (+) or below (— ) the normal monthly temperature. The mar- 
ginal letters represent the months of the year. The curved lines and shaded areas represent 
the frequency of the changes expressed as percentages of total number of months. In- 
crease in intensity of shading i-eprosents increase in the frequency of stated changes. For 
e.xample, changes of + 3 or —2 occurred in 10 per cent, of the total number of instances in 
March, 20 per cent, in May and August, 10 per cent, in December, etc. See also Fig. 23. 



FREQUENCY OF STATED SEASONAL DEPARTURES. 



Winter. . 
Spring... 
Summer. 
Autumn 



r 


2° 


3- 


4° 


5° 


6° 


7° 


8° 


9° 


% 


:% 


% 


% 


% 


% 


« 


% 


% 


19 


29 


n 


19 


10 


2 









32 


27 


26 


8 


4 


2 








1 


46 


32 


14 


6 


1 


1 








3:3 


34 


20 


8 


4 





1 







13 



winter SprluK Summer Antumii Average 



The arithmetical average departure 3.4' 

The most frequent departure 2.0'^ 



2.6' 
1.0' 



1.7' 
1.0' 



2.3' 
2.0' 



2.5° 
1.5" 



The PROB.A.BLE Error of the ^Monthly and Annual Means. 
Employing the formula given in a preceding paragraph for the deter- 
mination of the probable error of the daily mean temperature in order to 



102 



THE CLIMATE OF BALTIMORE 



arrive at the probable error of the monthly and annual means for the 
series of 87 years we obtain the following values : 

AVERAGE MONTHLY AND ANNUAL DEPARTURES AND PROBABLE ERROR OF 
THE MONTHLY AND ANNUAL MEAN TEMPERATURES. 



Average de- 
parture (V) 

Probable error 
(E) 



Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


3.4° 


3.9° 


3.1" 


2.5° 


2.1° 


1.8° 


1.6° 


1.6° 


2.0° 


2.4° 


2.5° 


2.9° 


.32 


.36 


.28 


.23 


.20 


.16 


.15 


.15 


.18 


.21 


.23 


.26 



Year 



1.1° 

.10 





















/ 


\ 


















/ 


\ 


















/ 


c^ 


\ 














/ 




'Vl 


ii. 














/ 




.' " 


v^ 


y 












/ 






\- 


\ 1 












/ 






' 


I b\ \ 


o\ 








/ 










\ \ 


k. 


\ 






/ 












\S 


^?r^ 


i^ 





/ 







SEASONAL- 
A 



ANNUAL- 

B 



Fig. 23. — Frequency of Stated Departures from the Normal Seasonal (A) and 
Annual (B) Temperatures, ia) Summer, (&) Autumn, (c) Spring, (d) Winter. 

Fig. 23 shows the frequency of stated departures from the normal seasonal (A) and annual 
(B) temperatures. The upper horizontal row of figures indicates the degree of change, and 
the vertical rows to the right and left of the diagi-am indicate the frequency of change, 
expressed as percentages of the total number of seasonal and annual changes from 1817 to 
1903. 

The probable error of the mean annual temperature is about one-half 
that of the mean monthly temperature ; the probable error of the latter is 



MARYLAND WEATHER SERVICE 



103 



about one-fourth that of the mean daily temperature. Hence the prob- 
able error of the daily mean is eight times as large as that of the annual 
mean. 

SUCCESSIOX OF THE SEASONS. 



It may be definitely stated, as the result of a study of the Baltimore 
temperature observations for 87 years, that there is no regular periodic 
recurrence of cold and warm periods. Some interesting questions in prob- 
abilities are, however, suggested by a classification of the decidedly cold 
and warm seasons, in connection with the character of succeeding seasons. 
This has been done in the following manner : A winter was regarded as 
cold or warm when the departure from the average winter condition 
equalled or exceeded 3° ; when less than 2° it was considered a normal 
season. The point of departure for the spring and autumn was 1.5°, 
for the summer and for the year 1.0°. This classification yielded from 20 
to 25 abnormal seasons of each class during a period of 87 years. For 
each of the abnormal winters the character of the succeeding spring, sum- 
mer, autumn and winter was then noted. Abnormal summers and ab- 
normal years were tabulated in a similar manner, with the following 
result : 

SUCCESSION OF THE SEASONS. 



Cold Winters 

(-2.0°+) 23 

Warm Winters 

(+2.0* + ) 22 




Summer 

(±1.0°+). 



Autumn 

(±1.5°+). 



Winter 

(±2.0°+). 






? be 



> §1 



6 11 



Cold Summers 

(-1.0°+) .. . 
Warm Summers 

(+1.0° + ) .. . 



Cold Years (—1.0°+). I 20 



Warm 



(+1.0°+).; 24 



Autumn 

(±1-5° + ). 



Winter 

(±2.0° + ). 



Spri ng 

(±1.5°+). 



Summer 

(±1.0° + ;. 



1.5 



Year. 



104 THE CLIMATE OF BALTIMORE 

Of 23 cold winters, 10 were followed b}' cold springs, 13 by average 
springs, and not one by a warm spring. Nine were followed by a cold 
summer, 11 by an average summer and only 3 by a warm summer. The 
succeeding winter Avas cold in 6 cases, average in 9 and warm in 8. These 
figures show a decided probability in favor of a cold or cool spring and 
summer following a cold winter, and of a cool autumn; there is no 
decided tendency as to the character of the succeeding winter. A similar 
tendency is shown in favor of a warm spring and summer after a warm 
winter. There is also a decided probability that a cold summer will be 
followed by a cold or a cool autumn, winter, spring, and next succeeding 
summer, and that a warm summer will be followed by a warm or an 
average autumn, winter, spring, and next succeeding summer. 

]\Iore cautious, and perhaps safer, is the negative statement that a cold 
winter is not likely to be followed by a warm spring or summer; that a 
warm winter is not likely to be followed by a cold spring and summer. 
In general it may be said that an extreme season is not likely to be 
followed by an opposite extreme. Such a conclusion may not be regarded 
as of much practical value for determining the probable character of a 
coming season, but a more definite statement does not seem to be war- 
ranted by the statistical record. 

Considering the average temperature for the entire year there were 20 
cases of a departure of 1° or more below the normal. Of these 8 were 
followed by cold years, 11 by years of an average temperature, and 1 by 
a warm year. Of 24: M'arm years 3 were followed by cold years, 11 by 
average years, and 10 by warm years. Here again the same tendency is 
shown against the occurrence of a succession of years of opposite character. 
That is, a decidedly cold year is not likely to be followed by a decidedly 
warm year, or a warm year by a cold year. All such classifications are, 
however, arbitrary and inferences as to the succession of the seasons should 
be accepted with caution Avhen based upon phenomena as variable in their 
nature as the climatic factors of the middle latitudes. 

Daily Exteemes or Temperature. 
Thus far only average temperatures for a day, month, or year, have been 
considered, with departures from the normal conditions based on many 



MARYLAXD WEATHER SERVICE 



105 



years of observations. The variability of a given climate is best illus- 
trated, however, by extremes of temperature within given limits of time, 
and by the frequency of occurrence of stated changes from day to day. 

TABLE XX.— DAILY EXTREMES OF TEMPERATTRE. (Spring.) 



Date. 



March. 



B ® 



i- ** ' 



April. 



O its 



May. 



® . 



1 72 1895 14 1884 

2 69 1882 15 1886 

:i 68 1871 12 1873 

4 74 1880 5 1873 

5 76 1880 9 1872 

6 72 1894 13 1901 

7 72 1878 12 1890 

8 a5 1878 12 1873 

9 67 1871 21 1885 

10 70 1897 17 1877 

11 71 1879 20 1892 

12 76 J890 12 1900 

13 75 1890 12 1888 

14 66 1903 14 1888 

15 '71 1886 21 1900 

16 68 1886 18 1893 

17 72 1898 15 1900 

18 68 1894 9 1877 

19 78 1894 12 1876 

20 69 1903 12 1885 

21 72 1897 12 1885 

22 82 1894 19 1885 

23 71 1871 16 1888 

24 66 1903 18 1896 

25 70 1904 21 1878 

26 66 1896 21 1878 

27 69 1903 20 1894 

28 77 18!K) 24 1894 

29 77 1902 23 1887 

30 f.9 1896 21 1887 

31 74 1888 29 1873 



78 1893 30 

82 1882 30 
80 1892 29 

83 1892 29 
77 1880 25 



75 1892 26 

75 1890 30 
85 1871 32 

82 1871 32 
8t 1887 32 

85 1887 30 

76 1899 27 

83 1890 29 

85 1896 34 
82 1891 32 

86 1896 36 
89 1896 27 
94 1896 26 
93 1896 24 

87 1896 27 



1887 
1876 
1899 
1904 
1881 

1898 
1898 
1896 
1885 
1894 

1882 
1874 
1874 
1885 
1904 

1893 
1875 
1875 
1875 
1904 



S3 1896 32 1875 

89 1902 32 1875 
88 1902 36 1875 
88 1886 38 1888 
86 1895 34 188:3 

88 1872 37 1883 

82 1891 39 1893 

81 1888 34 1898 

90 1888 33 1874 

91 1903 34 1874 



48 



87 1890 34 1876 

87 1894 37 1903 

84 1878 38 1882 

86 1892 40 1900 

84 1896 42 1875 



86 1880 40 

88 1872 44 

89 1900 42 

93 1896 40 
96 1896 42 

94 1896 45 

94 1881 44 

95 1881 40 

91 1900 46 
94 1900 42 

87 1900 43 
93 1896 44 

92 1896 46 
92 1877 47 
92 1903 48 



1891 
1882 
18ii8 
1898 
1900 

1877 
18a5 
1895 
1895 
1895 

1904 
1891 
1895 
1895 
1899 



88 1893 44 1895 

88 1903 42 1895 
90 Mt02 47 1892 

89 1884 46 1893 

90 1880 46 1877 

92 1880 4u 1886 

93 1880 48 1897 
90 1899 46 1902 
89 1895 43 1894 
95 1895 46 1884 
95 1895 50 1873 



53 
50 
46 
46 
42 

46 
44 
47 
53 
54 

49 
50 
55 
45 
52 

44 
49 
46 
45 
44 

44 
46 
43 
43 
44 

47 
45 
44 
46 
49 
45 



Table XX shows the highest and lowest temperatures recorded on each 
day of the year during 34 years from 1871 to June, 1904, with year of occur- 
rence, and the extreme range for the day. 



In Table XX, and in curves A, C, and D of Plate IV, the highest 
and lowest temperatures officially recorded upon each day of the year dur- 
ing a period of 33 years are shown, together witli the absolute daily range. 
In addition the table shows the year of occurrence of the extremes. A 
studv of the curves of Plate V will most clearly and quickly reveal the ex- 
tremely changeable character of the temperature from day to day, and the 
8 



106 



THE CLIMATE OF BALTIMORE 



relative variability of the seasonal changes. The changes in the average 
temperatures from day to day throughout the year have already been 
described in preceding sections of this report. As the average tempera- 
tures were derived from the daily extremes^ there must of necessity be 
a general agreement, with a difference only in the amplitude of change. 



Table xx Con't.— DAILY EXTREMES OF TEMPERATURE. (Summer.) 





June. 


July. 


August. 
































^9 










u 


S) be 




bi 




u 


« be 




u 




^ 


a, bo 






& 




a 


i- c 




c8 


fl 


es 


u c 




es 


c 


-^ 


.-• 7i 




93 


<s> 




« 






V 


•- 


o 


t^ cS 




O 




o 


■^ a 


Date. 


s 


>* 


g 


t>^ 


B 


S 


>< 


g 


>* 




S 


fcH 


S 


^ 


J< u 


1 


97 


1895 


47 


1894 


50 


103 


1901 


56 


1885 


47 


95 


1890 


57 


1895 


38 




95 


1895 


48 


1897 


47 


103 


1901 


59 


1891 


44 


92 


1879 


58 


1875 


34 


3 


97 


1895 


52 


1888 


45 


104 


1898 


.59 


1888 


45 


92 


1881 


59 


1895 


33 


4 


91 


1890 


53 


1888 


38 


100 


1898 


59 


1891 


41 


94 


1888 


58 


1886 


36 


5 


93 


1899 


52 


1886 


41 


96 


1881 


58 


1892 


38 


96 


1896 


59 


1874 


37 


6 


98 
96 


1899 
1899 


47 
47 


1894 
1894 


51 

49 


96 
96 


1901 
1900 


58 
60 


1891 
1891 


38 
36 


97 
lOo 


1900 
1900 


60 
62 


1894 
1897 


37 




38 


8 


96 
98 


1874 
1874 


47 
50 


1891 
1891 


49 
48 


98 
99 


1890 
1876 


55 

56 


1891 
1891 


43 
43 


99 
100 


1900 
1900 


58 
62 


1903 
1887 


41 




38 


10 


90 


1879 


52 


1904 


38 


97 


1880 


56 


1894 


41 


100 


1900 


56 


1879 


44 


11 


92 


1893 


50 


1904 


42 


96 


1876 


57 


1898 


39 


100 


1900 


58 


1879 


42 


12 


95 


1880 


52 


1887 


43 


96 


1876 


.■)i 


1895 


39 


99 


1900 


60 


1890 


39 


13 


94 


1902 


53 


1903 


41 


99 


1880 


57 


1888 


42 


98 


1881 


.56 


1902 


42 


14 


95 


1885 


53 


1873 


42 


95 


1887 


58 


1895 


87 


96 


1872 


Oi 


1893 


39 


15 


93 


1891 


53 


1884 


40 


96 


1900 


57 


1895 


39 


90 


1900 


59 


1887 


31 


16 


94 


1891 


52 


1884 


42 


101 


1887 


59 


1903 


42 


9h 


1SS8 


58 


1889 


38 


17 


93 


1887 


55 


1899 


38 


100 


19(K) 


59 


1892 


41 


9l' 


1900 


55 


1902 


37 


18 


94 


1887 


54 


1879 


40 


102 


1887 


60 


1892 


42 


yj 


1900 


60 


1874 


31 


19 


93 


1893 


56 


1886 


37 


96 


1878 


61 


1890 


35 


9:- 


1872 


59 


1896 


34 


20 


98 


1893 


00 


1879 


43 


98 


1885 


57 


1890 


41 


97 


18i.9 


54 


1896 


43 


21 


93 


1896 


56 


1897 


37 


99 


1885 


55 


1890 


44 


97 


1899 


55 


1876 


43 


22 


94 


1888 


54 


1897 


40 


96 


1899 


56 


1890 


40 


96 


1872 


56 


1876 


40 


23 


97 


1894 


55 


1898 


42 


95 


188:^ 


59 


1890 


36 


9:- 


1898 


00 


1888 


38 


24 


98 


1894 


54 


li^02 


44 


95 


1884 


59 


1876 


;w 


94 


1898 


51 


1890 


43 


25 


98 


1898 


55 


1902 


43 


97 


1892 


59 


1876 


38 


97 


1903 


56 


1879 


41 


26 


97 


1875 


61 


1893 


36 


99 


1892 


61 


1891 


38 


96 


1900 


52 


1874 


44 


27 


95 


1876 


Oi 


1893 


38 


97 


1892 


59 


1876 


■M 


92 


1900 


53 


1885 


39 


28 


94 


1898 


56 


1897 


38 


97 


1894 


59 


1893 


38 


9J 


1895 


53 


1885 


38 


29 


97 


1874 


55 


1888 


42 


97 


1892 


62 


1897 


35 


94 


1877 


52 


1874 


42 


30 


99 


1901 


57 


1899 


42 


95 


1903 


6(1 


1880 


35 


91 


1898 


54 


1896 


37 


31 












95 


1890 


55 


1895 


40 


95 


1898 


55 


1887 


40 







The greatest variability in extreme conditions occurs in the winter months, 
with a gradual decrease to more uniform conditions toward summer. The 
greatest change of temperature which has been recorded within a period 
of 24 hours during 33 years at Baltimore is 47°. This remarkable range 
between the highest and lowest temperature of a single day occurred on 
the 24th of February, 1900. When we consider extremes which liave 



MARYLAND WEATHER SERVICE 



107 



occurred upon a given date, without reference to the year of occurrence, 
the range is greatly increased. For instance, upon the 11th of February 
a maximum temperature of 72° was recorded in 1887, and a minimum of 
6° below zero in 1899, a range of 78°. Even in the months of least vari- 



Table x.v Con't.-DAILY EXTREMES OF TEMPERATURE. (Autumn.) 



Date. 



September. 






October. 





u 




c 


X 


« 


c 


c3 


ai 


e 






S 


tH 


S 


>H 














November. 






] 95 1898 

2 96 1898 

3 97 1898 

4 i 9L 1898 

5 1 91; 1880 

6 94 1900 

7 101 1881 

8 94 1873 

9 i 94 1894 

10 1 98 1884 

I 

11 97 1897 

12 9:1 1895 

13 93 1897 

14 89 1903 

15 92j 1901 

16 89 1897 

17 90 1886 

18 91 1898 

19 94 1896 

20 5)0 1895 

21 96 1895 

22 96 1895 

23 95 1895 

24 87 1881 

25 90 1881 

26 93 1895 

27 90 1881 

28 91 1886 

2« 87 1884 

30 1 88 1881 

31 ! ! 



56 1887 

50 1892 

51 1893 
50 1872 

50 1872 

51 1883 
51 1883 
55 1892 
51 1891 
46 1883 

49 1875 

51 1879 

53 1902 

48 1902 

40 1873 



1873 

1887 
1875 
1875 
1875 



45 1897 
44 1873 

46 1896 
43 1875 

42 1887 

40 1879 

43 1879 

44 1899 
43 1903 
39 1888 



89 1881 39 

88 1881 38 

89 1879 36 
87 1884 38 
85 1884 42 



89 1884 36 

81 1884 40 

82 1887 38 

84 1893 37 

85 1887 35 



79 1898 40 
8J 1889 33 

80 1884 35 
»-2 1883 34 

83 1897 ! 33 

90 1897 30 

»-Z 1879 36 

84 1881 36 
76 1899 35 

76 1884 36 

77 18.><4 39 

78 1901 34 

81 1901 36 

79 1900 34 

80 1903 33 

77 1891 30 

75 1899 35 

77 IS99 'M 

78 1874 31 
75 19113 30 
77 1896 . 31 



1899 
1899 
1899 
1888 
1901 

1893 
1893 
1876 
1896 
1895 

1881 
1876 
1876 
1875 
1876 

1876 
1893 
1876 
1880 
1900 

1900 
1899 
1889 
1889 
1879 

1879 
1903 
1898 
1873 
1873 
1893 



74 1903 31 1873 

76 1876 33 1873 

75 1903 33 I 1875 

75 1903 38 1879 

73 1896 25 1879 

74 1888 31 1892 
68 1896 28 1903 

73 1890 38 1886 

77 1895 29 , 1886 

74 1879 31 1874 

73 1899 31 1901 

78 1879 27 1894 

76 1902 28 1873 
68 1889 I 26 , 1873 

75 1902 28 1883 



75 , 1897 

75 1896 

71 1896 

74 1900 

68 1900 

79 1900 

71 1883 

68 1900 

70 1896 
65 1890 

71 ' 1896 
74 1896 
71 1896 

61 1879 

62 1899 



23 1883 

25 1883 

23 1891 
21 1891 
33 1879 

21 1879 

15 1880 

16 1880 

17 1880 

24 1881 

21 1903 

18 ltt03 
30 1903 
23 1875 
16 1875 



43 
44 
43 
47 

47 

43 
40 
45 
48 
43 

41 
51 

48 
42 
47 



ability the range is still largo. Tlio smallest range, namely 31°, is cred- 
ited to August 14 and 18. The higliest temperature of the year occurred 
on July 3, 1898, and the lowest on February 10, 1899. Although we have 
no systematic and official records of daily cxtromos of tomporature prior 
tc 1872, when self-registering ma.ximum and minimum tlicniidmeters 
were added to tlic (■(iiii|iiii('iit df the V. S. AW'atlicr liiireau stations, we 
have good reason to believe that the period from 1872 to 1903 comprised 



108 



THE CLIMATE OF BALTIMORE 



within its limits the warmest and coldest days experienced ar Baltimore. 
In the summer of 1898 all existing records of high temperature were 
broken during the hot spell of July 1-4 within the State of Maryland. 
In the following winter during the first decade of February all existing 

Table xx Con't.-DAILY EXTREMES OF TEMPERATURE. (Winter.) 



Date. 



December. 



1881 
1901 

1874 
1873 
1883 

1879 
189B 
1892 
1889 
1897 

1897 
187.3 
1889 
1881 
1893 

1877 
1877 
1877 
1900 
1877 

1885 
1889 
1891 
1893 
1893 

18S9 

1881 
1889 
1893 
1898 
1-84 



1875 
1886 
1-86 
1886 
1871 



16 1901 

15 1885 

10' 1882 

4 1.S76 

1 1876 

13 1880 
22 1895 

14 1895 
14i 1898 



s ® 

i^ C 



15 


1900 


13 


1876 


h 


1876 


12 


1875 


1(1 


18S4 


r 


1871 


5 


1871 


6 


1872 


17 


1872 


8 


1872 


8 


1872 


12 


1903 


11 


1903 


13 


1,H72 


4 


1880 


-3 


1880 


— 1 


1880 



January. 



61 



1885 
1876 
1876 
1874 
1890 

1890 

1874 
1898 
1876 
1876 

1891 
1890 
1890 
1892 
1871 

1901 

1885 
1876 
1876 
1880 

1901 

1874 
1874 
1894 
1879 

1878 
1890 
1876 
1876 
1896 
1880 



1881 
6 1899 

1879 

5 1879 

1877 

1904 

1884 
1878 

6 1875 
-2 1875 



1875 

1886 
1886 
1886 
1893 

1893 
1893 
1893 
1904 
1901 

1893 
1893 

1883 
1882 
1897 

1897 
1888 
1888 
1873 
1878 
1873 



13 



o be 

n a 



65 



65 



February. 





ii 




u 




cd 




cS 




0) 




e 


§ 


;>H 


S 


^ 


63 


1891 


8 


1900 


58 


1877 


4 


1881 



61 i 18813 
66 I 1903 



1895 

1886 



71 , 1890 I —1 1886 



68 < 1884 
64 : 1904 

57 : 1892 

58 1,H78 
1876 

1887 
1898 

66 1903 
63 1884 

67 j 1886 

67 1891 

73 1S91 

71 I 1891 

60 I 1887 
60 1887 

72 I 1874 

74 ' 1874 



II 1895 

61 1895 

2 1S95 

2 1S99 

—7 1899 

—6 1899 

5 1899 

6 1899 
6i 1899 
6 1899 



1875 
1896 
1903 
1903 
1896 



1H74 
1872 
1871 

1890 
18>h0 
1903 

1880 



15 



1885 
1896 
1889 
1873 
1900 

1900 
1900 

1888 
1884 



C 4) 

o be 
S a 



records of great cold were lowered. The variability of temperature con- 
ditions during the past five or six years has been phenomenal. Eecords 
of extremes of temperature which have remained undisturbed for many 
years were broken to a remarkable extent. In addition to the instances 
of absolute extremes of temperature just referred to, may be mentioned 
the high monthly average temperature of March, 1903, the warmest March 
in 87 years or more; the summer of 1903, the coolest in 87 jears or more; 
and the following winter of 1903-04 which was the coldest in a hundred 
years. 



MARYLAND AVEATIIER SERVICE 109 

ABSOLUTE EXTREMES OF TEMPERATURE 1871-1903. 



Absolute 






Absolute 






Absolute 


Max. 


Year. 


Day. 


Min. 


Year. 


Day. 


Range. 


73" 


1889 


26 


- 3° 


1880 


30 


76° 


73 


1890 


13 


— 6 


1881 


1 


'•.9 


78 


1874 


23 


— 7 


1899 


10 


85 


82 


1894 


23 


5 


1873 


4 


77 


94 


1896 


18 


24 


1875 


19 


70 


96 


1896 


10 


34 


1876 


1 


62 


99 


1901 


30 


47 


1891 


8 


52 


104 


1898 


3 


55 


1891 


8 


49 


100 


1900 


10 


51 


1890 


24 


49 


101 


1881 


7 


39 


1888 


30 


62 


90 


1897 


16 


30 


1876 


16 


60 


79 


1900 


21 


15 


1880 


22 


64 


78 


1874 


Feb. 23 


y 


1899 


Feb. 10 


85 


96 


1896 


May K 


5 


1873 


Mar. i 


91 


104 


1898 


July 3 


47 


1891 


.Tune 8 


57 


101 


1881 


Sept. 7 


15 


1880 


Nov. 22 


86 


104 


1898 


July 3 


— 7 


1899 


Feb. 10 


111 



December. 
January ... 
Febi'uary . . 



March. 
ApriL. 
May . . . 



June 

July 

August. .. 

September. 
October — 
November. 



Winter. . . 

Spring . . . 
Summer . 
Autumn . 



Year. 





Jan Feb Mcm Apr M«» June Jui» Aug Sept Oct Nov Dec j«n 




45* 
40' 
35* 
30' 
25 
20 
I? 






























4 5° 

4cr 

35° 
3(f 

25* 

20 






































































































































































































10' 
































0* 

































Fig. 24. — Greatest Daily Raiitje of Temperature. (See Table XXL) 



The Greatest Daily Eange of Temperature, 

In Table XXI the gi-eatest difference between the daily maximum and 
minimum temperatures, or the greatest daily range, is entered for each 



110 



THE CLIMATE OF BALTIMORE 



month and year from 1871 to 1903, together with the daily average range 
for each ten-year period. There is a marked uniformity in the size of the 
daily ranges throughout the year, the average monthly values ranging be- 



TABLE XXI.-GREATEST DAILY RANGE OF TEMPERATURE. 



1871. 

1873. 
1873. 
1874. 
1875. 

1876. 

1877. 
1878. 
1879. 
1880. 



J881. 
1882. 
1883. 
1884. 
1885. 

J886. 

1887. 
1888. 

18S9. 

1890. 



1891. 
1892. 
1893. 
1894. 
189,5. 



I89f). 
1897. 
189S. 
1899. 
1900. 

1901. 
1902. 
1903. 
1904. 



20 



Means. 

1871-1880 

1881-1S90 

1891-1900 

1871-1902 



27.5 
36.3 
23.9 
35.8 



19 
28 
32 
37 
33 

29 
33 
23 

28 
24 

24 
37 
29 
26 
36 

40 
28 
.30 
35 
34 

27 
31 

36 

28 
39 

31 
22 

37 
35 
47 

33 
19 

28 
20 



37.5 
38-9 
39.3 
28.1 



29 
25 
33 
28 
30 

29 
29 

24 
28 
32 

34 

38 
36 
26 
37 

39 

38 
34 
34 
33 

34 
38 
35 
39 
30 

32 
38 
29 
32 
31 

34 
3:3 
40 
30 



20 
29 
30 
31 
31 

29 
25 

37 

a3 

30 

25 
29 
33 
35 
37 

30 
24 
24 
30 
29 

30 
30 
30 
30 
31 

39 
31 
30 

28 
35 

30 
33 

29 
33 



17 
30 
31 
38 
26 

28 
36 
23 
30 
26 

23 
25 
oo 

38 

37 

33 
29 

38 
35 



36 
37 
26 
29 

37 

24 
35 
30 

27 
31 

29 
29 
26 

37 



29.1 37. 6i 38.5 35.5 

37.4! .30.91 37.5! 25.8 

31.1 31.8! 31.4 27.2 

29.3 30.3! 29.31 26.3 



33.4 

24.8 
26.3 
25.0 



27 
36 
36 
38 
30 

22 
36 
26 
29 



22.8 
23.5 
26.2 
24 



36 
24 

38.6 
36.5 



23 

24 
25 
29 

38 

27 
26 
25 
25 
23 

24 
23 
32 
38 
23 

28 
28 
25 
29 
21 

33 
33 

33 
30 
35 

27 
30 
28 
26 
31 

35 

27 
30 



25.5 
35.1 

30.4 

37.2 



24 

26 

24 
28 
24 

28 

27 

27 
37 
27 

81 

25 
26 

26 
27 

30 
31 

26 
24 
39 

31 

28 
28 



26.1 

36.3 

37.6 
36.8 



31 



36 
23 
42 
26 
29 

39 
23 

28 



25.4 
26.1 



26.5 






29 
35 
33 
31 
33 

38 
33 
35 
33 
33 

34 
31 
33 

30 
42 

40 
38 
34 
34 
33 

.34 
36 
35 
35 
35 



33.6 
34.8 
37.3 
35.3 



Table XXI shows the greatest daily range of temperature (the greatest 
■difference between the maximum and minimum of any day) for each 
month and year from 1871 to 1904. Also the average of the greatest daily 
ranges for the entire period of 32 years and for each 10-year period. 



tween 30.3° for April and 24.2° for August. The extreme daily ranges 
for each month and for the year are shown in the following tabular state- 
ment and in Fiff. 24. 



MAEYLAXD WEATHER SERVICE 



111 



EXTREME DAILY RANGE OF TEMPERATURE. 



January — 
Februarj'.. 

March 

April 

May 

June 

July 

August 

September , 

October 

November.. 
December. . 



Year. 



1871-1903. 



Day. 


Year. 


Range. 


Max. 


Min. 


17 


1885 


42=' 


65 


23 


34 


1900 


47'= 


55 


8 


7 


1873 


, 36° 


50 


14 


4 


1903 


40° 


71 


31 


9 


1896 


39° 


93 


54 


11 


1900 


31° 


92 


61 


18 


1887 


1 32° 


102 


70 


6 


1900 


' 30° 


97 


67 


16 


1873 


38° 


79 


41 


19 


1901 


35° 


75 


40 


13 


1876 


31° 


68 


37 


31 


1898 


42° 


59 


17 


Feb. 


1900 


1 ''' 


55 


8 



J 


% Fe 


e M 


H A 


n M 


Af JU 


NE JU 


LV Al 


&. Se 


PT 





:t Nf 


)V D 


C. Ja 


100° 
90° 






/ 












\ 












/ 
















\ 






8(f 




^ 


/ 




y 


^ 




"\ 






\, 






^ 


r^ 






/ 


^^- 




^ "^ 


s. 




> 






7d 








/ 


y 






N. 


\ 


\, 














/ 


/ 


^^ 




^ 


\ 


s 


V 






60' 







-7 


'-/ 


Y 






^ 


\ 


N 


d 






50' 

40° 


b 


^ 


z 


7^ 


^ 


^ 




\ 


-A 
\ 


<^ 


V^ 


^ 


30' 


-r- 


^ 




7- 












:^ 


\ 




V 


20° 






-f 
















^ 






10° 
0* 




/ 


J- 
















\ 




• 


y 




















\ 




lo' 





























Fig. '.^.5. — (a) Extreme .Moutlily Ma.ximum Temperature. 
(h) Mean '• " <i 

(c) Average " Temperature. 

(d) Mean " Minimum «' 

(e) Extreme " " " 
(See Tables XXII, XXIII aud XXIV.) 



112 



THE CLIMATE OF BALTIMORE 



1871. 
18:2. 
1873. 
1874. 
1875. 

1876. 
1877. 
1878. 
1879. 
1880. 

1881. 
1882. 
1883. 
1884. 
188.5. 



1886. 
1887. 
1888. 
1889. 
1890. 

1891. 
1892. 
1893. 
1894. 
1895. 



1896. 
1897. 
1898. 
1899. 
1900. 

1901. 
1902. 
1903. 
1904. 



TABLE XXII.-MONTHLY MAXIMUM TEMPERATURES. 







Xi 






o 


a 


^ 


bl 




® 


■* 


>-i 


fe 


g 



63 
56 
58 
69 
52 

71 

54 
57 
64 
65 

45 
£9 
50 
62 
65 

57 
65 
.50 
60 
73 

60 

58 
52 
57 
60 



61 
62 
78 
59 

65 
63 
63 

58 
67 

64 
59 
64 
68 
50 

67 

72 
60 

48 
74 

73 

.57 
61 
19 
62 

61 
56 
65 
60 
65 



Means. 



1871-1880 60.9 64.2 

1881-1S90 57.6 62.6 

1891-1900 58.7 61.9 

1871-1903 i 58.9| 62.3; 



69.2 

6' 

70.0 



91 



78.7 
82.8 
83.8 
82.1 




Table XXII contains the highest recorded temperature during each month 
and year from 1871 to 1904, together with the average of the monthly ex- 
tremes for the entire period of 32 years and for each 10-year period. The 
observations were obtained by means of a self-registering mercurial ther- 
mometer, excepting for the time from January, 1871, to July, 1872, during 
which period the 3 p. m. observations were employed. 



Monthly axd Axxual Extremes. 

The highest and lowest temperatures recorded during each month and 
year from 1871 to 1903 are shown in Tables XXII and XXIII, together 
with the average values for each ten-year period and for the entire period. 



MARYLAND WEATHER SERVICE 



113 



The absolute monthly extremes of temperature, the average maximum, 
and minimum, and the monthly normals are shown in Fig. 25. Fig. 26 
shows the absolute annual extremes and the mean annual temperature for 
each year from 1871 to 1903. 



TABLE XXIII.-MONTHLT MINIMUM TEMPERATURES. 



1871. 
1873. 
1873. 
1874. 
1875. 

1876. 
1877. 
1878. 
1879. 
1880. 

1881. 
1882. 
1883. 
1884. 
1885. 



1886. 
1887. 

lang. 

1889. 
1890. 

1891. 
1892. 
189;^. 
1894. 
1895. 



1896. 
1897. 
1898. 
1899. 
1900. 

1901. 
1902. 
1903. 
1904. 



Means. 



14 
11 

—4 
13 

—3 

17 
1 
6 


17 



10 
15 

15 
i 

13 

18 
20 
12 
15 

4 
23 

23 
10 
3 

— 1 

21 

11 

3 

33 

16 
14 
11 

8 
1 

6 
18 
10 

—7 
8 



1871-1S80 7.3 

KWl-lSflO 8.H 

1S91-19UJ 11.1 

1871-1902 9.6] 



36 

9 

5 

33 

19 

13 

9 
31 

34 
23 

2T 
26 
16 
14 
12 

15 
31 
12 
38 
12 

16 
30 
16 
20 
21 

16 
28 
27 
36 
12 



14 13 

13 30 

5 , 29 

5 I 20 



42 
38 
38 
37 
24 

30 
33 
43 
39 
30 

25 
29 
30 
34 
32 

34 
30 
33 
34 
31 

30 
32 
36 
30 
34 

31 
29 
26 
29 
30 

37 
35 
27 
27 



51 
48 
44 
41 
43 

34 
41 
43 
43 

38 

46 
38 
45 
45 
44 

45 
51 
41 
43 
43 

40 
46 
45 
43 
40 

47 
44 
40 
47 
40 

47 
43 
37 
43 



13. S 18.0 33.2 42.5 

11.9 18.3 31.2 44.1 

8.4 20.2 30.7 43.2 

ll.Oi 1».7| 38.0; 43.4! 



53.8 
r3.4 
52.5 
t3.2 



61 
55 
58 
55 



45 
50 
40 
53 
43 

45 
48 
47 
40 
50 

J9 
48 
46 
49 
46 

50 
42 
39 
46 
46 

51 
49 
44 
45 
46 

46 

45 
PS 
42 
60 

46 
48 
43 



62.4 58.6 
60.1 56.5 
57.9 57.7 
67. 6[ 



40 
38 
30 
35 
34 

30 
41 
35 
30 
35 

39 

44 
40 
35 

38 

36 
32 
36 
34 
36 

33 
34 
31 
36 
34 

36 
38 
34 
34 
36 

37 
34 
35 



28 
17 
23 
24 
16 

25 
35 
33 
30 
15 

34 

36 
33 
26 
33 

26 
25 
25 
38 
26 

18 
31 
22 
24 



15 

13 

—3 

34 
10 
17 
9 
15 

15 
16 
16 
23 
18 

17 

14 

18 



6 

—4 

13 



1 
1 
6 

-3 

—6 

11 

8 
3 

— 1 

7 

9 

3 

12 

16 
12 
1 

7 
1 



■'1 



46.1 34.8' 22.5' 

47.1 37.0 26.1 

47.0! 34.6 25.0 

46.81 35.51 24.81 



11.4 
16.3 
13.8 
13.9 



2.3 
5.3 
6.1 
5.0 



Table XXIII contains the lowest recorded temperature during each month 
and year from 1871 to 1904, together with the average of the monthly 
extremes for the entire period of 32 years and for each 10-year period. 
The observations were obtained by means of a self-registering alcohol min- 
imum thermometer, excepting for the time from January, 1871, to July, 1872, 
during which period the 7 a. m. observations were employed. 



114 



THE CLIMATE OF BALTIMORE 



The absolute range of temperature at Baltimore, the diflference between 
the highest (104°), and lowest (7° below zero) recorded readings of the 
thermometer, is 111°. The former occurred on July 3, 1898, and the 
latter on February 10, 1899. On both of these days records for extreme 
heat and cold were broken in many parts of the countr}'. 



871 






1875 










t880 






1885 








189C 














1895 










1900 




i9o: 








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4- 










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: 


















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Fig. 26. — (.4) Absolute Annual Maximum Temperature. 
(J5) Average " Temperature. 

(C) Absolute " Minimum Temperature. 

(See Tables XXII, XXIII and XXIV.) 



The Greatest Monthly Eange, 

The difference between the highest and lowest temperatures recorded 
during each month of the year is entered in Table XXIV for every year 
from 1871 to 1903. The extreme difference for each month during the 
entire period is shown in the table which follows, together witli the ex- 



MARYLAXD WEATHER SERVICE 



115 



tremes of temperature and the j^ear of occurrence. The extreme range 
is also shown in Fig. 27. 

JFMAMJJASONDJ 

70' 







1 


























bU 
50° 
40° 


















































































































































lU 































0° 
Fig. 27. — Greatest Monthly Raui^e of Temperature. (See Table XXIV.) 

EXTREME MONTHLY RANGE OF TEMPERATURE. 

1871-1903. 



January.. 
February. 

Marcb 

April 

May. 



June 

July 

August 

September . 

October 

November. . 
December . . 



Year. 


Range. 


Max. 


Min. 


1873 


62° 


58 


— 4 


]8St) 


B8° 


67 


- I 


18!tO 


65° 


77 


12 


19()3 


64° 


91 


27 


189.=) 


.5.5° 


95 


40 


ISitl 


51° 


98 


47 


1W8 


47° 


104 


57 


1874 


45° 


97 


52 


1873 


.53° 


93 


40 


]87!» 


59° 


89 


30 


1879 


58° 


78 


20 


1880 


59° 


56 


— 3 



Frequency of Days tvith Frost. 

As the frequency of occurrence of days with a temperature of freezing, 
and the distribution of such days especially in the autumn and spring 
seasons, is a matter of greatest practical importance in agricultural and 
commercial affairs, the subject is here given more than ordinary attention 
and space. For purposes of convenience all days during which a minimum 
temperature of 32° was recorded are classed as frost days. It is a well recog- 



116 



THE CLIMATE OF BALTIMORE 



nizecl fact of observation, however, that frost usually occurs before the 
temperature falls to the freezing point (32°) as ofiBcially recorded. The ap- 
parent inconsistency is of course explained by the method of exposure of 

TABLE XXIV.— ABSOLUTE MONTHLY RANGE OF TEMPERATURE. 



1871. 
lS'i-2. 
1873. 
1874. 

1875. 



1876. 
1877. 
1878. 
1879. 
1880. 



1881. 
1882. 

188:^. 

1884. 
1885. 



1886. 
1887.. 
1888. . 
1889. . 
1890., 



1891. 
1892. 
1893. 
1894. 
189.5. 



1896 

1897 

1898 

1899 

1900 



1901. 
1902. 
1903. 
1904. 



Means. 



49 
4.5 
62 
56 
54 

54 
£3 
53 
61 
37 

51 
53 
39 
44 
55 

55 

58 
40 
40 
53 

39 
46 
51 
.39 
51 



1871-1880 52.3 

1881-1890 48 

1891-1900 1 47.61 

1871-1903 1 49.01 



■S ' — 



f^ s 



56 
46 
E3 
63 
53 

53 
45 
43 
46 

63 

60 
36 
60 
58 
47 



46 
50 
51 
61 

56 
38 
55 
67 
57 

35 
46 
66 
59 



35 


43 


56 


50 


63 


37 


49 


41 


44 


50 


57 


45 


56 


48 


51 


37 


47 


54 


54 


50 


33 


59 


43 


53 


n 


44 



51.0 
53.51 
53.8 
51.71 



46 



44 


56 


45 


51 


46 


45 


63 


49 


51 


63 


53 


63 


44 


65 


.50 


55 


48 


51 


55 


54 


61 


49 


57 


54 


43 


64 


50 


53 


51.2 


45.5 



39 
41 
45 

48 
46 

54 
51 
43 
51 
55 

49 
45 
41 
44 
38 

43 
36 

46 
50 
44 

48 
41 
44 
44 
55 

49 
40 
53 
43 
54 



47 



48. 9j 
49.8! 
50.6 



51.6 
53.1 
50.3 



47.2 
43.6: 
47. 0| 
45. 7i 



37 
43 
43 
39 
88 

47 
40 
41 
51 
44 



47 
43 
43 

38 

47 
41 
35 

45 



41.4 
39.4 
43.3 
41.5 



32 
29 
34 
34 
34 

40 
39 
33 
39 
37 

31 
39 
34 
35 
43 

33 
35 
38 
33 
43 

34 
41 
38 
41 

40 

35 
33 
47 
37 
43 

39 
37 
37 
40 



34.1 
36. 3i 

38 



38 
41 
37 
45 
30 

35 
31 
33 
36 
30 

38 
33 
33 
35 
41 

34 
36 
41 

33 
44 

40 
35 
33 
36 
39 



36 
44 
53 
39 
49 

43 

40 
40 
45 
41 

42 
40 
35 
44 
40 

41 

46 
45 
38 
41 

39 
39 
44 
49 
50 



38 


41 


43 


47 


44 


43 


43 


47 


43 


50 


47 


51 


39 


43 


45 


38 


59 


58 


46 


47 


50 


47 


34 


47 


38 
54 


48 
45 



44 


48 


30 


53 


;56 


45 


39 


63 


39 


45 


.33 


46 


36 


44 


39 


46 


34 




34.6 


43.0 


36.7 


41.2 


37.1 


46.3 


36.0 

1 


43.6 



38 



53 
38 

48 
43 

53 
49 
63 
49 
40 

41 
53 
60 
44 
49 

44 
46 

48 



43 

47 
44 
49 
43 
47 

46 
49 
40 
46 
51 

46 

48 
43 
41 
51 

41 
44 

57 



44.fi 45.4 50 

44.1 45.8 45.0 

47. 9| 46.1 £0 

45.5 45.6 48 



47 
41 
43 
57 
49 

37 
43 

43 
50 
41 

50 
50 
49 
53 
47 

63 
50 
53 

68 
47 

54 
41 
41 



ei g 
a u 



68 
62 
50 
65 

57 
51 
53 
63 
61 



57.8 
69.0 
58.1 
58.3 



Table XXIV shows the greatest monthly range of temperature (the 
difference between the highest and lowest temperature recorded within the 
month) for each month and year from 1871 to 1904, also the average value 
of these monthly ranges for a period of 32 years, and for each 10-year 
period. 



the thermometer. Usually thermometers are placed in a " shelter " which 
shields the instrument from undue radiation from the ground ; the shelter 



MARTLAXD WEATHER SERVICE 



117 



is also usually mounted at a considerable distance above the ground, 
varying from four or five feet to a hundred feet or more. In a quiet atmos- 
phere with a clear sky, radiation from the ground is very rapid during the 
night and early morning hours. The temperature at the surface of the 
earth may fall considerably below that of the air but a few feet above 



TABLE XXV.-XUMBER OF DAYS WITH A MINIMUM TEMPERATURE OF »2° 

OR BELOW. 



Oct. 



Nov. I Dee. Jan. Feb. 



Mar. 



Apr. 



Season 



1871-3 . 
1872-3 . 
1873-4 . 
1874-5 . 
1875-6 . 



1876-7 . . 
1877-8 . . 
1878-9 . , 
1879-80 . 
1880-1 . , 



1881- 2 . 
1882-3. 
1883-4. 
1884-5. 
188.5-6. 



1886-7.. 
1887-8 . . 
1888-9 . , 
1889-90 . 
1890-1 . , 



1891-2 . . 
1892-3 . 
1893-4 . . 
1894-5 . . 
1895-6 . . 



1896-7 . 

1897-8 . 
1898-9 . 



1899-1900 . 
1900-1.... 



1901-2 . 
1902-:} . 
190:i-4 . 



1872-1881 . 
1882-1,><91 . 
1892-1 901 . 
1872-1903 



Means. 



0.4 
0.1 
0.1 
0.2 



18 
26 
12 

20 
15 

30 
9 
30 
14 
24 

9 
15 
15 
11 
17 

25 
19 
16 
8 
21 

10 
17 
20 
15 
14 

20 
13 
90 
19 
19 

19 
23 
25 



7.0 I 18.8 
4.8 i 15.6 
5.5 i 16.9 

6.1 I 17.5 



19 
31 
14 
29 
17 

29 
19 
27 
9 
36 

19 

35 
34 
20 
24 

24 
29 
16 
10 
18 

33 

28 
17 
26 
22 

32 
15 
21 
20 
23 



21.0 
20.9 
21.7 
21.6 



17 
18 
19 
24 
19 

19 
14 
25 
17 
30 

12 

18 
11 
34 



19.3 
16.6 
19.9 

18.8 



13.1 
13.8 
11.2 
13.3 



2.3 
1.3 
1.8 
1.9 



73 

94 
75 
104 

77 

103 
48 
85 
64 



61 
80 
78 
81 

87 
64 
114 



81.8 
73.1 
77.1 

78.4 



under such conditions. Thus we may have frost, especially in the low 
places, when the thermometer records a minimum temperature of 35° 
or 40°, according to the position of the instrument. In view of these 
facts the figures entered in the following tables to represent the fre- 
quency of frost days must be regarded as the lower limit of frequency ; the 



118 



THE CLIMATE OF BALTIMORB 



figures would be increased to a small extent by placing the thermometer 
nearer the ground. To enumerate frost days upon the basis of the actual 
observation of frost on the ground, introduces additional diflficulties as 
the production of frost depends not only on a temperature of 32° or 



TABLE XXVL- 



-LONGEST PERIOD OF CONSECUTIVE DAYS WITH A MINIMUM 
TEMPERATURE OF 33° OR BELOW. 

D^'ysf '^*™'' °* Occurrence. 



1871-3. 
1873-3. 

1873-4. 
1874-5. 

1875-6. 



1876-7.. 
1877-8.. 
1878-9 . 
1879-80. 
1880-1.. 



1881-3. . 
1883-3.. 
1883-4.. 
1884-5. . 
1885-6. . 

1886-7.. 
1887-8.. 
1888-9.. 
1889-90. 
1890-1.. 



1891-3. 
1893-3. 
1893-4 
1894-5. 
1895-6. 



1896-7.... 

1897-8 

1898-9.... 
.1899-1900. 
1900-1.... 



1901-3 . 
1903-3. 
1903-4. 



Means. 

1873-1881. 

1883-1891. 
1893-1901. 
1873-1904. 



15 
33 
14 

30 



37 
9 

10 
9 

13 

24 

9 

34 

15 

15 

15 
34 
13 

38 



Jan. 33-Feb. 5 
Dec. 9-Dec. 31 
Jan. 30- Feb. 13 
Dec. 30- Jan. 38 
( Dec. 13-Dec. 31 
(Jan. 30-Feb. 7 



Dec. 
Jan. 
Dec. 
Feb. 
Jan. 

Dec. 
Jan. 
Jan. 
Jan. 
Jan. 

Dec, 
Jan. 

Feb. 
Mar. 
Feb. 

Mar. 
Jan. 
Dec. 
Jan. 
Feb. 

Jan. 
Jan. 
Jan. 
Dec. 
Jan. 

Jan. 
Feb. 
Dec. 



1.5-Jan. 18 
38-Feb. 7 
16-Jan. 24 
1-Feb. 11 
23-Feb. 8 

30-Jan. 7 
2-Jan. 17 
3-Jan. 10 

10- Jan. 27 
6-Jan. 26 

3.5-Jan. 30 
9-Feb. 4 

19-Feb. 37 
l-Mar. 10 

37-Mar. 7 

11-Mar. 32 
3- J an. 36 
1-Dec. 9 

23-Feb. 25 
9-Feb. 23 

33-Feb. 6 
27- Feb. 10 
35-Feb. 17 
3.5-Jan. 5 
23-Mar. 1 

26-Feb. 31 
16-Feb. 37 
36-Jan. 31 



under at the place of formation, but also upon the relative amount of 
moisture in the atmosphere. The actual observation of frost has, however, 
been employed in the table in which first and last frosts of the autumn 
and spring respectively are recorded and a minimum temperature of 32° 
resorted to only in case of conditions unfavorable to the occurrence of 
frosts on account of a dry atmosphere. 



MARYLAND WEATHER SERVICE 



11& 



In making a comparison of the relative severity of winter seasons the 
frequency of occurrence of a minimum temperature of 32° or under is in 
some respects a more satisfactory test of the general character of the 
season than the usual one of the average temperature. 

In Table XXV, the frequency of occurrence of such days is shown for 
each month from October to May, and the total number for each year from 
1871 to 1904. The season having the greatest number of frost days from 
1871 to 1904, is that of 1903-04 when 114 were recorded. There were 104 
in 1874-75, and 102 in 1876-77. In 1877-78 there were but 48 ; in 1889- 
90, 49. The average number for the entire period of 33 3-ears is 78. 



LLJ I I I I I I I I — LI — I— 



Fig. :i8. — Longest Period of Cousecutive Days with a >[inimura Temperature of 
32° or Below. (See Table XXVI.) 



In Table XXVI the longest period of consecutive days with a minimum 
temperature of 32° is recorded for each year, with the time of occurrence; 
the duration of these periods is also presented graphically in Fig. 28. 
The cold periods of this class begin most frequently in the month of 
January, though many begin in December. In one instance the longest 
uninterrupted cold spell fell entirely within the month of March, namely 
in 1890, from March 1-10. 

The season credited with the longest period of consecutive days witli a 
minimum of 32° is that of 1878-79 when the minimum was 32° or below 
daily without interruption from December IG to January 24, or 40 days. 
In the winters of 1875-7G, 1881-82, 1883-84, 1888-89, 1890-91 and 1893- 
94. the longest period was 9 days. The average lengtli of uiiinti'rru])t('d 
periods of freezing weather is 19 days. 



130 



THE CLIMATE OF BALTIMORE 



The cold days occurring in Baltimore from 1871 to 1904 were also 
tabulated on the basis of a daily mean temperature below 32° and below 
14°. The results are shown in Fig. 29 for the entire season. The aver- 
age winter season contains 33 days with a mean temperature below freez- 
ing point. The winter season of 1903-04 contained 66, the highest num- 
ber recorded during any year of the period; the seasons of 1884-85 and 
1901-02 follow with 50 each. In 1877-78 there were but 17; in 1879-80 













1875-6 








1880-1 








1 885-6 








1890-1 








1895-6 








1900-1 








DAYS 




































A 


































60 
40 


























































































































































































































































































































































|0 


































































































B 



































.J 


L 






_l 


L_ 


_l 


U 








_l 


II 






_l 


L 








J 


1 


J 


L 










-J 


L. 





Fig. 29. — (A) Number of Days with Meau Temperature below 32° 

(j5) a a .1 u .. 140 

and 1881-82, 13; and in the winter of 1889-90, but 10, the lowest number 
on record. The average number for each 10-year period from 1871-1904 
is shown in the following table: 

AVERAGE NUMBER OF DAYS WITH A MEAN TEMPERATURE BELOW 32° 



1871-1880. . 
1881-1890. . 
1891-1900. . 


Decade. 


Nov. 

0.8 
1.3 
1.3 


Dec. 

8.4 
6.9 
7.6 


Jan. 

9.8 
12.. 5 
11.8 


Feb. 

8.8 
7.8 
9.1 


March. 

2.9 
4.8 
3.7 


April. 

0.1 
0.0 
0.0 


Season. 

30.8 
33.3 
33.5 










1871-1904. . 






1.3 


8.0 


11.9 


9.5 


3.6 


0.0 


34.3 










Greatest number (1903-4) . . . . 
Least number (1889-90) 


6 




15 
1 


23 
2 


20 
1 


2 
6 






66 
10 



MARYLAND WEATHER SERVICE 



121 



The frequency of days during which the highest temperature of the day 
fell below the freezing point is shown in Fig. 30. The average monthly 
and annual frequency for the entire period of 33 years and for each decade 
is given below. 



AVERAGE FREQUENCY OF DAYS WITH A MAXIMUM TEMPERATURE 

BELOW 32* 



Decade. Nov. Dec. Jan. 

1871-1880 0.3 3.; 4.7 

1881-1890 0.3 2.3 6.7 

1891-1900 0.1 3.5 6.3 

1871-1904 0.2 3.6 6.0 

Greatest number (1892-3) 10 19 

Least number 11877-8) 3 



Feb. March. Season. 



5.9 



0.8 


12.0 


1.6 


13.6 


0.9 


16.7 



4.3 



1.0 



15.1 











87>« 








880-1 








88 


D-D 








890-1 








89>6 








900-1 
















































































































































































































































































































































































































































































































J 




J 





















































Fig. 30. — Annual Frequency of Days with a Maximum Temperature below 32°. 



With an average annual frequency of 15 days, the number varied from 
3 in 1877-78 to 36 in 1892-93. The winter of 1903-0-1 contained 21. 

In Fig. 31, the annual frequency of cold days is shown on a basis of the 
occurrence of a daily minimum temperature of 20° in the months of De- 
cember, January and February and a minimum of 28° in November, 
March, and April. In the table below the average monthly and seasonal 
frequency of occurrence of such days is indicated for each ten-year period 
from 1871 to 1904, and for the entire period of 33 years. 



123 



THE CLIMATE OF BALTIMORE 













187 


« 








188 


OJ 








188 


5-6 








1890-1 








1895-6 








1900-1 








DAYS 
50 








-J 






























































40 








-J 






























































30 






























































































1 












































- 


10 








































- 



















































































_ 











Fig. 31. — Annual Frequency of Cold Days. 

20° or less in December, January and February. 
28° " " November, March and April. 

(See Tables XXVII and XXVIII.) 











187 


5-6 








18801 








1885-6 








1890-1 








1895-6 








1900-1 










1 
















1 


















































































































1 


































































1 

-Dec. 
1 


































































































































1 


















































1 




































































































































































































































































































































































Feb 




























































































































































i 




1 


















1 








































1 






1 




















1 










































1 
















1 






1 — 














































































































































































































































































- Apr 




































































































































1 1 










1 
































1 


l_ 






—i 


I— 


_l 


1_ 



Fig. 32. — Monthly Frequency of Cold Days. 

(a) 20° or less in December, Jauuary and February. 
(&) 28° " " November, March and April. 

(See Tables XXVII and XXVIII.) 



MARYLAND WEATHER SERVICE 



133 



FREQUENCY OF DAY^S WITH A MINIMUM TEMPERATURE OF 20° IN WINTER 

AND 28° IN SPRING AND AUTUMN. 

Decade. Nov. Dec. Jan. Feb. March. April. Season. 

1871-1880 2.8 5.0 0.0 5.2 6.1 0.5 24.8 

1881-1890 3.0 4.1 8.2 4.7 6.9 0.1 27.0 

1891-1900 2.3 4.2 7.1 6.6 6.6 0.1 26.9 

1871-1904 2.8 4.6 7.3 6.2 6.0 0.2 27.1 

Greatest number (1903-4) 11 7 14 18 4 1 55 

Least number (1881-2) 3 7 1 11 



TABLE XXVIT.-LIST OF COLD DAY'S.-January. 

(Minimum of 20° or below.) 

DAY OF MONTH. 



Year. 


1 
o 


2 
o 


3 
o 


i 
o 


6 



6 



7 



8 



9 


19 


1011 

1 


12 



13 




14 




15 


20 

ii 

16 
12 

is 
9 


16 


is 

20 
26 


17 



i3 

ie 
26 


18 


ii 

26 
i2 


19 


26 


202 


ii.' 

i8i 

20. 
26 i 


122 




23 


14 


24 

_ 


17 


25 


i9 


362 


15. 


7282 



« 30 



31 



g 


Is71 




17 





>i 






















4 18 
6—4 


11 

8 


4 


3 






















14 








1 


4 






















18. 




*) 


,5 


15 










18 






6 


2 


2 

17 
17 




19 


20 

i7 

20 
6 








H 


fi 


h'.'. 
!i9 

41<s 
.10 

6 '9 


ii 
11 

i.5 

17 

ii 


i9 

18 

'i 

IS 
12 


is 


ie! 




;? 




10 


13 


18 



6 
'5 


1 

20 
6 


10 
15 
14 


ii 

14 


6 


11 


13 
26 


V 


K 




9 19 
.. 




H 


9 




6 


ii 




7152 


q 


1,S,<| 




1 


1 


—6 


16 
19 


9 
18 


i7 


i9 










i7 

20 

15 


i7 

6 
12 
17 


20 

is 

4 
i4 


,', 


10 

7 


ii 












s 


4 










12 


8 


9 


20 


11 
14 


;io 

18 

i5 


16. 








V' 


.5 






18 




0141 


1 18 




s 








'9 
19 


iei 

20. 


11 






12 


7 


13 


__ 




20 


13 


26 i 


sioi 






10 


s 




11 


y 






















1 


1M90 


.20 

ii9 

3 8 




16 


ii 


191 

is! 


i!! ! 






1 


1 


































i2 
12 

20 


'2 

'" 

26 
11 


ie 
26 

16 

ii 


isi 

161 
26' 

so! 

14 




8 


3 








11 


17 
9 




19 
18 




16 


6 




13 


26 


is 




Ifl 


4 


17 


i9 




1 


.1 




i is 

•i 19 

) i-i 


20 

ii 

26 
10 

19 
20 


8 


i; 


'si 
m 


ejisi 

0182 
9..1 


4 










10 


H 

9 

1900 

1 


9 
15 


18 
6 
14 


it> 

20 


18 
19 


is 


20 
20 






ii 


iti 

19 

1 




'9 

is 






2 

12 

6 








io 






26 






201 


■ 19 


7 


3 










9 


4 




17 


8 


8 


2 


5 






.. 


6l.^l. 






.. ..1 


■17. 


18 


20 


14 
•49 










1 


1 




1 



Table XXVII contains a complete list of all days from 1871 to 1904 during 
which the temperature fell to 28° or lower in November, March and April, 
and to 20" in December, January and February; together with the minimum 
temperature recorded on the corresponding days, and the number of such 
days in each month. Fig. 31 shows the annual frequency of cold days, and 
Fig. 32 the monthly frequency. 



124 



THE CLIMATE OF BALTOIORE 



Cold days of the class described in the above table occur most fre- 
quently in the month of January, as is the case with the other classes 
tabulated. A peculiarity in the seasonal distribution is however revealed 



TABLE XXVir.-LI8T OK COLD DA YS.-Februaiiy. 
(Minimum of 20° or below. i 

DAY OF THE MONTH. 





1 



3 




3 


4 



5 


16 

is 

16 


6 


10 

26 
18 


7 


i2 


8 


io 


9 


ig 

15 
6 


10 


is 

'4 


11 



12 


is 


13 



14 


is 

.. 
20 

is 
26 

is 


IS 



17 
15 

io 

.. 

26 

18 


16 


'6 
26 




i2 

io 

• • 

ii 


IS 


■9 
26 

ig 

8 

i3 

13 


19 


is 

ii 

ii- 
ig 

ii 

is 
5 


20 


26 

io 
ig 

ii 

8 

is 

is 
12 

16 


21 


ig 

"s 
20 

12 
11 

ig 

19 


22 


ii 

18 


23 


i2 

13 

is 
i2 

16 


24 

'2 

12 
14 

'3 

is 

20 

'8 
16 


26 

30 

" 

ii 

"s 

*8 
19 

ig 


26 


ig 
26 

is 
is 


37 


17 

ig 

'g 

.. 


28 


is 
ii 

ie 

18 

i.3 
is 


2g 


io 


3 



H 


1871 


3 




n 

16 


17 
18 
20 

16 


13 


ig 


4 


3 


H 


4 





5 


20 


17 


6 


6 
1 


9 .... .......... [.........[ 






20 


20 












.. 


-• 






3 
5 


1880 




15 
4 


'' 


12 

8 


19 
14 

-1 


18 
7 


15 
26 


19 




17 

is 
12 


'3 


• • 
u 


13 

14 
26 


4 


1 

o 

3'.'.'.'.'..'.'.'.'.'..'.'.'.'.'.'.'.'. ".'.'.'.'. 
i 

5 


15 


8 


2 

1^ 


6 


18 


11 


8 


ii>'.'. 



6 


9 












18 

is 
i 


17 


18 



'4 


8 


1890 

1 








20 


18 


u 


26 
is 



9 










4 


3 










16 
14 
9 


[on 


•S 


4 










19 

20 
15 


5 

ig 
26 

i2 

6 


4 




17 




r 


16 


T) 


6 






18 
14 

8 
8 

14 


io 

14 

8 

19 


i2 
is 


ii 
is 


19 
13 


IS 
14 


16 


8 
ii 


19 


ig 


-6 

ig 


*5 
19 


6 

16 
18 


'e 

14 
16 


6 


1 


8 


Ii 


9 


11 


1900 


11 


1 


11 




1' 


3 






5 


4 


12 


11 


16 


17 


19 






..18 


14 


20 


IS 14 




18 


17 


18 
211 



in the comparatively high frequency of occurrence of such days in March, 
after making due allowance for the fact that the March minimum is 
8° higher than the minimum for the winter months. 

The distribution of cold days of this class by months and years is 
shown in Fig. 32; a complete list with temperatures recorded is con- 
tained in Table XXVII. 



MARYLAND WEATHER SERVICE 



125 



TABLE XXVIL— LIST OF COLD DAYS.— March. 
(Minimum of 28° or below.) 

DAY OF MONTH. 



Year. 


o 
27 


2 

o 

24 
27 


8 


i 


5 


6 


7 

o 

23 

14 


8 


9 
o 


10 




11 
o 


12 
o 

38 


13 
o 

23 


14 



35 

,. 

3« 


15 
o 

24 


16 

o 
37 

27 


IT 

o 

25 
20 


13 



23 

is 

9 


19 



12 
15 


20 
o 

ig 

24 
12 


21 

o 
2i 

26 

26 
27 


32 
o 

i9 
25 


23 




24 



23 

.. 

36 

23 

i9 

38 
37 

18 


25 



28 

2i 

22 
36 

25 


26 



2i 

28 

.. 
37 


27 
o 
23 

30 

37 


28 



28 
24 


29 
o 

23 
24 

38 


30 



3i 

28 

•• 


31 
o 


o 
H 


1871 


o 

24 

12 


o o 
is 9 



10 
9 

1 

8 
12 
3 
2 

4 

4 

1 
11 


3 

4 


51013 


5 

fi 


20 




20 


27 

37 


26 

28 


36 








i? 


21 


g 










9 

l^^iO 


24 






36.. 














28 

28 

ii 

25 
20 


24 

25 
12 

16 


26 

ii 

3^ 

24 

18 


24 

21 

25 

28 

25 
25 


23 

33 

23 

18 

28 


ii: 

37 

38 

24 
24 

27 

25 

27 
28 

ih 

28 


io 

26 
24 

24 

25 

27 

14 
23 


20 
23 

■■ 
20 


12 
26 


22 

i2 

3i 

3i 

33 

36 

38 


24 
i9 

ie 

23 
■»7 

26 


is 

24 
26 


1 

^ 


27 


27 




1 
..:28 

28 i9 
1823 


28 
27 


22 


ie 

35 

2i 




37 


2.5 
20 


4 


14 


24 


23 


S 
13 

3 

9 
13 

1 
10 

9 
11 

i 

5 


6 

S. ...'.'..'..... '".'.'.'.'.'.'.'. 

9 

1890 

1 

•> 

s'.'.'.'.'.'.'.'.'.'.".'.'.'.'.'.'.'.'..'.'.'. 

i 

.5 


19 

26 

20 


15 

24 

16 


23 

19 

24 

28 


27 
24 

34 


25 

20 

24 
16 
21 

9i 


is 

32 
34 


20 
i2 
27 








24 


1>3 


15 










\ 


9 


27 




__ 


..'.. 




2i; 


26 


28 




22 


is 


36 




21 


32 


1 
4 


1900 








1 




q 




28 






..19 


13 
36 


14 








<| 




3 


3 

4 




'■ 


'.'ki 


j6 



4 

307 



TAI5LE XXVII.-LL><T OF (OLD DAYS.-Apkil. 
(Minimum of 28° or below.) 

DAY OF THE MONTH. 



Year. 

Iy74 


1 







3 




4 




5 



38 


6 



36 


7 
o 

.. 


8 




9 
o 


10 




11 




12 



37 


13 
o 


H 
o 


15 




16 
o 


_ 



1. 

26 


19 



24 


20 
o 

37 


21 




22 




a' 




24 



.. 





26 




37 
o 


38 




2930 
o 


1 
o 


]ij7S 










8 


ly((l . 










26 


1 












1 


1903 










27 


1 


1904 










1 










































9 



126 



THE CLIMATE OF BALTIMORE 



The Fkequexct of Cold Waves. 

A cold wave, to come within the definition of the U. S. Weather 
Bureau, is a fall in temperature, in the horizon of Baltimore, of 20° 
within twenty-four hours, to a minimum of 20° in December, January, 



TAHLE XXVIL— LIST OF COLD DAYS— Novem her. 
(Minimum of 28° or below.) 

DAY OF THE MONTH. 



Year. 


1 



■■ 


o 




3 




4 
o 


5 




6 
o 


7 
o 


8 




9 
o 


10 
o 


11 

o 


12 
o 


13 

o 


14 




15 
o 


16 

o 


17 



28 


18 



27 


19 

o 

38 

23 


20 
o 
24 

33 


31 



26 
32 

2i 

28 


32 
o 

20 
15 


2334 


25 


26 


27 


28 


29 


30 
o 

ii 

25 
24 
16 

35 
35 

28 

37 

38 

18 
37 
25 


1 
o 


3871 


o 

28 

ie 




ii 

28 




,', 

24 

.. 

:'' 

28 
35 

36 

36 


o 

35 

28 

28 

33 
26 
2i 




22 

26 

24 
34 

is 


o 

28 
37 

26 

37 

35 
20 


o 
38 
20 

23 

26 

25 

27 
24 

24 

34 
24 


1 

5 


3 


























28 


26 
27 


28 




7 


4 


























9 


5 




























3 


fi 




.. 




28 


S5 


























1 


8 

!( 


1 



1880 








s 


1 






































3 












































38 






i\ 


3 






























38 


33 


25 


38 






4 


4 






























1 


5 

(i 
















38 


























35 
26 


J5 


.. 

25 
36 


21 



3 


















3 


}< 










































3 


9 

1890 

1 




































22 


21 




1 
3 

4 


o 




































3 


S 










































28 
26 


28 


4 


4 
























3" 
















26 


;i 


5 
























f, 


6 

8 












































1 
4 


9 

1900 

1 
































28 








37 
25 


25 


28 








1 

4 


3 ........ .........[[.[.[[.[ 














38 


















■• 




•'fi 


'>\ 



11 




















94 



and February, and to a minimum of 28° in ]March and November. 
The designated minimum must be reached not later than 12 hours after 
the expiration of the 24-hour period. Thus three events are essential 
for the technical verification of a cold wave, namely: (a) a fall of 20°; 
(b) the fall must occur within a period of 24 hours; (c) a designated 
minimum temperature must be reached witliin 36 hours. Cold waves 



MARYLAND WEATHER SERVICE 



127 



fulfilling all these conditions are not of frequent occurrence within the 
geographical horizon of Baltimore. It has been shown in the para- 
graphs dealing with diurnal variability of temperature to what extent 
the frequency of occurrence of given changes in temperature from day 



TARLE XXVII.-LIST OF COLD DAYS.— December. 

(Minimum of 20"^ or below.) 

DAY OF THE MONTH. 



Year. 


1 

o 


2 
o 


3 




4 
o 


5 



12 


6 


7 


8 


' 


10 


11 


12 


« 


14 


15 


16 


17 


18 


19 


« 


21 


22 


23 


24 


25 


26 


27 


2829 


30 


31 


■5 
1 


11*71 


o 
17 







is 
in 




i 




is 

ii 




20 

18 

is 


o 


o 


o 


o 

ii 


o 
13 




ie 

8 


o 

i2 
15 

ie 

17 


o 

ie 

10 

io 




7 

i2 

12 

9 
20 




5 
14 

ie 

20 
20 

14 


o 
16 
6 

11 


o 
i7 

is 
is 

is 

20 




's 

20 

ii 
is 

14 
16 




's 

18 

is 

■■ 

• ■ 
■■ 

i4 

18 
16 

ig 


o 

ii 

•• 

ie 

IS 
20 

20 

ii 

20 

20 

is 

12 


o 

ii 

is 
io 

20 

ii 

is 

20 

11 


o 

is 

18 
20 

ii 

is 
ii 

15 

is 
is 

19 




ie 

• • 

4 

ie 
is 

is 

'7 

20 
ie 

i9 



,', 

-3 
20 

17 

ig 

is 

16 

ii 



17 

-i 

26 
26 

17 

ii 
9 


5 

10 


1 
5 

le 


6 

9 


9 .... 


17 

ie 

18 


20 




.. 


3 

4 

5 


fi 

T 

8 

9 

1880 


1 














1=) 


ii'C. 



3 


3 

4 
















3 

6 


6 














l*)!"! 














IS 
.. 
19 


15 


16 


<> 


6 




17 


16 


18 


18 






20 


11 


8 


18 


4 


9 

1890 
















20 















5 


1 
















1 


o 




































10 


3 












18 


•• 












i4 

is 


20 

is 

i4 
i9 


is 
is 

19 


19 
17 

ig 


19 
18 
20 


20 


17 


1R 





4 












3 


6 








19 


















7 


8 








2 

1 


9 

1900 

1 










20 


16 


18 












6 
4 

11 


§!!!!!!!. .............. 










'w" 


•■ •■ i 










































154 



to day depends upon the method of determining the change. Basing 
the 20° fall upon the minimum temperatures, the 8 a. m. or 8 p. m. 
temperatures from day to day, the Baltimore records from 1871 to 190-1 
show a frequency of cold waves indicated in tlie table below. The table 
shows the extent of tlie fall, and the minimum temperature attained 
within ."50 hours. 



128 



THE CLIMATE OF BALTIMORE 
TABLE XXAail.-FREQUENCY OF COLD WAVES. 



1870-1 . 



1871-2. 



1872-3.. 
1873-4.. 
1874-5. . 

1875-6.. 

1876-7.. 

1877-8.. 
1878-9.. 
1879-80. 



1880-1. 
1881-2. 



1882-3. 



1883-4.. 
1884-5. . 



1885-6.. 

1886-7 . 

1887-8.. 
1888-9.. 
1889-90. 



1890-1. 
1891-2. 

1892-3. 

1893-4. 
1894-5. 



1898-9. 



1899-1900. 



1900-1. 
1901-2. 
1902-3 



Nov. 



Fall I Min. 




36 





26 





35 
22 



1895-6 21 

1896-7 

1897-8 



1903-4 21 



Total number 8 

Average number. . . 0.2 



Dec. 



Fall Min 



19 
0.6 



13 

7 

20 

8 



19 

15 

9 

13 



18 




12 


10 



IS 

15 

16 













Jan. 



Fall Min 



20 



23 
0.7 



Feb. 


March. 


Fall 


Min. 


Fall 


Min. 


Q 





^ 


^ 


21 
21 


15 

20 








26 


15 


21 


9 

19 


27 

24 


20 

16 


21 


25 


7 



26 


20 
25 


17 
16 














33 


23 












27 




25 




23 




28 
33 

28 


16 




12 

5 
10 


20 

23 
26 
22 





28 

18 
25 
23 





























24 

"o 


11 
3 




~0 



18 




22 


18 














21 


20 


25 
27 
23 
22 


12 

20 
13 

8 


23 





18 





34 


20 


5 

1^ 


25 




19 




24 


2 








25 


10 


28 


13 








26 


13 








22 


27 



30 
20 
20 




18 

8 

19 










25 
0.7 




17 

0.5 





Season. 



No. 



Gr. 

fall 



92 
2.7 



21 



20 






15 



20 

18 
12 

5 
14 

18 

13 
13 

16 

17 
13 

18 



Table XXVIII shows the frequency of occurrence of a fall of 20° or more 
in the temperature of two successive days, combined with the attainment of 
a minimum temperature of 20° or less in December, January and February, 
or a minimum of 28° or less in March and November. 



MARYLAND WEATHER SERVICE 129 

As a special chapter is to be devoted to an analj'sis of cold waves in 
Part II of this Eeport, reference is here made only to the frequency 
of their occurrence. In the period comprising 34 winters the total num- 
ber of occurrences fully satisfying the technical conditions imposed is 
92, or an approximate average of three per year. Since 1871 there 
were three seasons without a cold wave, namely, 1873-74, 1885-86, and 
1889-90. The greatest number occurring in any one season was 6, in 
1871-72, 1884-85, and in 1903-4. Of the total of 92 in 34 years, 8 oc- 
curred in JSTovember, 19 in December, 23 in Januar}^, 25 in February, 
and 17 in March. The greatest fall in temperature recorded within 
the prescribed time of 24 hours was 38°, which occurred in December, 
1901. Three cold waves occurred in each of the following months: 
December, 1871-72, January, 1898-99, February, 1903-04, and March, 
1882-83. 

Killing Frosts. 

A factor of the highest importance, especially to the agricultural and 
trucking interests of a community, is the average date of occurrence of 
the first " killing " or " black " frost in autumn, and the last in spring, 
and their variations in time of occurrence from year to year. Frosts are 
usually designated as " light," " heavy," or " killing." The term 
" light " is applied to frosts which are destructive only to tender plants ; 
" heavy " to copious deposits of frost, but which do not destroy the staple 
products ; " killing " to such as are blighting to the staple products of the 
locality in which the frost occurs. First and last killing frosts are 
tabulated below for each year from 1871 to 1904 for the vicinity of 
Baltimore. In the absence of a killing frost before a minimum tempera- 
ture of 32° was observed, the date of the first record of a freezing tem- 
perature was entered in the table. The interval in days between the 
last frost in spring and the first in autumn is likewise given in order 
to show the length of the period of safe plant growth. 

The average date of occurrence of the last killing frost in spring, based 
on observations of 34 years, is April 4. It has occurred as early as 
February 26, namely in 1903, and as late as May 3, as in 1882. The first 
killing frost in autumn has occurred, on the average on November 3. 



130 



THE CLIMATE OF BALTIMORE 



The earliest appearance is that of October 6, 1892, and the latest that 
of December 6, 1878. In the ordinary course of events, accordingly, the 
period of safe plant growth in the neighborhood of Baltimore, based 
upon the occurrence of killing frosts, is from April 4 to November 3, 



TABLE XXIX.-KILLING FROSTS. 



18T1 

lS-2.... 

1873 

1374 

1875 



1876. 

1877. 
1878. 
1879. 
1380. 

1881. 
1883. 
138-3. 
1884. 
1885. 



1886.. 
1887.. 
1888.. 
1S89.. 
1890.. 



1891 . 
1892. 
1893. 
1894. 
1895. 



1896 

1897 *Mar. 

1S98 

1899 

1900 



1901. 
1902. 
1903. 
1904. 



Average date 1871-1903. 

Earliest date 

Latest date 



Last in Spr 


mg-. 


First in 


Autumn. 


Interval in days. 






Min. 






Min. 




*Feb. 


23 


30° 


*Nov. 


28 


31° 




*Mar. 


25 


33 


* •> 


16 


30 




" 


31 


29 


Oct. 


29 


31 


OJ.7 


Apr. 


13 


29 


Nov. 


10 


31 


211 


" 


22 


3.' 


'* 


3 


33 


195 


.* 


» 


30 


Oct. 


15 


33 


196 


'• 


3 


33 


Nov. 


4 


37 


215 


Mar. 


2fi 


21 


Dec. 


6 


32 


255 


Apr. 


n 


32 


Oct. 


26 


30 


204 


■' 


12 


30 


Nov. 


8 


35 


210 


.. 


21 


39 


" 


27 


34 


220 


May 


3 


38 


" 


19 


30 


200 


Apr. 


25 


34 


" 


13 


32 


203 


Mar. 


30 


31 


" 


7 


30 


•;•» 


" 


IB 


31 


'^ 


1 


33 


2:30 


.. 


24 


29 


Oct. 


17 


36 


307 


Apr. 


6 


30 


*• 


31 


32 


308 


Mar. 


19 


30 


" 


22 


33 


217 


* n 


30 


28 


Nov. 


6 


35 




*Apr. 


2 


31 


Oct. 


31 


36 




» 


9 


36 


" 


;:9 


33 


303 


" 


15 


34 


" 


6 


36 


1:4 


" 


16 


36 


" 


17 


36 


184 


" 


11 


32 


Nov. 


12 


''V 


31. 


** 


11 


34 


Oct. 


29 


34 


301 


" 


8 


33 


Nov. 


14 


32 


320 


*Mar. 


29 


34 


Oct. 


31 


39 




Apr. 


6 


26 


" 


28 


34 


205 


Mar. 


25 


;!0 


Nov. 


4 


38 


224 


** 


22 


26 


" 


16 


38 


239 


* <i 


17 


30 


.1 


11 


31 




" 




31 


Oct. 


30 


34 


S37 


Feb. 


26 


29 


Nov. 


7 


28 


254 


Apr. 


17 


31 






■■ 




Apr. 


4 .... 


Nov. 


3 ... 


213 Average period. 


Feb. 


26, 1903 


Oct. 


6, 1892 


355 Longest period. 


May 


3, 


1883 


Dec. 


6. 


1878 


174 Shortest period. 



* No frost recorded ; first day in Autumn and last day in Spring with a minimum temi)er- 
ature of 33° or below. 



or approximately seven months. While this is the most probable length 
of the period, the interval may be considerably extended by a late autumn 
frost in conjunction with an early spring frost, or the period may 
be shortened by a late sjjring frost followed by an early 
autumn frost. The extent to which this important interval has 



MARYLAND WEATHER SERVICE 131 

varied in the past 34 years is shown in the above Table XXIX. The 
shortest interval namely, 5 months and 24 days, was that of 1892, 
extending from April 15 to October 6; the longest was that of 1878, 
extending from March 26 to December 6, or 8 months and 15 days. 
Calculating on the basis of a '34-year record, we find that the last killing 
frost in spring is likely to occur sometime within the first decade of 
April once in 4 years ; in the second decade once in 5 years ; in the third 
decade once in 11 years; the latest occurrence, as stated above, was 
May 3, in 1882. In the autumn the first killing frost has occurred but 
once in 33 years in the first decade of October, three times in the second 
decade, and ten times in the third decade. It fell within the first decade 
of Xovember nine times, within the second decade seven times, and 
within the third decade twice. The Litest in 33 years occurred on 
December 6, 1878. 

The First and Last Occurrence of a Minimum of 32°. 

Another measure of the period during which staple products are safe 
from tlio influence of low temperatures is the last occurrence of a re- 
corded temperature of 32° in spring and the first in autumn. As 
these records are determined by self-registering instruments they are 
not subject to the uncertain judgment of observers as to the character 
and extent of damage inflicted by the frost. Especially is this judg- 
ment liable to error in the case of observers stationed within large cities. 
With a fair exposure of the thermometer, a really injurious frost is not 
likely to occur with a recorded minimum temperature more than 2° or 3° 
above the freezing point. Tlie average of the minimum temperatures 
recorded at the time of occurrence of the " killing frost" in tlic preceding 
table, is 31°. It sometimes happens that a temperature several degrees 
below 32°, sufficient to form heavy ice and seriously injure vegetation, 
occurs many days and even several weeks after the last recorded '' killing 
frost." Such was the case in 1903, when the last killing frost occurred 
on the 2nth of February, wliilc a temperature of 27° occurred as late 
as April 5, but without frost. A deposit of frost requires not only a 
freezing temperature but a liigli iicirontagc of Inimidity in the layers of 



132 



THE CLIMATE OF BALTIMORE 



atmosphere resting upon the ground. Upon the basis of the last and 
first " freeze " the period of safe vegetable growth is lengthened by 
about 13 days as compared with the interval between the last and first 
" killing frost." Table XXX shows an average interval of 226 days 
between the last occurrence of a minimum temperature of 32° in spring 
and the first occurrence in autumn. AVith killing frosts the interval is 
213 days. In the exceptionally warm year of 1871 the interval of safe 





^^L 




__^_.^_ 








1 1 1 I 11 


___^___ 












r 






^^^^^ 








1 1 1 1 






-'--'— 




_. 


_ 


^r 




^ 


^^— 


p- 


















- 


-J 


-f 




^ 


T 






tlTS 














1876 
































1 














1 ' ' 












^^^^ 


-p 






1 








































' r 


. . 












^^^^^^^ 


^ 




















1 1 1 1 1 




























j 










































1 ' ^^^^ 












^^^^H 1 


i ' ' 




' 


















; *^^ 














! 1 
























































^C ^^M 




































































' 


■ 
















. ' 




_ 




J 


- 


^ 




H 


- 






















- 




- 


- 






A 


- 




-i 
















1 i 


1891 
1896 


2 


- 
- 




: 






-J 


- 




-1 


^^ 














. - 

1 — 


1901 


~- 


\ 


\ 


] 


\ 


I 


: 
: 


■ 
- 






m 






— 


=j= 


-T- 


























, : i . . 






I . 1 1 




' , 


^^^ 










' ; 1 ' 




. , 

















Fig. 33. — Interval Between Last and First Occurrence of a Minimum Temperature 
of 32° (Heavy Frost). 

Fig. 33 shows the time of occurrence of the latest freezing- temperature in spring and the 
first in autumn; also the intervening interval in months and days, for each year Irom 1871 
to 19j3. The lowest line, marked "mean" shows the average date of occurrence and the 
avei-age length of the intervening period. iSee Table XXX.) 



growth wafi lengthened to 278 days. In 1875 it was only 195 days. 
These figures indicate that under the least favorable conditions during 
the past 33 years the period of entire safety to all excepting tender plants 
near Baltimore was six months and a half; under the most favorable 
conditions a trifle over nine months; while the average period is seven 
months and a half. (See also Fig. 33.) The probability of given 
departures from the average, or normal, period is shown by the follow- 
ing figures : 



MARYLAXD WEATHER SERVICE 



133 



FREQUENCY OF STATED DEPAKTURE.S FROM THE AVERAGE LENGTH OF THE 
PERIOD OF SAFE VEGETABLE GROWTH. 

(Number of days.) 



Departure. 1-5 


ft-10 


11-15 


16-30 31-25 


1 1 
36-30 31-35 .36-10 


41-45 '4&-5O 51-55 Total 


Frequency 10 

In percentage ••• 30 

Cumulative percentage 30 


5 
15 
45 


10 
30 
75 


3 1 
10 3 
85 88 


2 10 

6 3 

9t i 97 .... 

i 1 


1 i 33 

3 ' 100 

1 100 t ■••• 

1 1 



TABLE XXX.-LAST AND FIRST OCCURRENCE OF A MINIMUM TEMPERATURE 

OF 32° 



Tear. 



Last in Spring. 



Feb. Mar. Apr, 



1871. 
1872. 
1S73. 
1874. 

1875. 

1376. 
1877. 

1878. 
1879. 
1880. 

1881. 
1882. 
1883. 

1884. 
1885. 

1886. 
1887. 
1888. 
1889. 
1890. 

1891 . 
1892. 
1893. 
1894. 
1895. 

1890. 
1897. 
189.S. 
I89f». 
1900. 

1901. 
1902 
l!)0;!. 
1904. 

Average date, 1871-1880. 
•• ls^l-1890. 
" 1891-1900. 
" 1871-1903. 



31 



30 
29 



First in Fall. 



Oct. Nov. Dec 



13 
12 

U 
1- 
:.'4 
13 
14 

11 

29 I 
6 



278 
237 
243 
211 
195 

237 
232 
255 
204 
218 

231 
221 
226 
321 



228 
208 
S3: 
231 
233 

212 
210 
215 
215 

228 

220 
213 
231 
223 
217 

£39 
2.55 
215 



231 
236 
219 
226 



®.5 



+.53 
-1-11 

-1-17 
—15 
—31 

-hll 

+ 6 
+29 

o*> 

— 8 

+ 5 

— 3 


— 5 

— 4 

+ 2 
—18 
-i-11 
+ 5 

+ 7 

-14 
-16 
-11 
-11 

+ 2 

— 6 
-14 
+ 5 

— 3 

— 9 

-i- 1 
^2'.» 
-11 



The average departure from the normal period of 226 days is about 
12 days. In the year 18T1 the last freezing temperature occurred 53 



134 



THE CLIMATE OF BALTIMORE 



days before the average date; the nearest approach to this excessive 
deviation was a departure of 31 days in the opposite direction in 1875. 
The figures in the above table indicate that a departure exceeding one 
month is extremely improbable, having occurred but twice in 3-1 years. 
In 28 out of a total of 34 years the limit of variability was under 30 
days; in 25 it was under 15 days; in 15 under 10 days; and in 10 under 
5 days. 





I 






, 
















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- 









^ ; 


























^^* 






! 1 1 
















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^ , : 1 


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J_ 
















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j 
















[ 




































1 




















































1 
















■ ^^ 






































1886 


._ 


_ 






_ 


-\ 


4S 






^■^^M 










I 










^ 






_ 







— 


-^ 






-- 


^ 


\ — 


























- 






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1 ■ 












^^1 1 


















































^; 1 






































1 ^1 












nss 


* 




































! ' '. 




















































i 1 i! 












■: : 1 








































MJJ 












■¥W 








































Til 




















































^^ 












^Z 


















1 




















iffi 












L. j_ 


















1 




















T 




































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1^1 






























































^^^■1 








































^^ 












■^■21 








































"□"•" 








, ' . , 




■ H^^E ■ 


















































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1 , 1 


i 1 1 
























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M 




















1 1 j 1 


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1 . . i. ^. 






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MEAN 
























1 1 ! ! 1 






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il 



McH. Apb- May June J"' v Aug. Sept. Oct. Nov. Dec. 

Fig. 34. — Interval Between Last and First Occurrence of ^Minimum Temperature 
of 40° (Light Frosti. 

Fig-. 34 shows the time of last occurrence of a minimum temperature of 40° in spring, and 
that of the first occuri-ence in fall, together with the length of the intervening period in 
months and daj's. The lowest line marked " mean" shows the date of average occurrence 
and tbe average length of the intervening period. (See Table XXXI. i 

The probability of the occurrence of an injurious freeze some time 
within the month of April may be expressed by 65 on the basis of a 
possible 100, a temperature of 32° or less having occurred at some time 
within the month 22 times in 34 years. The probability of occurrence 
from the 1st to the 10th of the month is 41 ; from the 11th to the 20th, 
21; from the 21st to the 31st, 3. It has occurred 15 times in 34 3'ears 
on some day between the 1st and the l(^th of April; 7 times from the 
11th to the 20th; and 1 time after the 20th. These figures represent 
the chances of an injurious freeze in the first, second and third decades, 
respectivel}', of the month of April. 



MAKYLAXD WEATHER SERVICE 



135 



Light Frosts. 

A light frost, injurious onh' to tender plants, may, and frequently 
does, occur with a recorded minimum temperature of 40°. Hence the 
period of safe growth for the frailer varieties of vegetation is reduced 

TABLE XXXI.— LAST AND FIRST OCCURRENCE OF A MINIMUM TEMPERATURE 

OF 40°. 



Last in Spring^. 



Year. 



Mar. Apr. May 



ISTl 
1ST3 
1873 
1874 
1875 

1876 

1877. 
1878 
1879 
1880 

138L 
1S82 
1883 
1884 

1885. 

1886 
1887 
1888. 
1889 
1890 

1391 
1892 
1893 
1894 

1895 

1896 
1897 
1898 
1899 
1900 

1901 
1903 
1903 
1904 

Average date, 1871-1880 
" I8fl-1890 
" 1891-1900 
" 1871-1903 



13 



13 



First in FalL 


_ o6 










Sept. 


Oct. 


Nov. 


Sa i 

5"^ t 




31 




30(1 




13 




179 


is 






145 




15 




168 




13 




140 




8 




157 






4 


203 






39 


244 


36 






161 




35 




177 






15 


308 
184 




itJ 




lf,9 




33 




194 




23 




l-ifi 
■.'10 




3 




160 




9 




185 




•2S 




191 




18 
3 




165 
160 




17 




173 




15 




185 




9 




149 




9 




183 




18 




180 




17 




171 




1 




167 




17 




160 




18 




189 




29 




196 




19 




170 




17 




178 




23 




186 




12 




169 




18 




179 



a": 



+37 

-»4 
—11 
-39 



+34 
+6.-. 
—18 



+39 
+ 5 
—10 
+15 



+31 

— 3 
—19 
+ 6 
+ 13 

-14 
-19 

— ti 
+ 6 
-30 

+ 3 

+ 1 

— s 
—13 
-13 

+10 
+17 



— 1 

+ 7 
-10 



still more. A minimum of 40° has been recorded at Baltimore as late 
as May 26, namely in 1875; but this is an exceptional case. The last 
spring mini iiui 111 of 40° has occurred as early as March 29. The 
dates of the last in spring and first in autumn, witli the length of the 



136 



THE CLIMATE OF BALTIMORE 



intervening interval in days, is shown in Table XXXI. This interval 
is also shown graphically in Fig. 34. 

The Period of Effective Temperatures for Plant Growth. 

It is generally conceded that every plant requires a certain tempera- 
ture in order to develop successively the leaf, bud and fruit ; that there is 
a minimum temperature below which physiological activity in the plant 
ceases, and hence that only temperatures above this limit are effective 
in carrying the plant forward from the sprouting of the seed to the 
maturity of the fruit. This "critical" point in the history of plant 
growth is assumed to be a mean daily temperature of 43° F. In order 













1875 








1880 








1885 








1890 








189b 








1900 








DAYS 


- 








































































L 




































































































































- 


200 


















































































































- 








- 




- 


- 




- 
































































































_ 





















DAYS 
300 



Fig. 35. — Annual Number of Days with Mean Temperature above ■42°. 

Fig. 35 shows the total annual number of dajs having a mean temperature of 43° or 
above, the degi-ee of heat which marl£S the beginning- of physiological activit)- in plants. 

to determine the number of days in the year during which this " effec- 
tive " temperature prevails in the vicinity of Baltimore, the days with a 
mean daily temperature of 43° or above from 1871 to 1903 have been 
tabulated. The normal number of such days for each month and for 
the year is shown in the following table, while the variation in the total 
annual number is represented in Fig. 35. 

PERIOD OF effective TEMPERATURES FOR PLANT GROWTH. 
(Average number of days.) 



Means. 


Jan. 


Feb. Mar. Apr. 


May 


.Tune July 


Aug. 


Sept. Oct. Nov. Dec. Year 


1871 1902 


5.3 


0.5 14.1 27.0 


31. n 


30.0 ; 31.0 


31.0 


30.0 30.0 19.4 8.2 264 















MARYLAXD WEATHER SERVICE 



137 



The first appearance of a daily mean temperature of 43° in spring 
normally falls upon the 25th day of March, and the last upon the 27th 
day of November, an interval of 245 days, or about eight months. How- 
ever, such days occur throughout the year and are probably effective in 
directly or indirectly promoting physiological activity in the plant. 
Hence to the period above mentioned must be added the days of the 
winter and early spring months before the permanent appearance of 
the daily mean of 43°. This will materially increase the annual period 
of " effective " temperatures, as a considerable proportion of the winter 
days fall within the prescribed limit. Calculating on this basis the 
average period comprises 264 days. The longest period, namely, that 













1675 








1880 








1885 








1890 








1895 








1900 








DAYS 


















































































































































































































10 







































































































































Fig. 36. — Annual Number of Days with Maximum Temperature of 90° aud over. 
(See Tables XXXII and XXXIII.) 

of the year 1878, contained 293 days, and the shortest, 244 days in 1886. 
The ten year average values of the three decades from 1871-1900 varied 
only from 261 days to 268 days, showing a remarkably constant average 
length for this period. 



The Frequency of Warm Days ix Summer. 

It is a well recognized fact of observation that the temperatures above 
the normal heat of a locality fluctuate to a less degree than the tempera- 
tures below the normal. In other words the extent of variability in the 
temperature for a given locality is generally determined by the cold 
days and not by the warm. The extreme maximum temperature in 

the United States has a range of about 40°, or from 80° to 120° ; the 
10 



138 



THE CLIMATE OF BALTIMORE 



extreme miniiiuim varies from (15° below zero to about 40° above, a 
range of 105°. While variability in the temperature of warm da3-s is of 
less importance in agricultural and mercantile life than that of cold days, 
it is a faetoi- of much concern in personal comf(jrt, especially to the 
dwellers in large cities. 

TABLE XXXII.-LIST OF WARM DA YS.-Aprii.. 
(Temperature of 90° or above.) 



Year. 


' 


2 


3 


4 
o 


5 
o 


6 
o 


7 
o 


8 
o 


9 
o 


10 




11 
o 


12 
o 


13 

o 


14 
o 


15 




1617 




181920 

o o o 
9493;; 


21 




22 




2334253637 
00000 


3829 


..90 


30 



9i 


1 




1888 

1898 


o 


o 


o 


1 


1903 






























1 
4 



May. 





1 









3 




4 



5 



6 




7 



8 



9 




10 




11 



13 



13 



14 



15 


16 


17 


18 


19 


20 


•21 


22 



33 



90 


34 




35 

90 

•■ 


26 



■■ 
92 


37 

93 


38 


90 


29 



30 


95 


31 


94 

95 





1877 












92 

90 
92 



93 




9i 

•• 
92 








1879 

18S0 














•• 




91 




9i 

94 


94 


!? 






•• 


90 
93 


1 
7 


18S1 




















1 


3 


1889 


















90 
93 


93 
96 





1895 

1«!)6 














__ 




:: 




3 
5 


189S 


















1 


1899 

1900 
























.. 


9i 


9i 


94 










1 
3 


19U-,' 


























1 


1903 . 








































92 






1 










































39 



Table XXXII contains a complete list of all days from 1871 to 1903, during 
which the temperature rose to 90° or above; together with the maximum 
temperature recorded on such days and the monthly frequency of occurrence. 
Fig. 36 shows the annual frequency of days upon which the maximum tem- 
perature equalled or exceeded 90°. 



The frequency of occurrence of days with an excessively high tempera- 
ture hence plays an important part in the composition of local climates. 
We can provide against extreme cold in Avinter; from the hot and ener- 
vating summer weather there is no escape for the great numbers who 
are compelled by circumstances to remain in the large cities beyond the 
reach of mountains or seashore. 

A temperature below 90° is not especially uncomfortable or unwhole- 
some unless accompanied Ijy a high degree of humidity and a stagnant 



MAEYLAND WEATHER SERVICE 



139 



atmosphere. Defining a hot day as one with a temperature of 90° or 
above, the Baltimore statistics show a frequency and distribution indi- 
cated in Table XXXII. 

TABLE XXXII.-LIST OF WARM DATS.-June. 
(Temperature of 90° or above.) 



Year. 


1 







3 




4 




5 
o 


6 
o 


7 
o 


8 
o 


9 

o 
90 


10 

o 


11 




13 



90 


13 

o 


14 

o 


15 




16 
o 


17 

o 


18 
o 


19 



93 


20 
o 


21 


9i 


22 



23 


95 
96 


24 


93 
96 

90 

95 
9i 

93 

98 

(V) 
94 


25 


95 

90 

92 

93 
97 

92 
93 

92 

9S 


36 
.° 

9i 

97 

94 
95 

90 
92 

93 
91 

95 
92 


27 


94 

95 

92 
92 
91 

90 

9;i 

92 


38 


91 
92 

93 
93 

93 

92 
94 

92 

94 
9i 

93 


29 


97 
90 

93 

93 
91 



94 

.. 

95 
90 

99 




1871 


1 


3 


















^ 


4 














90 


96 


98 


















90 


9 


5 














7 


6 














































5 








90 














90 




95 


92 




90 








93 

91 
9i 


9i 
90 

yi 
90 


92 

.. 
92 
91 

90 
90 

93 




93 

93 

93 

97 
91 


4 


H . 

9 


94 




1 

5 


1880 


9 


1 
























3 


o 






























90 






90 


n 


;j 












90 


















1 


4 












4 


5 










91 


















95 


90 


94 


9S 


94 


4 


6 













n 






























92 


94 


90 


s 


9 

































1890 








91 


90 
















93 


94 


93 


94 


90 
91 

9i 


90 


9i 
9:i 


4 


1 






90 


n 








R 


3 

4 








90 














92 
90 


90 


5 


5 


9t 


95 


97 


5 


6 


H 


7 

8 


















90 




93 


9i 

92 
91 


94 


90 


93 






•• 




93 


2 

7 


9 










93 


98 


96 


95 
91 


8 


1900 










n 


1 
















7 








93 


















4 


3 







147 



Such days do not generally make their appearance until the month of 
June and disappear in the first week of September. The average num- 
ber for the entire season is 21, and the monthly distribution is as 
follows: June 4, July 10, August 5, and September 2. They occasion- 
ally occur in May (about one in two years on the average), and have 
occurred on two occasions in 33 years as early as April and once as late as 
October. There is considerable difference in their total frequency during 



140 



THE CLIMATE OF BALTIMOEE 



a period of 10 years: From 1871 to 1880 there were 204; from 1881 to 
1890, 161 ; from 1891 to 1900, 243. The annual frequency has varied 
from 8 in 1871 to 43 in the memorable summer of 1900, as shown in 
Fig, 36. It is remarkable that the summer containing the highest total 



TABLE XXXII.— LIST OF WARM DAYS.— July. 

(Temperature of 90° or above.) 



Year. 


1 


2 


3 


4 


5 



93 
91 

92 

92 
92 


6 
o 

94 
90 
92 


7 

o 
90 

90 


8 
o 

92 

97 
93 


9 

o 
91 

92 

99 
91 
91 

93 
90 


10 

o 
91 
93 

96 

97 

94 
93 
97 

93 


11 

o 

90 
96 

oi 

95 

94 

'M 


13 

° 

96 

90 


13 

o 
91 

90 
95 

'M 

Itfi 

9i 

96 

9i 
94 
97 

93 

93 


14 

o 

92 
94 
92 

91 
91 

90 
it;.' 

95 
90 

90 
91 

9i 


15 



90 

9i 
94 

91 

92 

94 
Itl 

9i 

90 

9i 
96 

92 


16 

o 
92 
91 
90 

9i 

90 
91 

99 
94 

91 
93 

loi 

92 

92 

92 

100 

90 


17 



9i 

93 

90 

9i 
91 
90 

95 
99 
97 

90 
90 

100 
9fi 


18 

o 

95 

96 

96 
93 
98 

96 
102 

92 
93 

98 

90 
99 


19 
o 

93 
96 

93 

94 
93 


20 



9i 

97 
93 

98 
90 

97 

93 

90 

9i 

90 
95 


21 



95 
99 

95 

92 
94 
96 

92 


33 
o 

90 

96 
93 

■■ 

9i 

96 
93 


33 



90 

93 

95 
93 
92 

90 

• • 
94 

94 


o 
92 

90 
91 

95 
92 

90 

93 
90 


25 
o 
94 

92 

90 
90 

96 

97 
92 
91 

9i 

94 

9i 


26 



94 
93 

94 

93 
91 

90 
93 
94 

99 
98 
93 

■• 
94 


37 



93 
90 

9i 

97 
92 

96 

92 


38 
o 

92 
93 

95 
97 

92 

90 


29 



90 
93 

97 
90 
95 

94 
94 

96 
92 


30 
o 
9i 

92 

91 
92 

92 

9i 

92 
94 

95 
95 


31 



90 

95 
90 

• • 
90 


o 


1871 




96 


o 

97 
93 




96 
96 




93 
93 
92 


s 




1" 


3 


15 


4 




7 










1 


6 




94 


93 


95 

92 

98 


IS 




93 


^ 


H 


16 


9 






95 


10 


1880 






10 


1 






91 


93 


96 


96 


91 




11 








8 


3 




91 


92 


94 




94 


93 




7 


4 




s 


5 














91 

93 
94 


93 
90 

93 
98 


94 

93 
90 


93 


90 
90 


92 
90 


If) 


g 














i 


7 

8 


90 








9i 




10 
5 


9 










») 


1890 








91 








8 


1 









10 


3 










91 








90 

93 
95 


90 

96 


90 
94 
95 


95 

92 

9i 
93 


q 


4 


91 








11 


5 


5 


6 






90 

104 
90 
92 

97 
97 
95 


l66 
90 
97 

96 


94 

94 
95 


90 

96 

96 
95 


96 
90 


9i 

93 

92 
90 


10 






91 
97 


<) 


8 


100 


10 


9 


8 


1900 






15 


1 


103 


103 


19 


3 


10 


3 


93 


95 


1'^ 




307 



number of excessively hot days in a period of 33 years did not have an 
average temperature sufficiently high to class the season as excessively 
warm. The " hot-spell " occurred late in July and in August and was 
followed by an exceptionally warm autumn. This period will receive 
further attention in Part II, in the discussion of summer weather. 
Inspection of Fig. 36 reveals a strikingly uniform increase in the num- 



MARYLAND WEATHER SERVICE 



141 



ber of days with a maximum temperature of 90° and above since the year 
1889. Starting with a frequency of 9 in the latter year, the number 
rose steadily to 43 in 1900 with but one marked interruption and two 
or three minor ones. Since 1900 there has been a steady fall, repre- 

TABLE XXXII.-LIST OF WARM DAYS.-AuGUST. 
(Temperature of 90° or above.) 



Year. 


1 

o 


2 

o 


3 

o 


4 
o 


5 

o 


e 

o 


7 




8 




9 
o 


10 
o 


11 




12 




13 




14 

o 


15 

o 
90 


16 

o 
9J 


17 




18 



90 
9i 


19 



93 

92 
92 

92 


20 

o 
93 
97 

92 
94 

97 


21 

o 

gi 

96 

" 
•• 

91 
90 

97 
90 


22 

o 
96 

90 
90 


23 
o 
90 

90 
93 


24 



94 

94 
94 

93 


25 



90 
92 

90 
97 


26 



92 

90 
96 


27 



92 
90 
92 


28 



90 

90 
9i 


29 
o 

94 
9i 

92 


30 
o 

9i 


31 



91 

96 
93 


"5 
o 


1871 


o 






















92 


92 


94 


95 


96 


90 


1*? 


3 

4 


94 


91 


90 














4 

3 


5 

6 














90 


92 


92 


9i 

93 






98 








90 



















g 


8 


91 


90 




90 
94 


90 
94 


90 
93 


90 


(f 


9 


qoqo 


r, 


1880 

1 






92 


2 

8 








1 


3 














p 


4 






































3 


5 


























90 
92 


90 

oi 


90 
90 


96 

90 
9i 


90 
90 
9i 

92 
92 


9i 


3 


6 






















90 
91 

94 

92 


9i 

91 
90 

9i 

97 


4 














90 
90 


93 


94 


90 

92 
95 


94 
95 


s 


8 






90 


94 


93 


10 


9 

1890 


95 




1 

9 


1 


5 




















3 


3 


















9 


4 
















93 
91 

94 


90 
94 

97 


96 
90 


95 
96 


o 


6 
















10 


6 








91 


96 


94 


98 


10 










1 


8 








93 
91 


93 


97 


93 
100 


92 
99 


l66 


100 
93 


i66 

90 


99 


gi 

92 


9 


9 

1900 




90 


90 


8 
17 


1 












8 


•J 

3 




90 


91 


90 










•• 


4 
2 

m 



sented by the figures 43, 30, 20, 16. The numbers for the rising branch 
of the curve of frequency are: 9, 14, 11, 19, 16, 23, 28, 29, 12, 35, 
27, 43. 

The real discomfort of a hot summer depends not so much on the 
actual number of hot days as the length of the periods of uninterrupted 
hot weather. A month, for example, with 10 scattered days having a 
temperature exceeding 90° would be far more comfortable than one with 
an equal number of consecutive days with the same degree of heat. In 



14S 



THE CLIMATE OF BALTIMORE 



fact, liot spells of tlie latter description are of comparatively rare occur- 
rence in Baltimore, not having occurred more than five times in 33 years, 

TABLE XXXIL-LIST OF WARM DAYS.-September. 

(Temperature of 90° or above.) 



Year, 


1 


2 


3 


4 


5 


f) 


7 


8 
o 


9 
o 


10 




11 

o 


12 




13 

o 


14 

o 


15 




16 




17 




18 
o 

90 
9i 


19 
o 

■■ 
94 


20 
o 

90 


21 

o 

96 


o 
96 


23 
o 

95 


o 


25 
o 

90 


26 
o 

90 
93 


27 
o 

90 


28 
o 

91 


29 




30 
o 


-2 


1871 


o 


o 





o 


o 


o 


o 





2 
















94 
90 

93 


92 
92 


98 




90 
' 1 










o 


3 


P3 








90 






o 


4 










o 


5 






92 


90 
9i 


9i 


90 
93 

90 


101 
90 


>) 


6 

7 

8 

9 

1880 










1^ 


1 








R 


2 

3 

i 
















5 













6 


































90 

•• 


t 


7 

8 

9 

1890 

1 







































1 


2 

3 

4 


















94 


94 


90 
97 
95 


93 


93 




92 


[ ' 


■■ 





o 


5 


















7 


6 






91 






93 
94 
94 


93 
9i 


90 


9i 


91 

92 


3 


7 






4 


8 


95 


96 


97 


91 


90 


8 


9 


o 


1900 






90 






4 


1 






1 


9 


92 




" 
























1 


3 


n 










































m 



October. 



1897. 



1011 



192021222324252627 



293031 



while the former condition has occurred about 25 times. A list of the 
longest periods of consecutive days with a maximum temperature of 90° 
or above for each year from 1871 to 1903 is published in Table XXXIII. 
The table likewise shows the dates of beginning and ending of the periods 



MARYLAND WEATHER SERVICE 



143 



and the maximum temperatures attained. Their annual average length 
is a little less than six days, with limits of variations hetween 2, as in 
1871, 1886 and 1889, and 14 in 1900. The season with the longest hot 
spell on record likewise contained some of tlie highest temperatures ever 

TABLE XXXIII.-LONGEST PERIOD OF CONSECUTIVE DAYS WITH A MAXIMUM 
TEMPERATURE OP 90° OR ABOVE. 



1871 

1872 

187.3 

1874 

1875 

1876 

1877 

1878 

1879 

1880 

1881 

1883 

188;^ 

1884 

1885 

1886 . .. 

1887 

1888 

1889 

1890 

1891 

1893 

1893 

1894 

1895 

1896 

1897 

1898 

1899 

1900 

1901 

19f)2 

1903 

Average 



Began. 



July 9 
June 30 
July 14 
June 7 
June 23 

July 8 
July 25 
July 4 
Aug. 2 
July 9 

July 3 
July 34 
July 2 
June 19 
July 16 

July 7 
July 11 
Aug. 3 
July 8 
July 30 

Aug. 9 
July 34 
June 19 
July 2.i 
May 30 

Aug. 4 
Sept. 9 
Aug. 30 
.lune 5 
Aug. 6 

June 26 
Aug. 2 
July 8 



Enrled. 



July 



Aug. 1 

12 
31 
23 

29 
•lune 3 

13 
11 

Sept. 1 
8 
19 

July 7 

4 

11 



Length. 



Days. 



Max. temp. 



8 
4 
11 

* 2 
4 

* 2 

* 3 



10 

3 

9 

* 4 

14 

12 
3 
4 

5.8 



91° 



93 
94 
92 
99 

96 
93 
94 
93 
99 

92 
96 
94 
93 
95 

94 
99 
98 
97 
97 



97 
97 

98 
100 

103 
91 
96 



♦Two periods of equal length ; the period with the highest maximum temperature selected. 



recorded in Baltimore. On six successive days the maximum tempera- 
ture ranged between 99° and 100°; the month contained 17 hot days, 
and was preceded by a iiiiuitli with I't. This was doubtless the most try- 
ing period in tlic history of Pialtimore summers. A more extended ac- 
count of tliis roniarkal)l(' hot s])ell will l)e given in V^vi IF of this Report. 



144 



THE CLIMATE OF BALTIMORE 



-EEE:::::!;i|^ 


1 


5 

i 


I 


il: 


■; 1 ' 1 - 


^ 



Fig. 37. — Time of Occurrence of the Lowest aud Highest Temperature of the 
Year. 

Fig. 37 shows the time of occurronce of the lowest temperature of each winter season, 
trom 18a to 1904, and of the highest temperature of each succeeding' summer season, from 
18/1 to 1903; also the length of the intervening period in months and days. The lowest line 
marked ' mean " shows the average time of occurrence and the average length of the inter- 
vening period. The line for 1904 shows only the time of occurrence of the lowest tempera- 
ture. (See Table XXXIV.) 













^ 






















/ 


// 


,- 


\ 


















// 


// 




■^^ 


\ 














/ 


1 1 






















// 


1 1 
11 

1 










\ 










1 


I 
// 
1 












\ 










/ 


II 
1 
1 
'/ 












\ 








/ 


// 






















/ 


b /' c 
















\ 




/: 




















s 


V 


■"^ 






















"*> 



Fig. 38.— (a) Air Temperature at 2 p. m. (Harbor). 

(6) Temperature of Surface Water, 2 p. m. (Harbor), 
(c) << " Water at depth of 10 ft. (Harbor). 

Fig. 38 shows the mean monthly temperature of the water in the harbor, and of the air at 
2 p. m., at the foot of youth street, (a) Air temperature ; (70 temperature of the water at 
the surface ; (c) temperature of the water at the bottom (10 ft). See Table XXXV. 



MARYLAND WEATHER SERVICE 



145 



Time of Occurrence of Annual Minimum and Maximum 
Temperatures. 

The dates of occurrence of the winter minimum and the summer maxi- 
mum temperatures, and the length of the intervening period, are ex- 



table xxxiv.-time of occurrence of annual minimum and maximum 

temperatures. 





Min. 


Date of minimum. 


Date of maximum. 


Max. 


Summer. 




Dec. 


Jan. 


Feb. 


Mar. 


June 


July 


Aug. 


Sept. 


1870-1 

18T1-L' 

\^-,o 3 


10° 
5 
—4 

13 
-2 

13 

I 


13 

—6 

7 

11 

8 

3 

—1 

7 
9 
3 
13 

16 
13 

1 
8 

1 

5 

8 

10 

8 

13 

11 

5 


si 

18 
10 

37 

23 

5 
8.04 


30 

16* 

10 

's 

3 

1 
24 
23 

6 

■3 

23 

i7 
16 

26 

6 

15 
4.08 


6 

ii 

5. 
24 

25 
6 

17 


10 

1 

io 

11 
4.01 


"7 
3 

6 

3 
13.07 


'9 
27 

26 
25 

26 
24 
3 

"e 

8 
B7.«2 


16 


18* 
9 

is 

16 
13 

23 

24 
21 

7 
18 

'9 

8 

26 
'3 

It 

18 

19 
97.°9 


ie 
11 

7 

25 

4 
96.03 


7 

ii 

3 
99.00 


930 

97 

96 

98 

97 

99 
95 
98 
99 
99 

101 
97 
96 
95 
99 

93 
102 
96 
93 

98 

94 
99 
98 
98 
97 

98 
97 

104 
98 

100 

103 
99 
97 


1871 

1872 
1873 


l}Jt73 4 


1874 


l»T4-5 

1875-6 


1875 
1876 


1S76-7 


1877 


1877-8 


1878 


1878-9 


1879 


1879-80 . . 

1880-1 

lt<8l •' 


1880 

1881 
1883 


1}<H'> 3 


1883 


1883-4 

1884-5 


1884 
1885 


1885-6 


1886 


18iS6-7 


1887 


1887-8 

1888-9 

1889-90 


1885 
1889 
1890 


Iy90-1 


1891 


1891 3 


1893 


189"^ 3 


1893 


1893-4 

1894-5 


1894 
1895 


1895-6 


1896 


1896-7 


1897 


1897-8 

1898-9 

1899-1900 

1900-1 

1901-" 


1898 
1899 
1900 

1901 
1903 


iyo-_'-3 


1903 


1903-4 


1904 


No. of occurrences 


•"•• 



* On other days also. 

ceedingly variable quantities. While the lowest winter temperature 
usually occurs in January, it has on several occasions appeared in 
December, frequently in February, and occasionally in March, in the 
past 34 years. The earliest occurrence was on the 10th of December 
in 187G, the latest on the 7th of March in 1890. 



146 



THE CLIMATE OF BALTIMORE 



TABLE XXXV.-MEAN TEMPERATURE OF AIR AND OF SURFACE WATER IN 

THE HARBOR AT 3 P. M. 



Date. 



n 



Air 
Water 



10 
II 

W 

15;- 

16^ 



18- 



20 



28 



Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec 



Monthly Average Air ... «« 
Water. Si 



35 



36.3 

SA.3 
35.3 

Si.O 
40.5 

i^ii.e 

35.2 

Si.6 
37.0 
54.6 
89.3 
Si.l 
45.0 
Si.O 
43.7 
Si.O 
43.1 
Si.i 
41.0 
Si.S 
35.7 
SJ,.5 
43.7 
Si.S 
46.7 
35./, 
45.6 
36.1 
45.6 
56.6 
43.3 
36.3 
44.9 
56. i 
41.9 
36.5 
41.0 
56. :? 
35.5 
5,5.7 
38.3 
56.0 
38.0 
36.9 
38.3 
,36.6 
36.8 
56.6 
44.3 
56.7 
40.0 
.1)6.6 
43.3 
36.3 
43.3 
57.5 



4 40. 

.8 35. 



42.1 

37.1 
46.3 
57.7 
43.6 
.37.5 
43.7 
56. S 
42.4 
57.2 
43.3 
37.9 
41.7 
5S.0 
35.7 
5«.3 
39.3 
58. i 
46.6 
5«.5 
43.7 
55.7 
48.1 
3S.S 
45.4 
39.5 
47.0 
59.9 
.50.9 
40.5 
46.8 
40. i 
45.3 
40.2 
46.9 
40.6 
49.9 
40.9 
41.9 
40.9 
45.3 
40.5 
41.2 
40.6 
39.3 
/.1.2 
47.3 
4i.7 
49.2 
42.5 
53.6 
42.6 
53.5 
45.0 
53.8 
45.9 
46.7 
45. i 
48.6 
42.S 
53.3 
45.1 



54.1 

iS.O 
58.4 
45.7 
62.9 
46.0 
51.0 
46.0 
55.1 
46.6 
55.3 
47.0 
66.8 
47.6 
55.5 
47.9 
61.8 
47.6 
53.5 
iS.l 
48.3 
iS.7 
.54.9 
45.5 
58.1 
49.5 
57.3 

50.4 

54.3 

50.6 
57.5 
5i.4 
59.3 
5i."4 
63.9 
52.0 
6.). 2 
55.2 
64.6 
54. i 
63.6 
5/t.5 
63.8 
55.3 
61.5 
55.5 
65.0 
55.9 
59.9 
56.5 
63.1 
56. i 
65.8 
57.0 
67.5 
57.5 
61.4 
57.4 
63.5 
57.6 



45.8 68.7 
40. -Z 51. i 



63.8 

57.6 
68.3 
58.3 
65.8 
59.0 
69.7 
.59.5 
69.0 
59.2 
65.9 
59.2 
61.6 
59.4 
63.9 
59.6 
73.3 
59.6 
69.0 
60.8 
64.6 
60.6 
63.0 
6i.O 
69.0 
60.5 
60.9 
60.5 
68.1 
60.8 
67.1 
6?.0 
66.2 
6i.O 
69.0 
61.6 
69.1 
62.5 
71.5 
62.8 
73.3 
64.^ 
73.1 
64.4 
73.3 
66.1 
71.9 
65.6 
70.0 
65.0 
71.6 
65.2 
72.9 
65.5 
73.8 
65.7 
69.7 
66.2 
70.4 
66.8 
73.2 
67.0 



62.7 



78.1 
67.4 
74.4 
67.7 
76.0 
67.9 
76.8 
68.3 
79.9 

69.2 

77.9 

70.1 
79.9 

70.6 
79.2 
7i.2 
77.7 
7i.O 
81.8 
71.8 
77.6 

's.k 
73.6 
79.9 

75.7 
77.0 

79!3 

75.9 
80.5 
74.5 
79.3 
75.3 
80.6 
75. i 
80.0 



76.4 
81.5 
76.2 
80.3 
76. i 
79.8 
75.7 
83.1 
76.0 
83.0 
76.1 
77.9 
76.5 
77.5 
76.3 
79.3 
76.7 
80.7 
76.9 
76.7 



79.1 

75.4 



78.3 
75.5 
78.1 
75.9 
82.8 
76.5 
80.9 
76.5 
81.3 
77.0 
83.6 
77.5 
85.3 
77.8 
84.7 
77.5 
81.5 
77.5 
85.6 
77.6 
83.3 
77.7 
82.6 
77.9 
80.3 
77.7 
79.4 
77.9 
81.1 
75.0 
83.2 
75.2 
83.1 
75.2 
84.0 
79.4 
84.6 
79. i 
83.0 
79.1 
83.5 
79.0 
87.6 
79.4 
87.9 
79.5 
86.7 
79.5 
84.0 

85!o 
79.5 
83.5 
79.5 
83.1 
79.7 
82.9 
79.9 
83.3 
79.5 
82.6 
79.7 



81.4 
79.7 
80.3 

79.5 
79.0 

78.9 
80.6 
75.6 
80.5 
79. i 
78.6 
75.7 
76.4 
75.0 
79.3 
75.2 
78.8 
75.2 
79.0 
75. i 
81.6 
75.5 
81.3 
78.1 
84.7 
75.5 
83.5 

78.8 
75.2 
78.4 
75.0 
83.5 
75.2 
81.6 
75.1 
83.8 
75.4 
83.4 
75.5 
84.5 
75.4 
81.3 
75.5 
80.8 
75.4 
83.0 
75.2 
83.0 
75.2 
80.1 
75.0 
77.5 
77.6 
77.5 
77.5 
76.0 

76!9 

76!9 

77.1 

80.3 

75.5 



77.1 

76.8 
75.3 
76.7 
78.3 
76.6 
79.0 
76.6 
78.8 
76.5 
77.2 
76.7 
77.7 
76.4 
78.5 
76.5 
77.7 
76.4 
77.3 
76. i 
73.3 
75.7 
73.3 
75.2 
73.3 
74. i 
76.7 
7i.l 
76.4 
74. i 
78.0 
74.0 
79.1 
74.5 
73.8 
74.4 
76.4 
74.0 
76.8 
75.0 
73.7 
72.6 
68.8 
72.0 
70.6 
7i.6 
73.0 
71.1 
73.4 
70.9 
69.6 
70.6 
73.0 
70.5 
77.8 
71.0 
73.5 
70.5 
73.7 
70.0 



75.5 
74. ( 



70.7 
69.5 
66.5 
69.0 
66.4 
65.7 
68.1 
65.6 
68.6 
65.5 
67.1 
67.5 
67.7 
67.6 
66.8 
67.-? 
67.9 
67.5 
69.1 
67.5 
71.5 
67.2 
70.9 
66.5 
72.5 
66.5 
69.4 
66.3 
63.7 
66.0 
60.7 
65.2 
64.3 
64.6 
67.8 
65.4 
66.6 
65.0 
68.0 
64.6 
61.5 
64.2 
61.4 
63.0 
57.8 
62.9 
57.8 
62.5 
60.9 
62.6 
.59.4 
61.9 
59.5 
61.2 
59.7 
67.0 
60.1 
60.6 
68.9 
60.2 
58.6 
59.7 



65.i 



61.2 

59.2 
60.3 

55.7 
.^3.4 
57.7 
55.5 
57.4 
59.1 
57.1 
57.0 
.56.7 
53.0 
56.3 
.53.9 
.^1.3 
54.3 
54.9 
56.6 
5i.7 
59.7 
.54.5 
55.1 
54.5 
53.9 
53.6 
47.6 
,55.0 
48.4 
.'■>2.5 
47.8 
,5-?. 5 
63.6 
51.5 
48.7 
51.3 
47.9 
50.5 
48.9 
,'-)0.2 
49.7 
49.7 
.51.6 
49.5 
66.5 
49.5 
46.8 
49.2 
43.1 
45.4 
43.8 
47.4 
44.7 
47.0 
45.6 
46.5 
43.4 
46.5 
43.0 
46.5 



64.8 51.3 40.1 



52.4 



43.0 

/,6.2 
40.7 
45.5 
38. 4 
44.5 
40.3 
44.1 
42.9 
45.6 
44.2 
45.4 
38.4 
42.7 
41.3 
42.4 
43.7 
41.9 
45.5 
41.7 
44.5 
41.5 
41.6 
40.4 
43.0 
40.4 
43.6 
iO.2 
37.3 
.19.9 
34.7 
39.7 
36.3 
55.7 
38.0 
55.1 
34.8 
55.2 
32.2 
38.0 
43.0 
.S7.3 
43.9 
.S7.S 
38.3 
57.5 
40.8 
37.6 
36.7 
37.3 
33.7 
36.5 
36.4 
55.5 
39.3 
56.5 
40.2 
56.2 
39.8 
35.7 
46.9 



39.8 



Annual average : Air 60.3; Water 57-1. 



Table XXXV shows the average daily temperature of the surface water in 
the harbor, and of the air, at 2 p. m., for the period of five years from 1882 
to 1886. The roman figures show the air temperature, and the italic figures 
the water temperature. 



MARYLAND WEATHER SERVICE 147 

The highest annual temperature occurred in July in the great major- 
ity of cases in the past 34 years; it has never appeared earlier than 
June 3; on two occasions it fell in the months of September, on the 
7th in 1881, and on the 11th in 1887. The average interval between 
the occurrence of the lowest and highest temperatures of the year is 
181 days, from January 25 to July 15. The longest was 250 days, from 
January 1 to September 7, 1880; the shortest was 116 days, from 
February 10 to June 6, 1899. 

The details of occurrence, together with the minimum and maxi- 
mum temperatures recorded, are shown for each year since 1871 in 
Table XXXIV. The length of the intervening period is represented 
graphically in Fig. 37. 

Temperature of the Water in the Harbor. 

From September 1, 1881 to March 31, 1887, observations were made 
daily at 2 p. m. of the temperature of the surface water in the harbor, 
from the wharf at the foot of South Street. At the same time the tem- 
perature at the bottom (a depth varying from 9 to 12 feet according to 
the tide) and the temperature of the air were also noted. The average 
values of the surface water temperature and the air temperature for the 
five years from 1882 to 1886 are presented in Table XXXV. Fig. 38 
also shows graphically the mean temperature of the water at the surface, 
and at the bottom, and of the air at 2 p. m. The temperature of the 
surface water is approximately 5° to 6° cooler than the air temperature 
from February to July. The difference diminishes gradually from July 
to October. In October the temperatures are approximately equal, and 
i-emain so until December when there is again a gradual divergence. The 
difference between the temperature of the surface water and that at a 
depth of ten feet is very small at all seasons of the year, averaging about 
five-tenths of a degree. 



11 



148 THE CLIMATE OF BALTIMORE 



HUMIDITY. 



Introduction. — Two distinct gaseous envelopes surround the earth; 
one is the dry air, with small quantities of other relatively permanent 
gases; the other is the vapor of water which may be condensed into 
visible forms of dew, frost, cloud, rain or snow, under ordinary conditions 
of temperature and pressure. 

It is of the highest importance, in the consideration of climatic con- 
ditions, to understand the functions and the distribution of the element 
of water in the atmosphere, in its great variety of forms and proportions. 
Water is being changed into invisible vapor from the ice and snow of the 
frozen north no less constantly, though less abundantly, than from the 
warm ocean waters of the tropics. As a result, the atmosphere is never 
free from the vapor of water. It may be present in small quantities 
only, as in the dry desert regions of the earth, or in the cold zones of the 
north and south, or in the rare and cold atmosphere of the mountain 
tops. The vapor capacity of a given space increases rapidly with in- 
crease in temperature. A cubic foot of vapor at a temperature of 50° 
F. at normal sea-level pressure, and at saturation, weighs about 4 grains ; 
at 70° it weighs 8 grains; and at 100°, about 20 grains; hence with an 
increase in temperature from 50° to 70° the quantity of moisture at 
saturation is doubled. 

The invisible vapor of water in the atmosphere is generally referred 
to as the humidity of the atmosphere. When the amount of vapor is 
actually weighed in grains, ounces, or pounds, or when it is measured 
in terms of pressure, as so many inches of mercury, it is referred to as 
absolute humidity. When it is measured in terms of percentage of the 
total amount which can exist in a given portion of the atmosphere, it is 
referred to as the relative humidity. For example, as stated above, the 
total quantity of invisible vapor which may be contained at ordinary 
pressure, in a cubic foot at 70° temperature, is 8 grains. The atmosphere 
is then said to be saturated, and the relative humidity is 100 per cent. 
Suppose the amount of vapor to be reduced to 4 grains, or one-half the 
full capacity, the temperature remaining the same, the percentage of 



MAEYLAND WEATHER SERVICE 149 

the relative hiimidity would then be 50. The point of saturation is also 
called the dew point. 

As the temperature of the atmosphere diminishes rapidly from the 
earth's surface upward, the capacity for water vapor diminishes, and at 
a more rapid rate. Calculations have been made of the amounts of 
aqueous vapor which the atmosphere can hold in suspension at different 
temperatures and below given altitudes. In the following table by Fer- 
rel, the figures given show the depth in inches of water which would result 
if all of the vapor which it is possible for the atmosphere to hold in 
suspension under the given conditions were condensed to water. 

AMOUNT OP AQUEOUS VAPOR IN SATURATED ATMOSPHERE OF STATED 
TEMPERATURE AND DEPTH. 



Elevation. 


80° F. 


70° F. 




60° F. 


50° F. 


6,000 


feet. 


1.3 inch. 


1.0 lE 


ich. 


0.1 inch. 


0.5 inch. 


12,000 




2.1 


1.5 




1.1 


0.8 


18,000 




2.5 


1.8 




1.3 


0.9 


24,000 




2.7 


2.0 




1.4 


1.0 


30,000 




2.8 


2.1 




1.5 


1.1 



The conditions assumed probably never occur in nature, as the rapid 
decrease in temperature with elevation would preclude the possibility 
of an average temperature sufficiently high, or at unifonn saturation, 
to the given elevations. Probably under the most favorable conditions 
of a moist atmosphere in the tropics, the amount of vapor from the 
earth's surface to the upper limits of the atmosphere, if condensed, would 
measure less than two inches. 

The decrease in the amount of vapor in the atmosphere with decrease 
in temperature may be stated in another form. At the equator the 
amount of vapor may reach about 11 grains per cubic foot, or about 20 
tons per cubic mile; at latitude of 40° it may reach about 5 grains or 
about 9 tons per cubic mile, assuming a temperature of 55° as an average 
for the year; at latitude 70° with an average temperature of 30°, it may 
attain about 2 grains per cubic foot, or about 3.5 tons per cubic mile. 
It has been estimated that one-half of the total quantity of vapor in the 
entire atmosphere lies below an elevation of about (>500 feet, or below the 
summit of !Mt. Washington ; hence the decrease in quantity is very rapid 



150 THE CLIMATE OF BALTIMORE 

and we may easily realize how mountains of moderate elevation, and 
even the higher hills, may afi'ect the rainfall and cloudiness of a locality. 

As the absolute humidity of the atmosphere at any given place depends 
upon the local temperature and a local water supply, there is a steady 
decrease in the amount of water vapor from the equator to the poles, 
from the surface of the earth upward, and from the oceans toward the 
interior of the continents. This general law of distribution may, how- 
ever, be modified, and even completely reversed, by the direction and 
character of the wind movement over a given area. While the vapor of 
the atmosphere is taken up by evaporation from a variety of surfaces, 
such as lakes, rivers, moist fields, or the foliage of the forest, the great 
source of supply must always be the surface waters of the equatorial and 
tropical oceans. From these it is carried up by the winds and distrib- 
uted to all parts of the globe. 

This invisible moisture of the atmosphere has a most important func- 
tion to perform in tempering the heat and cold. Where it is found in 
abundance, extremes of temperature are unlikely. Its absorbing power 
is great compared with that of dry air, and the earth's surface is pro- 
tected against the heat of the sun by day, and the rapid cooling by 
radiation during the night. Its abundance in the tropics is largely 
responsible for maintaining the uniformly high temperature of those 
regions. Its absence in limited areas of the tropical and sub-tropical 
zones is marked by great diurnal fluctuations in temperature, as in the 
arid regions of the southwest. Most important of all, it is the great 
source of supply of the rainfall and snowfall of the world; without 
first passing into the form of vapor, the oceans and rivers would have 
but little effect in watering the fields and forests of the earth. 

While the benefits of the atmospheric vapor are numerous and appar- 
ent, it may also be a source of much personal discomfort. When the 
relative humidity is high, that is, when the vapor is near the saturation 
point, or dew point, evaporation from the body becomes sluggish, or 
ceases, the air begins to feel muggy, when the temperature is high, or 
raw when it is low. A temperature which would be considered mod- 
erate with a dry air becomes oppressively hot when the humidity ap- 



MARYLAND WEATHER SERVICE 



151 



Mdt 3 6 9 Noon 3 6 9 Mot Mdt. 3 6 9 Noon 3 6 9 Moi 



























JAN. 









66 



^ 








' 






/ 








/ 








/ 


APRIL 




kJ 





MCT, 3 



9 Noon 3 



9 Mdt. 





















— ~ -N 












/- 


































YEAR 








-^ 









Mot 16 9 Njon 3 6 9 Mdt Mdt 3 6 9 Noon 3 6 9 Mdt 



^ 








\ 




/ 








I 








j 


JULY 




J 











^ 








J 








1 








' 


OCT. 




J 





Fig. 39. — Mean Hourly Relative Humidity. 

Till- mean hourly relative humidities for the months of January, April, July, October and 
for the year are oxiircssed as pcn-entii^'os, coniplctc saturation boiu},'- reprcsouted by 100 per 
cent. The curves are based on the 'M months' record of a Richard hytjrograph. 



153 



THE CLIMATE OF BALTIMORE 



preaches or exceeds 80 per cent. On the other hand, a cold of 15° or 
20° above zero with a humidity of 80 per cent, a condition which is 
common in the Atlantic coast states, will cause much suffering, while 
temperatures of 20° below zero in the northwest, with a humidity of 
25 or 30 per cent, are described as comfortable and exhilarating. 

The atmosphere does not lose in transparency with increasing humidity 
either relative or absolute; on the contrary, a high humidity is often 
accompanied by greater clearness, and an unusual transparency has 
come to be regarded as a sign of rain. When, however, the vapor reaches 
the dew point, just beyond the point of saturation, we have a series of 
phenomena of the highest interest and importance to us, resulting from 
condensation into the visible forms of dew, fog, clouds, rain, snow, frost 
and hail. The particular form of condensation is primarily a function 
of temperature, modified by local conditions of topography, elevation 
above the earth's surface, and the movements of the atmosphere. 

Hourly Variations in Humidity. 



Continuous automatic records of the variations of relative humidity 
are available for about two years and a half at Baltimore. While this is 
table xxxvi.-mean hourly relative humidity. 

(1902-1904.) 



1 A. M. 

2 

3 

4 

5 

fi 



8. 



10 

11 

Noon... 

1 P. M. 

2 

3 

4 

5 

. 6 



10 

11 

Midnight . 



Jan. Feb. Mar 



Averag-e. 



62 
61 
59 
58 
58 
60 
62 
64 
66 



er 



70 
71 
71 
72 
73 
74 
76 
75 
71 
70 
66 
64 
62 
60 
58 
59 
60 
62 
64 
66 
67 
68 
70 
70 



78 



.2 67.4 70.6 62.6 



Apr. 



76 



May June July Aug. Sept. Oct. 



76 



80 
80 
77 
70 
66 
60 
55 
51 
50 
47 
46 
46 
48 
60 
54 
60 
64 
6S 
71 
73 
75 



.4 67.4 70.4 72.1 72.4 



Nov. Dec 



72 
72 
72 
74 
74 
74 
74 
72 
66 
61 
56 
63 
50 
50 
51 
62 
66 
60 
62 
65 



70 
70 
70 
70 
70 
70 
70 
70 
66 
62 
69 
56 
64 
54 
55 
56 
69 
61 
63 
64 
66 



5 64.3 64.0 67.4 



An'l 



MARYLAND WEATHER SERVICE 



153 



a much shorter record than that utilized in the discussion of tempera- 
ture, pressure, wind direction and other factors, it is still of sufficient 
length to enable us to establish firmly the form of the diurnal curve. 
The diurnal variation in the relative humidity is represented by a simple 
curve with its maximum point at about 5 a. m. for the year, but varying 
between 4 a, m. and 7 a. m. according to the season. The time of maxi- 
mum follows closely the time of sunrise. The minimum point, or the 
drj^est time of day, occurs between 1.30 p. m. and 3.30 p. m. The 




70 s H. 70 



Fig. 40. Mean Hourly Relative Humidity. 

As in Fig. 39, the hourly humidities are expressed as percentages, 100 per cent repre- 
senting complete saturation. The light shades represent the lower humidities, or the 
dryer portions of the day and year ; the heavy shades, the time of higher humidities. 
The dotted lines, S. R. and S.S indicate the time of sunrise and sunset respectively. 
The diagram is based on the 30 months' record of a Richard hygrograph. 



details of the hourly variation are shown statistically in Table XXXVI, 
and graphically in Fig. 39 and Fig. 40. The seasonal distribution is 
revealed at a glance in Fig. 40, in which the light shades represent the 
lower humidities, or the dryer portions of the day, and increase in the 
density of the shades shows an increase in the humidity. It will be 
observed that the dotted line representing the time of sunrise- (S. K.) 
passes through the areas of heaviest shading, approximately in their 
central portions. The values in both tables and figures are shown in 
percentages, total saturation of the atmosphere being represented by 100. 



154 THE CLIMATE OF BALTIMORE 

The actual values for relative humidity from hour to hour during the 
day fluctuate rapidly on days with unsettled weather. The changes are 
particularly marked during the course of a thunderstorm. There is 
frequently very little resemblance between the curve showing actual con- 
ditions and that representing average conditions for a month or more. 
Every change in the direction of the wind, or in the temperature, is 
reflected in the form of the actual curve. In Plate VIII some typical 
relative humidity curves made by the self-recording instrument are repro- 
duced and they may thus be compared with the curve in Fig. 39 repre- 
senting average values. 

Direct observations of relative humidity were made at varying hours 
of the day from time to time in past years. The continuous record 
enables us to determine the corrections to be applied to any of the com- 
binations of hours employed in the past in order to obtain a correct 
daily mean based on 24 hourly observations. A daily mean derived 
from any of the series of three observations per day, one in the morning 
between 7 and 9, one at 3 or 4 in the afternoon and the other between 
9 and 11 at night, gives a value so nearly equal to the true mean based on 
24 hourly observations that the corrections to be applied fall within the 
limits of probable error of observation, and hence may be neglected. For 
the series of observations made at 8 a. m. and 8 p. m. the departures 
from the true average are large enough to require the application of 
the necessary corrections shown in the following figures : 

CORRECTIONS TO OBTAIN TRUE DAILY MEAN HUMIDITY. 

(Expressed in percentages.) 

Hours of Jan. Feb. Mar. Apr. May June Julj- Aug-. Sept. Oct. Nov. Dec. Year 

observation. 
( 8a.m. + 8 p.m. ) -3.3° -3.1° -1.9° -1.9° -1.6° -0.6° -1.6° -1.9° -2.6° -4.0° -4.2° -3.0° -2.6° 
2 

Phases of the Diurnal March of Eelative Humidity. 

The time of occurrence of the maximum and minimum values for the 
day, and the time, in hours and minutes, when the humidity is the same 
as the mean for the entire day, are sho^^^l in the following table and in 
Fig. 41 : 



MARYLAND WEATHER SERVICE 155 

PHASES OF THE DIURNAL MARCH OF RELATIVE HUMIDITY. 



S s ^,S 



§ 


>> 


3 


3 


1-5 


>-s 



OQ O 



O o a> 



Max. (a.m.) 

First mean (a. m.) — 

Min. (p.m.) 

Second mean (p. m.). 



7.30 7.20 5.40 4.30 4.30 4.00 4.30 5.30 5.00 4.30 5.30 4.30 5.00 

10.00 10.40 9.20 8.50 8.30 8.30 8.20 8.40 8.40 8.50 9.20 9.30 9.30 

2 30 3.20 3..30 3. 00 2. 30 2. 00 3. 00 2. 30 1.30 2. 30 1.30 1.30 2. 80 

7'.00 9.00 8.40 8.40 7.50 8.20 7.50 8.00 7.20 7.20 7.40 8.00 7.60 



JFMAMJJASONDJ 



6 A. M. 











































































/ 


"^ 




\, 


^ 


A 












% 


r 














"^ 
















































































.^ 


, 








B 














/ 






'^ 


s.^^ 


^ 


"^ 


\^ 


y 


N^ 




y 
















V 


y 


•^ 


— ' 


^ 


















































^ 


^ 
























V 


S 
















^ 










— 1 


■ — 


C 


, 


■ 






, 


^. 




















I 




N 


















N, 


i 






\ 








/^ 


-^^ 




,^ 






^V 


' 


^^ 


0^ 


y 






'^ 


V 


/ 

































































































6 P. M. 



Fig. 41. — Phases of the Diurnal Variations in Relative Humidity. 

B shows the variations in the hour of occurrence of the dryest time of the day from 
month to month ; D the time of occurrence of the dampest portion of the day ; A and C 
show, respectively, the afternoon and the morning hours when the mean humidity of 
the day is most likely to occur. 



156 



THE CLIMATE OF BALTIMORE 



Mean Monthly and Annual Eelative Humidity. 
The observed values of the relative humidity for the entire period 
from 1871 to 1903 were reduced to true mean monthly and annual values 
based upon hourly observations; these corrected figures are contained 





i F M fi 


^ 


J 




A s o ^ 


D J 


70 


























N 


N, 






/^ 


^-^ 


/ 




■\ 


V 


^ 


/ 


60 






V 


y 


r — 








































50 



























7Q 



50 



Fig. 42. — The Mean Monthly Relative Humidity. 



The diagram is based on direct observations at two or three stated periods of the 
day during a period of 30 years; the average values were corrected for the diurnal 
variation, and expressed as percentages of total saturation. 



IB/S 1880 1885 lo 


»0 1895 1900 






























V»._^ 




/\ 


f\y\ 


/■\ 




'^Vy 










V 



































Fig. 43. — Variations in the Mean Annual Relative Humidity. 
(Expressed as percentages of total saturation.) 

in Table XXXVII, together with the ten-year monthly averages and the 
normals for the entire period. The monthly normals and the variations 
in annual means are also graphically shown in Fig. 43 and Fig. 43 
respectively. 



jVIAEYLand weather service 



157 



The normal amount of moisture in the atmosphere for the entire year 
is about two-thirds of the total capacity of the atmosphere for water 
vapor, namely, 66.5 per cent. The mean monthly amounts vary from 
season to season, being greatest in the month of September (70.4) and 

TABLE XXXVII.— MEAN MONTHLY RELATIVE HUMIDITY. 



Year 


Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


An'l 


18T1 


68 
63 
66 
75 
73 

68 
74 
72 
71 
73 

71 
74 
74 
67 
65 

76 
69 
67 
67 
66 

67 
76 
72 
69 
70 

63 
66 
70 
66 
71 

69 
70 
66 


66 

68 
60 
73 
68 

66 
66 
68 
70 
66 

66 
66 
66 
68 
63 

69 
71 
68 
61 
70 

69 
75 
69 
67 
68 

66 
71 

63 

73 
68 

69 

73 
62 


82 
68 
64 
66 
68 

66 
70 
64 
64 
63 

63 
63 
66 
66 
66 

63 
60 
64 
61 
64 

70 
71 
65 
61 
60 

69 
63 
67 

72 
66 

73 
76 
73 


66 
50 
59 
66 
60 

56 
66 
64 
57 
56 

69 
60 
65 
57 
56 

69 
69 
51 
63 

60 

65 
63 

68 
68 
63 

62 
65 
57 
61 
69 

63 

68 
54 


66 
60 
65 
69 
51 

64 
64 
63 

58 
68 

66 
70 
59 
59 
65 

71 

68 
68 
66 

67 

59 
68 
66 
67 
66 

66 
60 
68 
63 
69 

72 
64 
66 


68 
60 
64 
64 
63 

66 
67 
64 
64 
63 

69 
60 
68 
66 
63 

71 
66 
64 
73 
64 

71 
76 
73 
66 
69 

69 
64 
61 
67. 
71 

68 
66 
75 


65 
60 
63 
63 
65 

63 
68 
66 
61 
64 

61 
66 
67 
65 
63 

73 
71 
66 
73 
62 

77 
71 
65 
64 
64 

68 
70 
66 
68 
63 

74 
73 
65 


69 
65 
78 
62 
76 

69 

62 
73 
70 
71 

60 
74 
63 
69 
69 

71 
70 

68 
70 
70 

78 
73 
65 
68 
64 

63 
68 
73 
70 
68 

76 
69 

76 


65 

66 
76 

72 
66 

73 
71 

73 
70 
67 

68 
74 
70 
63 
67 

70 
70 

74 
75 
74 

78 
69 
67 
73 
67 

66 
65 
65 
69 
83 

74 
75 
70 


66 
62 

73 
66 
67 

66 
71 
66 
68 
67 

67 
76 
73 
64 
76 

66 
64 
65 
64 
68 

71 
63 
71 

67 
63 

63 
70 
69 
71 
80 

66 
74 
63 


65 
67 
69 
63 
66 

70 
68 
67 
63 
66 

67 
65 
64 
61 
69 

61 
60 
67 
71 
63 

70 
71 
68 
60 
70 

65 
66 
67 
67 

72 

66 

73 
66 


63 

58 
71 
66 
74 

71 
66 
68 
69 
69 

70 
65 
67 
68 
62 

71 

68 
62 
67 
62 

67 
71 
68 
68 
68 

63 
72 
66 
64 

73 

77 
70 
57. 


67.4 


1872 


58.8 


1873 


66.1 


1874 


66.4 


1^75 . ... 


66.3 


1876 

1877 


66.3 

67.7 


1878 


67.2 


1879 


66.4 


1880 

1881 

1882 

1883 


65.0 

65.6 
67.7 
65.9 


1884 


64.4 


1885 


64.3 


1886 

1887 

1888 

1889 


69.0 
66.3 
65.3 
67.5 


1890 

1891 

1893 

1893 


66.8 

69.3 
70.5 
68.1 


1894 


65.7 


1895 


64.2 


1896 


64.3 


1897 


65.8 


1898 

1899 .... 


65.8 
67.5 


1900 


69.3 


1901 

1902 


69.7 
70.1 
65.2 






Average, 1871-80 

1881-90 

1891-1900 


70.2 
69.5 

ra.o 


66.9 

66.8 
67.6 


66.3 
61.4 
65.4 


59.8 
69.9 
60.0 


59.8 
65.9 
64.3 


64.3 
66.3 
68.7 


63.8 
66.6 
67.5 


69.3 

68.4 
68.8 


69.7 
70.5 
70.3 


67.1 
68.0 
67.6 


65.4 
64.7 
67.6 


67.5 
66.3 

67.8 


65.7 
66.2 
67.0 


1871-1903 


69.6 


66.6 


64.9 


eo.i 


63.6 


66.6 


66.4 


69.3 


70.4 


67.6 


65.8 


67.2 


66.6 



least in the month of April (60.1). For individual years, the monthly 
averages vary considerably. For example, the average humidity for the 
month of March has been as high as 82 per cent and as low as 54 per 
cent, a range of 28. A similar range has been experienced in the month 
of October. The month possessing the smallest range in the monthly 



158 THE CLIMATE OF BALTIMORE 

average value is January, with a maximum of 76 per cent and a minimum 
of 62 per cent, a range of 14. 

The range in actual conditions of humidity, as distinguished from 
average conditions for a considerable period, shows, of course, much 
greater fluctuations. As the upper limit, namely, 100 per cent, is reached 
at all seasons of the year during misty or rainy weather, the lower limit 
is the index of variability. The lowest values are most likely to occur 
during the clear, cold days of winter or early spring. During the two 
years and a half in which continuous automatic registration was main- 
tained at Baltimore, the humidity during the afternoon hours occasion- 
ally fell to 25 per cent, or one-fourth the moisture capacity. The occa- 
sions upon which the moisture content fell below this percentage were 
rare. A few of the exceptionally dry days during this period are here 
cited : 

EXCEPTIONALLY DRY DAYS. 
January 31, 1903, minimum humidity was 20 per cent. 



April 5, 1904, 


" 11 •• • 


May 4, 1904, 


" 15 " 


August 17, 1902, 


" 24 " ' 


October 30, 1903, 


" 14 " 


November 9, 1902, 


" 22 " 



The limits of variability in the annual average humidity during 33 
years were 70.5 per cent in 1892, and 58.8 per cent in 1872, a range of 
11.7 per cent. The ten-year averages have only varied from the normal 
for the entire period by the following small departures: 

1871 to 1880 0.8 below normal ; 

1881 to 1890 0.3 below normal ; 

1891 to 1900 0.5 above normal. 

Absolute Humidity. 

Expressing the humidity in the terms of the actual weight of the water 
vapor in the atmosphere at different hours of the day throughout the 
year, the distribution is shown in the following table. These figures 
show the average amount of water present in the atmosphere at the hours 
specified during the five years from August, 1881, to July, 1886. As 
the amount of moisture in the atmosphere is primarily a function of 
the temperature of the air, we find a steady increase in the absolute 




SELECTED RELATIVE HUMIDITY CURVES. 



MARYLAND WEATHER SERVICE 



159 



humidity from January, the coldest month, to July, the warmest month 
of the year. 

MEAN ABSOLUTE HUMIDITY. 
(Weight of the vapor of water in grains per cubic foot.) 



Hours of 
Observation 



1.41 
1.47 
1.60 
1.6" 
11 p.m 1.54 



7 a. m 

11 a. m . 

3 p. m . 

7 p.m 



Jan. 



Feb. 



1.58 
1.66 
1.71 
1.72 
1.72 



aiar. 



Average 1.619 1 



1.78 
1.78 
1.84 
1.91 
1.82 



Apr. 



2 72 
2171 
2.78 
3.01 
2.93 



1.827 2.828 3.997 5 



May 



3.89 

3.80 
4.02 
4.20 
4.08 



June 



5.56 
5.50 
5.51 
5.79 

5.82 



July Aug. Sept. 



6.50 
6.12 
6.11 
6.68 
6.71 



5.98 
6.06 
6.16 
6.38 
6.38 



5.29 
5.53 

5.48 
5.79 
5.61 



Oct. Nov. Dec. 



3.83 

3.87 
4.06 
3.96 
4.03 



6.425 6.193 5.539 3.951 2.411 1.836 3.267 



2.26 
2.36 



2. 47 

2.48 



1.74 
1.82 
1.91 

1,88 
1.83 



Year 



3.082 
3.222 
3^267 
3.437 
3.327 



Mean Vapor Pressure. 
The average monthly values of the tension of the vapor of water in 
the atmosphere, based upon observations of temperature of the air and 
the temperature of the wet-bulb thermometer at 8 a. m. and 8 p. m. 
from 1892 to 1903, are shown in the following table expressed in frac- 
tions of an inch of mercury: 



Jan. Feb. 


Mar. 


( 
April May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 
.152 


Year 


.136 .133 


.193 


.252 


.394 


.549 


.631 


.614 


.512 


.334 


.222 


.344 



PRECIPITATION. 
Introduction, 

The German meteorologist. Dove, has aptly compared the atmosphere 
to a huge still, of which the sun is the furnace, and the sea the boiler, 
while the cold air of the higher elevations and of the temperate zones 
plays the part of the condenser; we, on a wet day, catch some of the 
liquid which distills over. 

The condition and method of formation of dew, fog, cloud, rain, snow 
and hail are admirably and concisely stated in the following extract from 
one of Dr. Hann's recent treatises : * 

' Hann, J. Allgemeine Erdkunde. I. Abtheilung. 8° Wien, 1896. pp. 173 
et seq. 



160 THE CLIMATE OF BALTIMORE 

" Condensation of moisture gives rise to numerous phenomena which 
are collectively called liydrometeors. It takes place whenever the tem- 
perature falls below the dew point. Hence whatever favors a lowering 
of the temperature of the air favors the production of dew, fog, cloud, 
rain, snow and hail. 

" The ground cooling rapidly during a clear, calm night by radiation, 
lowers the temperature of the air resting upon it; we then have dew. 
With a temperature below freezing we will have frost. Warm, moist 
air mixing with colder air near the earth's surface will give us fog; 
thus we see fog formed over rivers in the early morning. Over the Banks 
of New Foundland we have a cold body of water over which pass warm, 
moist southerly winds, producing almost continual fog. With a tem- 
perature below freezing point the fog collects on trees and shrubs in 
the form of hoar frost. Mixing of air currents at different temperatures 
high above the earth, and rising and cooling moist air, produce clouds. 
Fog and clouds are composed of small drops of water. In winter and 
at high altitudes they are composed of ice crystals. High cirrus clouds 
all the year round are composed of ice crystals. At an elevation of 3500 
meters (11,483 feet) in the middle latitudes the air temperature is 
below the freezing point throughout the year. The presence of ice 
crystals in the higher clouds is shown by the colored rings about the 
sun and moon. These ice crystals and drops of water float in the air 
because of the great amount of surface exposed to the resistance of the 
air in comparison with their weight. 

"As fog and cloud increase in density the drops coalesce and become 
larger, until they become too large and heavy to be supported by the 
resistance of the air; they then go over into rain. Should the rain pass 
through drier strata of atmosphere it may be reabsorbed. In winter 
condensed water crystallizes and falls as snow. In stormy weather and 
a temperature near freezing, the snow packs and balls and falls as snow 
pellets. As these become firmer and pass alternately through layers of 
air above and below freezing point they become coated with water in the 
warmer layer and with ice in the colder layer, and thus form hail. 

" With an increasing quantity of vapor in the atmosphere the entire 



MARYLAND WEATHER SERVICE 161 

heavens become of a whitish dimness and lunar or solar halos appear. In 
most cases, however, the condensed vapor is not uniformly distributed 
in the atmosphere, but is collected in masses which float in the air, re- 
flecting the light and throwing shadows. We then call them clouds." 

The Causes of Precipitation. 

The theory that rainfall is primarily a result of the mixing of moist 
air currents at different temperatures has lost much in faVbr among 
meteorologists of to-day. Calculations have shown that the rain result- 
ing from such a cause must be comparatively light, and would not at 
all account for the abundant precipitation of the tropics, or the heavy 
falls connected with the movement of storms. Clouds may undoubtedly 
be accounted for upon the supposition of a mixture of air currents, with 
high humidity, especially the stratified forms, but even here it is neces- 
sary to find a more abundant source of condensation. This is found in 
the agency of a rising current of air. As already stated, the atmosphere 
contains at all times a considerable amount of moisture, especially in 
the lower layers of the warmer climates. Conditions are favorable for 
an upward movement of the air wherever there are opposing currents or 
where there is a considerable difference in temperature over adjacent 
areas. Such conditions are always present within storm areas, or, on a 
larger scale, in the equatorial region where we have the northeast and 
southeast trades meeting and causing a slow upward movement of the 
atmosphere at all times. Eising currents with the attendant decrease in 
temperature, combined with the presence of moisture, are capable of 
accounting for the heaviest rainfalls recorded. 

The Geographical Distribution of Rainfall. 

The rainfall of a locality being primarily dependent upon the -quantity 
of moisture in the atmosphere, that is, the absolute humidity, and this 
being in turn dependent upon the temperature, we find a steady decrease 
in the- amount of precipitation from the equator to the poles. In the 
equatorial regions the annual rainfall averages about 75 inches, while 
within the Arctic circle it is reduced to less than 10 inches. The decrease 



163 THE CLIMATE OP BALTIMORE 

is not at all uniform, as other factors enter into the problem of distri- 
bution to such an extent as to overbalance the effect of the temperature. 
The prevailing distribution of atmospheric pressure, the prevailing wind 
direction, and topography, may separately or in combination be the 
determining factors in the distribution of rainfall over any given area. 

The Influence of Wind Direction. 

The effect of wind direction may readily be inferred from what has 
already been stated concerning the sources of atmospheric moisture. The 
oceans being the great source of supply, and the winds being the carriers 
and distributors of moisture, it follows that rainfall is most copious along 
the coasts, and decreases with distance from the coasts when the wind 
direction is from the oceans inland. In the United States there is a 
steady decrease from the Gulf and Atlantic coasts toward the central 
portions of the continent, being 50 inches to 60 inches in the southeast, 
and about 15 inches in the Eocky Mountain region. On the Pacific coast 
a similar decrease obtains but is here greatly modified by the presence of 
high mountain ranges near the coast. The same is true in a general 
way over all of the continental areas. 

The Influence of Topography. 

The distribution of rainfall along the Pacific coast from California 
northward affords an excellent illustration of the influence of mountain 
ranges crossing the path of rain-bearing winds. During the rainy season 
the moist and comparatively warm winds from the Pacific are first forced 
up over the Coast Eange and again over the Sierra Nevadas to altitudes 
varying from a few thousand to over ten thousand feet. The moisture is 
cooled below the saturation point and rains are copious. The winds 
descend upon the eastern slope of the Sierra Nevada Mountains com- 
jjaratively dry. While the annual rainfall on the Pacific coast decreases 
from about 50 inches to 10 inches in passing over a distance of about two 
hundred miles inland, a similar decrease from the Atlantic and Gulf 
coasts extends over 1500 to 2000 miles. 

In northern India the elevated range of the Himalaya Mountains lies 



MARYLAND WEATHER SERVICE 163 

directly in the path of the southwest monsoon winds. The winds, blow- 
ing for days over the warm waters of the Indian Ocean, reach the moun- 
tains heavily laden with moisture, and are forced up the southern slope 
to elevations of 20,000 feet and more before they can proceed further on 
their way to the deep and persistent barometric depression which is cen- 
tered to the north of the mountains during the warm season. Among 
the foothills on the southern slope of the mountains, just north of Cal- 
cutta, and at an elevation of about 4500 feet, lies the station of Cherra 
Poongee, which has the heaviest annual rainfall in the world. The aver- 
age annual fall for 25 years approximates 475 inches; the annual amount 
has varied from about 300 inches to over 900 inches, nearly all of which 
falls in the six months from April to September. 

The Influence of Atmospheric Pressure. 

As previously pointed out, conditions which favor the production of 
rising air currents are favorable to the production of rain. Areas over 
which the barometer is relatively high are apt to be poor in rainfall, 
and areas with a low barometer in comparison with adjacent areas are 
apt to be comparatively rich in rainfall, other conditions being equal. 
This broad generalization may be verified by almost any daily weather 
chart which may be consulted, and is familiar to all who have occasion to 
study the weather charts issued by the United States Weather Bureau, 
or those of any other nation issuing such charts. When we see upon 
these charts an enclosed area of low barometer, or a "Low," as it is 
familiarly called, cloudiness and rain prevail within this area; where the 
barometer is high, relatively, the skies are prevailingly clear or partly 
clouded. " High area " weather is proverbially " fine " weather ; " low 
area " weather is generally " bad " weather. As the winds flow toward 
the center of an area of low barometer from all sides, the air at and near 
the center must necessarily rise, and rising, it is cooled. If it rises high 
enough, cloud formation and rain follow. On the other hand, where 
the barometer is relatively high, the air descends and the winds blow out 
from the central portions of the high area in all directions. We have 
seen that ascending air is cooled by expansion and radiation as it rises; 
12 



164 THE CLIMATE OF BALTIMORE 

conversely, descending air is warmed by compression. As it warms, its 
capacity for moisture increases, and not having an opportunity to take 
up more moisture in its descent, it becomes relatively drier; its relative 
humidity is decreased. Clouds which may be within this area at the 
beginning of its formation tend to dissolve, and near the center where 
the descent of air is most active, the skies are apt to be clear. These 
areas of low and high pressure (cyclones and anticyclones as they are 
technically termed) move from west to east in rapid succession in the 
middle latitudes and constitute the distinguishing feature of our weather. 
In equatorial regions there is a belt of varying width in which the 
pressure is constantly lower than it is to the north or south. Within 
this belt, situated between the northeast and southeast trade winds, the 
air has, in addition to its westward drift, an upward movement, produc- 
ing the "cloud belt" or doldrums with its almost daily copious showers. 
To the north of the northeast trades and south of the southeast trades 
there are broad belts, most regularly developed over the southern hemi- 
sphere where the surface conditions are more uniform, in which the 
pressure is relatively high. Here the air has a descending tendency and 
these areas are characteristically dry. Within them are the great desert 
regions of the globe — the Sahara, the Arabian desert, the arid regions of 
Australia, as well as those of our own Southwest. Over the oceans they 
are known as the " horse latitudes " with their light winds and scanty 
rainfall. 

The Seasonable Distribution of Eainfall. 

Climates are often classified according to the manner in which the 
rainfall is distributed through the year. We have regions of perennial 
rainfall, as in the United States east of the Mississippi Eiver, where there 
is a fairly uniform precipitation throughout the year. This is a condi- 
tion which prevails with limited exceptions between latitudes 35° and 
60°. Within the tropics, and over limited areas elsewhere, as in the 
upper Missouri Valley, there are large areas in which most of the annual 
fall of rain occurs in the summer months, with light rain in winter and 
spring. In other regions, as in the Pacific coast states, nearly all of the 
precipitation occurs in the winter months with little or no rain in sum- 



MARYLAND WEATHER SERVICE 



165 



mer. Lastly there are the arid regions of the world which are nearly 
free from rain throughout the year. 

Hourly Amount of Eaixfall. 

A diurnal period in the relative amounts and frequency of rainfall is 
most distinctly revealed in tropical countries, but is still clearly shown 
in the summer months of the middle latitudes. The precipitation which 
occurs in connection with the movement ot a general barometric depres- 
sion has a fairly uniform distribution throughout the day; that occur- 




FiG. 44. — Average Hourly Precipitation. 

The average amount of rainfall or snowfall during each hour of the day, for every 
month of the year, is shown by the heavy black lines and the shaded areas. The light 
shades show the time of day and year when the precipitation is usually lightest. The 
figures attached to the curved lines show the amount of the precipitation In hundredths 
of an inch. The values are based on the ten years' record of a tipping-bucket rain- 
gage and on eye observations. Only days with an appreciable amount of precipitation 
were considered In determining the average amount of precipitation for the day. 

ring in connection with thunderstorms is restricted mostly to the after- 
noon hours, and is intimately associated with the diurnal variation in 
temperature and pressure. 

The most conspicuous feature of the diagram representing the hourly 
quantity of rainfall (see Fig. 44) is the uniform distribution of the pre- 
cipitation throughout the day during the winter and spring months. 
This is doubtless explained by the fact that the winter and spring snows 
and rains occur in connection with the more or less regular succession of 



166 



THE CLIMATE OF BALTIMORE 



the cyclonic disturbances of the middle latitudes whose eastward progress 
is but slightl}', if at all, affected by the diurnal variations of temperature 
and pressure. On the other hand, the intensity of summer rains has a 
distinct diurnal period, being light in the forenoon, increasing rapidly to 
noon, or 1 p. m., and then more slowly to a maximum at about 



TABLE XXXVIII.— TOTAL HOURLY RAINFALL PER MONTH AND YEAR. 
(In hundredths of an inch.) 





Hours. 


i 




05 


p. 


^ 


0) 

a 

3 


^ bi 

3 1 3 





+3 




> 





oil 

S 

3 








n 


.16 


.16 


< 


S 


>-i 


>-> 

.09 


<! 


cc 



.15 


12; 

.17 


Q 
.11 


03 

1.70 


§ 


Md't. 


to 1 A.M 


.11 


.11 .11 .OS 


.35 .10 


.14 


1 


tk 41 






.09 


.13 .18' .09: .18, .06 


.03 .12 .06 


.15 


.15 


.14 


1.38 


.12 


2 


" 3 " 






.09 


AV .16 .15! .16 .05 


.13| .10 .09 


.15 


.13 


.09 


1.40 


.12 


3 


" 4 " 






.111 .11 .13; .15 .12 .06 


.OS; .02 .16 


.14 


.12 


.12 


1.32 


.11 


4 


" 5 " 






.06 .13 .13 .20 .11 .06 


.03 .06 .05 


.09 


.10 


.08 


1.10 


.09 


5 


" 6 " 






.Os! .16' .13 .17i .12! .06 


.04 .07 .09 


.16 


.09 


.07 


1.24 


.10 


6 


" 7 " 






.07i .14 .15 .13 .12 .10 


.05 .10 .11 


.09 


.12 


.11 


1.29 


.11 


7 


" 8 " 






.08 .15 .12 .17J .09 .04 


.04 .08 .06 


.20 


.08 


.15 


1.26 


.10 


8 


" 9 " 






.07i .18 .13 .14 .08 .04 


.04 .04 .04 


.10 


.07 


.14 


1.07 


.09 


9 


" 10 " 






.10 .22| .13, .14, .10 .02 


.08 .02 .08 


.11 


.10 


.15 


1.25 


.10 


10 


" 11 " 






.13 .171 .111 .12 .07 .05 


.06' .08 .08 


.20 


.08 


.16 


1.31 


.11 


11 


" Noon 






.12 .14 .121 .10 .0.5 .05 


.04 .28 .08 


.15 


.07 


.15 


1.35 


.11 


Noon 


" 1 P. M 






.15 .14' .10 .11 .13 .08 


.21 .17 .15 


.14 


.11 


.15 


1.64 


.14 


1 


" 3 " 






.16 .14' .12! .14' .19 .10 


.16 


.13 .17 


.10 


.10 


.14 


1.66 


.14 


3 


" 3 " 






.121 .12 .13 .12, .16 


.23 


.36 


.18 .24 


.08 


.10 


.14 


1.98 


.16 


3 


" 4 " 






.11 .13 .19 .15 .12 


.22 


03 


.26 .27 .09 


.18 


.14 


2.08 


.17 


4 


" 5 " 






.11 .1.5 .12 .18 .12 


!62 


'.SI 


.34 .14 .09 


.09 


.13 


2.30 


.19 


B 


" 6 " 






.12j .16 .10 .14 .24 


.24 


.22 


.10 .40 .06 


.08 


.14 


2.00 


.17 


fi 


" 7 " 






.12! -17 .08' .13 .17 


.18 


!25i .18 .28' .08 


.07 


.15 


1.86 


.16 


7 


" 8 " 






.12] .16 .08| .13 .27 .14 


.17! -11 -371 .15 


.23 


.15 


2.0s 


.17 


8 


" 9 " 






.12i .1.5 .12 .12 .23 .06 


.27, .30 .26! .09 


.15 


.18 


2.06 


.17 


9 


'• 10 " 






.151 .13! .lo! .10 .18 


.06 


.21 .22 32 


.10 


.14 


.14 


1.85 


.16 


10 


" 11 " 






.13 .12 


.09 


.16 .26 


.09 


!24l !l3i -33 


.16 


.15 


.14 


1.98 


.16 


11 


" Md't. 






.10 .10 


.15 


.14 .13 


.14 


.13 .21 .26 


.18 


.14 


.13 


1.81 


.15 



Table XXVIII. Total hourly precipitation per month and year. The 
figures expressing hundredths of an inch of rainfall or melted snow, 
indicate the amount of precipitation which was recorded on an average per 
month and year during each hour of the day. For example, an average of 
10 hundredths of an inch of rain fell from noon to 1 p. m. during the month 
of March from 1893 to 1902; 21 hundredths in July, and 164 hundredths on 
the average per year during that hour. The figures are based upon the ten 
years' record of a self-recording rain gage. 

5 p. m., then decreasing to midnight or early morning. The influ- 
ence of the thunderstorm is distinctly seen in this distribution of 
rainfall. The intimate connection existing between the intensity of rain- 
fall and the occurrence of thunderstorms becomes strikingly apparent 
when Fig. 44, showing the hourly distribution of rainfall throughout the 
year, is compared with Fig. 77, showing the hourly distribution of 



MARYLAND WEATHER SERVICE 



167 



thunderstorms. The great majority of these local storms fall within the 
period from 2 p. m. to 8 p. m. in June, July and August. 

The early morning hours of June, July and August have the smallest 
amount of rainfall (see Table XXXVIII), while the hours from 3 p. m. 
to 7 p. m. of the same months show the heaviest average falls. For the 
month of September the heaviest rains are apt to occur somewhat later in 
the day, between 6 p. m. and 11 p. m. 



i 

\ Jan. 



Fig. 45. — Average Hourly Amounts of Precipitation in January and July. 

The amounts are expressed in hundredths of an Inch. In obtaining the mean amounts 
only such days were considered upon which rain fell to the amount of .01 Inch or more. 

In Fig. 45 the average monthly amount of rainfall for each liour of the 
day is shown graphically for the months of January and July. The 
comparatively uniform distribution throughout the day in January is in 
striking contrast with the small rainfall in the morning hours, and the 
rapid increase in the early afternoon hours in July. 

Hourly Eaixfall Frequency. 

The graphical record of rainfall during a period of ten years at Balti- 
more, together with a careful record of direct observations of beginnings 



168 



THE CLIMATE OF BALTIMORE 



and endings of rainfall and snowfall, enable us to make a careful sinidy 
of the actual and relative frequency of precipitation at different hours 
of the day and night. A table was prepared (see Table XXXIX) show- 
ing the number of times precipitation was recorded during each hour 
of the day from January, 1893, to the close of December, 1902. 



TABLE XXXIX.-AVERAGE MONTHLY FREQUENCY OF RAINFALL FOR EACH 

HOUR OF THE DAY. 







a 


X5 


(i (i 


>, 


§ 


>. 


bfi 


4i 


4J 


> 


d 


a 

a 


■^ 00 

eg 




Hours. 


O 


,2 8* 


<a 


3 


2 


3 


o 


y 











^ 3 






3.1 


4.3 


4.0 3.8 


3.2 


•-> 

2.0 


1-5 

2.8 


1.7 


CO 
2.4 



3.0 


3.8 



3.0 


3.1 


<^^ 


Md't. 


to 1 A. M 


37.1 


1 


" 3 " 


3.0 


3.7 


3.7 3.5 


3.7 


1.9 


3.4 


1.6 


1.6 


3.0 


3.8 


3.0 


2.9 


34.9 


2 


" 3 " 


3.0 


3.5 


3.6 3.6 


3.4 


1- 7 


3.1 


1.3 


1.3 


2.9 


3.6 


2.6 


''.7 


33.5 


3 


" 4 " 


3.3 


3.8 


3.9 3.4 


3.6 


1.8 


1.9 


1.3 


1.3 


2.8 


3.4 


2.9 


2.S 


33.4 


4 


'• 5 " 


3.2 


4.1 


3.8 3.S< 


4.1 


1.7 


l.H 


0.9 


1.4 


2.5 


3.4 


3.3 


2.8 


34.0 


6 


'• 6 " 


3.6 


4.3 


4.6 4.7 


4.3 


1.6 


1.9 


18 


1.8 


3.0 


3.7 


3.3 


3.2 


38.5 


6 


" 7 " 


3.7 


4.3 


5.2 4.6 


4.8 


2.0 


1.8 


1.8 


2 2 


3.2 


3.5 


3.3 


3.4 


40.4 




" 8 " 


4.4 


5.4 


5.7 5.5 


6.0 


2.9 


2.4 


2. '^ 


3.5 


4.1 


3.7 


4.2 


4.0 


48.6 


8 


" 9 " 


.5.4 


5.6 


5.4 4.6 


6.6 


2.5 


2.9 


3.4 


3.9 


3.9 


4.1 


4.6 


4.2 


49.8 


9 


"10 " 


5.4 


5.3 


5.2 4.r, 


3.9 


2.2 


3.6 


1.7 


3.3 


3.5 


4.5 


4.4 


3.8 


45.5 


10 


"11 " 


4.8 


4.4 


5.1 4.5 


3.8 


2.3 


3.8 


1.9 


3.7 


3.5 


4.3 


4.2 


3.7 


44.3 


11 


" Noon 


5.5 


4.6 


5.3 3.6 


3.9 


2.7 


3.3 


3.0 


3.6 


3.3 


4.1 


4.3 


3.S 


46.3 


Noon 


" 1 P. M 


5.7 


4.6 


.i.2 4.4 


3.9 3.1 


3.3 


2.4 


2.7 


3.0 


4.5 


4.3 


3.9 


47.1 


1 


'* 3 " 


5.0 


4.2 


6.1 4.3 


4.0 3.1 


3.5 


2.6 


3.0 


2.8 


4.9 


4.2 


3.9 


46.8 


2 


" 3 " 


4.T 


3.9 


6.4 5.0 


3.8 


3.1 


3.5 


3 .0 


3.2 


3.0 


4.3 


4.1 


3.9 


46.4 


3 


" 4 " 


4.7 


4.3 


5.3 :>.2 


4.3 


3.5 


3.0 


3!i 


3.5 


3.8 


3.9 


4.3 


4.0 


47.8 


4 


" 5 " 


5.7 


4.7 


6.1 4.6 


5.0 


4.8 


4.4 


2.6 


4.1 


3.7 


4.0 


3.9 


4.5 


.53.6 


5 


" 6 " 


5.0 


4.3 


6.0 4.9 


4.7 


3.7 


4.3 


3.3 


4.0 


3.2 


4.6 


3.9 


4.2 


50.8 


6 


" 7 " 


5.3 


5.1 


5.0 4.8 


5.3 


4.1 


4.4 


3.9 


3.3 


3.3 


4.4 


4.7 


4.4 


53.4 


7 


" 8 " ....".'..'.'.'....'...'.'...'.'. 


5.4 


5.1 


5.2 4.S 


5.4 


4.1 


5.0 


3.7 


3.3 


3.6 


3.9 


4.7 


4.4 


53.3 


8 


•' 9 " 


5.4 


4.7 


5.3 4.2 


5.6 


3.4 


4.8 


3^7 


3.4 


3.0 


4.3 


4.5 


4.3 


61.3 


9 


"10 " 


5.1 


4.8 


5.3 4.6 


4.5 


3.0 


3.3 


3.3 


3.8 


3.0 


4.3 


4.2 


4.0 


48.0 


10 


"11 " 


5.3 


6.2 


5.3 5.3 


3.9 


2.3 


3.5 


2.7 


3.0 


3.1 


4..") 


4.1 


4.0 


48.1 


11 


" Md't 


4.9 


6.1 


4.9 4.8 


3.8 


2.3 


3.3 


2.0 


3.7 


3.0 


4.0 


3.5 


3.7 


44.3 



Table XXXIX. Average monthly and annual frequency of precipitation for 
each hour of the day. The figures indicate the average number of times rain 
or snow fell per month and year during each hour of the day. The results 
are based on the record of a self-registering rain gage for ten years, from 
1893 to 1902. 



The distribution of precipitation throughout the day is quite uniform. 
During the period of ten years there were but few months in which rain 
or snow was not recorded at all hours of the day at least once. Precipi- 
tation has most frequently occurred between 4 p. m. and 5 p. m. in the 
month of March, namely, 61 times in 10 years. The hour of least fre- 
quency is from 4 a. m. to 5 a. m. in the month of August, with a total 
number of 9 times in 10 years. On the average there is a fairly well 



MARYLAND WEATHEK SERVICE 



169 



defined diurnal period in the frequency of precipitation. Beginning 
with a minimum in the early morning hours there is a rise in the average 
annual frequency to a maximum at about 5 p, m., followed by a return to 
the minimum in the early morning. This periodic movement is most 
readily seen in the July curve (see Fig. 46). In this month the mini- 
mum frequency (1.8) occurs at about 6 a. m. and the maximum (5.0) 
at 8 p. m. There is a comparatively rapid increase in frequency between 
7 a. m. and 9 a. m., especially in the winter months. The most uniform 



DT 














Mot 






















. 


\J 


A 


A 




V Jan 


\ 




/ 


\J 


V 




/' 




t 


/-^ 


^ 


/ 


_^ 


\J 




\ JULV 


\ 


■v 


/ 


■\y 











































Fig. 46. — Average Houi'ly Frequency of Precipitation. 

The curves show the average frequency, for each hour during the months of January 
and July, of the occurrence of precipitation to the extent of .01 inch or more. The 
values are based on the ten years' record of a tipplng-bucket ralngage, supplemented by 
eye observations. 



distribution occurs during the cold months when the rains and snows are 
associated with the regularly recurring cyclonic disturbances. In the 
summer months the diminished effect of cyclonic disturbances is particu- 
larly noticeable in the reduced frequency of early morning rains, while 
the increasing afternoon rains are due to the summer thunderstorms, 
which reach a maximum frequency between 3 p. m. and 5 p. m. 

The month of March exhibits the most uniform hourly distribution of 



170 



THE CLIMATE OF BALTIMORE 



precipitation frequency, especially from about 7 a. m. until midnight. 
Precipitation is more frequent at all hours of the day during the winter 
and spring months than during the summer months. In Fig. 46 the 
January curve of frequency is well above the July curve throughout the 
day. This seasonal and diurnal distribution of precipitation frequency 
is graphically shown in Fig. 47, in which an increase in the intensity 
of shading represents an increase in the frequency of rains or snows. 
The figures attached to the heavy black lines show the average frequency 




Fig. 47. — Average Hourly Frequency of Precipitation. 

The heavy shades show the time of most frequent occurrence of precipitation during 
the day for every month of the year. The small figures attached to the irregular curved 
lines show the average number of times precipitation was recorded per month at the 
times indicated. 

of occurrence per month for each hour of the day and month of the year. 
The hourly and seasonal distribution is shown more accurately in Table 
XXXIX, in which the average frequency is recorded to tenths for all 
hours and months of the entire yesLT. 



Duration of Precipitation. 

Precipitation is not as continuous as it is generally supposed to be. If 
an automatic record of a rainy day be carefully examined, it will be found 
to be made up of numerous showers, some of them perhaps extending over 
an hour or two, but most of them lasting less than half an hour. The 



MARYLAND WEATHER SERVICE 



171 



duration and continuity depend mostly upon the position of the locality 
with reference to the center of the cj^clonic disturbance which is the occa- 
sion of the precipitation. As the character of the storm and the position 
of its path depend largely upon the season of the year, the duration of 
the accompanying precipitation is found to vary with the season. 

An automatic tipping-bucket rain gage has been in use at the Balti- 
more office of the Weather Bureau since January, 1893. The ten years' 
record enables us to obtain accurate values for the duration of the pre- 
cipitation. These records were supplemented by direct observations 
during the daytime. In a later chapter of this report some attention 
will be devoted to an analysis of the character of the rainfall accompany- 
ing different t3'pes of storms. In the table below an effort has been made 
to arrive at average values only for storms of all kinds. It has been 
found desirable to compute the average duration for three different con- 
ditions. The first line of figures of the following table includes " traces " 
of rainfall, i. e., amounts perceptible but too small to measure accurately 
by the ordinary methods. In the second line, only rains of measurable 
amounts have been considered, or precipitations equalling or exceeding 
one-hundredth of an inch in depth. The third line relates only to pre- 
cipitations which amounted to less than one-hundredth of an inch, or to 
" traces." 

AVERAGE DURATION OF rRECIPITATION. 
(In hours and minutes.) 



Class. 


Jan. 


Feb. Mar. 


Apr. 


May 


June 


Jul y| Aug. Sept. Oct. Nov. 


Dec. 


Year 


A. Including 
" traces." 


10.00 


1 
13.10 !l0.20 


11.00 


7.30 


4.30 


4.00 4.00 6.30 8.30 10.50 


10.00 


8.20 


B. Excluding 
" traces." 


11.10 


13.10 8.45 11.00 


7.00 


3.30 


4.00 3.30 5.15 8.05 


9.25 


9.20 


7.50 


C. Traces only. 


3.16 


3.00 1 3.20 1 4.05 


3.00 


2.10 


1.40 1 1.40 1 2.30 3.45 

1 1 


3.45 


2.66 


3.00 



Class A contains what may be regarded as the most trustworthy fig- 
ures for the average duration of precipitation, as these averages express 
the entire period of precipitation in connection with a passing storm. The 



172 



THE CLIMATE OF BALTIMORE 



rains of the winter, spring and autumn months show the influence of the 
more frequent cyclonic storms, while the rains of the summer months 



A S O N D J 



























/ 


\ 






















/ 


























/ 
































/ 


^ 
















1 




I. 


y 














/ 




/ 




\ 


/ 


, 










i 


f 












\ 










\ 
























/ 
















\ 






\ 


/ 
















\ 


B 




/ 












^ 


/' 


\ 


V 


y 


\ 


J 


/ 


/^ 


\ 


^^ 










\ 


c 




y 


/ 


















V 





y 



































Fig. 48. — The Average Duration of Precipitation. 

The upper curve (B) shows the variation in the average duration of rain and snow 
storms during which the precipitation amounted to .01 inch or more. The duration is 
expressed in hours and tenths of an hour ; the values are based upon a ten years' record 
of a tipping-bucket raingage supplemented by eye observations. The lower curve (C) 
shows the duration of light sprinkling rains, or light flurries of snow, with amounts 
too small for accurate measurements. 

are mostly of the kind accompanying thunderstorms. The former have 
a duration averaging more than double those of the latter. 



r^IAETLAXD WEATHER SERVICE 173 

An examination of the figures in the table above will show that the 
average duration of rainfall or snowfall is a little less than eight hours, 
when only such storms are considered as yield an appreciable amount of 
precipitation. When " traces " are included, the average duration is 
somewhat above eight hours. The summer rains are less than half the 
duration of those of the winter, spring and fall; they are obviously of 
the thundershower type, while the rains of the winter, spring and fall 
occur mostly in connection with the cyclonic depressions. The entire 
interval between the beginning and ending of precipitation of each storm 
was considered as the duration of rainfall or snowfall, regardless of inter- 
ruptions in the continuity of the fall. 

The duration of the actual period of precipitation is something quite 
different from the duration of the general storm, or local atmospheric 
disturbance in connection with which the precipitation occurs. Dur- 
ing the passage of a storm over any given locality, there are likely to be 
many beginnings and endings of precipitation with intervals of a few 
minutes or a few hours with no precipitation, or of so small an amount 
as to be negligible. There is a certain t}^e of storm which is of com- 
paratively frequent occurrence in the Middle Atlantic states — the 
" northeaster ; " this storm is generally accompanied by heavy and per- 
sistent rainfall. But even in this class the intensity of rainfall varies 
greatly from hour to hour and it is generally made up of severjil showers 
with intervals of several hours without appreciable rainfall. A list of 
storms accompanied by an uninterrupted rainfall exceeding 24 hours in 
the city of Baltimore during the past ten years would not be a very long 
one, the number probably not exceeding three or four per year. 

The following list comprises rains of exceptionally long duration, 
which have occurred during the ten-year period ending with 1903. In 
these storms the rainfall was practically continuous, although in most 
of them there were intervals of a few hours during which only light 
sprinkling, or misting, rains were recorded. 



174 



THE CLIMATE OF BALTIMORE 



RAIN AND SNOW STORMS OF LONG DURATION. 

Am't. 

1893, Apr. 19-21 44 hotus of actual precipitation. 1.14 In. 

1894, Apr. 10-12 52 " " " 1.95 " 

Dec. 10-12 49 " " " 2.01 " 

1895, Jan. 8-10 55 " " " 1.55 " 

Apr. 27-May 1 102 " " " 3.69 " 

Nov. 24-25 42 " " " 0.13 " 

1897, Dec. 3-5 44 " " " 1.18 " 

1898, Feb. 18-21 47 " " " 1.18 " 

Dec. 3-4 44 " " " 1.27 " 

1899, Feb. 11-13 54 " " " 1.50 " 

1900, Feb. 16-17 41 " " " 0.40 " 

1902, Feb. 20-22 51 " " " 2.53 " 

Nov. 24-26 44 " " " 1.60 " 

1903, Apr. 13-15 43 " " " 1.68 " 

The long-continued rains generally occur in connection with our 
'' northeasters/' depressions originating over the Gulf of Mexico, or in 
the West Indies, and moving northeastward directl}^ over ]\Iaryland, or 
following the Atlantic coast line. The most notable case in the list of 
long-continued rain storms is that of the spring of 1895, when rain, 
though sometimes very light, fell for practically 103 consecutive hours, 
beginning at 8 a. m., April 27, and ending at 2 p. m., May 1. The total 
precipitation for the entire period (3.69 inches) was not very large, 
though the rate of fall was at times excessive. There was no well-defined 
storm area near Baltimore at any time during the period. The bar- 
ometer was high over the New England states, while there was a shallow 
and ill-defined depression over the Gulf of Mexico which moved slowly 
northward and eastward some distance off the south and middle Atlantic 
coast, causing a steady northeast wind at Baltimore. 

Frequency of Precipitation of Stated Amounts. 

A table of monthly and annual precipitation as usually compiled may 
lead to erroneous inferences as to its agricultural value. The beneficial 
effects of rainfall depend not only on the quantity, but often to an equal 
extent upon the time of occurrence and the rate of precipitation. A 
given amount falling rapidly is of less value, agriculturally, than an equal 
or even less amount falling more slowly, as a rule. The greater portion 
of an excessive rain is apt to find its way to the streams immediately, 
while the lighter rains will soak into the ground to be utilized later in 



MARYLAND WEATHER SERVICE 



175 



the processes of plant life. It is of great importance to know the exact 
seasonal distribution of rainfall in order to determine to what extent it 



TABLE XL.-NUMBER OF 



DAYS WITH PRECIPITATION OF 
OF AN INCH OR MORE. 



ONE HUNDREDTH 



Year. 


a 

>-> 






<! 




o 

a 

s 

1-5 


3 
1-5 


bo 

D 
< 


t 

02 


o 


o 
I? 


c5 


c 

a 
< 


1871 

1873 

1873 

1874 


5 
8 
13 
12 
11 

11 
14 
13 
9 
15 


7 
8 
13 
8 
8 

14 
6 
9 
13 
13 


10 
14 
9 
8 
13 

13 
15 
11 
15 
IS 


6 
10 
14 
15 
11 

7 
14 

■J 

14 


6 
9 
14 
10 

8 

10 
9 

13 
8 
5 


8 
9 
8 
8 
11 

11 
9 

13 
8 

14 


16 
10 
11 
9 
14 

15 
13 
11 

7 
13 


13 

18 
21 
10 
17 

13 

16 

li 


4 
9 
14 
9 

8 

15 
8 
9 

10 
9 


10 
8 

11 
2 

9 

14 
13 
9 
5 
10 


10 
8 
9 
6 

11 

13 
14 
10 
8 
14 


10 
11 
11 
12 
16 

9 

8 
9 
16 
17 


104 
133 

147 
109 


18T6 


136 


1876 

1877 

1878 

1879 

1880 


144 
129 
133 
119 
154 


1881 

1883 

1883 


14 
17 
19 
15 
11 

16 
10 
11 
13 
13 

13 
13 
8 

13 
16 

4 
13 
12 
14 
11 

6 
10 
13 

11.1 
13.8 

n.5 


10 
11 
13 
19 
15 

8 
16 
13 
11 
13 

16 
14 
14 
15 
6 

13 
13 
6 
17 
13 

4 
8 
10 

9.7 

13.8 
13.5 


14 
14 
8 
19 
13 

13 
13 
14 
13 
16 

18 
13 
11 
8 
13 

14 
13 
19 
13 
13 

13 
11 
13 

13.5 
14.5 
13.3 


13 
11 
15 
11 
9 

7 
13 

9 
16 
13 

10 
13 
15 
13 
11 

8 
10 
13 

4 
10 

14 
9 
9 

11.1 
11.7 
10.4 


10 
21 
9 
13 

13 

17 
9 
17 
16 

18 

14 

15 
14 
18 
13 

9 
13 
13 
13 

7 

14 

8 
7 

9.3 
14.3 

13.8 


14 

8 
16 
10 

8 

13 
13 

7 
14 
6 

11 
13 
14 
11 

10 

13 
13 
6 

7 
11 

7 
11 
14 

9.8 
10.8 
10.8 


8 
10 
14 
13 

10 

13 
13 
8 
18 
10 

14 

9 
11 

9 

9 

15 
14 

8 

10 

9 

11 
15 
13 

11.8 
11.5 
10.8 


7 
13 

7 

10 
15 

8 

7 
13 

9 
15 

13 
10 

6 

8 

6 

8 

8 
11 
13 
10 

11 

7 
14 

13.9 
10.3 
9.3 


8 
13 
9 
2 

8 

8 
10 
16 
17 
12 

5 
9 
8 
10 
5 

10 

4 

4 
10 

7 

13 

13 

5 

9.5 

10.3 

7.3 


9 
13 
13 

9 
11 

7 

9 
15 
13 
16 

9 
3 
9 
11 
4 

6 
12 
13 

6 
10 

6 

7 
8 

9.0 
11.4 
8.3 


10 
10 
10 
8 
13 

10 

8 
10 
16 

9 

11 
11 
11 
9 
10 

11 

15 
14 

5 

8 

4 
9 

7 

10.3 
10.4 
10.6 


15 

8 
12 
13 

6 

16 

10 

7 

10 
14 

9 

8 
11 
11 
13 



15 
9 

13 
15 
9 

11.9 
11.1 
9.4 


132 
149 
145 


18S4 

1885 


142 
130 


1886 


134 


1887 


129 


1888 

1889 


138 

164 


1890 


155 


1891 


143 


1892 

1893 


129 
133 


1894 


134 


1895 


114 


1896 


115 


1897 

1898 

1899 

1900 


141 
125 
118 
115 




112 


1902 


122 




121 


Averajfes, 1871-1880. 
1881-1890. 


129.7 
141.8 
126.6 


1871-l!l03. 


11.9 


11.3 


13.3 


11.0 


11.8 


10.5 


11.5 


11.1 


9.1 


9.3 


10.0 


10.9 


131.4 



Table XL shows the frequency of occurrence of an appreciable amount of 
rain or snow (.01 inch) for each month of every year from 1871 to 1903, 
also the total annual frequency, and the average frequency for each ten year 
period and for the entire period of 33 years. 



may be counted upon at the critical stages of plant growth, A statement 
of monthly amounts will not reveal these important facts with sufficient 
accuracy. 



176 



THE CLIMATE OF BALTIMORE 



Tables have been prepared to show the total monthly and annual fre- 
quency of appreciable amounts of rainfall in each year from 1871 to 

TABLE XLL— ANNUAL NUMBER OF DATS WITH PRECIPITATION OF STATED 

AMOUNTS. 



Tear. 



1871 

1872 

1873 

1874 

1875 

1876 

1877 

1878 

1879 

1880 

1881 

1883 

1883 

1884 

1885 

1886 

1887 

1888 

1889 

1890 

1891 

1893 

1893 

1894 

1895 

1896 

1897 

1898 

1899 

1900 

1901 

1903 

1903 

Average 
Greatest 
Least 



Less than 
.01 inch. 



24 
10 
13 
31 

24 
31 

37 

25 
30 

43 
43 
56 
43 
44 

56 
45 
54 
41 

46 

60 
50 
55 

39.0 

60 
10 



.01 to .10 
inch. 



37 

66 
63 
50 

57 

63 
61 
63 
63 
73 

50 
69 
64 
61 

58 

57 
54 
63 
74 
67 

56 
50 
67 
63 
44 

53 
59 
45 
38 
50 

43 
46 
50 

56. 
74 
37 



.11 to .25 
inch. 



23 

27 
31 
20 
28 

35 
18 
16 
24 



30 

28 
SO 
26 

25 
20 
33 
21 

35 

33 
16 
23 
21 
23 

21 

37 
34 
24 



15 

25.1 

38 
15 



.26 to .JO 
inch. 



25 
16 
23 
20 
20 

22 

22 
19 

20 

23 

28 
22 
28 
33 
18 

23 
26 
17 
31 



18 
29 
17 
2fi 
23 

12 

20 

23 
26 
21 

21 
19 

23 

22 1 

33" 

12 



.51 to 1.00 
inch. 



11 

18 
23 
12 
21 

14 
18 

33 
13 
11 

15 

18 
16 
20 
16 

14 
16 
14 
21 
21 

24 
25 
17 
13 
14 

20 
24 
17 
23 
14 

16 
19 
21 

17.5 

25 

11 



Over 1.00 
inch. 



11 
16 
13 

10.6 
17 
5 



.01 inch 
or more. 



104 
123 
147 
109 
136 

144 
129 
133 
119 
154 

132 
149 
145 
142 
130 

134 
129 
138 

164 
155 

143 

129 
133 
134 
114 

115 
141 
135 
118 
115 

113 
133 
121 

131.4 

164 

104 



Table XLI shows the number of days for each year from 1871 to 1903 upon 
which rain or melted snow was recorded to the depth indicated by the figures 
at the top of each column. Amounts less than .01 inch were not recorded 
until 1882. 



1903 (Table XL), and of the total annual frequency of falls of stated 
amounts (Table XLI). These tables, together with those referred to 
later showing the rainfall frequency and amounts for each day of the 



MARYLAND WEATHER SERVICE 



IV 4 



year and for each successive pentad and decade, give most of the facts 
necessary for a detailed study of the influence of rainfall upon plant 
growth in the vicinity of Baltimore. Some of the more conspicuous de- 
ductions from these tables are here summarized. Considering only days 
with an appreciable amount (0.01 inch or more), there are on the average 
131 per year. The limits of variability are 164 and 104, occurring in 
1889 and 1871 respectively. Such days are the least frequent in Sep- 
tember and October, and most frequent in March. With an average 



I l_ _ _ _l 



Fig. 49. — Variations in the Annual Frequency of Days with Appreciable 

Precipitation. 



maximum frequency of 13.3 in March, there is a steady decrease to 9.1 
in October, followed by an almost uniform increase to March. With 
normal conditions, the rainfall is ample at all periods of the year. Dis- 
astrous droughts are of rare occurrence. The most pronounced dry 
periods of the past 33 years will be referred to in subsequent pages. The 
variations in the total annual frequency of rainy days from year to year 
are confined within quite narrow limits (see Fig. 49 ). The successive 
ten-year averages from 1871 to 1900 are 130, 142, 127, respectively. 
Since 1895 the annual frequency has been continuously below the normal. 



178 



THE CLIMATE OF BALTIMORE 



with the single exception of 1897; from 1880 to 1891 it was almost con- 
tinuously above. 

In addition to the days with an appreciable quantity of rainfall 
referred to in the above paragraphs, there are nearly forty per year, on 
the average, during which light sprinkling rains or mists are recorded. 
Their distribution throughout the year follows closely that of the days 
with appreciable rain. While the individual effect of these light rains 
is small, their aggregate annual value to vegetation cannot be neglected. 

The most frequent quantity of rain or snow, and hence the most prob- 
able quantity to be expected in all months of the year, is some amount 
from 0.01 inch to 0.10 inch. The average daily rainfall for the year is 
0.32 inch, neglecting traces. Hence, as already pointed out in the dis- 
cussion of the temperature observations, the average value is not the most 
probable. The average monthly and annual frequency of stated amounts, 
based upon a record of 33 years, is shown in the following table : 

FREQUENCY OF PRECIPITATION OF STATED AMOUNTS. 
(Average for 33 years.) 



Precipitation in 

hundredths of 

an inch. 


Jan. 


Feb. 


Mar. 


April 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


Year 




3.4 
5.4 
3.4 
2.1 
1.5 
0.5 
11.9 


2.5 
4.8 
2.0 
2.1 
1.6 
0.8 
11.3 


3.6 
5.0 
3.5 
1.9 
1.7 
0.9 
13.3 


2.9 

4.7 
2.7 
1.8 
1.3 
0.6 
11.0 


4.6 
4.8 
2.5 
2.2 
1.6 
0.8 
11.8 


3.0 
4.1 

1.7 
2.0 
1.7 
0.8 
10.5 


4.0 
5.0 
2.2 
1.5 
1.4 
1.4 
11.5 


3.6 

4.7 
1.9 
1.8 
1.5 
1.2 
11.1 


2.2 
3.8 
1.4 
1.5 
1.4 
1.2 
9.1 


2.8 
4.7 
1.3 
1.5 
1.2 
0.7 
9.3 


2.9 
4.1 
2.1 
1.8 
1.5 
0.7 
10.0 


2.8 
5.3 
1.8 
1.9 
1.0 
0.9 
10.9 


38.3 


0.01 to 0.10 

0.11 to .25 

0.26to0.50 

0.51tol.00 

Over 1.00 

.01 and over 


56.3 
25.4 
23.1 
17.4 
10.4 
131.4 



♦Average for 20 years. 



Average Daily Eainfall. 
In the following consideration of the question of the daily amount of 
rain or snow which falls throughout the year no account was taken of 
days upon which less than 0.01 inch was recorded. The period covered 
in determining the average daily rainfall was that from 1871 to 1900, or 
thirty years. The total amount of precipitation for each day was then 
divided by the number of days upon which precipitation occurred to the 



MARYLAND WEATHER SERVICE 



179 



amount of .01 inch or more, and tliis value regarded as the average daily 
rainfall and recorded in Table XLTI. 



TABLE XLIL— AVERAGE AMOUNT OF PRECIPITATION ON DAYS WITH RAIN 

OR SNOW. 
(.In hundredths of an inch.) 



Date. 


Jan. 


Feb. 


Mar. 


Apr. 


1 
May June 


July 


Aug-. 


Sept. 


Oct. 


Nov. 


Dec. 


Ann'l 


1 


.24 
.16 
.15 
.32 
.25 

.35 
..30 
.34 
.33 
.29 

.33 
.21 
.39 
.24 
.39 

.14 

.27 
.13 
.35 
.21 

.33 
.25 
.13 
.43 
.30 

.24 
.27 
.11 
.18 
.20 

.26 

0.26 


.08 
.35 
.34 
.23 
.25 

.48 
.19 
.37 
.51 
.28 

.154 
.38 
.42 
.20 
.23 

.61 
.21 

.28 
.27 
.27 

.31 
.41 
.44 
.34 
.36 

.26 
.17 
.24 
.45 

0.33 


.29 
.25 
.35 
.46 
.23 

.14 
.34 
.31 
.48 
.33 

.46 
.37 
.14 
.19 
.25 

.39 
.13 
.35 
.45 
.37 

.31 
.35 
.16 
.15 

.37 

.34 
.61 
.31 
.33 
.40 

.32 

0.31 


.17 
02 

.51 
.23 
.35 

.33 
"2 

'.h 

.47 
.34 

.23 
.17 
.35 
.21 
.18 

.13 
.30 
.28 
.19 
.26 

.35 
.29 
.14 
.19 
.50 

.48 
.45 
.43 
.36 
.24 

0.29 


.34 
.18 
.28 
.31 
.33 

.25 
.49 
.61 
.22 
.16 

.17 
.44 
02 
!34 
.43 

.37 
.18 
.25 
.39 
.41 

.41 
.33 
.33 
.33 
.24 

.37 
.26 
.21 
.21 
.21 

.32 

0.30 


.35 
.20 
.30 
.40 
.41 

.24 
.33 
.33 
.49 
.21 

.44 
.25 
.25 
.47 
.44 

.86 
.30 
.72 
.23 
.57 

.29 
.49 
.44 
.39 
.33 

.40 
.54 
.70 
.17 
.18 

0.37 


.36 
.71 
.39 
.29 
.36 

.17 
.33 

'.I2 
.38 
.40 

.63 
.33 
.33 
.39 
.54 

.34 
.40 
.39 
.33 
.47 

.69 
.49 
.19 
.36 
.41 

.54 
.36 
.38 
.39 
.65 

.54 

0.41 


.35 
.33 

.60 
.11 

.38 

.55 
.78 
.46 
.11 

.56 

.30 
.47 
.50 
.13 
.18 

.43 
.50 
.31 
.11 
.36 

.53 
.35 
.37 
.33 
.56 

.15 
1.03 
.14 
.54 
.20 

.28 

0.38 


.16 

.28 
.48 
.17 
.57 

.84 
.36 

..17 
.33 
.34 

.60 
.39 
.26 
.45 
.68 

.80 
.74 
.13 
.93 
.23 

.16 
.30 
.57 
.34 
.47 

.28 
.35 
.20 
.28 
.12 

0.41 


.15 
.28 
.14 
.55 
.26 

.37 
.15 
35 
.10 
.57 

.06 
.33 
.38 
.17 
.21 

.17 
.08 
.04 
.49 
.60 

.30 
.23 
.86 
.27 
.46 

.37 

'.h 

.09 
.18 
.44 

.39 

0.30 


.53 
.33 
.31 
.39 
.48 

.31 
.36 
.43 
.29 
.24 

.14 
.19 
.30 
.35 
.37 

.07 
.21 
.35 
.39 
.17 

.20 
.16 
.50 
.64 
.23 

.37 
.35 
.38 
.29 
.11 

0.30 


.29 
.15 
.39 
.30 
.42 

.34 
.70 
.11 
.11 
.60 

.18 
.25 
.29 
.39 
.33 

.16 
.46 
.23 
.22 
.41 

.33 
.29 
.11 
.33 
.14 

.30 
.28 
.16 
.37 
.37 

.19 

0.29 




3 

S:;:;:::;.;;-;;::::: 

6 




H 




9 




10 

11 

1--' 

13 




U 

15 

16 

IT 

18 

19 

20 




21 

22 




23 

24 

25 

26 

2- 

28 




29 








31 


0.33 







Table XLII. In determining the average daily amount of precipitation in 
the above table, the total precipitation of the month was divided by the 
number of days upon which rain fell to the depth of one hundredth of an inch, 
or snow to the depth of one tenth of an inch. The record is based upon daily 
observations during the period of 30 years from 1871 to 1900. 

The results are interesting, among other reasons, as showing the great 
variability in the amounts for adjacent days, even in values representing 
an average for thirty years. The average amount for any particular day 
is at any time likely to be materially altered by the occurrence of a single 



180 



THE CLIMATE OF BALTIMORE 



heavy rain. The heaviest rains occur in the warm months of June, July, 
August and September, while the smaller amounts are confined, in the 
main, to the colder months. The influence of a single heavy rainfall on 
the average amount for thirty-one years is clearl}^ shown in the excep- 
tionally high average fall for the 27th of August. The average for all 

TABLE XLIir.-AVERAGE AMOUNT OF PRECIPITATION 
BY PENTADS AND DECADES. 

Pentads. 
(Pentads ending on stated dates.) 





January. 


February. 


March. 


ApriL 


May. 


June. 




5th .22 


4th .25 


1st .26 


5th .28 


5th .25 


4th .29 




10 .32 


9 .34 


6 .29 


10 .34 


10 .34 


9 .m 




15 .31 


14 .36 


11 .36 


15 .21 


15 .33 


14 .32 




20 .22 


19 .32 


16 .27 


20 .23 


20 .32 


19 .41 




25 .30 


24 .35 


21 .30 


25 .29 


25 .;« 


24 .43 




30 .20 




26 .25 
31 .37 


30 .39 


30 .35 


29 .43 




July. 


August. 


September. 


October. 


November. 


December. 




4th .39 


3rd .49 


2nd .29 


2nd .21 


1st .33 


1st .26 




9 .31 


8 .46 


7 .48 


7 .29 


6 .32 


6 .32 




14 .39 


13 .39 


12 .43 


12 .28 


11 .37 


11 .34 




19 .38 


18 ..SI 


17 .57 


17 .20 


16 .36 


16 .28 




24 .44 


23 .30 


23 .35 


23 .31 


21 .26 


• 21 .33 




29 .42 


28 .44 


27 .40 


27 .44 


26 .36 


36 .21 
31 .27 



Decades. 





Jan. 


Feb. 


March 


April 


May 


June 


July 


'Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


1st Decade — 


.27 


.30 


.31 


.31 


.30 


.33 


.37 


.42 


.40 


.29 .34 


..34 


2nd " 


.26 


.34 


.31 


.23 


.32 


.40 


.39 


.32 


.51 


.24 .25 


.29 


3rd 


.25 


.33 


.30 


.34 


.28 


.38 


.45 


.40 


.31 


.35 .30 


.25 



Table XLIII. The average amount of precipitation recorded during each 
successive five-day period and ten-day period throughout the year is shown 
in the above tables. The figures are based upon a 30-year record, and repre- 
sent approximately the most probable amount of precipitation to be 
expected within the same pentads and decades in coming years. 



August days is 0.38 inch, while for the 27th it is 1.03 inch. On the 27th 
of August, 1882, the amount recorded was two inches. The total num- 
ber of times rain occurred on the 27th of August from 1871 to 1901 was 
but 5, while the average frequency for all the days of the month was 11.4. 
The variable character of the rainfall from day to day is more readily 
seen when represented in graphical form as in Plate IX. Some of the 



MARYLAND WEATHER SERVICE 



181 



smallest daily amounts belong to the month of October. The average 
fall for tlie 17th of this month is but .08 inch, and that for the 18th but 
.04 inch. The general average for the entire year is 0.32 inch per day. 

TAI5LE XLIV.-FKEQUENCY OF PRECIPITATION ON EACH DAY OF THE YEAR 

FROM 1871 to 1901. 

(Precipitation of .01 inch or more.) 



Date. 


Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


An'l 


1 


15 


11 


10 


11 


9 


13 


10 


16 


4 


6 


8 


9 




2 


l.S 


9 


14 


10 


9 


12 


11 


16 


4 


8 


9 


8 




3 


4 


16 


13 


9 


10 


12 


r, 


8 


6 


8 


13 


7 




4 


10 


15 


13 


11 


13 


13 


11 


13 


7 


7 


7 


14 




m 


16 


14 


14 


9 


14 


11 


16 


11 


5 


8 


6 


11 




« 


15 


13 


10 


10 


18 


11 


13 


10 


13 


10 


9 


9 




7 


10 


17 


11 


11 


13 


9 


10 


9 


11 


13 


11 


7 




8 


8 
15 


15 
9 


13 
15 


13 
14 


11 
11 


11 

9 


11 

6 


11 
9 


8 
9 


10 
5 


13 
16 


9 




9 




10... 


12 


13 


13 


15 


11 


13 


7 


10 


13 


4 


13 


8 




11 


18 


13 


13 


10 


13 


11 


13 


11 


13 


~ 


11 


10 




rj 


15 


14 


16 


14 


9 


14 


10 


14 


13 


13 


11 


11 




13 


13 


16 


. 15 


10 


11 


13 


13 


14 


13 


13 


8 


9 




14 


12 


14 


13 


8 


13 


7 


11 


13 


9 


13 


8 


13 




15 


11 
15 


9 

10 


13 
13 


14 
15 


14 
14 


7 
14 


11 
9 


15 
11 


13 
11 


6 

5 


9 
6 


13 
5 




16 




17 


15 


13 


13 


11 


7 


17 


8 


9 


13 


6 


13 


i'Z 




18 


13 


14 


8 


13 


14 


9 


10 


13 


9 


6 


13 


10 




19 


12 


13 


16 


11 


13 


9 


11 


9 


8 


6 


10 


9 




20 


13 


13 


16 


13 


11 





13 


8 


9 


9 


11 


9 




21 


12 


13 


15 


7 


16 


11 


8 


11 


8 


13 


9 


13 




oo 


9 


11 


13 


9 


13 


11 


10 


15 


10 


13 


i-z 


14 




23 


9 


6 


13 


13 


14 


7 


K 


15 


11 


13 


17 


13 




24 


13 


s 


13 


13 


14 


(; 


11 


14 


7 


11 


13 


11 




25 


14 


19 


13 


13 


14 


k; 


13 


13 


9 


7 


8 


11 




26 


9 


14 


14 


13 


13 


11 


17 


9 


13 


13 


14 


20 




07 


10 
15 


11 

10 


13 
15 


13 

18 


13 
10 


10 

11 


19 

18 


5 
6 


6 

7 


13 
16 


7 
13 


13 
11 




2S 




29 


13 




14 


15 


9 


13 


14 


10 


9 


16 


13 


13 




30 


8 




8 


6 


11 


8 


10 


13 


14 


11 


8 


10 




31 


14 




13 




13 




13 


13 




13 




17 






13.1 
1« 


13.5 
19 


13.9 
IS 


11.6 
15 


13.2 

18 


10.8 

17 


11.4 
19 


11.4 
16 


9.4 
14 


9.5 
16 


10.5 
17 


10.9 
20 


11.3 


Greatest 


20 




4 


6 


8 


5 


7 


6 


ti 


5 


4 


4 


6 


6 


4 



Table XLIV. This table indicates the number of times in thirty-one 
years that rain or melted snow was recorded to the depth of one hundredth 
of an inch or more upon each day of the year. Thus we learn that rain or 
snow fell but 4 times in 31 years on the 3rd of January, that precipitation was 
recorded 20 times on the 26th of December during the same period, etc. The 
days of most frequent and least frequent preoipitation are also shown for 
each month. 

July and September have the highest average, with 0.41 inch, and Janu- 
arv, with 0.2G inch, the lowest. 



183 



THE CLIMATE OF BALTIMORE 



As the daily averages are so variable in character, they have been 
grouped in periods of five and ten days, and average values determined 
for each pentad and decade of the year (see Table XLIII) . By this pro- 
cess of smoothing out accidental irregularities, we obtain values which 
represent more nearly the most probable precipitation to be expected 
upon any day of each pentad or decade (see also Plate IX). 





J F M * 


M J 


, 


A £ 





N 


D J 










/ 


\ 




r 


\ 










1 5 






V 


/ 


\ 


/ 


1 


\ 


V 




^ 


J 


1 




/ 


\ 


^ 


\ 


y^ 


-^ 


\ 






^ 


y 


5 




/ 






^ 


^ 


"N 


\ 




^ 


.^^ 






/ 




^ 








\ 


^-> 


/ 


/ 


A 


n 





Fig. 50. — Monthly Frequency of Precipitation. 

The maximum monthly frequency, the mean frequency and the least monthly frequency 
of occurrence of days with an appreciable amount of precipitation are shown, respec- 
tively, by the upper, the middle and the lower curves. 

Daily Eaixfall Frequency. 
A matter of considerable importance to agricultural and commercial 
interests is the frequency of the occurrence of rain or snow in appreciable 
quantities. This has been determined with great accuracy for the 
vicinity of Baltimore, especially since 1871, when systematic observations 
were begun bv the Weather Bureau. Here asfain as in the determination 



MARYLAND WEATHER SERVICE 183 

of the average daily quantity of precipitation, the average values for 
adjacent days vary remarkably. Thus the average frequency for Janu- 
ary 2 is 13, while that of January 3 is but 4. For June 24 and 25, the 
values are 6 and 16 respectively. The day upon which rain or snow fell 
the greatest number of times from 1871 to 1901 is December 26, namely, 
20 times in 31 years. The lower limit, namely, 4 times, belongs to 
January 3, September 1 and 2 and October 10. (See Table XLIV and 
Plate IX.) 

The table throws an interesting side-light on the mooted question of 
the occurrence of " equinoctial storms." Are rains any more frequent 
on March 21 and September 21 than on the days immediately preceding 
and following? On March 21 rain fell 15 times in 31 years; on March 
19 and 20, 16 times; on 22 and 23, 12 and 13 times respectively. The 
average for all days in March is 13 times. On September 21 rain fell 
8 times in 31 years; the average for all days of the month is 9.4 times. 
That is, an appreciable amount of rain fell on only 26 per cent of the 
September equinoctial days of the 31 years, or 4 per cent below the 
average for all days of September. These figures show that rain is not 
as likely to occur on these days as on many other days of the month. 

The Probability of Eain. 

If we divide the actual number of occurrences of rain on any given 
day by the number expressing the total number of years under considera- 
tion (in this case 31), we obtain an expression which in a rough way 
represents the percentage of expectancy of rainfall, or the rainfall prob- 
ability, for that day. Rainfall in the middle latitudes is too erratic in 
its occurrence to place much reliance upon this percentage as a forecast 
for any particular day ; if, however, we have a long series of observations 
and take the average value for 5 successive days, we arrive at a figure 
which more accurately represents the most probable percentage of occur- 
rences of rainfall for any one of the five days. This has been done in 
Table XLV and the results graphically represented in Plate IX. The 
curve based on 5-day means shows some periods of the year to be de- 
cidedly freer from rain tlian others, although there is a fairly uniform 



184 



THE CLIMATE OF BALTIMORE 



distribution of precipitation throughout the A'ear. The period from the 
middle of September to the middle of October, for example, has shown 
in ol years from ISTl to 1901, an average rainfall frequency of about 
28 per cent. The month of March, on the other hand, shows a record 
of about 42 per cent. The last week of July shows a probability of 52 
per cent, the highest for any week in the year. The average daily proba- 

TABLE XLV.— RAINFALL PROBABILITY BY PENTADS AND DECADES. 

(In perceiitag-e of possible frequency.) 

Pentads. 

(Pentads ending on stated dates.) 





January. 


February. 


March. 


ApriL 


May. 


June. 




5tli 37.4 


4tli 


41.8 


1st 43.7 


5th 32.0 


5th 35.4 


4th 40.3 




10 38.6 


9 


43.8 


6 40.6 


10 39.8 


10 41.0 


9 32.6 




15 40.6 


14 


44.6 


11 41.8 


15 36.0 


15 38.0 


14 36.6 




20 43.8 


19 


38.0 


16 45.2 


30 39.8 


20 37.8 


19 36.0 




25 86.8 


24 


32.2 


21 43.4 


25 33.6 


25 45.8 


24 26.0 




30 35.4 






26 40.8 
31 40.0 


30 45.8 


30 36.0 


29 39.3 




July. 


August. 


September. 


October. 


November. 


December. 




4tli 29.4 


3rd 


40.8 


2nd 29.6 


2nd 28.2 


1st 40.8 


1st 31.6 




9 36.0 


8 


30.6 


7 26.8 


7 29.0 


6 37.6 


6 31.4 




14 34.0 


13 


37.2 


12 35.6 


12 25.6 


11 40.6 


11 29.6 




19 31.4 


18 


38.6 


17 37.4 


17 25.8 


16 27.0 


16 33.2 




24 34.2 


23 


37.3 


23 38.4 


22 29.0 


21 35.4 


21 33.6 




29 52.2 


28 


29.8 


37 38.8 


27 36.0 


26 44.4 


26 44.3 
31 40.6 


Decades. 




Jan. 

38.0 


Feb. 


March 


April 


May 


June 


1 
July lAug 


. Sept. Oct 

i 


1 
. Nov. 


Dec. 


1st Decade — 


42.2 


40.2 


35.9 


38.2 


36.4 


32.3 


36.4 


26.0 


25.1 


33.2 


30.2 


2nd 


42.2 


41.3 i 43.7 


37.9 


37.9 


34.0 


34.4 


37.3 


35.5 


26. C 


31.8 


28.2 


3rd 


36.9 


39.1 41.1 


37.4 


41.0 


33.3 


42.5 


35.9 


29.8 


39.6 


36.5 


42.1 



Table XLV. The figures in this table represent approximately the proba- 
bility of rain or snow upon any one of each of the stated five- and ten-day 
periods throughout the year, expressed In terms of percentage of the total 
number of similar days in thirty-one years, or of the possible frequency. For 
example, the probability of the occurrence of rain upon the 15th of March 
(or any stated day from the 11th to the 20th) is expressed by 43.7%; for the 
15th of October, by 26.0%. If rain had occurred upon every 15th of March 
and 15th of October in the thirty-one years from 1871 to 1901 the percentages 
of probability of rain upon these days would have been represented by 100. 

bility for the entire year is 36 per cent; the highest for any one day is 
64 per cent, namely, for December 26, and the lowest is 13 per cent, for 
January 3, September 1 and October 10. The probability of rain is less 



VOLUME 2, PLATE I 




b 



PRECinTATinS I'DOBADILITY. 

A. Precipitation ti-onnoncy for each day of the year. fEipressp,] , 

B. rrecipitati.m freqiieiicy for each successive 5-day pcrloa oi i ™ as u^rcenlages of the possible freqiie 

C. Average amount of precipitation for each day of tl 
U. Average amount of precipitation for each successlv 



MARYLAXD WEATHER SERVICE 



185 



than 20 per cent on but few clays of the year, and does not often exceed 
50 per cent. 

The probability of precipitation at Baltimore on the following days 
may be of special interest : 



Jan. 1, 48 per cent. 

Feb. 22, 35 

Mch. 4, 42 

Mch. 21, 48 

Apr. 30, 16 

May 1. 29 



May 30, 35 per cent. 

July 4, 35 " " 

Sept. 1, 13 " 

Sept. 12, 42 " 

Dec. 25, 35 " 

Thanksgiving Day 35 per cent. 

S O N D J 









- 




/ 




y 


) 










f\ 






^ 








\ 




/ 


^ 


V. 


J 
































/■ 














^ 




■\ 


^ 










\ 




- 


■- 








-- 






N 


^ 








y 



Fio. 51. — The Monthly Amount of Precipitation. 

The upper line indicates the variations in the maximum monthly rainfall from 
month to month ; the middle line shows the mean monthly rainfall based on 30 years of 
observations ; the lower line shows the least monthly precipitation recorded during each 
month in 30 years. 

The Monthly Precipitation. 
The usual method of representing the precipitation of any given 
locality is by means of monthly and annual amounts. - Owing to the 
great variability in the character of rainfall and snowfall, a great many 
years of continuous observations made under practically imchanged 
exposure of the gauge are required. The vicinity of Baltimore has to its 



186 



THE CLIMATE OF BALTIMORE 



TABLE XLVI.-TOTAL MONTHLY AND ANNUAL PRECIPITATION FOR 87 YEARS. 



Year. 



1817.. 
1818. 
1819. . 
1820.. 



1821. 
1822. 
1823. 
1824. 

1825. 



1826. 
1827. 
1828. 
1829. 
1830. 

1831. 
1832. 
1833. 
1834. 
1835. 

1836. 
183". 
1838. 
18.39. 
1840. 

1841. 

1842. 
1843. 
1844. 
1845. 



1846... 
1847... 

1848... 
1849... 
1850... 

18.51... 

18,52. . . 
1853. . . 
1854... 
1855... 

18.56.., 
1857... 
1858... 
18.59... 
1860. . . 

1861... 
1862.., 
1863... 
1864.., 
1865.. 

1866... 
1867.. 
1868.., 
1869. . 
1870. . 



".25 
0.90 
0.70 
2.80 



3.30 
1.80 
5.60 
2.30 

0.62 

0.81 
i.03 
1.1,0 
5.50 

i.n 

U.38 
3.21, 
'2.81 
1.11 
1.96 

3.94 
3.10 
2.10 
3.. 50 
2.30 

6.10 
1.80 
1.60 
3.65 
3.40 

2.83 
3.92 

1.58 
1.03 
3.58 

1.70 
3.60 
1.30 
4.40 
2.50 

2.11 
3.50 
1.83 
7.06 
2.29 

3.70 
0.60 
3.33 
0.80 
1.19 

2.50 
0.90 
2.56 
3.4" 
2!o6 



2.80 
2.00 
1.90 
2.20 

5.40 
4.80 
0.70 
5.90 

2.87 

1.85 
3.13 
2.1,1 
4.40 
1.79 

2.1S 
2.33 
1.05 
1.93 

1.57 

3.41 
3.10 
2.90 
3.60 
3.30 

1.40 
3.35 
2.20 
1.45 
3.59 

1.83 
3.43 
0.94 
1.15 
2.43 

2.90 
3.60 
3.40 
4.90 
4.00 

0.50 
0.66 
1.61 

5.74 
2./,2 

1.79 

i.n 

A. 15 
0.14 
1.21 

4.90 
3.80 

2.21 
2.85 
1..50 



•S • - 



4.. 50 
3.00 
4.55 
3.30 

1.70 
1.30 
7.10 
4.30 
/,.53 

5.70 
1.13 
3.25 
9.10 I 

A. 02 j 

2.87 I 
1.80 
2.12 I 
1.92 
3.73 

1.64 
6.30 
4..=)0 
4.00 
3.70 

5.95 
2.40 
3.80 
3.00 
1.70 

3.54 
2.38 
3.70 
3.63 
5.90 

5.70 
3.90 
2.70 
4.70 



2.47 
2.30 
1.31 

6.36 
1.32 



S.tS 
5.79 
4.23 
3.62 

1.40 
3.90 

3.20 
3.64 
1.90 



1.50 
2.10 
3.70 
1.10 

2.10 
3.10 

1.80 
4.70 
0.67 I 

S.il 
2.U7 I 
3.37 
4.40 
1.57 I 

U.61 
2.61 
0.56 

2.1,7 
3.82 

4.23 
2.10 

2.80 
9.10 
4.30 

4..'i0 
4.. 30 
2.90 
1.60 
1.49 

2.38 
0.41 
0.81 
0.87 
3.85 

4.70 
7.80 
3.10 
7.20 
0.39 

1.48 
1.84 
4.33 
6.96 



5.13 

5.67 
6.25 
3.. 54 

2.83 

2.50 
1.50 
1.65 
0.50 
3.03 



2.60 9.00 

6.45 1.15 

4.10 1.30 

4.40 4.60 



5.10 
1..50 
2.10 
2.95 
1.59 



1.80 
1..50 
1.60 
5.03 
3.08 



0.21 3.86 

2.29 ! 1.80 

3.18 2.28 

3.40 1 8.50 

3.i2 I U.92 



1.01 
U.90 
5.33 
3.21 

1.83 

4.10 

4.20 
4.30 
4.50 
3.90 i 

2.75 ! 

4.00 

3.55 

4,00 

2.36 

5.77 
1.19 
2.96 
4.18 
3.08 

4.60 
1.70 
4 .30 
5.20 
0.91 

1.19 
3.23 

9.08 
2.74 
3.i8 

4.76 

2.12 
.',.10 
3.39 
4.92 

0.20 
0.63 
3.80 
1.38 
3.53 



1.37 
A.S6 
3.32 
5.15 

9.20 
4.90 
4.70 
4.10 
5.10 

4.35 
2.65 
0.90 
1.70 
2.93 

1.78 
3.36 
4.34 
\.IQ 
1.66 

1.20 
2.70 
0.60 
4.80 



0.92 
7.45 
4.90 
1.16 

2.1,1, 

2.38 

5.71 
3.53 
0.82 
3.93 

2.. 50 
3.00 
.1.03 
1.76 
3.37 



3.50 10.00 
4.10 
3.20 
2.20 



7.50 
4.35 
3.60 
3.37 

1.7i 



2.50 
/,.08 
4.64 
3.55 

S.6U 
2.2U 
3.62 
3.80 
5.78 

2.35 
4.30 
1.90 
5.60 
1.85 

1.35 

3.70 
5.40 
3.90 
1.26 



2.. 51 
4.42 

2.06 
3.10 

4.20 
5.70 
3.30 
2.60 
2.62 

1.82 
2.47 
3.23 

6.20 
0.77 

7.06 

2.11 
5.29 
0.41 
1.74 

2.15 
2.03 
5.05 
0.30 
0.35 



3.30 
2.00 3.20 
4.30 , 3.00 
8.00 1.50 



0.30 lO.'iO 
0.80 I 2.25 
4.10 5.80 
4.50 2.94 
3.21 , 2.U7 



2.1,0 
A. 95 
1.35 
3.98 
3.35 

k.6U 
/,.90 
2.9/, 
0.59 
1.81 

6.70 
5.10 
9.10 
2.20 
3.35 



1.92 
0.83 
i.28 
1.93 
2.16 

U.92 
1.38 
3.56 
3.33 
2.1,9 

3.15 

3.80 
4.50 
1.90 
3.80 



4.00 1 2.30 
4.40 ! 1.00 
7.82 10.. 50 
0..31 ; 4.47 
1.51 



7.20 
3.97 
3.34 
2.55 
4.70 

3.30 
4.60 
4.70 
3.00 
2.50 

4.88 
4.43 
3.37 
3.76 

7.25 

3.31 

0.85 
1.30 
2.36 
1.90 

1.92 
3.. 52 
1.33 
0..50 
1.68 



3.88 j 
5.. 55 ' 
1.64 
1.90 
4.70 

0.50 
2.20 
2.40 
4.10 
2.30 

2.83 
1.40 
4.44 

7.05 
2.69 

1.80 
3.70 
0.91 
2.19 
1.81 

4.20 
1.00 
4.4S 
3.15 
1.76 



1.80 
3.10 
0.70 
7.80 

3.40 
3.. 50 
3.80 
1.77 

0.88 

U.5U 
U.60 
0.99 
1.72 
3.32 



2.60 
1.92 
2.51 
0.85 

4.00 
3.10 ' 
3.10 

1.60 
4.50 

3.80 
1.40 
1.97 
3.03 
3.73 

1.30 
3.38 

7.35 
6.37 
3.10 

3.20 
3.60 
4.40 
7.10 
3.70 

0.77 
3.89 
2.34 

2.38 
3./,9 

3.96 

3.69 
1.8/, 
1.33 
3.68 

1.55 
2.60 

0..50 
5.08 
3.00 



3.70 
2.00 
1.10 
2.70 

5.60 
5.10 
3.10 
2.27 
l.~23 

1.62 
3.95 
5.51 
3.32 
l,./,2 

1.65 
2.21 
1.89 
2.55 
2.69 

4.80 
3.40 
3.70 

2.80 
3.15 

3.30 
3.75 
4.25 
1.85 
1.23 

7.17 
2.. 54 
1.44 
l.Ofi 
4.30 

5.60 
7.90 
3.. 50 

7.:« 
1.20 

1.85 
1.87 
3.97 

3.20 
5.05 

5.48 
3.91 
2.30 
2.41 
2.. 50 

1.10 

2./,9 
3.. 50 
1.86 
0.28 



3.60 
2.60 
2.20 
1.90 

3.30 
1.20 
6.25 
2.25 

3.. JO 

1.16 

2.95 
0.25 
1.37 
/,.6S 

1.09 
U.69 
5.12 
2.11 

2./,2 

7.10 
3.60 
4.50 
8.80 
3.25 

5.10 
3.35 
3.90 
3.. 50 
3.43 

2.10 

2.38 
3.10 
4.44 
4.40 

1..50 
6.20 
2.. 30 
3.90 
3.60 

2.05 
6.33 
5.65 

3.15 
2.99 

1.36 

1.50 
/,.1S 
1.40 
5.90 

2.. 50 

\.'m 

3.90 
1.04 



48.55 
32.60 
28.75 
43.. 50 

50.20 
29.20 
44.. 55 
42.28 
36.35 

30.68 
33.69 
33.01 
53.26 
38.97 

37.40 
34.27 
41.38 
29.51 
34.10 

.54.63 
45.00 
47.10 
51.70 
37.. 50 

43.90 
35.10 

48.79 
32.46 
28.39 

46.66 

as. 01 

34.42 

30.63 
44.80 

38.10 
.51.. 50 
.36.00 
.59.20 
29.31 

22.87 
38.37 
46.06 
.55.64 
37.54 

43.. 55 
35.48 
43.97 
33.03 
33.32 

37.48 
33.90 
33.63 
27.34 
22.43 



:maryland weather service 



187 



TABLE XLVI CONT.— TOTAL MONTHLY AND ANNUAL PRECIPITATION FOR 

87 YEARS. 



Year. 



1871. 
1872. 
1873. 
1874. 
1875. 

1876. 
1877. 

1878. 
187P. 

1880. 



1881. 
1882. 
1883. 
1884. 
188.5. 



1886.. 

1887.. 
1888.. 
18S9. . 
1890.. 



1891 

1893 

1893 

1894 

189.5 



189fi. 
1897. 
1898. 
1S99. 
1900. 



1901.. 
19ft?. . 
190:i. . 



1.55 
0.88 
4.27 
2.22 

sisi 

1.67 
3.80 
4.. 51 
2.. 59 
2.26 

4.84 
5.38 
3.16 
4.81 
3.07 

4.48 
2.. 57 
3.35 
4.22 
1.80 

4.89 
6.42 
1.78 
1.46 
4.67 

2.62 
2.05 
2.99 
3.. 50 
2.11 

2.45 
3.05 
3.81 



Averages. 



l821-]H.30... 
1831-1840... 
1841-l8ro... 
18.51-1,'W)... 
1861-1870... 



1.38 


a.fts 


1.46 


3.06 


4.74 


3.02 


3.18 


1.41 


2.91 


4. 72 



2.96 
1.87 
3.31 
1 . 55 
1.96 

5.68 
3.73 
4.69 
6.69 
4.40 

5.49 
4.69 
2.. '3 
2^53 
4.80 

5.. 52 
2.41 
4.43 
3.. '3 

0.8:5 

7.07 
5.13 
1.32 
5.47 
4.65 

0.65 
4.68 
5.43 



2.4B 3.33 

2.81 2.43 

2.75 2.18 

2.98 2.97 

2.00 2.67 



6.37 
3.60 
4.74 
1.65 

4.82 

7.59 
3.43 
3.68 
6.37 
1.60 

4.85 
3.49 
4.62 
5.71 
4.07 

7.94 
7.20 
1.38 
1.19 
2.94 

4.70 
2.40 
2.. 58 
4.93 
3.17 

3.58 
3.41 
4.40 



4.21 
3.16 
3..';0 
3.. 35 
3.40 



S -. 



1.90 
3.06 
2.77 
6.65 
4.27 

1.90 
3.30 
4.19 
3.69 
3.07 

2.00 
2.14 
3.20 
2.65 
1.37 



2.03 
1.44 
6.31 
1.92 
1.49 

4.94 
2.2.3 
5!38 
2.74 
1.23 

2. .30 
3.42 
1.^2 
3!i7 
4.50 

7.07 



2.06 

2.44 2.. 57 

2.11 4.22 

8.70 6.82 

3.94 5.98 



2.48 
3.15 

3.. 52 
3.80 
7.42 

1.44 
3.19 
1.84 
1.89 
2.06 

5.53 
2.90 
3.29 



2.66 
3.66 
2.. 31 
4.11 
3.07 



3.11 

6.35 
3.78 
7.26 
3.04 

1.61 

6.88 
3.86 
3.29 
1.00 

3.67 
1.62 
3.33 



2.. 57 
3.73 
3.. 38 
3.64 
3.28 



2.82 
4.16 
0.94 
1.11 
2.85 

4.09 
3.53 

4.09 
3.92 
5.48 

7.81 
2.30 
8.08 
2.. 51 
6.31 

5.64 
4.44 
3.22 
6.17 
2.42 

5.45 

4.87 
2.26 
3.29 
2.83 

3.94 

2.57 
1.06 
2.16 
4.;34 

0.90 
4.30 
5.01 



3.44 
4.. 52 
2.51 
2.90 
2.96 



1817-1870 1 2.52 2.68 3..55 I 3.03 [ 3.40 3.32 

1871-1880 2.63 2.53 3.64 3.48 2.97 3.30 

1881-189<I 3.77 4.55 4. ."4 3.06 4.13 4.89 

1891-1900 3.25 4.04 3.84 3.08 4.02 3.28 

1871-1903 i 3.£0 I 3.'.0 3.99 3.27 i 3.63 1 3.78 



is z. 



6.15 

1..58 
2.90 
4. .30 
4.78 

5.64 
4.60 
4.66 
3.16 
6.47 

1.40 
4.ft2 
3.10 
9.43 



8.08 
8.32 
2.82 
li!03 
3.61 

7.79 

4.07 
1.8-< 
1.73 
3.40 

6.32 
6.93 
3..-1 

1.64 
1.51 

6.18 
2.45 
7.65 



3.92 
3..-1 
3.46 
3.29 
2.65 

3.34 

4.42 

5.45 

3.88 



3.41 

4.. 59 
9.49 
3.47 

8.67 



S.22 
5!06 
3.70 
4.83 
3.62 



1.76 10. .52 

0.64 5.27 

4.82 0.82 

6.71 2.72 

4.44 1.78 



2.15 
5.10 
o . 7*^ 

L74 

7.78 

3.94 
4.15 
6.17 
1.40 
6.44 

4.24 
\.i3 
1.81 
1.41 
2.43 

1.93 
4.71 
6.09 

4.86 
2.91 

6.73 
4.31 

5.88 



2.89 
4.03 
4.00 
4.18 

1.87 



2.98 
9., 38 
3.49 
0.09 
1.30 

1.90 

2.80 
4.90 
4.59 
4.76 

5.46 
2.36 

1.80 
4.75 
6.01 

4.14 
2.17 
1.56 
7.09 
4.26 

2.10 
7.19 

l.no 



3.. 59 
3.18 
3.74 
2.99 
2. JO 



3.11 

4.08 
6.21 
0.16 
1.44 

2.79 I 

5.22 

4!41 

0.75 

2.64 

4.06 
0.86 
2.83 
1.42 

6.51 

1.39 
1.06 
2.99 
4.12 
5.73 

2.76 
0.26 
3.44 

3.. HO 
2.20 

1.11 

3.67 
3.97 

2.09 
1.68 

l.f3 
6.85 
3.54 



2.65 
3.37 
3.43 
3.19 
2.52 



3.24 
3.17 

4.05 
2.48 
4.86 

2.74 
6.85 
3.55 
1.30 

2.86 

2.41 

0.65 
1.37 
3.09 
4.04 

4.09 
2.ft2 
3.04 
6.45 
0.74 

1.33 
3.85 
3.78 

1.98 
1.86 

3.34 
4.39 
4.34 
2.27 
1.81 



1.90 

o!97 
1.90 
3.14 

1.32 
2.23 

5.61 
5.33 
4.89 

5.90 
1.70 
2.9S 
3.91 
3.49 

3.12 
5.04 
3.26 
0.61 



3.24 

2 . 28 
2!29 
4.13 
3.t'4 

0.37 
3.40 
3.34 
1.40 
2.07 



3.26 7.07 
S.'O 5.66 
0.73 2.19 



3.61 2.68 

2.68 4.17 

2.99 3. .57 

4.14 3.77 

2.59 3.. 59 



3.59 3.17 3.06 i 3.14 3.30 



4.F0 
4.16 
3.23 



4.05 
3.63 
3.96 



3.08 
3.10 
2.50 



3..51 I 2.94 
2.79 3.17 
2.90 2. .54 



4.66 4.20 i 3.85 2.99 2.99 3.07 



33.74 
.34.76 
49.-37 
33.63 
45.26 

46.70 
43.14 
50.09 
36.01 
41.90 

49.13 
43.11 
40.. 53 

45.8'^ 
40.04 

.53.11 
43.-59 
43.53 
63.-35 
46.96 

.54.31 
45.05 
33.15 
38.. 33 
40.47 

3S.59 
47.49 
36.46 
40.. 59 
31.. 57 

43.04 
-50.13 
46.26 



38.01 
41.25 
37.83 
41.46 
:i-2.W 

38.13 

41-36 
47.32 
40.49 

43.:J4 



Table XLVI is a record of the total monthly and annual precipitation at 
Baltimore from 1817 to the close of 1903, including rain and melted snow. 
From 1817 to 1824 the record is that of Capt. Lewis Brantz; from 1836 to 
1870, that of the U. S. Army Medical Department at Fort McHenry; from 
1871 to 1903 that of the U. S. Weather Bureau. All figures in italics are 
interpolated values based upon the record of the Pennsylvania Hospital, 
Philadelphia, after applying the proper corrections to reduce the record to 
the Fort McHenry series. No attempt has been matle in this table to reduce 
the entire record to a single uniform series. 



188 



THE CLIMATE OF BALTIMORE 



credit a particularly long series of observations made under careful super- 
vision by trained observers. From 1836 to 1870 the record contained in 
Table XL VI is that of the U. S. Army Medical Department kept at Fort 
McHenry. This is followed by the record of the U. S. Weather Bureau 
from 1871 to 1903. The observations from 1817 to 1824 were made by 
Capt. Lewis Brantz, in what was, in his time, West Baltimore. To 



I — ■ — ' ' 

i — — — — — — — — — — I — ■ — 

2 

I — ^ 

^_ 1 M 



Fig. 52. — Mean Monthly Precipitation. 



complete the record from 1817 to 1903, it was found necessary to inter- 
polate the monthly and annual amounts for the years 1825 to 1835. 
This was done by computing the normal precipitation at the Pennsylvania 
Hospital of Philadelphia and applying the monthly and annual de- 
partures from this normal value to the normal amount for Fort McHenr3\ 
The records from 1817 to 1870 are fairly comparable. No attempt was 
made to reduce this series to that of the U. S. Weather Bureau from 1871 
to 1903 in Table XLYI. The results of the two series may be compared 



MARYLAND WEATHER SERVICE 



189 



with entire safety by employing the departures from their respective 
normal values. This has been done in Fig. 54, in which the departures 
are graphically shown by months in regular chronological order for the 
entire period from 1817 to 1903. In Plate X the monthly, seasonal and 
annual departures are shown separately for the entire period. The Fort 
McHenry series may be reduced to the Weather Bureau series by adding 
the following differences to the monthly and annual normals of the 
former. These differences were computed from an overlapping record 
of twenty-one years from 1871 to 1891. 

Jan. Feb. Mar. Apr. May June July Aajr. Sept. Oct. Nov. Dec. Year 
0.88 0.55 0.78 0.5t 0.03 0.29 0.S4 0.05 0.0:5 0.47 0.48 0.49 5.4-3 

The record from 1871 to 1903 was made with great care and without 
any interruption. The average monthly and annual values computed 
from this series may be accepted with entire confidence as reliable aver- 
ages for Baltimore. We have then the following figures to express the 
normal precipitation : 

NORMAL MONTHLY rRECIPITATION. 
(In inches and hundredths.) 

Jan. Feb. Mar. Apr. May June July Aug'. Sept. Get. Nov. Dec. Year 

3.20 3.70 3.99 3.27 3.63 3.78 4.66 4.20 3.85 2.99 2.99 3.07 4^3. .34 

While the above figures represent the best average values available, they 
do not represent the most probable values to be expected during any 
future month or year. As precipitation is the most variable of all the cli- 
matic factors, a long .series of accurate observations is necessary to estab- 
lish a normal average, or an average which would not be materially altered 
by succeeding observations. The degree of variability at Baltimore is 
indicated by the figures in the following table of extremes during the 
period of 87 years from 1817 to 1903 : 

EXTREMES OF PRECIPITATION. 
(1817-1903.) 





.Ian. Feb. 


Mar. 


Apr. 


May 

5.68 
1858 
3.20 
1866 
8.88 


June 


July 


Aug. 

6.41 
1817 
3.5(5 
1877 
9.97 


Sept Oct. Nov. 


Dec. 


Year 


.\l)(j\e 


normal 


1 

4.54 3.87 
18.59 1896 
2.32 3.05 
1873 1901 
6.86 6 43 


6.55 

1820 
2.80 
1894 
8.36 


6.04 
18:» 
2.67 
18.55 
8.71 


5.8S 
18:W 

2.SH 
1901 
8.76 


6.. 37 
18H9 
3.26 
18KI 
9.63 


7.53 4.86 4.76 
1831 18.33 1853 
3.76 2.H3 2.8»> 
1884 1874 1870 
11.29 7.69 7.62 


5.50 
3.05 

1.H2.H 

8 55 


21 .07 

1S64 


Helow 
Year.. 
KanKe 


normal 


15.70 

1870 

36.77 



190 THE CLIMATE OF BALTIMORE 

The extreme monthly ranges are observed, from the above table, to be 
more than double the average monthly amounts, while the extreme annual 
range closely approaches the mean annual precipitation. Another inter- 
esting fact brought out by the above table is the ratio existing between 
excessive and deficient monthly precipitation. In every instance, except- 
ing the month of February, the excessive amounts are in round numbers 
about double the deficiencies. As, in the long run, the sum of the ex- 
cessive amounts must equal those of the deficient amounts in order to 
produce the normal value, it follows that a precipitation below the normal 
is the most frequent, and hence the most probable. 

It is the excessive rainfall which disturbs average values to the greatest 
extent. A single heavy rainfall will occasionally materially change the 
average value for a long series of years. A case in point may be cited. 
In 1897 the average precipitation for the month of July for the southern 
part of Anne Arundel County, Maryland, based on a series of observa- 
tions covering 7 years was 5.67 inches. On the 26th of July, 1897, dur- 
ing a local thunderstorm of great intensity, a rainfall of nearly 15 inches 
was recorded. This single fall changed the average July rainfall from 
5.67 inches to 7.45 inches. 

The Seasonal and Annual Precipitation. 

The great variability in the total annual precipitation is best seen by 
an inspection of Fig. 53, in which the 3'early amounts are presented 
graphically from 1817 to 1903 after reducing all observations to the 
Weather Bureau series. The dotted horizontal line represents the 
average height for the entire 87 years. The fluctuations from year to 
year are so irregular and vary so greatly in amount that it is difficult to 
detect any periodic movement. There are suggestions here and there 
in the diagram which point to possible periodic swings. There are 
groups of years during which the precipitation remained constantly above 
the normal value, and others with a persistent deficiency. Take, for 
instance, the period from 1850 to 1861 (see Fig. 53), when there was a 
decided annual excess with the exception of two or three years in the 
middle of the period ; this was followed by 13 years of deficient pre- 



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192 



THE CLIMATE OF BALTIMORE 



cipitation; from 1873 to 1889 the anuual values fluctuated a great deal, 
but on the whole there was a gradually increasing excess, culminating in 
1889 in one of the heaviest annual falls recorded ; in the following years 
there was diminishing rainfall w ith an average Ijelow the normal to the 
present time. 

TABLE XLVII.-TOTAL SEASONAL PRECIPITATION FOR 87 YEARS. 









u 


C 1 








h 


c 








t; 


a 


a 




bo 

c 


s 


a 


B 
O 


u 


bo 

C 


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< ; 






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1817 

1817-8 
1818-9 
1819-20 

1820-1 
1821-3 
1822-3 
1823-4 
1824-5 

1825-6 
1826-7 

1837-8 
1828-9 
1829-30 

1830-1 
1831-2 
1832-3 
1833-4 
1834-5 



6.50 11.55 
5.20 11.35 
7.20 ! 8.80 



132.58 
' 7.25 

7.80 . ,. 
114.80 112.00 



8.80 
8.30 



1845-6 8.08 11.69 15.87 12.35 

1846-7 i 8.44 '■ 3.98 i 8.84 111.47 

1847-8 ' 4.90 ' 6.47 11.90 10.43 

1848-9 5.27 8.68 6.11 9.23 

1849-50 10.45 12.83 9.46 12.10 



10.60 8.90 9.60 19.70 

9.90 4.90 6.65 ; 9.85 

7.50 11.00 9. .30 11.70 

14.45 11.95 12.90 6.98 

5.74 6.79 ' 8.03 4.58 

6.03 ; 9.33 9.46 8.08 

6.33 I 5.89 9.31 9.3S 

6.83 ' 9.80 8. .31 10.78 

10.15 16.90 17.13 6.97 

4.33 9.01 11.83 10.50 



jll.19 

6.66 
! 8.55 

I 8.83 
I 5.64 



8.49 11.36 10.05 

9.31 8.51 i fi.19 

8.01 10.92 13.37 

7.60 I 7.71 I 8.39 

9.38 12.74 ' 6.03 



1835-6 9.77 , 9.97 18.35 11.95 

1836-7 !l3.30 13.60 14.30 10.30 

1837-8 7.60 11.60 15.70 10.30 

1838-9 11.60 17.60 11.90 ' 6.30 

1839-40 13.40 10.90 9.30 ' 9.45 

1840-1 10.75 13.20 9.70 8.40 

1841-2 110.25 10.70 10.75 5.15 

1843-3 7.15 ,10.25 14.12 16.72 

1843-4 I 9.00 8.60 5.91 9.35 

1844-5 9.49 5.55 6.96 6.46 



1850-1 
1851-3 
1852-3 
18.53.4 
1854.5 

1S55-6 
1856-7 
1857-8 
1858-9 
1859-60 

1860-1 
1361-3 
1863-3 

1863-4 
1864-5 

186.5-6 
1866-7 
1867-8 
1888-9 
1869-70 

1870-1 
1871-2 
18T2-3 
1873-4 
1874-5 



9.00 15.00 8.70 8.30 

7.70 13.40 13.00 13.70 

10.90 10.10 8.60 10.30 

11.60 17.10 10.40 18.50 

10.40 4.10 7.91 7.30 



6.31 
6.21 

9.77 
18.45 

7.86 



5.14 7.63 5.45 
7.37 14.35 6.16 

14.73 11.50 10.75 

15.96 11.13 13.61 

8.15 10.46 11.23 



8.48 13.71 12.75 

6.07 ' 9.24 ! 8.67 

8.98 16.14 10.12 

5.13 11.16 i 3.59 

3.80 10.36 7.57 



10.34 
11.33 
5.05 
5.93 
6.99 



13.30 4.16 
7.30 13.09 
7.24 7.71 
6.93 5.53 
7.40 i 7.45 



6. .57 6.85 

7.55 I 6.09 
10.01 8.48 

3.56 10.09 
5.40 I 5.04 



3.97 I 6.96 12.38 , 8.57 
4.24 7.56 10.33 12.31 
11.23 11.60 13..S3 13.96 
6.37 9.98 8.88 7.47 
7.32 10.48 16.30 9.92 



1875-6 

1876-7 
1877-8 
1878-9 
1879-80 

1880-1 
1881-3 

1883-3 
1883-4 
1884-5 

1885-6 
1886-7 
1887-8 
1888-9 
1889-90 

1890-1 
1891-3 
1893-3 
1893-4 
1894-5 

189.5-6 
1896-7 
1897-8 
1898-9 
1899-00 

1900-1 

1901-2 

1902-3 

1817-70 

1871-03 



7.77 13.21 11.49 

6.99 ! 9.13 8.77 

10.05 14.31 13.57 
9.75 8.08 13.79 
9.45 9.13 16.39 

15.41 11.89 11.. 36 

15.01 i 8.99 11.42 
9.55 8.10 13.90 

14.48 13.19 13.68 

11.38 I 7.47 16.76 

13.46 13.98 17.66 
10.38 8.50 16.91 

11.23 10.95 13.21 

10.01 21.23 18.60 

7.21 13.99 13.47 

13. OS 13.53 17.48 

13.07 16.70 10.77 

8.49 8.68 : 5.95 
7.38 12.25 6.43 
9.62 13.40 8.66 

13.53 7.75 13.19 

7.55 13.47 14.21 
7.71 8.28 10.66 

13.31 10.11 ; 8.66 

8.16 ■ 6.33 ' 8.76 

5.17 13.78 '13.81 
14.80 7.93 11.06 
14.90 11.03 18.54' 

8.50 10.01 10.35 
9.97 10.89 12.64 ; 



16.05 
17.34 

8.78 
4.77 
7.28 

9.45. 

10.89 
7.69 
4.60 

11.85 

7.38 

5.88 

10.93 

15.16 

11.23 

9.55 

6.47 

9.02 

10.53 

10.07 

8.59 
10.33 

9.87 
11.45 

7.75 

6.28 
17.76 
5.27 
9.37 



Table XLVII. For the character of the record in the above table see foot- 
note to table XLVI. 



The greatest annual precipitation of the entire period of 87 years, 
making due allowance for the permanent differences between the Fort Mc- 
Henry record and that of the U. S. Weather Bureau, was that of 1854, 
with a fall of 64.63. The mean annual amount for Baltimore based on 
the Weather Bureau observations for 33 years is 43.34 inches. The 
rainfall and snowfall of 1889 followed close behind that of 1854 with a 



MARYLAND WEATHER SERVICE 



193 



Jan. Julv Jan. julv Jan. Julv J 




Fio. 54a. — Departures from Moan Monthly Precipitation (1817-1859). 



191 



THE CLIMATE OF BALTIMORE 



Jan. July ■>*" Jul/ J'" J"i- 



JULv Jan Julv Jan. 



4^44-186 








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Fig. 54b. — Departures from Mean Monthly Precipitation (1860-1904). 




1875 1880 1885 1390 1895 1900 

XRTCRKS FROM THE NnRMAL PRECIPITATION ( 1817-1904'). 



MARYLAND WEATHER SERVICE 195 

total of 63.35 inches. The smallest annual precipitation recorded was 
27.86 inches, reduced value, in 1870. These figures show a total range 
in the annual precipitation of 36.77 inches. The average departure of 
the annual precipitation from the normal quantity is 6.68 inches, a 
figure which attests the great variability of this climatic element. 

If we compute from Table XLVI average annual values for each 
10-year period from 1820 to 1900, we find a fairly close agreement, show- 
ing a strong tendency to return to a certain normal value in spite of 
great fluctuations in individual years. The most conspicuous departure 
is the great deficiency recorded from 1861 to 1870. 

DEPARTURES OF TEN-YEAR AVERAGES FROM THE NORMAL FOR 87 YEARS. 

Decades. Departures. 

1821-1830 -0.11 inches. 

1831-1840 +3.13 

1841-1850 -0.30 

1851-1860 + 3.34 

1861-1870 -6.02 

1871-1880 -1.98 

1881-1890 + 3.88 

1891-1900 -2.85 

Monthly and Annual Departures. 

The chief characteristics of the monthly and seasonal departures for 
successive years are clearly shown in Fig. 54 and Plate X. The most con- 
spicuous feature of these curves is the irregular, short-period fluctuation,? 
above and below the line representing the average amounts for a long 
series of years. The extreme irregularity in the fluctuations makes it 
entirely impracticable to employ this method as a basis for making long- 
range forecasts. When the precipitation is charted by months in regular 
chronological sequence, as in Fig. 54, the same characteristic fluctuations 
are noted. They are irregular in amount and period, but with a general 
tendency to return to the normal level, and with occasional evidence of a 
long period of excessive or deficient precipitation. 

An inspection of Fig. 54 and Plate X will show at a glance that the 

exact average precipitation for the months, seasons or the year is an 

unusual occurrence. As sliown in the discussion of temperatures, the 

arithmetical mean amount is not the most probable amount. The pre- 

14 



196 



THE CLIMATE OF BALTIMORE 



cipitation actually recorded is, in most cases, well above or below the 
normal value. The following figures show the average departure above 
or below the mean value, based on observations for 87 years, and disre- 
garding the sign of the departures : 

AVERAGE DEPARTURES FROM THJ: MEAN PRECIPITATION. 
(In inches and hundredths.) 



Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year 



Plusorrainus 1.07 1.25 l.m 1.33 1.40 1.54 1.64 1.72 1.57 1.33 1.27 1.32 



The most frequent, and hence the most probable departure, differs from 
the figures above representing the average departure, as shown by the 
following table : 

FREQUENCY OF PLUS AND MINUS DEPARTURES FROM THE NORMAL 

MONTHLY PRECIPITATION. 

(In percentage of total frequency.) 





Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 

46 
54 

8 


Dec. 

45 
54 

9 


Year 


Plus departure . . . 
Minus departure.. 


40 
60 
20 


48 
63 
4 


46 
54 

8 


40 
60 
20 


46 
53 


47 

53 

5 


45 
54 
9 


46 
54 

8 


39 
61 
22 


45 
54 
9 


46 
54 

8 















In all months of the year the departures below the normal are in excess 
of those above the normal ; in January, April and September as much as 
20 per cent of the total number of months in 87 years. The average 
monthly difference is nearly 11 per cent. Hence the monthly precipi- 
tation is most likely to be below the normal amount, but the minus de- 
partures are likely to be smaller than the departures above the norma). 

The most probable departures from the normal monthly amounts are 
indicated by the following figures: 



FREQUENCY OF STATED MONTHLY DEPARTURES. 



Departures 
(In inches). 

0—1 

1—2 

2—3 

3 — 4 

4—5 

5—6 

6—7 

7—8 



Plus. 



Minus. 



12.3 ' 


19.5 •• 
10.1 " 
1.4 " 
0.0 " 
0.0 " 
0.0 " 
0.0 " 


5.1 ' 


3.8 ' 


1.8 ' 


1.0 ' 


0.4 ' 


0.2 ' 



44.6 per cent. 



55.1 per cent. 



maryland weather service 197 

Excessive Eains. 

The heaviest rainfall recorded in Maryland occurred at Jewell, in the 
southern portion of Anne Arundel County, on July 27, 1897, when the 
local voluntary observer measured 14.75 inches, all of which fell in the 
course of 18 hours, and most of it in six hours, during a severe thunder- 
storm. 

On September 13, 1904, a storm of marked intensity developed over 
the Atlantic Ocean east of the coast of the South Atlantic states. It 
increased rapidly in intensity during the 14th and with rapid movement 
passed northeastward on the 14th and 15th with its center following the 
coast line. The precipitation in the path of the storm was of unusual 
intensity. The center of the cyclonic system passed just to the east of 
Baltimore during the night of September 14-15 accompanied by de- 
structive winds and torrential rains. The rain began about 8 a. m. and 
continued with but slight interruptions until about 3 a. m. of the 15tli. 
The total fall during these 19 hours was 5.06 inches; between 2 a.m. 
and 4 a. m. of the 14th, .02 inch fell, making a total fall in 24 hours of 
5.08 inches. This is the greatest fall in 24 consecutive hours recorded 
at Baltimore since the establishment of the local office of the TJ. S. 
Weather Bureau in 1871. The greatest rate of fall during this storm 
occurred between 8.25 p. m. and 9.10 p. m., Avhen l.iU inches were re- 
corded in 45 minutes. Some of the more interesting records of tor- 
rential rains or " cloud bursts " mentioned by Professor Henry in his 
report ^ on the rainfall of the United States are cited here : 

"A cloud burst passed over the edge of the little town of Palmetto, 
Nevada, in August, 1890. A rain gauge that was not exposed to the full 
intensity of the storm caught 8.80 inches of water in an hour. At Tri- 
delphia. West Virginia, 6.90 inches fell in 55 minutes on July 19, 1888. 
At Campo, California, 11.50 inches fell within an hour, " and some of 
the fall was lost!" 

'Henry, A. J. Rainfall of the United States. Bull. D.. U. S. Weather 
Bureau, 4to. Wa.sh., 1S97. 



198 



THE CLIMATE OF BALTIMORE 



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MARYLAND WEATHER SERVICE 199 

In his " Climates and Weather of India/" * Mr. H. F. Blanford cites 
the following, among others, as the heaviest rainfalls on record in that 
locality : 

TORRENTIAL RAINFALLS IN INDIA. 

Place Amount in ,,.. 

^'''^®- 34 hours. ^'™®- 

Cherra Poongee f Assam i 40.8 June 14, 1876. 

Purneah (Bengal) 35.0 Sept. 13, 1879. 

Nagina 32.4 Sept. 18, 1880. 

Danipur 30.4 Sept. 18, 1880. 

Rewah (Central India) 30.4 June 16, 1882. 

In connection with the above, Mr. Blanford writes : " These exces- 
sive falls are always the result of cyclonic storms. Not such as are of 
destructive violence, but the long-lived cyclonic storms in which the 
barometer is not greatly depressed, and only recognizable in their true 
character when the barometer readings and the winds are laid down on 

the charts Another noteworthy point is that they have frequently 

occurred in years of partial drought Thus in 1875, although in 

the Punjab it was one of the wettest years on record, the rainfall was very 
deficient in Bengal and all over the southern half of the Peninsula. . . . 
Scarcely less remarkable is the occasional heaviness of the falls even at 
places where the average rainfall is not by any means excessive. That 
upwards of 40 inches in 24 hours should have been recorded at Cherra 
Poongee will perhaps hardly be surprising, but falls nearly as great, from 
30 to 35 inches, in the same interval have occurred on more than one 
occasion on the plains of the Ganges Valley, at places where the average 
for the whole year is not more than from 40 to 65 inches; and even in 
the extremely arid province of Sind as much as 20 inches fell in one day 
in 1866, at Doprbaji, where the annual average is probably less than six 
inches." 

Greatest Kainfall in 24 Hours. 

In Table XLVIII the greatest precipitation in any 24 consecutive 
hours of each month and year from 1871 to 1903 is recorded, together 
with the amount of the fall and the date of occurrence. The greatest 

- Blanford, H. F. Tlip Climates and Weather of India, 12mo, London, 
1889, pp. 77 et seq. 



200 THE CLIMATE OF BALTIMORE 

during the entire period for each month is also graphically shown in 
Fig. 55. Falls equalling or exceeding two inches in 24 hours have 
occurred in all months of the year, but the heaviest have been recorded in 
the summer and early fall months. The greatest fall recorded in the 
table, namely, 4.76 inches, was that of September 6, 1895. During the 
present year (1904), however, this record was broken by the excessive 
rainfall in connection with the severe coast storm of September 14-15, 
when 5.08 inches fell in 17 hours. 

An inspection of Table XLVIII shows that the heaviest precipitation of 
the month may be very small, sometimes falling below half an inch, but 
such instances are comparatively infrequent, especially during the months 
of active plant growth. The following list comprises all months without 
a fall of half an inch or more in 24 hours during the 33 years from 1871 
to 1903 : 

MONTHS WITH A MAXIMUM RAINFALL OF LESS THAN HALF AN INCH IN 

24 HOURS. 

January 1S71, 1872, 1890. 

February 1805. 

March None. 

April 1881, 1898. 

May 1872, 1896. 

June 1901. 

July 1894, 1900. 

August 1876, 1877, 1889, 1894. 

September 1878, 1884. 

October 1874, 1882, 1884, 1892. 

November 1882, 1890, 1903. 

December 1871, 1873, 1874, 1875, 187G, 1889, 1896. 

Fig. 55 shows the extent to wliich the heaviest precipitation in 24 
consecutive hours occurring in each year from 1871 to 1904 has varied 
from year to year. The amounts range from a minimum of 1.47 inches 
in August, 1898, to a maximum of 5.08 inches in September, 1904. The 
tendency to a periodic fluctuation embracing a group of years is more 
marked in this diagram than in those representing the total seasonal or 
annual fall. Especially interesting and instructive, as well as striking, is 
the gradual and steady increase in the intensity of maximum rainfalls 
from the smallest of the entire period of 34 years in 1898 to the greatest 
of the period in 1904. It would seem to be a safe inference from these 



MARYLAND WEATHER SERVICE 



201 



facts that we have arrived at a maximum for this particular periodic 
swing, and that during the following two or three years there will be a 
diminishing intensity of precipitation in individual storms. This view 
finds additional confirmation in the grouping of excessive rainfalls of 
2.50 inches and over as sho\^ai in Fig. 56. There seems to be no fixed 
relation between the annual amount of rainfall and individual rains in 
any given locality. Eegions with a high annual or seasonal precipitation 
do not necessarily have excessive rates of fall for shorter periods. While 
some of the phenomenal rains have occurred in the tropics, where the 
seasonal rainfall is generally greatest, there are many instances where 
record-breaking downpours have occurred in comparatively dry regions. 



H 



Fig. 55. — The Heaviest Precipitation in any 24 Consecutive Hours. 
(E.ypressed in inches anfl fractions of an inch.) 

In Table XLI will be found a record of the annual number of occasions 
on which the rainfall of a 24-hour period equalled certain stated amounts 
under and exceeding one inch. The precipitation records of the past 3-i 
years have been further examined for all days upon which the rainfall 
exceeded 2.50 inches (see Table XLIX). Eainfalls of the latter amount 
may be considered excessive for all but a few limited regions. They are 
not of frequent occurrence in the vicinity of Baltimore; since 1871 
there have been but 42 all told, most of which occurred in the montiis 
from June to September. Their total monthly frequency in 34 years is 
shown by the following figures : 



an. 


Kcl.. 


Mur. 


Apr. 


May 


•luni" 


July 


Aujf. 


Sei)t. 


Oct. 


\ov. 


Dec. 


^'(•al■ 





o 


■> 


1 


1 


a 


9 


3 


9 


li 


1 


:! 


42 



202 



THE CLIMATE OF BALTIMORE 



In Fig. 56 their frequency and intensity are also graphically shown by 
months and years. The manner in which these excessive rainfalls are 
grouped is interesting. There are apparently three groups in the entire 
period of 34 years, of which the j^ears 1876, 1887 and 1901 are the 



JA'. 


18 
■ I 1 


7t 18 
1 1 1 1 


80 18 
1 1 1 1 


85 IJ 
1 1 1 1 


90 IS 
1 1 1 1 


95 19 
1 1 1 1 


00 

1 1 1 1 


Feb. 








1 




1 




MCH 






1 


1 








Apn 








1 








May 








1 








June 






1 1 


ll 








Juiv 




II 


1 


1 II 


1 




1 


AJG 


1 












1 


Sep- 


1 


1 






1 


ll 


1 1 1 


Oct. 


1 1 


II 










1 


Nov. 




1 












Dec. 


-I 1 


nil 


1 1 1 1 


nil 


1 1 1 1 


1 i 1 1 


ilM 



Fig. 56. — Rainfalls Equalling or Exceeding 2.50 Inches in a Day. 

The frequency and seasonal distribution of rainfalls of 2.50 Inches in 24 consecutive 
hours are shown in this diagram. The total amount of the fall is roughly indicated 
by the length of the heavy vertical lines, the shortest representing 2.50 inches. 



central years. These years coincide with considerable exactness with the 
minimum sunspot period of approximately eleven years. Further atten- 
tion will be given in later pages to the relation existing between rainfall 
and this well-known period of solar activity. 



MARYLAND WEATHER SERVICE 



203 



TABLE XLIX. — DATES UPON WHICH PRECIPITATION EQUALLED OR 
EXCEEDED 2.50 INCHES IN 24 HOURS. 



.Januarj-. 


February. 


March. 


April. 


May. 


June. 


u 
o 


1 1 






s 

a 
C 


03 


i^ 1 » 




f 1 © 

S ai 

< n 


Year 
Am't 
Date 


r- 

1880 
1883 

1885 
1SS6 
1900 


a 

< 

2.66 
2.66 
4.47 
3.18 
2.62 


1 






1886 
1896 


2.60 
3.48 


10-11 
5- 6 


1881 
1889 


3.. 51 8-9 
2.71 3-4 


1889 
.... 


3.58 25-26 


1886 2.99 7-8 


11 

26-27 

28 

22-33 

16-17 


July. 


August. 


September. 


October. 


November. 


December. 


1875 
1876 
1880 
1884 

1887 

1889 

1891 
1903 


2.70 1.5-16 
3.14 30 

3.71 20 
3.75 11 
2.77 20-21 

3.63 1- 2 
4.02 30-31 
2.. 59 8 
3.99 13-13 


1873 

1885 
1901 


4.36 
3.35 
3.28 


13-14 
2- 3 
6- 7 


1874 
1876 
1891 
1895 
1899 

1900 
1902 
1904 


3.15 
3.94 
4.00 
4.76 
2. CO 

2.90 
3.61 
3.82 
5.08 


15-16 

16-17 

5- 6 

6 

19-20 

25-26 
1.5-16 
25-26 
14-15 


1873 

187.! 
1877 
187S 
1890 

1902 


3.42 19-20 
2.64 27-28 

2.74 4 

2.75 22-2-3 
3!04 "" 23 

2.79 4- 5 


1877 

.... 


2.85 


23-24 


18T8 
1888 
1901 


2.85 
2.. 56 

2.88 

.... 


10 
16-17 
28-29 



Table XLIX shows all periods of 24 consecutive hours during which rain 
fell to the depth of 2.50 inches or over, from 1871 to 1904. The day and year 
of occurrence are likewise shown, and the total amount which fell within the 
24 hour period. 



1 I- 


1 1 1 1 


MM 


MM 
1 


MM 


J ...,. , , 


I 1 1 r 

1 






1 


1 


II 


1 






1 


1 






II 


1 1 


1 






III 




III 


ml 










1 1 


1 1 


-I 1 


Mil 


MM 


MM 


MM 


<l<< 


Mil 



Fig. 57. — Rainfalls Equalling or Exceeding One Inch per Hour. 

The frequency and seasonal distribution of rainfalls equalling or exceeding one inch 
In an hour are indicated In the above diagram. The exact amount of the rainfall is 
roughly Indicated by the length of the short and heavy vertical lines. The double lines 
indicate the oceurrenro of two such falls on the same month. 



204 



THE CLIMATE OF BALTIMORE 



A class of rainfalls of somewhat greater intensity than those just 
referred to in Table XLIX is shown in Table L, which contains all 
occurring from 1871 to 190-1 in which the rate of fall equals or exceeds 
one inch per hour. These rains have occurred almost entirely in the 
warm months of the year. None are credited to January, February, 



TABLE L.-DATES UPON WHICH PRECIPITATION EQLTALLED OK 
EXCEEDED ONE INCH IN ONE HOUR. 



May. 


June. 


July. 


August. 




-»:> 


® 




.^ 


.4^ 


« 


9 


^ 


+j 


» 


4) 


u 


-w 


e 




« 


e 


6 


eS 


a 


E 


a 


« 


OS 


a 


fc3 




£ 


s 


d 




^ 


< 


H 


o 


>* 


< 


H 


Q 


V* 


< 


H 


r- 


>• 


< 


H 


(5. 


1889 


1.20 


1-0 


20 


1881 


1.00 


0-45 


£0 


1877 


1.28 


0-55 


24 


1873 


1.30 


1- 


10 


1903 1.49 


0-37 


24 




1.16 


1-10 


£0 


1884 


1.40 


1-15 


31 


1875 


1.41 


1-15 


la 






1888 


1.10 


0-35 


33 


1895 


1.05 


1- 


•1 


1887 


1.74 


1-10 


22 






li-91 


1.15 


1- 


4 


1896 


l.EO 


1- 


21 


1888 


1.03 


1- 


8 








1893 


1.S3 


1- 


27 


1897 


1.36 


0-23 


17 


" 


1.13 


1-0 


5 
















1901 


1.77 


0-25 


35 


1890 


1.96 


1-10 


21 
















1903 


2.87 


0-33 


12 


1898 


1.43 


0-25 


1 












*' 


1.00 


0-24 












September 






October. 




:::: 








1899 
1901 


1.59 
1.00 

l.Ofi 


1- U 
0-44 

0-35 


36 
6 


1896 ' 1.00 0-40 


19 


1897 


1.38 


1- 


13 


13 


1899 1.00 0-53 


2.i 


















1903 


1.40 


0-39 


5-6 


1900 i 1.62 1 1- 


1.1 


















" 


1.23 


0-25 


37 


1904 1 1.48 i 0-40 


14 
















' 







Table L is a list of all occurrences of rainfall equalling or exceeding one 
inch in one hour, from 1871 to 1904. The year, month and day of occurrence 
are shown, and also the amount recorded and tlie duration of the excessive 
rate of rainfall; the latter in the column marked "Time," expressed in hours 
and minutes. 



March, April, jSTovember or December. The distribution through the 
season is as follows : 

May 



June 


July 


Aug. 


Stpt. 


Oct. 


Year 


5 


8 


13 


4 


1 


as 



The monthly distribution here indicated associates this class of exces- 
sive rainfalls at once with the thunderstorm. Their frequency and 
intensity, arranged by months and years, are also graphically shown in 
Fig. 57. The grouping referred to above in the discussion of the rain- 
falls of 2.50 inches and over is here also evident, though less clearly. 



IMARYLAND WEATHER SERVICE 



205 



Excessive Eates of Precipitatiox. 
The rate of rainfall, or the quantity which falls per hour, or part of 
an hour, in the case of excessive precipitation, is one of great importance 
in large centers of population, as it involves the engineering problem of 
providing adequate means for carrying oi¥ the surplus water without 
damage to proj^erty or interruption to traffic. Especially is it desirable 
in this connection to know the maximum rate of fall. Hence particular 
pains have been taken to tabulate and chart excessive rainfalls under a 
variety of conditions. To facilitate the study of such practical problems 
in engineering. Table LI has been prepared, showing all the necessary 

TABLE LI.-EXCESSIVE RATES OF RAINFALL IN CUMULATIVE 
FIVE-MINUTE PERIODS. 



59 :>> 



Total 
duration. 



Begin-! End- 
ning. I ing. 



Excessive 
rate. 



Excessive periods in minutes. 



o Begin- 
H ning. 



?n^~ J ®i 6 I 10 , 15 20 I 25 I 30 j 35 



10 



45 



60 



1894. 6 6.6.5p 
S.lOp 
S.lOp 

" 20 8.20p 

8.20p 

1S94.23 ll.ir,!i 
1897.21 1.43p 
" 24 6.2Sp 
1898. Ki 4.08p 
1899.1(5 «.50i) 



T.OOp 
8.45p 
lO.Oop 

9.22p 



7.30p 1 .25 

DN 1.47 

DX L4- 

21st 

9.15a 1.53 

2lst 

9.15a 1.53 10.52P 

12.30p .«) l:.'.00n 

2.35p .72 ].4!tp 

9.15p .85 (i.KJp 

fi.09p 1.20 5.09p 

8.20p .74 7.0.^1) 



1901 . 24 10. 15p DX .02 10.25p 
1902.25 n.3.-)p 6.20p .51 ,'i.37p 
1903.,24 2.80a 4.20a .l-OS^ 2.53a 



7.04p T 1.25 ...... 

8.57p .20 .30 1.35 |.40 ' .. 
10.27p .GO .20 .30 .45 .55 



,15 i.30 .30 

,35 1.40 I .. 



9.64p T .15 
11.06p j.60 .15 



l.>.2'.p .05 .20 .45 .60 .70 

2.00p .01 .57 .67 .70 .71 

7.01p .01 .11 .29 .42 .44 

ri.S'lp .24 .29 .57 ..59 .59 

7.2<ip T .23 .42 .51 .53 



10.40p .01 .11 .;50 .47 .48 
5..52p T .27 .to .45 .47 
3.80a .03 .21 .58 ,.60 .87 



40 



.75 



.45 



56 



.84 



Aver.. 
Great. 



Duration. 

h. m. 
3-45 
12-65 



11.03 
1.63 

1 I 



Duration. 

li. m. 
0-19 
0-37 



13 



28 .40 

.67 .67 



.93 1.33,1.441.611.64 



.66 .67 
,87 .93 



.83 .941.5i;i.e4 
1.321.441.511.64 



60 



80 











January. 








1896.26 


25th 
6.45p 


7.20a 


1.85 

1 


3.30a 


3.53a 


0.70 


.05 


.10 


.35 


.45 


















March. 


1S99. d 8.1.5p 


8.55p 


.34 


8.23p 


8.29p 


.01 


.01 


.26 


.29 


















• 












M.\Y. 





206 



THE CLIMATE OF BALTIMORE 



TABLE LI CONT.— EXCESSIVE RATES OF RAINFALL IN CUMULATIVE 
FIVE-MINUTE PERIODS. 





P 


Total 
duration. 


s 

S3 
1 


Excessive 
rate. 

Begin- End- 
uing, ing. 


■2® 






Excessive 


periods in 


minutes. 




o 


Begin- End- 
ning. ing-. 


5 


10 


16 


20 


25 


30 


35 


40 


45 60 60 


60 



June. 



1894. 

1895. 
1896. 

1896. 
1897. 

iroo. 

1902. 

1902. 
1903. 


13 

24 

27 
8 
16 

21 

4 

25 
14 

7 

26 

6 

8 


4.28p 
4.10P 
3.00P 
4.04P 
4.17P 

4.47p 

4.33p 
3.50p 
4.10p 
2.13p 

6.27p 

8.20p 
3.03p 


5.00p 
6.30p 
7.35p 
6.20P 
7.03p 

5.25p 

5th 
8.18a 
4.07p 
8.10p 
4.55p 

26th 
4.00a 
7th 
7.10a 
3.45p 


0.47 

.70 

.65 

.95 

1.21 

.45 

.82 
.30 
.62 
.88 

1.13 

1.05 
.73 


4.40p 
4.57p 
3.02p 
4.07p 
4.20p 

4.56p 

4.48p 
3.5.3P 
4.20p 
2.13p 

11.25P 

1.46a 
3.18p 


4.54p 
5.08P 
3.08p 
4.36p 
4.40p 

5.14p 

5.05p 
4.03p 
4.40p 
2.43p 

11.50p 

2.04a 
3.35p 


T 

T 
T 
T 

T 

T 

.05 
T 
T 


.33 

.25 
T 

.05 
.33 


.33 
.25 
.25 
.30 
.35 

.25 

.16 
.10 
.04 
.13 

.15 

.10 
.32 

.21 
.35 


.42 
.35 
.30 
.50 
.55 

.36 

.40 
.30 
.10 
.23 

.32 

.24 
.64 

.35 
.55 


.47 
.40 

.65 
.65 

.40 

.49 

.36 
.35 

.30 

.33 

.70 

.46 
.70 


.80 
.70 

.46 

.50 

.52 
.60 

.45 

.40 
.73 

.67 
.80 


.85 

.58 
.75 

.63 

.68 


.90 

.87 
.56 

.78 
i.90 

1 





— 





■■ 


_' 





•; 

V 
V 

V 

V 
V 

V 
V 


ver.. 
reat. 


Duration. 

h. m. 
4-23 
15-45 


.75 
1.21 


Duration. 

b. m. 
0-18 
0-30 





July. 



1894. 
1896. 



1897. 



1898.19 

" 120 

" !28 

1901.25 

1902. 20 



3.35p 

13.30p 

4.30p 

10.23p 
12.25P 

7.25p 

9.15p 
7.45p 
2.10p 
12.25P 

1.45p 
8.25p 
2.17p 
6.00p 
1.27p 



5.00p .44 4.41p 
1.30p 1.0512.21P 
4.5.5p .66 4.33p 
5th 

5.10a .33 10.28p 
12.55p .50 12.26P 



7.43p 


.49 


22nd 




1.50a 


2.05 


8.38p 


.56 


3.25p 


.30 


5.40p 


1.76 



9.19p 
8. lip 
3.17p 
3.28p 



1903.12 IS.Olp 
" I ", 5.40p 
" 30 4.10p 



4.50p 
12.46p 
4.52p 

10.32p 
12.43p 



r.28p ; 7.36p 



4.25p 1.01 2.0"p 

I)N .51 8.30p 

3.15p .88i 2.23p 

7.00p 1.95 6.00p 

1.65p .56 1.29p 

l.lOp 2.87 12.04p 

7.60p 1.02 6.20p 

6.10p 1.04 4.33p 



Aver.. , 
Great. 



Duration. 

h. m. 
1-66 
6-47 



1.00 

p.87 



10.39p 
8.24p 
2.23p 
3.48p 

2.35p 
8.44i> 
2.43p 
6.3.5P 
1.42p 

13.37p 
6.40p 
4.65P 



Duration. 

h. m. 
0-21 
1-20 



.27 
.20 



.40 



T .25 i.45 



T .25 1.35 

T .30 .45 

,03 .25 .26 

,38 .26 .6b 



.13 .36 

.15 .34 

.3? .66 

.31 .80 

.31 .50 



.45 



.50 



.99 .99 



991.04 



1.05 



.60 



1.17 



.65 



.81^ .91 



.65 .70 



.95 



.58 

.50 .. 
.79j .86; 
1.181.541.771.851.911.94 

.55 .56 ..':.... .. 



75 .86 



1.20 



.33 .98 1.722.23i2.52!2.693.87 
.34 ..54 .61 .71 .72 .. .. 
.03 .18 .64 .9311.00, .. .. 



.03 .25 :.50 
1.38 1.37 .98 



.74 .88 



1.32 



1.732.232.622.692.87 



1.63 



1.23 .94 .951.201. 
1.9411.041 .9511.201. 



MARYLAND WEATHER SERVICE 



2or 



TABLE LI CONT. 



-EXCESSIVE RATES OP RAINFALL IN CUMULATIVE 
FIVE-MINUTE PERIODS. 







Total 
duration. 


6 




Excessive 
rate. 




1- 


Excessive periods in minutes. 




Begin- 
ning. 


End- 
ing. 


Begin- End- 
uing, ing. 


5 


10 


15 


20 


35 


30 


35 


40 


45 


50 


60 


80 





August. 



1895 


31 I 4.18P 


5.00p 


.78 

























.78 








1 


1897 


9-10 11.30P 


5.10a 


l.«0 


1.30a 


2.20a 


.40 


.03 


.07 


.13 


.19 


.25 


.43 


.52 


.62 


.77 


.83 








" 


23 11.0:3a 


11.5-<a 


.74 


11.33a 


11.55a 


.02 


.14 


.24 


.48 


.71 




















" 


25-61 9.50p 


l.lua 


.83 


11.26p 


11.37p 


.24 


.23 


.36 


.37 




















V 


1898 


1 1 3.10p 


4.00p 


1.44 


3. lop 


3.40p 


T 


.46 


.78 


1.16 


1..36 


1.43 
















V 


1893 


4-5 11 40p 


DX 


1.47 


11.50p 


13.25a 


.03 


.05 


.29 


.48 


.78 


.94 


1.03 


1.09 


.13 


1.17 










1899 


13 


2.1.5p 


2.40p 


.53 


2.18p 


3.33p 


T 


.23 


.43 


.52 


.53 


















}■' 


" 


21 


7.29p 


9.10p 


.78 


8.10p 


8.24p 


.35 


.21 


.32 


.38 


.39 


















y 


" 


26 


8.10p 


DN 


1.63 


8.15p 


O.OOp 


T 


.04 


.35 


.49 


.60 


.83 


1.00 


1.39 


.51 


l.SH 








) 


1900 


16 


2.20a 


3.00a 


.36 


3.21a 


2.30a 


T 


.18 


.32 


.34 




















1 


1900 


21 


DN 


7.30p 


1.48 


5.47a 


fi.20a 


.25 


.10 


.37 


.58 


.72 


.85 


.91 


.93 


.94 


.97 


1.00 


1.06 






1901 


6 DN 


r2.25p 


ZA>b 


11.15a 


11.45a 


1.08 


.05 


.09 


..33 


..5"? 


.77 


.9'7 
















" 


12 


10.37a 


2.05P 


1.43 


11.4.5a 


12.20p 


.09 


.OP. 


.10 


..31 


.43 


.78 


1.00 


1.10 












V 


" 


27 


6.35p 


7.30p 


.81 


6.40p 


6.55p 


.02 


.:50 


.62 


.76 


.78 


















V 


1902 


3 


9.30p 


9.50p 


.tit) 


9.33p 


9.43p 


T 


.m 


.64 


.66 




















]' 


1902 


5-6 11.40p 


D.V 


1.43 


11.46p 


16.25a 


.01 


.10 


.30 


.49 


.56 


.64 


.89 


1.34 


1.40 










Y 


" 


11 :5.11p 


4. CM) 


.55 3.46p 


3.58p 


.14 


.17 


.35 


.41 




















y' 


" 


27 4.29P 


5.30p 


1.2: 


4.3:p 


5.02p 


T 


.10 


.47 


.89 


1.12 


1 .-i'i 


1.24 














y 


1903 


26 9.45a 


10.3op 


.52 


9.53p 


lO.llp 


T 


.11 


.37 


.49 


.63 




















*^ 


U\nA5p 


8.05a 


1.53 


4.30a 


4.41a 


.98 


.03 


.37 


.49 


.60 






















Duration. 




Duration. 
































h. m. 




h. m. 






























Aver... 


2-48 


1.09 


0-24 


.18 


.15 


.35 


..51 


.65 


.85 


1.2;^ 


1.35 


1.13 


1.05 


.91 


1.06 






Great.. 


12-50 


2.05 


0-50 


1.08 


.46 


.78 


1.16 


1.36 


1.43 


1.24 


1.39 


1.51 


1.56 


1.00 


1.06 







September.* 



1894 


8 


1.27p 


1896 


3 


2.46p 


'* 


19 


4.66p 


1897 


16-7 


10.38p 


1898 


26 


6.20p 


1899 


2 


5.30a 




25 


5.20p 


1900 


15 


1.20p 



1.55p 
3.431 



1.51p 
3.57p 



.i.i.ip ..in ^.»op a.oip 

5.55pl.05 5.1.5p 5.39p 

1.1.5a .71 li).45p 10.56p 

6.45p .35 C.20p 6.35p 



8.25a 
8.05p 
DN 



2.15 
3.61 



Aver. . . 
Great.. 



Duration. 

h. m. 
1-35 
3-66 



1.16 
13.61 



C.20p 

5.54a 
5.43p 
10.45P 



6.14a 
6.30p 
11.30p 



Duration. 

h. m. 
0-33 
0-45 



T 

T 
T 

.03 

.11 

1.87 


.30 
.10 
.30 
.30 
.14 

.13 
.09 
.18 


.50 
..30 
.65 
.39 
.31 

.36 
.31 
.60 


..35 
.90 
.43 
.35 

.39 

.38 
.86 


LOO 

.48 

.40 

1.03 


I'Oo 

.50 

.53 

1.18 


;75 

1.33 


!87 
1.46 


ioo 

1.49 


!92 
1.55 


L53 


1.61 


" 


.35 

1.87 


.17 
.30 


.40 
.65 


.51 
.90 


7'' 


.83 
1.18 


1.04 
1.32 


1.16 
1.46 


1.20 
1.49 


1.24 
1.55 


1.53 
1.53 


1.61 
1.61 





October. 



1897. 12 
1900. 8 
19a3. 8 


5.35a 10.05a 

12.45p 1.35p 

9.10a 2.06p 


1.93 6.59a 8.00a 

.40 12.54p 1.05p 

1.07 9.11a 9.16a 


.14 
T 
T 


.17 
.20 
.25 


.36 
.36 


.31 

.38 


.48 

.48 
.48 


.53 

.53 
.53 


.67 

.67 
.67 


.83 

.82 
.82 


.93 

.92 
.93 


1.05 

1.05 
1.06 


1.21 
1.21 


1.35 

1.35 
1.35 


1.45 

1.46 
1.45 


V 


Aver.. 
Great. 


Duration. 

h. m. 
3-25 
4 55 


1.13 
19.2 


Duration. 

h. m. 
0-28 
1-01 


.06 
.14 


.21 
.26 


.31 

.38 


.34 

.33 





• Sept. 6, 1895, rain began 2.10 a. m., ended 6.45 p. m.. amount 4.76 inches. The gauge was 
not working, hence onlj' stick measurements were possible, and it is estimated that 1.06 
inches rain fell between 4.00 and 6.00 p. m. 



208 



THE CLIMATE OF BALTIMORE 



TABLE LI CONT.— EXCESSIVE RATES OF RAINFALL IX CUMULATIVE 
FIVE-MINUTE PERIODS. 



u 


P 


Total a 
duration. es 


|2 
Excessive o ^ 
rate. || 


Excessive periods in minutes 










Begin- 1 End- | 
ning. ing. p 


Begin- 
ning. 


End- la© 
ing- i*^ 


5 10 j 15 


20 


25 


30 35 40 1 45 50 


60 80 




November. 


1896 '^^'' '' ^^^ ' ^ ^^" i Q.ti 3 nnn 


4.00p .03 




_. i . _. 








.93 










1 ! 


i 











Table LI contains a complete list of all occurrences of excessive rainfall 
from 1894 to 1903, arranged according to months and years. The scale 
of excessive rates of precipitation adopted by the U. S. Weather Bureau, 
and employed in the above classification, is shown in the text. The above 
table shows the year, month and day of occurrence of the excessive rainfall, 
the time of the beginning and ending of the entire rain period, and also 
of the period of excessive rate of fall, the total amount of fall, and the cumu- 
lative amounts which fell in the successive 5-minute periods. The symbol 
{ \/) in the last column indicates the occurence of a thunderstorm in con- 
nection with the rainfall; •.• indicates the occurrence of a thunderstorm on 
the same day at some other hour. 

details of every instance of excessive rate at Baltimore occurrintj since 
1893, at which time the automatically recording raingage was installed 
at the Baltimore office. 

The arrangement is by months and years and shows the following 
facts: the year, the day and hour of occurrence, the total amount of the 
rainfall during the entire progress of the storm, the time of beginning 
and ending of the excessive rate of fall, the amount of rainfall before 

TABLE LII.-SDMMARY OF EXCESSIVE RATES OF PRECIPITATION IN 
CUMULATIVE FIVE-MINUTE PERIODS. 



E 


xcessive 
rains. 


o® 

V 






Excessive periods. 






d 


Dura- 
tion. 

Am'ts. 


5 10 


15 


20 25 30 35 40 45 

1 


50 


1 
00 ' 



Averages. 



January 

March 

May 

June 

July 

August 

September. 
October — 
November . 

Average — 





h. m. 


in. 


min. 


























1 


12-35 


1.35 


22 


.05 


.10 


.25 


.45 


















1 


0-40 


.34 


6 


.01 


.26 


.29 




















la 


3-45 


1.(12 


19 


.23 


.40 


.51 


..56 


.6'; 


.83 


.94 


1.51 


1..T4 








i:{ 


4-23 


.75 


18 


.21 


.35 


.46 


.."17 


.fit- 


.78 














l.H 


1-56 


i.on 


21 


.35 


.m 


.74 


.88 


1.2S 


1.43 


1.62 


1.23 


.94 


.95 


1.20 


1. 


30 


2-48 


1.09 


24 


.15 


.35 


..51 


.65 


.85 


1 23 


1.25 


1.13 


1.05 


.91 


1.06 




8 


1-35 


1.16 


92 


.27 


.40 


.51 


.72 


.82 


1.04 


1.16 


1.20 


124 


l.5h 


1.61 




3 


3-25 


1.13 


36 


.21 


.31 


.34 


.48 


.52 


.67 


.82 


.93 


1.05 


1.21 


1.35 


1 


1 


3-10 


.98 
























.93 




78 


3-49 


.98 


20 


.16 


.38 


.45 


.62 


.79 


1.00 


1.16 


1.20 


1.16 


1.16 


1.23 


1 



80 



45 



:marylaxd weather service 



209 



TABLE LII CONT.- 



-SUMMARY OF EXCESSIVE RATES OF PRECIPITATION IN 
CUMULATIVE FIVE-MINUTE PERIODS. 





Excessive 
rains. 








Excessive periods. 


50 


60 






Dura- 
tion. 

Am'ts. 


Si 


5 10 


15 


20 


25 30 35 40 45 


SO 



Greatest. 



Ih. m. I in. |h. m. 

January 12-35 1.35 0-22 

March 0-40 .34 0-6 

May 11-55 1.63 0-37 

June 15-45 1.21 0-30 

July 6-47 2.87 1-20 

August 12-50 2.05 0-59 

September 2-55 3.61 0-45 

October 4-55 1.92 1-01 

November 3-10 .98 



.05 
.01 
.57 
.35 
.37 
.46 
.30 
.25 



Greatest. 
Year 



Month 
Day.... 



Hour of beginning.. . 



15-46 



2.87 1-20 



.57 .98 



.46 



1.7" 2 
L161 

.901 

.36 



,80 
.232 
,36:1 
.021 



1.22 



1.722.2312.522 



1.44 1.51 1.64 



1.39 
1.46 

82 



s : 
i "^ 

1.49p 



1903 
July 

12 

12.04 p. m. 



1.94 

1.51 

1.49 

.92 



1.94 
1901 



26 



1.04 
1.56 
1.55 
1.05 



1.56 
1899 



.951.20 
1.001.06 
1.5S!1.61 
1.211.35 

.. I .93 

1.581.61 

1900 

Sept. 

15 



1.80 
l!45 



1.80 
1896 



n 



6.03p H.15p 10.45p 9.19p 



Table LII contains a summary of facts contained in Table LI. 

the excessive rate began, and the amounts recorded in cumulative five- 
minute periods during the continuance of the excessive rate. When the 
rain fell in connection with a thunderstorm this fact is also noted. It 
is a matter of record that in almost every instance these excessive rain- 
falls occur in connection with thunderstorms. In Table LII there is a 
summary of the preceding table, containing the average amounts recorded 
during cumulative 5-minute periods, and also the greatest amount for 
the same intervals during the entire period of ten years. The average 
and maximum rates of precipitation are also shown graphically for the 
entire year in Fig. 58. 

Excessive rates of rainfall as defined by the U. S. Weather Bureau, and 
employed in its published records, are indicated in the following table : 

TABLE OF RATES CONSIDERED EXCESSIVE. 



Amount. 


Time. 


0.23 


Inch 


In 5 


minutes. 


0.30 




'• 10 


•• 


0.35 




" 15 


•' 


0.40 




•• liO 


•• 


0.45 




'• 25 


" 


0.50 




•' 30 


•• 


0.55 




'• 35 


•• 



Amount. 


Time. 


0.60 inch in 


40 


rtJinutes. 


0.G5 " 


45 




0.70 " 


50 




0.75 " 


60 




0.80 " 


80 




0.90 " 


100 




1.00 " 


120 





210 



THE CLIMATE OF BALTIMORE 



An inspection of Table LI will show that excessive rates of fall as 
defined above are confined almost entirelv to the warm months of the 





10 


20 







40 


50 60 80 100 MiN. 














^ 


















\ 














Inches 
2.50 








/ 


/ 




\ 


















/ 


// 






\ 














2.O0 






/ 








\ 


















■/ 


/ 








1 


I 












1-50 




// 


/ 










\ 






) 




y 


l\ 


7 












\ 






A 


y^ 


X 




/ 


/ 








X 


^^c 


■~-«.\__ 




^.^ 








r.oo 


\ 




/ 


7 


r 




y 












y' 






^ 


0.50 


/ 




^y 


/ 






D 


--- 












/ 


^>^ 


^ 

























^ 



























Fig. 58a. — Excessive Rates of Rainfall. 

A. The curve A represents the maximum rate of precipitation attained in any con- 
secutive 5, 10, 15, etc., minutes during the heavy rainstorm of July 12, 1903. 

B. Represents the rate during the first 5, 10, 15, etc., minutes after the beginning of 
the excessive precipitation of the storm of July 12, 1903. 

C. Represents the average rate in 78 cases of excessive rates of fall. 

D. Represents the lower limits of rates considered excessive by the U. S. Weather 
Bureau. 

year. Of a total of 84 instances recorded in eleven years, the seasonal 
distribution is as follows : 

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year 
10 1 1.3 14 19 21 9 5 1 S4 

Over 90 per cent of all occurrences are credited to the months of May 
to September. None have been recorded in February, April and Decem- 



MARYLAXD WEATHER SERVICE 



21] 



ber, while January, March and Xovember have but one each. Over 90 
per cent of all excessive rains have occurred in connection with thunder- 
storms in the immediate vicinity of the station of observation. The 
average rate of fall based upon all excessive rains during a period of 10 
years is indicated by the following figures for the five-minute cumulative 
intervals from 5 to 80 minutes. 



E 

F ^*«s,„^ 



10 20 30 40 50 60 80 100 MiN, 

Fig. 58b. — Excessive Rates of Rainfall. 

E. Represents the rate of precipitation per hour during the heaviest 5, 10, 15, etc., 
minutes of rainfall in the storm of July 12, 1903. 

F. Represents the average rate per hour for 78 cases of excessive rates of fall. 

AVERAGE AMOUNTS OF EXCESSIVE RAINFALL. 
(In inches and hundredths.) 

Number of minutes from beginning of excessive rate. 
5 10 15 20 25 30 35 40 45 50 60 80 

Amount of fall Ki .33 .45 .02 .79 1.00 I.IG 1.20 1.16 1.16 1.23 1.62 

Selecting from Table Lll tlic maximum rate of fall for each of tbe 
periods indicated, irrespective of the month in which it occurred, we 
have the figures below, whicli represent the maximum observed rates for 
the first 5, 10, 15, etc., minutes after the beginning of the excessive 
rate of fall. 
15 



212 



THE CLIMATE OF BALTIMORE 



MAXIMUM RATES OF RAINFALL. 



Minutes. 



During first ; 6 

Amount of fall i .57 

Month May 

Tear l 1897 [ 



15 
1.72 



20 I 25 
2.23 2.53 



30 



36 

2.87 



July 
1903 



40 
1.94 
July 
1901 



45 
1.56 
Aug. 
1899 



50 60 80 
1.58 1.61 , 1.80 



September 
1900 



July 
1896 



Amount. 


Rate per hour. 


0.80 


Inches. 


9.60 


inches. 


1.3.5 




8.10 


" 


1.92 


•' 


7.68 


" 


2.32 


" 


6.96 


" 


2.58 


" 


6.19 


" 


2.75 


" 


5.50 


" 


2.87 


" 


4.31 


" 


2.87 


" 


3.44 


" 


2.87 




2.87 


" 


2. ST 


•• 


1.44 


" 



The destructive storm of July 12, 1903, which swept over Baltimore 
and vicinity, was accompanied by a downpour, the rate of which was 
probably never equaled in the annals of Baltimore weather. The rate 
of precipitation as measured at the local office of the U. S. Weather 
Bureau is indicated by the following figures, and graphically shown in 
Fig. 58: 

RATE OF RAINFALL IX STORM OF JULY 12, 1903. 
For any period of 

5 consecutive minutes 0.80 inches 

10 " •' 

15 '■ ■• 

20 " " 

25 " '• 

30 " " 

40 " " 

50 " " 

60 " " 

120 " " 

Table LII, Maximum Cumulative 5-^linute Periods, shows a different 
value, namely, the precipitation of the first period of 5, 10, 15, 20, etc., 
minutes after the he ginning of the excessive rate of fall. The storm of 
July 12, 1903, showed a maximum rate for every period from 5 minutes 
to one hour. 

A rough calculation has been made of the amount of water which fell 
within the limits of Baltimore City during the storm of July 12, 1903. 
It was probably one of the most severe storms ever witnessed in the city. 
While the area of destruction in this type- of storm is fortunately of 
extremely limited extent, it may be safely assumed that practically all 
of the city had a rainfall approximately equalling the amount recorded 
by the official gage. The central path of the storm was about a mile dis- 
tant from the office of the Weather Bureau ; nearer the center of the path 
the rainfall was probably heavier than in portions of the city beyond its 
area of destructive winds. Hence we may assume the officially recorded 



MARYLAND WEATHER SERVICE 



213 



fall to be a safe estimate of the average for the entire city. Assuming 
the area of Baltimore City to be 31.15 square miles, we may readily 
compute the amount of water which fell during the progress of the storm : 



WEIGHT OF RAINFALL IX STORM OF JULY 12, 1903. 
(Within the limits of Baltimore City.) 

Depth Gallons 

per acre. 



o( fall. 



During the first 

5 minutes 33 inch. 

10 ■• 98 " 

15 •• 1.72 " 

30 " 2.69 " 

35 " 2.87 " 

During the heaviest 

5-minute fall 0.80 " 



7,466 
22,172 
38,913 
60,859 
64,931 

18,099 



Tons within 
City Limits. 

745.428 
2,213,696 
3,885,162 
6,076,369 
6,482,967 

1,807,098 



Frequency of Consecutive Days with Eain or Snow. 
In a large percentage of instances when rain or snow falls, it is con- 
fined to a single day. The exact percentage depends largely upon what 
is regarded as a day with rain. Including a light sprinkling rain, or a 
flurry of snow, in our calculations we find that in the past 33 years, the 
precipitation was confined to one day in 36 per cent of the total number 
of days with rain or snow. Considering only measurable quantities of 
precipitation (0.01 inch or more) the percentage is increased to 45. In 
28 per cent of all cases the rain or snow extended into two consecutive 
days, and in 16 per cent, three days, when we take account of " traces." 
Counting only appreciable quantities, the percentages are respectively 
31 and 13. The instances of precipitation on more than three days 
decrease rapidly with each successive day added. In the table below, 
the frequency, excluding " traces " of rain or snow, is shown for each 
month and for the entire year to the maximum period, namely, 14 days. 
frequency of consecutive days with rain or snow. 

(Expressed as percentages of the total number of cases of appreciable rainfall in 

33 years.) 



Days. 



Jan. 


Feb. 


42 


41 


3.5 


35 


13 


13 


7 


8 


O 


1 


2 


1 




1 


1 






"i 



Mar. Apr. 



88 
33 
18 
6 
4 
1 
1 
1 



May June ' July Aug. Sept. Oct. Nov. ' Dec. Year 



10 



45 

31 

13 
6 
3 
1 

0.4 
0.1 
0.2 
0.1 
0.0 


0.1 
0.0 



214 



THE CLIMATE OF BALTIMORE 



Inchiding days with '' traces " of precipitation, the annual frequency 
is indicated by the following figures : 

ANNUAL FREQUENCY OF COXSECUTiyE DAYS WITH RAIN OR SNOW. 
(Including "traces.") 



Number of days. . . 1 


2 3 4 1 5 


6 1 7 
2.5 0.9 


8 1 9 ! 10 


11 


13 13 14 


Percentage of pos- 
sible occurrence. 36 


I 
28 16 9 5 


0.9 


0.4 0.2 


0.3 


0.1 0.0 0.1 



In the past 33 years there has been no single instance of rain or snow 
on more than 1-i consecutiye days, and but few in which the rain occurred 
on more than six consecutiye days. There haye been longer periods of 
" unsettled weather," but these will be found, upon inyestigation, to 
haye been interrupted by one or more days without eyen a " trace " of 
rain. (See Table LIV, Wet Spells.) 

Dry Spells. 

While the rainfall is quite eyenly distributed throughout the year, 
there are at times periods of many days without appreciable precipitation, 
or at least of amounts insufficient for the requirements of plant growth. 
During some seasons of the year this scarcity of rain is of comparatiyely 
little importance; during periods of critical crop growth, howoyer, it 
becomes a question of serio'us moment. What constitutes a dry spell is 
largely a matter of arbitrary judgment. It is not alone the number of 
days without rain ; pre-existing conditions enter largely into the problem, 
as well as the stage of deyelopment of yegetation. In the classification 
of dry spells noted in Table LIII, the selection includes, as a lower 
limit, all periods exceeding two weeks during which the precipitation was 
less than one-tenth of an inch. While this limit is a purely arbitrary 
one, a period of 14 days with either no rain or less than one-tenth of an 
inch is a long interyal considering the ayerage frequency of rains in this 
vicinity. For periods longer than two weeks, a proportionately larger 
quantity of rainfall was allowed, keeping in yiew the desire to select only 
such dry spells as fell yery far short of the normal quantity of rain for 
the dry interyal. In the course of 33 3^ears there haye been 58 periods 
answering the requirements of the definition, ayeraging a little less than 
two per year. A drought of this character cannot be regarded as seyere, 
but it may be followed by considerable loss to the farmer or trucker 



:marylaxd weather service 



215 



during certain critical periods. Ordinarily there are from ten to twelve 
days per month with rain to the extent of .01 inch, giving a ratio of one 
day with rain to two without. These are not uniformly distributed 
through the month, but are very likely to occur in groups of two or three 
days. 

The total monthly frequency of periods of this class, together with the 
average, the maximum and the minimum number of days included, is 
shown in the following tabular statement : 

DRY SPELLS IN 33 YEARS. 
I With less than one-tenth of an inch of rainfall in two weeks or more.) 





Jan. 


Feb. 


Mar. 


Apr. 


May June 


July Aug.' Sept. 


Oct. 


Nov. Dec. 


Year 


Total frequency 


" 


3 


3 


3 


7 


3 


3 3 


6 


12 


5 8 


58 


Max. duration.. 

Min. 

Aver. 


21 
16 
18 


31 
18 

22 


31 
19 
26 


22 
U 
19 


36 
14 
21 


29 
20 
25 


25 
23 


45 
23 
30 


34 
17 
25 


51 
15 

27 


48 
14 
33 


51 
20 
29 


51 days 
14 " 
25 " 


Aver. rainfaU .. 


.05 


.17 


.12 


.15 


.11 


.21 


.10 


.28 


.12 


.20 


.17 


.30 


.18 inch 



J.. 


18 
1 1 1 1 


75 18 
1 1 1 1 


8C 18( 

— r-i , 1 


5 18 
1 1 1 1 


)0 18 

— 1 — 1 — i— r 

1 


)5 19C 

-Till 




1 1 ■] I 

1 


Fts 




1 


1 








1 


MC" 










ll 




1 


APn 


1 


1 




1 








»,. 


1 1 


1 




1 1 






1 


JUVt 








1 




1 




Juit 


1 










1 




Ayr, 






1 


1 






1 


St>.T 


1 


1 


1 


1 1 




1 




Oct 


1 


ll 


1 1 




1 


III 1 1 


Nov 


1 








1 


1 II 1 


Otc 


1 , , , 


II, , 


\l 


1 1 : 1 


III 


ill 



Fiu. 59. — Dry Periods. 
The diagram shows the frefjuency of occurrence and the seasonal distribution of all 
periods of two weeks or longer with a precipitation of one-tenth of an Inch or less from 
1871 to 1004. The length of the period Is roughly shown by the length of the heavy 
vertical lines. The exact duration of the period is shown in Table LIII. 



216 



THE CLIMATE OF BALTIMORE 



TABLE LIII— DRY SPELLS. 



Tear. 



!2;b 



1871. 
1872. 
1873. 

1874. 
1875. 

1876. 

1877. 
1878. 
1879. 
1880. 

1881. 
1882. 
1883 

1884. 
1885. 

1886. 
1887. 
1888. 
1889. 
1890. 



31 



9 
.23 
18 



0.25 
23 



SI 
.08 
21 

17 
.01 
14 



.03 

ir 



.29 
29 



.02 
19 



35 
.04 
16 



SO 
.18 
14 



i 
.23 

28 



.05 
15 



.3: 
20 



.24 
22 



U 
.05 
20 



5 
.17 
34 



9 
.16 
33 



SO 
.17 
19 



9 
.04 
17 



6 
.10 
23 

20 
.30 
36 

2i 
.59 
38 



.05 
19 



21 
.39 
51 



.27 
36 



16 

.10 
21 



17 
.03 
20 

29 
.06 
^3 



9 
.66 
41 
18 
.31 
22 



is 

1 
3 
1 

2 
3 
1 
2 
1 



MARYLAND WEATHER SERVICE 



217 



TABLE LIII CONT.— DRY SPELLS. 



Year. 



189L 
1892. 
1893. 
1894 

1895. 

1896. 
1897. 

1898. 
1899. 
1900. 



1901 J. 

1902 ■! 

1903 < 

No. of dry spells. 



2S 
.04 
16 



20 
.06 
31 



20 
.12 
31 
1 
.16 
19 



.09 
27 



.36 
36 



18 
.07 
23 



.26 
38 



.13 
20 



.10 
16 



7 
.10 
20 



12 



3 
.26 
22 



SS 

.0 

14 



.13 
39 



.27 
29 



S7 
.09 
23 



51 



^i; 



58 



Table LIII contains a list of all of the more pronounced dry periods 
occurring near Baltimore from 1871 to 1903. No strict definition of a dry 
spell has been adhered to in the selection of these periods, but the 
table contains all periods exceeding two weeks during which the precipita- 
tion amounted to less than one-tenth of an inch. The length of the dry 
spell in days is Indicated by the figures in heavy face type, the date of ending 
by italic figures, and the total precipitation during the period by roman 
figures. 

These dry spells are most frequent in the month of October, and hence 
after the harvest season. Their occurrence in May has been compara- 
tively frequent. Coming at a time when soil moisture is a matter of the 
highest importance, the dry spells of this period are serious matters. The 



218 



THE CLIMATE OF BALTIMORE 



seasonal distribution of these periods of deficient moisture is shown 
graphically in Fig. 59 for each year from 1871 to 1903. 

In another classification of dry spells, all periods of ten or more con- 
secutive days were included in which precipitation was less than .01 



— r -r- 1 1 


1 I I 1 

1 


1 1 I 1 


1 1 1 1 


1 I 1 1 

1 


1 1 1 I 

1 


T-r 1 1 

1 




1 








1 i 


II 


1 






1 






1 


1 


II 




II 


1 


II 


1 


II 


1 




1 






1 


1 


1 1 


1 




1 


1 1 






h 




ii 




11 1 


1 


1 1 




1 


1 




1 


II 1 


1 1 


II 


III 


II 


III 


1 1 I 


1. 


1 il 


1 




iliiii 


II 


1 

1 


il 


II 1 


III 


II 1 




1 


1 1 






liilll 


1 , . 


,1,, 


1 1 1 1 


1 

l< ,1, 


1 1 1 1 


1 1 1 1 1 


1 1 1 1 



Fig. 60.— Dry Periods. 

The diagram shows the frequency of occurrence and the seasonal distribution of 
periods ^ith less than one-hundredth of an inch of precipitation in ten days or more 
from 1871 to 1004. The relative length of the period is approximately shown by the 
length of the heavy black vertical lines. 

inch. The total number of such periods, with the average and greatest 
duration, is shown in the following figures: 

DRY SPELLS IN 33 YEARS. 
(With less than one-hundredth of an inch of rain.) 





a 

•-s 

4 

14 
18 


fa 

6 

13 
14 


s 

3 
11 
11 


< 

9 
12 
14 


6 
14 
21 


a 

11 
12 


<-> 


1 


4J 
0. 

o 


O 

O 


> 

o 




Year 


Total number 


8 

12 
18 


8 
11 
12 


17 
12 

22 


17 
13 
19 


12 
14 
29 


5 
14 
17 


103 
13 days 

og 




Greatest du ration 







:\rARYLAXD WEATHER SERVICE 219 

The above table reveals the interesting fact that the longest period 
experienced by Baltimoreans in 34 years without rain was 29 days. This 
occurred in Xoveniber, 1874. September and October share the distinc- 
tion of having 17 periods each of this class of dry spells out of a total in 
34 years of 102. The average duration of such periods is only 13 days. 
These facts are not in accordance with popular impressions. Scarcely a 
summer passes without some reference to periods of five or six weeks 
'' without a drop of rain." These statements, upon investigation, are 
generally reduced to " no rain of consequence." The dry spells of this 
class are graphically shown in Fig. 60. There seems to be no apparent 
periodic grouping of these periods of deficient rainfall, either in Fig. 59 
or in Fig. 60. The following list comprises seasons with a marked defi- 
ciency in rainfall : 

SEASONS WITH DEFICIENT rRECiriTATION. 

(Departures below the normal in inches and liiuiciredths.) 

Winter. Spring. Summer. Autumn. 

1829-30. .. .4.17 1822 5.11 1844.... 4. o4 1819.... 4. 57 

18G4-65 4.70 1827 4.12 1849.... 4. 14 1825.... 4. 79 

1870-71 6.00 1845 4.46 1864 6.66 1842 4.22 

1871-72 5.73 1847 6.03 1869 7.(19 1863 4.32 

1900U1. . . .4.80 1855 5.91 1870 4.85 1870 4.33 

1856 4.87 1893 6.69 1879 5.06 

1866.... 5. 85 1894 6.21 1884 5.23 

1869 4.49 1903 4.56 

1900 4.66 

Wet Spells. 
While rain or snow storms do not usually exceed two or three days in 
duration, there are frequently periods of much more extended rainy or 
unsettled conditions. The more conspicuous "' wet spells " occurring since 
1871 are emiiiu'rated in Table LIV, which contains a list of all jioriods 
of 10 days or less during which the rainfall or snowfall was equal to or 
exceeded the mean monthly amount. Longer periods were included when 
the precipitation was proportionately excessive. The last day of the wet 
spell and the duration in days are indicated in the table, together with 
the total amount of the precipitation. Such periods have been recorded 
on an average of less than two times per year, or, more accurately, 51 
times during 33 years; the limits of variability are and 5. They occur 
in all months of the year, with a decided preponderance, however, in the 



220 



THE CLIMATE OF BALTIMORE 



warm months of July, August and September. Their frequency of 
occurrence and seasonal distribution are shown graphically in Fig. 61. 

One of . the most remarkable periods of unsettled weather was that 
accompanying the northeast storm of April 19-25, 1901. Rain began 
early in the morning of the 19th and continued during the greater part of 



I I 1 1 


1 1 1 1 


1 1 1 1 
1 


1 1 1 1 


1 1 1 1 


1 1 1 1 


1 1 1 1 






1 






1 






1 


1 


1 








1 






1 








1 


1 






1 1 








1 


1 


1 






1 






1 


h . 


1 


1 


1 


. ■ 1 . 


1 


II 




1 




1 


h 


1 


1 


■ 


1 


1 1 


I 


1 1 










1 1 




1 


1 


1 








1 1 1 1 


1 ii 


1 I 1 1 


1 1 1 1 


1 1 1 1 


1 1 1 1 


1 1 1 1 _ 



Fig. 61.— Wet Periods. 

The diagram shows the frequency and seasonal distrilnition of periods of ten days or 
less in which the average monlhJij amount of precipitation was recorded. The amount 
recorded is roughly indicated by the length of the heavy vertical lines ; the exact amounts 
and the length of the periods are shown in Table LIV. 

seven days, or 162 hours between the beginning and ending of precipi- 
tation. The rainfall was not continuous, however, scattered showers 
falling on April 21, 22, 23 and 25. The total amount of rainfall recorded 
was 2.03 inches, not a large amount considering the great duration of 
the storm. Another noteworthy rainstorm was that of May 16-26, 1894. 
During this period of eleven days the rainfall was scattered and at times 



MARYLAND WEATHEPx SERVICE 



221 



heavy. The actual duration of precipitation was ahout 63 hours, 
total amount recorded durinij the entire storm was ■i.45 inches. 



The 



TA.BLE LIV.— WET SPELLS. 



"" 5 

1873 } 

i 

1873 \ 

1874 -j 

1875 -j 

187fi -j 

1877 -j 

1878 ■/ 

1879 

1880 

1881 \ 

1882 j 

1883 -j 

1884 - 

i 

1885 "I 

1886 ^ 

1887 I 

1888 ■< 

1889 ^ 

1890 ' 



li 
11 
3.46 



19 
10 
4.61 



13 
5.4S 



19 
17 
3.71 

13 
11 
5.65 



52 
3.83 



29 
23 
6.39 



30 
6.31 



16 

4.25 

11 
11 
4.07 



i 
17 
7.63 



1 
7 
4.33 



23 
14 

S.81 



29 
9.61 



23 
4 
14.73 



31 
9 
5.20 



6.37 



11 
6.9S 



4 
4 
4.70 



26 
12 
5.76 



8 
5.60 



13 

n 

4.30 



9 
5.52 



fi 
4.19 



2i 

18 

10.10 

15 

10 
5.23 



27 

7 

4.61 



IS 

12 

4.53 



26 

5 

3.79 



4 

12 

1.09 



13 
3.39 



3.77 

S6 
21 
5.17 






222 



THE CLIMATE OF BALTIMORE 



TABLE LIV CON'T.— WET SrELLS. 



Tear. 



1891 

1893 < 

1893 \ 

1894 -j 

1895 } 

1896 ^ 

1897 \ 

1898 I 

1899 - 

1900 \ 

1901 ^ 

1902 

1903 < 

No. of wet spells. 



9 
9 
5.13 



21 
11 
4,S1 



2S 
13 
4.55 



s 

8 
14.43 



16 
11 
4.0a 



■29 
13 

6.31 



20 
10 
5.98 



13 

19 

7.15 



4.S6 



t6 
6.03 



26 5 

5.29 4.11 



z"5 



[' 
[» 
[' 

ll" 

![' 
If- 

30 \\ 
34 



3 61 



Table LIV contains a list of all of the more pronounced wet spells occuring 
near Baltimore from 1871 to 1903. The table includes all periods of 10 days 
or less during which the average monthly amount was recorded. Longer 
periods were included when the precipitation was proportionately excessive. 
The last day of the wet spell and the duration in days are indicated by 
figures in Italics and Roman respectively; the bold face type indicates the 
amount of precipitation recorded during the stated period, in inches and 
hundreths. 

Table LIV comprises the more conspicuous wet spells of the past 34 
years based upon excessive amounts of rain. Another table was pre- 
pared, but is not published in full, in which the basis of selection is the 
duration of unsettled, rainy weather. It includes periods of six or more 



MARYLAXD WEATHER SERVICE 



233 



consecutive days with a " trace " or more of precipitation, 8 days with 
not more than one day without rain or snow, or 10 days with not more 
than two days without precipitation. ^More extended periods were in- 
cluded when precipitation occurred on two in each successive period of 
three days. The total number of " unsettled periods '' of this descrip- 
tion comprised within the Si years is 164. The distribution throughout 
the year is indicated in the following summary; the last line indicates 
the number of intervening days without rain : 

PERIODS OF UNSETTLED WEATHER. 
(Six or more consecutive days with rain.) 



Total frequency 

Ma.xiraum duration in days 

Number of davs without rain 


d 

03 
11 

19 
i 


15 

17 

4 


S3 

20 

23 

2 


a 
< 

14 
13 

1 


oj 

23 

23 

4 


c 

3 
►^ 

13 

18 
3 


"3 

1-5 

12 
15 


si 

3 
< 

17 

19 
4 


P. 
$ 

9 
12 




§ 

7 
10 



> 

o 

is 

11 

10 

1 


o 

P 

12 
6 


03 
<D 

164 
33 

4 







SEASONS WITH EXCESSIVE PRECIPITATION. 





( Depai 


tures above the normal 


in inches 


and bun 


Wi 


nter. 


Spring-. 


Summer. 


1823-24 . 


. . .5.95 


1820 6.89 


1817. 


. . .12.25 


1839-40. 


. . .4.90 


1839 7.59 


1820. 


. . . 4.55 


1858-50. 


. ..9.95 


1851 4.90 


1829. 


. . . 6.87 


1865-66. 


...4.80 


1854 7.09 


1836. 


. . . 8.00 


1880-81 . 


. . .5.44 


1858 4.71 


1837. 


. . . 4.05 


1881-82. 


. ..5.04 


1859 5.95 


1838. 


. . . 5.45 


1883-84. 


. . .4.51 


1863 6.13 


1846. 


. . . 5.62 


1901-02. 


. ..4.83 


1890 10.34 


1856. 


. . . 5.02 


1902-03 


. ..4.93 


1892 5.81 


1857. 
1885. 
1889. 
1891. 
1903. 


. . . 4.10 
. . . 4.12 
. . . 5.96 
. . . 4.84 
. . . 5.90 



Autumn. 


1821. . . 


10.33 


1833 . . . 


4.00 


1843... 


7.35 


1854. .. 


9.13 


1873. . . 


4.13 


1876. . . 


6.22 


18r7. . . 


7.51 


1889. . . 


5.33 


1902.. . 


7.92 



The Distribution of Precipitation in Xormal, Dky and Wet Years. 

The comparatively uniform distribution of precipitation throughout 
the year in the vicinity of Baltimore is well shown in Fig. 62 and Fig. 63. 
In Fig. 62 the total monthly amounts are shown for the dry year of 
1900, when but 31.57 inches were recorded, 12 inches below the normal 
amount, and for the year 1889, which had an excess of nearly 19 inches. 
For purposes of comparison, a normal year is placed between the 
typical dry and wet years. In a similnr manner llie distribution of pro- 



224 



THE CLIMATE OF BALTIMORE 











































































































































































































































































































































































































































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226 



THE CLIMATE OF BALTIMORE 



cipitation by days and months is shown for the same years in Fig. 63. The 
depth of rainfall is indicated by tlie length of the heavy vertical lines. 
The dry year (1900) was deficient in rainfall frequency as well as in 
amount. The normal year (1888) had 154 rainy days; the dry year 
(1900) had 115, and the wet year (1889) had 164. The rainfall of the 
dry year was only about half that of the wet year, the amounts being 
31.57 inches and 62.35 inches, respectively. The normal precipitation is 
43.34 inches. The great excess in the wet year was due to the heavy 
spring and early summer rains of 1889. 



TABLE LV. 



-SUMMARY OF PRECIPITATION DATA. 

(1871-1903.) 



January . 
February 

March 

April 

May 

June 

July 

August — 
September 
October .. . 
November. 
December . 



Means. 



3.20 

3.70 
3.99 
3.27 
3.63 
3.78 
4.6r. 
4.20 
3.8f) 
2.99 
2.99 
3.07 



Mean 
depart. 



Year 43.34 



J. 11 

1 47 
1.44 
1.19 
l.fil 
1.45 
2.03 
1.80 
1.83 
1.48 
1.12 
1.28 



5.36 12 



Monthlj' and annual amounts. 



Greatest. 



Least. 



lA. S O 



1^ S 



fi.42 1892 

7.07 1896 
7.94 1891 

8.70 1889 

7.26 1894 

8.08 1883 
11.03 1889 

9.49 1873 

10.. ')2 1876 

fi.85 1902 

6.85 1877 

7.07 1901 

62.35I 1889 



201 
194 
199 
266 
200 
215 
241 
229 
267 
231 
224 
228 



0.88 
0.65 
1.19 
1.37 
1.00 
0.90 
1.40 
0.64 
0.09 
0.16 
0.65 
0.37 



1872 
1901 
1894 

1885 
1900 
1901 
1881 
1877 
1884 
1874 
1882 
1896 



144 31.. 57 ! 1900 



No. of days* 
with pre- 
cipitation. 



131 164 104 

in in 

1 1889 1871 



h. m 
7:18 
8:14 
6:1:^ 
6:25 
4:23 
2:45 
3:03 
2:42 
4:0:^ 

n-.m 

6:15 
6:46 

5:18 



Great- 
est in 
24hr8. 



1.951896 
3.481896 
3.511881 
3.581889 
2.991886 
4.471885 
4.021889 
4.361873 
4.761895 
.3.421^73 
2.851877 
2.881901 



4.76 



1895 



* Omitting days with only a " trace " of rainfall or snowfall. 

Table LV contains a summary of the principal facts relating to precipitation 
and published in full in preceding tables. The first column of figures shows 
the normal monthly precipitation based on 3.3 years' observations; the second 
column shows the proportion of the annual precipitation which falls in each 
month; the third column shows the average amount by which the actual 
monthly fall differs from the normal monthly fall, either above or below; 
the next following column of figures shows the same fact expressed as a 
percentage of the normal monthly precipitation. 



marylaxd weather service 227 

Snowfall. 

There are many difficulties in the way of securing accurate measure- 
ments of the amount of snowfall, difficulties which are inherent in the 
conditions attending precipitation in general, together with the additional 
one introduced when the temperature is at or near the freezing point of 
water. The method of exposure of the snow-gauge is of highest im- 
portance even under favorable atmospheric conditions for securing all 
the falling snow. When the wind is high, and especially when it blows 
in gusts, the snow is drifted and blown about to such an extent as to make 
it impossible to catch any but a small percentage of the total fall in the 
gauge. Under such circumstances it is necessary to resort to a different 
method of measurement. In an open and exposed area several measure- 
ments are made of the actual depth in inches of snow on the level ground 
at points which, in the estimation of the observer, represent most nearly 
the average depth in the vicinity of his station. The average of thc^o 
measurements is then accepted as the true depth of snowfall. In order 
to secure the equivalent depth in melted snow, the several measured depths 
are melted and the average depth of water obtained is computed. The 
amount of water yielded by a given depth of snow varies greatly with the 
temperature of the snow and the conditions under which it falls. A light 
fluffy snow may require 15 to 20 inches for one inch of water : on the other 
hand, a wet soggy snow of 4 or 5 inches may melt to an inch of water. 
In rough measurements, under average conditions, the ratio is about ten 
to one, and this is the relation generally assumed. With a slight change 
in temperature at or near the freezing point, the snow melts as it falls, 
or after falling for some time it may change to rain. These are some of 
the difficulties encountered in an effort to secure reliable snowfall data. 

The record of fairly accurate depths of snowfall at Baltimore begins 
with the year 1883. The record of frequency of snowfall begins much 
earlier, dating from the opening of the Weather Bureau Station in 18T1. 
The actual monthly and seasonal amount of snowfall recorded during 
each month and year from 1883 to 1904 is shown in Table LVI, together 
with the monthly and seasonal average amounts for the entire period of 
21 seasons. The seasonal variations are shown in Fig. 6-1. The average 
16 



228 



THE CLIMATE OF BALTIMORE 



u^ ^ 



_ o 



- 



in S 

<1 



MARYLAND WEATHER SERVICE 



129 




230 



THE CLIMATE OF BALTIMORE 



TABLE LVI.-MONTHLY AND SEASONAL SNOWFALL. 
(In inches and tenths.) 



Season. 



1883-4.... 

1884-.").... 
1885-6... 



1886-7.. 
1887-8.. 
1888-9 . 
1889-90. 
1890-1.. 



1891-2. 
1892-3. 
1893-4. 

1894-5. 
1S95-6. 



1897-8.... 
1898-9 ... 
1899-1900. 
1900-1.... 



1901-2 

1902-3 

1903-4 

Average (1884-1904). 

Greatest 

Year 



Oct. 


Nov. 


Dec. 


Jan. 


Feb. 


Mch. 


Apr. 


May. 


Sea- 
son. 








4.4 


14.2 





8.7 


8.0 





35,3 





0.6 


3.8 


1.9 


17.2 


5.9 


2.0 





31.4 











13.0 


15.3 


2.0 








30.3 





T 


10.2 


2.5 


6.0 


6.8 


1.1 





25.6 








12.0 


8.8 


3.9 


7.4 








32.1 





1.1 


T 


2.6 


5.3 


T 


T 





9.0 


T 








0.1 


2.5 


2.3 


T 





4.9 


T 


T 


10.6 


1.3 


3.5 


20.5 





T 


35.9 





T 


T 


14.5 


4.2 


25.6 


T 





44.3 





o 3 


4.3 


8.1 


11.7 


4.0 





T 


30.3 





0.2 


3.1 


1.0 


11.7 


T 


5.0 





21.0 





T 


3.0 


5.0 


9.3 


0.6 








17.9 


T 


T 


0.2 


1.0 


2.8 


13.8 


T 





17.8 





3.0 


3.2 


4.7 


0.7 


T 








11.6 





T 


2.6 


5.4 





2.4 


0.1 





10.5 





9.7 


0.6 


!.3 


33.9 


1.6 








.M.l 








0.7 


2.5 


13.0 


9.5 


T 





25.7 





T 


T 


6.5 


2.1 


0.1 








8.7 





0.1 


0.6 


6.7 


1.0 


5.0 








13.4 








7.0 


6.8 


6.0 











19.8 


T 


1.0 


3.8 


16.6 


2.5 


2.0 








25.9 


T 


0.8 


3.3 


5.6 


7.5 


5.8 


0.8 


T 


23.8 


T 


9.7 


12.0 


16.6 


33.9 


25.6 


8.0 


T 


51.1 




1898 


1887 


1904 


1899 


1892 


1.HH4 




1898-9 



depth of snow recorded during each month and the greatest and least 
monthly amounts recorded, are as follows: 



MONTHLY SNOWFALL, 
fin inches and tenths.) 



Average . 
Greatest. 

Y''ear 

Least — 
Year 



(1884-1904) 



Oct. 


Nov. 


Dec. 


Jan. 


Feb. 


March 


April 


May 


*T 


0.8 


3.3 


5.6 


7.5 


B.8 


0.8 


T 


T 


9.7 


12.0 


16.6 


33.9 


25.6 


8.0 


T 




1898 


1887 


1904 


1899 


1892 


1884 













0.1 
1890 



1898 



1903 









Season 



23.8 
51.1 
1898-9 
4.9 
1889-90 



* T represents a trace of snow. 

February is the month of greatest snowfall, followed by March, with 
January third in the order of depth. The annual fall has varied from a 
minimum of about 5 inches in the season of 1889-90, to a maximum of 
51 inches in 1898-9. Every month of the year excepting January has at 
some period since 1882 been entirely free from snow. The greatest 
monthly snowfall occurred during February, 1899, when about 3-i inches 



MARYLAND WP:ATHER SERVICE 231 

were recorded. Over half of this amount fell during the great blizzard 
of that month. 

Expressed in terms of the percentage of the total annual precipitation, 
the average annual snowfall at Baltimore is 5.6 per cent; that is, about 
one-eighteenth of the amount representing the total annual precipitation 
falls in the form of snow. The percentage has varied from 1 per cent 
in the calendar year 1889, to 11 per cent in 1892. Computing the rela- 
tive amounts which fall as snow and rain in the season of snowfall only, 
we have the following figures : 

RAINFALL AND SNOWFALL OF THE WINTER SEASON. 

(In percentage of total monthly precipitation.) 

Rainfall. Snowfall. 

November 97 per cent. 3 per cent. 

December 89 " " 11 " 

January 83 " " 17 " 

February 82 " " 18 " 

March 8G " " 14 " 

April 98 " " 2 " 

Average 89 " " 11 " 

Even in the mid-winter months of January and February, the amount 
of snowfall is generally less than one-fifth the total precipitation for those 
months. 

Dates of First and Last Snow. 

The first snow of the season usually falls about the 15th of Xovember, 
and the last about the first of April ; hence the average length of the 
season of snowfall is about four months and a half. These first and last 
snows are, however, usually only light fiurries. This is particularly true 
of the first autumn snows. In the 34 seasons since 1871, snow flurries 
have occurred as early as October 9, as in 1895 and 1903. The first snow 
of the season has occurred as late as December 17, as in 1883 and 1887. 
The early snows were not followed by either an abnormal amount or by 
an abnormal frequency of snows. The last snow of the season has occurred 
as late as May 6, as in 1891, and as early as February 22, as in 1903. 
Table LVII contains a record of first and last snows for each season 
from 1871 to 1904. 



232 



THE CLIilATE OF BALTIMORE 



TABLE LVIL-DATES OF FIRST SNOW IN AUTUMN AND LAST IN SPRING. 
(Including " traces " of Snow.) 



Year. 



1871 

1872 

1873 

1874 

1876 

1876 

1877 

1878 

1879 

1880 

1881 

1882 

1883 

18^4 

1885 

1886 

1887 

1888 

1889 

1890 

1891 

1892 

1893 

1894 

1895 

1896 

1897 

1898 

1899 

1900 

1901 

1902 

1903 

1904 

Earliest. 
Latest .. 
Average 



First in Fall. 



Last in Spring. 



Nov 


29 


Mar. 4 


" 


29 


•• 0'> 


" 


13 


" 21 


" 


13 

8 


Apr. 1 
^ 18 


Oct. 


15 


Mar. 2 


Nov 


10 


" 29 


Dec 


5 


Feb. 25 


Nov 


5 


Apr. 5 


** 


13 


Mar. 29 


Nov 


24 


Apr. 4 


" 


26 


" 11 


Dec. 


17 


Mar. 31 


Nov 


3 


Apr. 9 


" 


23 


" 11 


Nov 


13 


Mar. 8 


Dec. 


17 


Apr. 5 


Nov 


24 


Mar. 25 


Oct. 


23 


Apr. 6 


" 


19 


" 1 


Nov 


28 


May 6 


" 


9 


Apr. 15 


" 


15 


May 4 


" 


30 


Apr. 12 


Oct. 


9 


Mar. 20 


Nov 


13 


Ai)r. 7 


" 


33 


Mar. 14 


" 


24 


Apr. 28 


Dec. 


4 


" 16 


Nov 


9 


4 


Nov 


18 


Mar. 6 


Dec. 


5 


" 31 


Oct. 


9 


Feb. 22 


Nov 


13 


Mar. 28 


Oct. 


9 


Feb. 22 


Dec. 


17 


May 6 


Nov 


15 


Apr. 1 



Year. 



1871 
1872 
1873 
1874 
1875 

1876 
1877 
1878 
1879 
1880 

1881 

1882 
1883 

1884 
1885 

1886 
1887 
1888 
1889 
1890 

1891 
1892 
1893 
1894 
1895 

1896 
1897 
1898 
1899 
1900 

190] 
1902 
1903 
1904 



The Frequency of Days with Snowfall. 
The frequency of days with snow, including " light flurries," varies 
greatly from year to year. The average number for a series of years is, 
however, fairly constant. Dividing the entire j^eriod of 30 years from 
1871 to 1900 into three periods of ten years each, the average annual 
frequency was as follows : 

AVERAGE ANNUAL SNOWFALL FREQUENCY. 
(Including traces.) 

1871 to 1880 16.4 days. 

1881 to 1890 24.0 " 

1891 to 1900 26.1 " 

Mean (1891 to 1903) 22.0 " 



MARYLAND WKATIIER SERVICE 
Oct. Nov. Dec. jan. Fes Mch. Apr. Ma 



233 



Inches 30 



20 











\ A 
































\ 




/ 




^ 




B 


\ 






" ^^ 


^ 








^^ 


\; 


i^ 




















c\ 






























\ 








/ 


^ 


s. 


\ 








/^ 




N 


N \ 




/ 








E ~^^ 




k 


^ 




^ 








A 



30 Inches 



Oct. Nov. Dec. Jan. Feb Mch. apr May 

Fig. 65. — Monthly Frequency and Amount of Snowfall. 

A. The greatest monthly frequency of days with appreciable snowfall. 

B. The average frequency. 

C. The greatest monthly amounts of snowfall. 

D. The average monthly amounts of snowfall. 

E. The least monthly amounts of snowfall. 

With an average seasonal frequency of 22. the numher has varied 
from 5 as in 1875-6 to 40 as in 1892-3. The average monthly and 
seasonal frequency for 34 seasons, including light flurries of snow, or 



234 



THE CLIMATE OF BALTIMORE 



" traces," is indicated in the following table. The variations in the 
seasonal frequency are shown in Fig. 65. 



FREQUENCY OF DAYS WITH SNOW. 
(Including "snow flurries.") 



Average 
Greatest 
Least — 



Oct. 


Nov. 


0.2 


l.r, 


3 


5 









4.1 
10 




Jan. 


Feb. 


Mar. 


Apr. 


May j Season 


5.8 
18 



5.6 
14 



3.8 

8 




0.8 

3 




0.1 23 days 
1 140 •' 

j 5 " 



If we do not take into account da3's with light flurries of snow but 
only days during which a tenth of an inch or more fell, or days with 



TABLE LVIII.— NUMBER OF DAY^S WITH SNOWFALL EQUALLING OR 
EXCEEDING 0.10 INCH. 



Season 


Nov. 


Dec. 


Jan. 


Feb. 


Mar. 


Apr. 


Season 


1370 1 


'6 

5 
4 



6 

1 
3 

6 
o 


3 
3 


9 
3 



8 

3 

i 
1 

3 

1 
1 


3 
3 
6 

3.0 
3.6 
2.2 

2.3 


3 

5 
2 
o 

3 


3 
1 

1 


4 
4 
9 
3 

4 
3 

6 

2 

1 

3 
4 

7 
3 

5 

1 
6 
3 
4 
3 

o 

3 
3 

8 

2.0 
3.8 
3.6 

3.1 


3 
3 

5 
5 
3 

2 
2 
1 
6 
3 

3 
3 
o 

6 

13 
3 



3 

5 
3 

2 
3 

7 
8 
4 

3 
2 

9 
4 

3 
1 
2 
4 

3.1 
3.5 

4.3 

3.4 




5 
1 

3 

3 


3 

2 


3 

3 

5 
6 

1 
3 
4 



4 
6 
3 


1 

6 


1 
1 
4 

1 
2 

3 

1.8 
2.6 
2.5 

3.2 





1 
1 







1 

1 



1 

1 


3 




1 




3 





1 









0.2 
0.7 
0.3 

0.4 


6 


1871 2 





12 


187" 3 





13 


1873 4 





13 


1874-5 

1875-6 







10 

4 


1876-7 

1877 8 


1 




15 
3 


1878 9 





10 


1879-bO 

1880 1 .... 




4 


6 
18 


1881 2 





11 


188" 3 


4 


18 


188.3-4 

1884-5 

1885-6 

1886 7 









13 
26 

8 
19 


1887 8 





16 


1888 9 


1 


8 


1889-90 





7 


1890 1 





16 


1891-3 . . 

1893-3 

18^3 4 




4 

1 


13 
23 

15 


1894 5 





11 


1895-6 

1896 7 




1 


11 
11 


1897 8 • . 





8 


1898-9 . 


3 


18 


1899-1900 

1900 1 







11 
5 


1901 3 . . 


1 


9 


1903 3 





7 


1903 4 


1 


23 


Average. 

1871-1880 

1881-1S90 

1H91 1900 


0.1 

1.1 

0.9 


9.3 
14.3 
13.7 


1871 1903 


0.7 


13.0 









MARYLAND WEATHER SERVICE 



335 



what may be reaardeil as '' appreciable '' snowfall, the monthly and 
seasonal frequency is reduced considerably below the figures shown in 
the preceding paragraphs. A detailed list of such days is contained in 
Table LYIII, which gives a more satisfactory index of the snowfall con- 
dition of a season than the figures which include " traces." Basing our 
calculations upon "'' appreciable " snowfalls, Ave have an average seasonal 
frequency of 1"3 days. The season of 18T7-S contained but 2. while 26 
were recorded in 1884-5. The variations in the seasonal frequency are 
shown in Fig. 65. The average per month for the S-i seasons since 1871 
is as follows: 

FREQUENCY OF DAYS WITH SNOW. 
(Excluding traces.) 



Nov. Dec. Jan. Feb. 



Average.. . 
Greatest . 
Least 



2.2 

P 





3.1 



3.4 
12 




Mar. Apr. 



2.2 

e' 





0.4 
2 





Season 



13.0 days 
26 



Heavy Snowfalls. 

The heaviest snow noted in the oflficial records of the local otHce of the 
Weather Bureau fell during the great " blizzard "' of February, 1899. 
The fall occurred in connection with an Atlantic coast storm which 
reached Maryland at a time when the Middle Atlantic states w^ere in 
the embrace of the severest cold wave of the past 30 years. The ground 
was already covered by snow to the depth of about 10 inches, which fell 
from the 5th to the 8th, and to this layer 5 inches were added on the 
12th and 15.5 inches on the 13th. At the close of the storm of the 12th 
and 13th, the depth of snow on the ground measured 30 inches in the 
city of Baltimore. Greater depths were reported from other parts of 
Maryland. The wind was high and the temperature was extremely low, 
rancriutl between 5° and 20° below zero within the state. As a result, 
the dry snow was very much drifted and settled in places to depths of 
10 to 20 feet. The city was snoAvbound and all local traffic was 
blocked for two or three days. 

The greatest depth of snowfall for any 24 consecutive hours during 



236 



THE CLIMATE OF BALTIMORE 



this storm was 15.5 inches, according to the official measurements. 
Single snowfalls equalling or exceeding 10 inches in 2-i hours are ex- 
tremely rare in the vicinity of Baltimore. There was one on December 
17, 1887, another on the 3d of February, 1886, and another on the 18th 
of March, 1892, in the 21 years since 1884. In Table LIX will be found 
a record of the heaviest 2-i-hour snowfall for each month and season from 
1884 to 190-i. 

TABLE LIX.-GREATEST SNOWFALL IX 24 CONSECUTIVE HOURS. 



Season. 



1883-4. 
1884-6. 



1885-6.. 
1886-7.. 
1887-8.. 
1888-9.. 
18^9-90. 



Oct. 



1890-1 T 19 

1891-a .. 

1892-3 1 .. 

1893-4 

1894-5 



189.5-6.... 
1896-7.... 
1897-8 ... 
1898-9.... 
1899-1900. 



1900-1. 
1901-2 
1902-3. 
1903-4. 



Greatest. 



Nov. 



Dec. 



Jan. Feb. 



0.5 



2.9 

1.0 





3.1 6 

10.6 17 

T 19 





3.0 15 
1.0' 28 



T 
T 
1.2 
0.2 15 

T 30 

T i 20 
3.0 30 



6.5 9 13.0 3 

1.5 5 4.0, 26 

3.2 9 1.8 13 

2.5j 20 I 2.3; 27 

G.li 23 I 1.5: 2 



T 

0.1 29 



1.0 29 



4. 511898 10. 6 IBS' 



3.0 26 

0.2 13 
2.3 22 

2.0 26 
0.6 12 

0.71 27 

T I 21 
0.5 23 
4.0 5 
1.6 2 



1.0 25 

7.0 15 

4.8, 12 

0.5 27 

2.6| 29 

1.0 19 

3.6 27 

3.0 31 

2.8 1 

1.5 28 

4.0 25 

5.6 29 
4.0 34 
6.0, 29 



3.0 26 

4.0 6 

7.8! 17 

3.5! 25 

4.3| 7 

1.0! i 
0.5 8 
T 25 
15.. 5, 13 
6.0 17 

I 
2.0 3 
1.0 17 
5.0 17 
1.6 19 



Mar. 


5.0 


5 


1.5 


13 


2.0 


8 


3.6 


4 


3.5 


5 


T 


7 


3.4 


31 


9.5 


27 


13.fl 


18 


3.6 


4 


'1' 


26 


0.6 


11 



April 



0.7 



6.0 11 

T 14 

2.4 2 

1.6 7 

4.5 15 

0.1 
4.0; 



1.5i 18 



.0!l892|l5.5!l899:i2.0l892 



8.0 



1884: 



May 



Season. 



8.0 Apr. 
5.5 Feb. 



13.0 Feb. 

4.0 Feb. 
10.6, Dec. 

2.5! Jan. 

3.4 Mar. 



9., 5 Mar. 
13.0 Mar. 
7.8! Feb. 
4.0 Apr. 
4.3 Feb. 

6.0 Mar. 
3.0) Nov. 
3.0 Jan. 

15.0! Feb. 
6.0 Feb. 



4.0 Jan. 
5.6 Jan. 
5.0 Feb. 
6.0 Jan. 



15.5 Feb. 



The first column shows the amount of snowfall, and the second the date of 

occurrence. 



Duration of Snowfall. 

An effort has been made to obtain a value for the average duration of 
snowstorms in this vicinity. For this purpose the records were carefully 
examined for times of beginning and ending of snowfall for the period 
from 1884 to 1889, and the period from 1893 to 1902. Nq g^eat accu- 
racy can be claimed for the results, as there is no method in use for 
automatically recording beginnings and endings of snowfall. However, 
the figures given are based on a tabulation of 266 cases of snowfall during 



MARTLAXD WEATHER SERVICE 237 

TABLE LX.-SUMMARY OF SNOWFALL DATA. 



Means. 


■S 




OD 








_j- 


.Mii- 


5 




^ 


C O. 


o . ®_ 


= 5 , =i =i 


<»2 1 t.3 


^ ^^ 


o z 



Greatest 
monthly am'ts. 



Number of clays with snow. 



Greatest 

snowfall 

in 24 hours. 



1871-1903. 



1883-1903. 



Omitting traces. ^^^^^S^ 



1883-1903. 



0-5 





. o 




*3o3 




a e 


u 


§a 


a 


t,CW 


o 




>< 


- 



< ' c - 





.® 


^ 




« 




^ 


« 




c 




s 






s 






a 




o 


o 


> 




03 


a 


'^ 


< 


'K 




< 



October T 

November 0.8 

December 3.3 

January 5.6 

February 7.5 

March 5.8 

April 0.8 



May. 
Season . 



T 
33.8 



T 

9.7 
12.0 
14.5 
33.9 
25.6 

8.0 

T 



1895* 

1S9S 

1897 

1S92 

1899 

1892 

1884 

1893t 



1212 
364 
259 
452 
441 

1000 



61.1 1898-9 215 












0.2 


2 





T 


0.7 


4 





1.6 


5 





4.5 


2.2 


9 





4.1 


10 





10.6 


3.1 


9 





5.8 


13 





7.0 


3.4 


12 





5.6 


14 





15.5 


o o 


6 





3.8 


8 





12.0 


0.4 


o 





O.S 


3 





8.0 











0.1 


1 


T 


12 


26 


o 


22 


40 6 


15.5 




in 


in 




in in 






1884-5 


1877-8 




1892-31875-6 





1895* 

1898 

1887 

1892 

1899 

1892 

1884 

1893t 

1899 

in 

Feb. 



T Indicates a " trace ' 

• Also 1889 and 1890. 



of snow ; an amount less than 0.1 inch. 

+ Also 1891. 



16 years, and hence are fairly relial)le. Trace? of snow, or snow '' flur- 
ries," were included in the calculations. 

DURATION OF SNOWFALL. 
(Including traces.) 





Nov. 


Dec. 


Jan. 


Feb. 


March 


April 


Season 




11 
60 
5.30 


45 

2;» 

5.20 


99 
428 
4.20 


67 
395 
5.50 


34 

169 

5.00 


10 

47 

4.45 


266 


" duration in hours 

Average duration (in hrs. and miu.) 


1S^8 
5.00 



Fogs. 

Fogs in the vicinity of Baltimore are confined mostly to the fall, win- 
ter and early spring months. The record contained in Table LXI 
applies only to dense fogs surrounding the local station of the U. S. 
Weather Bureau office. Their frequency is doubtless greater as the 
harbor or the bay is approached. A fog has been regarded as dense 
when it obscured objects at a distance of about 1000 feet, and it has 



238 



THE CLIilATE OF BALTI:M01{E 



been recorded only when it hung abont the station for one hour or more. 
The table includes only such fogs as have been described and which 
occurred since 1891, the earlier records being regarded as less reliable. 

TABLE LXI.— FREQUENCY OF DENSE FOGS.* 



Year. 


Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


Ann'l. 


1891 


3 

i 

3 

1 

1 
1 
5 
2 

1 

i 

4 

23 

1.8 


1 

i 

4 
1 
1 

i 

4 

17 
1.3 


1 

i 

3 
3 

3 

1 

'8 

18 
1.0 


'i 

1 
1 

i 

4 
0.3 


1 
1 

2 
0.2 


1 

1 
0.1 






i 
i 

o 

0.2 


2 
1 

2 

6 
0.4 


i 
1 

3 

"b 

7 

3 

1 

20 
1.6 


2 

"i 

3 
5 

1 

i 

i 

3 
6 
2 

37 
2T1 


6 

"b 

1 
3 

2 

1 
2 

i 

3 
6 

31 
2.4 


13 


1893. 

1893 



9 


1894 

1895 


10 
15 


1896 


13 


1897 

1898 

1899 

1900 


8 
9 

13 
14 


1901 


13 


]<i02 


15 


1903 


19 


Total (13 yrs.) 

Aver, per jear ... 


150 
11.5 



* Fog about station for one hour or more, and too dense to see objects at 1000 feet. 

During the 13 years from 1891 to 1903 there were 150 dense fogs re- 
corded, or about 12 per year. The percentage of occurrence in the differ- 
ent months of the year is shown by the following figures : 



Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year 



Percentage of total 
annual number 



15 



13 



18 



100 



Arranging the months in the order of frequency of occurrence of fogs, 
we have: December, ISTovember, January, October, March, February, 
September, April, May and August, June, July. These fogs are not of 
long duration, rarely continuing any considerable portion of the day. 
When they do occur, however, they are a serious menace to the shipping 
interests in the harbor. A complete list of the dates of occurrence of all 
dense fogs about the local station of the U. S. Weather Bureau is given 
in Table LXII. The annual number since 1891 has varied from 19 in 
the year 1903 to none in the year 1892. None have been recorded in 



^iIARYLAXD WEATHER SERVICF 



239 



TABLE LXII.-DATES OF OCCURRENCE OF DENSE FOGS.* 



1S91 


1894 


1896 


1899 


1901 


1903 


Jan. 1 


Jan. 


11 


Jan. 29 


Jan. 


14 


March 


25 


Jan. 1 






16 


Feb. 1 




24 


May 


24 


27 


11 




24 


4 


Feb. 


18 


June 


14 


28 


Feb. 21 


Feb. 


9 


" 5 




21 


Oct. 


10 


29 


March 9 


Aug. 


30 


29 


March 


4 


" 


31 


Feb. 2 


Nov. 21 


Oct. 


20 


March 26 




6 


Nov. 


1 


4 


" 22 


Nov. 


2 


" 30 


" 


18 




16 


11 


Dec. "3 




21 


April 6 


Oct. 


11 




22 


28 


" 2'^ 




23 


Nov. 26 


" 


1.5 


Dec. 


1 


March 4 


23 


Dec. 


12 


Dee. 7 


" 


17 




2 


8 


24 






30 




26 




9 


" 11 


25 


189.=) 






Dec. 


~i 


" 


13 
29 


17 


26 




19 








1897 










" 20 
















21 
" 24 


1892 


Jan. 


21 




1900 




1902 






March 
Sept. 


5 


Jan. 3 
Feb. 6 










April 8 

Nov. 10 

23 

















11 


March 19 


Jan. 


19 


Jan. 


18 




Oct. 


2 


21 


Feb. 


8 


Feb. 


27 


1904 






'' 


April 6 
Nov. 5 


Aug-. 
Oct. 


30 


May 


28 
19 


1893 




26 








Nov. 


6 


21 




23 


Sept. 


3 


Jan. 22 






8 


Dec. 9 


" 


24 


" 


20 


Feb. 7 




Jan. 1 
April 29 




IT 




>' 


35 


Oct. 


19 


JIarch 2 




19 
25 
18 


1S98 


" 


26 


Nov. 


1 



3 


Oct 1'' 


Dec. 




" 


28 




3 




Nov. 2 
Dec. 7 

9 

10 
23 

28 




31 


" 


19 


Jan. 6 


Nov. 


25 


** 


5 


April 1 


" 


28 


11 


Dec. 


19 


" 


14 


9 






12 




20 


" 


15 


June 4 






13 


" 


22 


Dec. 


3 


Sept. 14 






20 
Feb. 10 








16 


Oct. 10 
Nov. 3 








Nov. 5 










4 








Dec. 20 










24 








21 










Dec. 27 



* Fog about station for one hour or more, and too dense to see objects at 1000 feet. 

the month of July during this period of 13 years, and but one in the 
month of June, two each in May and August. 



SUXSHINE AXD CLOUDINESS. 

Sunshine. 

In connection with a discussion of the amount of sunshine recorded 
at Baltimore, it is important to know the metliod employed in obtaining 
the record. The instrument in use at the local office of the Weather 
Bureau since 1893 is of the kind known as the electrical thermometric 
recorder. The essential parts of the instrument are the two glass 
bull)S, one of which is covered with lamp-black. The two Inilbs are 
joined by a tube, in the middle portion of which are the terminals of an 
electric circuit. The direct rays of the sun falling upon the Itlack bulb 



240 THE CLIIMATE OF BALTIMORE 

will raise the temperature of the air within to a higher degree than that 
within the bright bulb. This difference in temperature sends a column 
of mercury to the terminals in the connecting tube. When the sun 
passes behind a cloud or below the horizon, or, in other words, when the 
direct rays of the sun do not fall upon the bulb, the temperature in both 
is presumably the same, the mercury column remains below the terminals 
and the circuit remains open within the instrument. A recording de- 
vice is placed in the electric circuit at some convenient point in the ob- 
serving station. While the sun shines upon the black and bright bulbs, 
a characteristic line is drawn by a pen upon the revolving drum of the 
recording instrument. While the circuit is open a straight line is pro- 
duced. The clock which forms part of the recording device closes the 
circuit every minute of the day and night. In this manner we obtain a 
record of sunshine or no sunshine once every minute between sunrise 
and sunset. At the close of the day we may then add up the number 
of minutes of sunshine. With these figures and knowing the exact 
number of hours and minutes between sunrise and sunset, we may obtain 
the percentage of possible sunshine for each day. 

The hourly records for ten years show that there is a steady increase 
in the amount of sunshine in all months from sunrise to a maximum 
at about noon. The maximum hourly amount increases from 04 per 
cent in January to 81 per cent in September, and then again decreases 
to a minimum in January. The hourly distribution is shown in terms 
of percentages of the possible amount for each hour and month of the 
year in Table LXIII. The same distribution is graphically shown in 
Fig. 66, in which increase in the intensity of shading rei^resents an in- 
crease in the amount of sunshine. In Fig. 67 the average increase from 
hour to hour for the entire year is indicated by means of a single curve ; 
this shows a rapid and very uniform increase from sunrise to noon, and a 
similar decrease to sunset. This law of variation is common to all months 
of the year. The fact should not be overlooked that the amount of sun- 
shine recorded as described above is not a complement of the amount of 
cloudiness. Sunshine may be, and frequently is, recorded when the sky 
is, to a great extent, clouded. The instrumental record only indicates 



MARYLAND WEATHER SERVICE 



241 



TABLE LXIII.-AVERAGE HOFKLY DURATION OF SUNSHINE. 



Hours. 




» 


2 


a, 


>> 




G 


"3 


3 


+3 

a 





> 




§' = 




•^ 


tn 


i 


< 


S 


»-5 


HS 


< 


E» 





•^ 


~' "1 


4- 5 a. m 










29 


40 


44 












9 


5-6 '• 






33 


41 


.S2 


42 


42 


38 


49 








23 


6-7 " 




33 


31 


44 
,54 


38 
.50 


49 
61 


48 
61 


41 

55 


48 
57 


38 
43 


28 


.. 33 


7-8 " ... 


9H 


29 44 


g^- 9 " 


m 


4fi 


.51 


64 


.58 


70 


68 


66 


67 


53 


40 


38 55 


9-10 '• 


+9 


m 


63 


68 


65 


75 


77 


73 


74 


64 


,52 


54 64 


10 IJ •• 


6.S 


68 
74 


67 
70 


72 


71 
70 


78 


79 


79 
SO 


80 
81 


69 
70 


61 
64 


61 ; 70 


11- Noon 


65 : 72 


Noon- 1 p. m 


fi+ 


7"' 


72 


74 


71 


79 


80 


80 


80 


72 


65 


65 1 73 




62 


71 


70 


■j-o 


70 


7-* 


80 


77 


77 


72 


64 


63 ; 71 


2-3 •• 


!SX 


66 


66 


68 


66 


75 


75 


72 


74 


68 


57 


56 , 67 


3-4 " 


JO 
.ST 


61 

f,0 


63 
53 


62 

,56 


.56 
49 


70 
60 


67 
59 


66 

54 


70 
.59 


60 
48 


43 
34 


41 
31 


59 


4-5 " 


49 


&- 6 " 


m 


43 


39 


43 


m 


46 


47 


:{8 


47 


47 






35 


6- 7 " 






36 


83 


25 


m 


30 


27 


46 








19 


7-8 ■• 


•• 








18 


31 


27 


30 










» 


Mean dailj- number of hours of sunshine. . 


4.9 


6.4 


6.8 


7.9 


7 7 


9.2 


9.2 


8.4 


8.4 


6.7 


5.0 


4.9' 7.1 


possible numlier 


9.8 


10.7 


12.(1 


13.2 


14.314.E 


14.6 


13.7 


12.511.2 


10.1 


9.512.2 


Percentage of possible numV)er 


.50 


.59 


57 


eo 


.54 


02 


63 


61 


67 


60 


M 


51 58 



Table LXIII shows the average duration of sunshine for each hour from 
sunrise to sunset, expressed in percentage of the possible amount of sunshine. 
The values are based on the continuous record of a self-registering thermo- 
metric sunshine recorder during the ten years from 1894 to 1903. The 
average daily duration is also given in hours and tenths and in percentage 
of the possible number of hours. 




Fig. M. — Mean Hourly Sunshine. 

The diagram show.s the mean hourly sunshine during each hour of the day and mouth 
of the year, expressed as a percentage of the liighest possible amount for the season. 
It is based on the ten years' record of a self-registering thermometric sunsliine recorder. 
The dotted lines S. R. and S. S. show the time of sunrise and sunset, respectively. The 
heaviest shading shows the time of occurrence of the higliest percentage of sunshine. 
The curved lines mark intervals of 10 per cent in the amount of sunshine. 



242 



THE CLIMATE OF BALTIMORE 



U'hetlier the face of the sun is or is not obscured at the moment of record- 
ing. It is only approximately an index of cloudiness. 

There is, in all seasons of the year, an almndance of sunshine at this 
station. The amount varies considerably in different months, but in all 
months the average is above 50 per cent of the possible amount. Janu- 
ary and December have the smallest amount in actual number of hours 




Fig. 67. — Mean Hourly Sunshine for the Year. 

The diagram is based on the ten years" record of a self-registering thermometric sun- 
shine recorder. The figures to the right and left of the diagram show the amount of 
sunshine, expressed as percentages of the highest possible amount. 



as well as in percentage of the possible amount. The amount increases 
from 4.8 hours in December to a maximum of 9.2 hours in June, per 
day. September, with but 8.1 hours of sunshine, has a higher per- 
centage than June, the value for the latter being 62 per cent, and the 
former 65 per cent. 

The average monthly and annual amounts of sunshine are indicated by 
the following- frg-ures : 



MARYLAND WEATHER SERVICE 



243 



Average Daily Sunshine. 



Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


Year 


Average in hours. . 4.9 
" percen- 
tage of possible 
amount 50 


6.4 
69 


6.8 
57 


7.9 
60 


7.7 
54 


9.2 
62 


9.1 
62 


8.6 
63 


8.1 
65 


6.8 
60 


5.5 
61 


4.8 
50 


7.2 
58 



The sunshine of an}^ given month may vary greatly, however, from 
that indicated by the average figures given above. In the following 
table the months of the period from 1893 to 1903, during which the 
greatest and least amount of sunshine prevailed, are indicated, together 
with the monthh' ranges. The years in which these amounts were 
recorded may be found by consulting Tables LXIY and LXV. 



TABLE LXIV.— AVERAGE NUMBER OF HOURS OF SUNSHINE. 
(By months and years.) 



Year. 



Jan. Feb. Mar. Apr. May June July Aug-. Sept. Oct. Nov. Dec. Ann'l 



1893 .... 

1894 5.7 

189.5 1 5.2 

1S96 4.0 

1M97 3.7 

1898 5.5 

1.^99 6.4 

1900 i 5.2 

1901 ! 4.4 

1902 4.6 

1903 4.6 

Average 4.9 



6.2 

8.4 

4.8 
4.3 
7.1 
7.8 

5.8 

7.0 
6.3 
6.3 



7.9 

8.4 

6.2 
6.0 
6.8 
8.8 
5.7 

6.1 

6.4 
5.9 



6.4 , 6.8 

I 



9.0 

6.2 

7.1 

8.7 

11.0 



5.5 
7.4 
7.4 

7.9 



4.6 
8.2 
9.1 
5.8 
8.1 

5.4 
8.8 
9.7 

7.7 



11.8 
9.0 



7.3 
12.4 
9.5 



10.0 
9.8 
6.1 

9.2 



12.4 
10.1 

7.1 

6.0 

8.8 

10.3 

9.3 

8.0 
8.9 
10.8 

9.1 



9.0 
9.9 
11.0 


6.5 
7.2 
9.7 


8.7 
6.6 
9.2 


6.7 
9.2 
9.9 



7.3 

8.0 

8.4 
7.6 
6.9 

8.6 



7.7 

7.9 
6.7 
9.1 

8.1 



7.0 
6.5 
8.0 

5.0 

5.8 
7.5 
7.0 
4.8 

8.7 
7.3 
6.1 



5.8 
7.0 
4.5 

2.9 

5.6 
5.9 
10.0 
4.0 

4.9 
4.3 
5.6 

5.5 



6.2 
3.4 

3.5 
5.1 

5.7 
4.8 

4.8 

4.8 
4.6 
5.7 

4.8 



8.2 
8.0 

5.5 
6.2 
8.0 
8.1 
6.8 

fi.8 
6.9 
7.0 



TABLE LXV.— PERCENTAGE OF POSSIBLE SUNSHINE. 
(By months and years.) 



Year. 



1894. 
1H95. 

1896. 
1H97. 

iMOtl. 

1S99. 
1900. 

1901. 
1902. 
1903. 



Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Ann'l 



Average 



60169 67 60 6462 62 63 66 60 



64 



17 



244 



THE CLIMATE OF BALTIMORE 



HIGHEST AND LOWEST MEAN MONTHLY SUNSHINE. 
(In percentage of the possible amount.) 







p 


ti 


C 


^ 


e 

c 


>, 


to 


4a 


+5 


> 


d 


§ 






» 


as 


P. 


cS 


a 


3 


P 




u 


o 


» 


1.® 




>-> 


^ 


S 


<! 


3 


1-5 


i-s 


< 


m 


O 


!< 


fi 


rH 


Highest mean.. 


64 


79 


74 


84 


68 


84 


85 


81 


80 


78 


70 


66 


67 


Lowest mean.. 


38 


40 


48 


41 


32 


41 


41 


49 


52 


43 


29 


36 


45 


Range 


26 


39 


26 


43 


36 


43 


44 


32 


38 


35 


41 


30 


22 



The months of least sunshine show an average of over 40 per cent, 

only the winter months and the month of May having at any time during 
the eleven years fallen below this value. The monthly range varies 
from 26 per cent in January and March to 44 per cent in the month of 
July. The average for the entire year has varied from 67 per cent in 
1894 to 45 per cent in 1896, a range of 22 per cent. 





TABLE LXVL-SUNSHINE PHASES. 
(Local time). 










U 

C 


1st Mean. 


Maximum. 


2d Mean. 


43 




Time. 
Hours 
ending 
a. m. 


Value 


Time. 
Hours 
ending 
p. m. 


Value 


Time. 
Hours 
ending 

p. m. 


Value 

in 
hours 


O 

s 


January 

February 

March 


7.17 
6.. 52 
6.11 
5.25 
4.47 
4.33 
4.45 
5.13 
5.41 
6.10 
6.43 
7.12 

5.54 


10.10 
10.00 
9.30 
8.40 
8.30 
8.10 
8.20 
8.30 
9.00 
9.40 
10.00 
9.50 

9.15 


50 
59 
57 
60 
54 
62 
63 
61 
67 
60 
50 
51 

.58 


1.00 
12.20 

1.10 
12.30 
12.50 

1.10 
12.20 
12.20 
12.20 
12.50 

1.00 

1.10 

12.45 


64 
74 

72 
74 
72 
79 
80 
80 
81 
72 
65 
65 

73 


4.00 
4.10 
4.30 
4.20 
4.20 
4.. 50 
4.30 
4.30 
4.20 
4.00 
3.30 
3.20 

4.10 


4.9 
6.4 
6.8 
7.9 
7.7 
9.2 
9.2 
8.4 
8.4 
6.7 
5.0 
4.9 

7.1 


5.02 
5.37 
6.07 
6.37 


May 


7.05 
7.26 


July 


7.25 
6.. 55 


September 

October 


6.09 
5.22 
4.46 


December 

Year 


4.39 
6.06 



Table LXVI shows the time of day when the maximum amount of 
cloudiness is most likely to occur, and the time which most nearly represents 
the time of occurrence of the average daily cloudiness in the morning and 
afternoon. 

Sunshine Phases. 

Table LXVI indicates the hour of day during which the maximum 

amount of sunshine has occurred most frequently in the past eleven 

years; also the hours of the morning and afternoon during which the 

sunshine is most likely to be equivalent to the average amount for the 

day. These facts are of importance in selecting the best hours for 

observino^ and recording sunshine. 



MARYLAND WEATHER SERVICE 



245 



Cloudixess. 

Recording the amount of cloudiness has always been an important 
feature of a regular observation in the work of the U. S. Weather Bureau. 
A careful record of the extent of cloudiness has been maintained at the 
local station of the Bureau since the establishment of the Service in 
Januar}', 1871. At the present time, two direct observations per day 
are made, one at 8 a. m. and the other at 8 p. m. At various times 
since 1871 the following constituted the recjular hours of observation : 



1 1 


1 1 


I 1 


1 1 


1 1 


1 1 


1 1 


1 1 
























^^P 














^^H 


^^^ 








J 


r 


^^!S^ 




^^^ 




B 




B 







Fig. 68. — Average Hourly Cloudiness. 

The diagram Is based on a record of five years of direct observations at 12 stated 
hours of the day from 7 a. m. to 11.30 p. m. The cloudiness is rated on a scale from 
to 10, the former figure representing a clear sky and the latter an overcast sicy. The 
dotted line is based on interpolated values from midnight to 7 a. m., no direct observa- 
tions being available for the period. 

7 a. m., 8 a. m., 11 a. m., noon, 2 p. m., 3 p. m., 4.30 p. m., 7 p. m., 

8 p. m., 9 p. m., 10 p. m. and 11 p. m. The amount of cloudiness is 
noted in terms of the number of tenths of the sky covered at the time 
of observation. In order to arrive at the law of increase and decrease 
in the diurnal amount of cloudiness, the average extent of cloudiness 
was determined for a period of five years at each of the 12 hours of 
observation mentioned above. These twelve periods of the day afford 
ample material for accurately noting the daily march of cloudiness 
between the hours of 7 a. m. and 11 p. m., but leave a serious gap 



246 THE CLIMATE OF BALTI:M0RE 

between midnight and early morning which coukl onl_v be l)ridged over 
by interpolating the most probable values. 

The following figures show the average values for the extent of cloud- 
iness at the stated hours of the day: 

AVERAGE CLOUDINESS. 

(On a scale of one to ten.) 

Hours of Observation. For the Year. 

7.00 a. m 5.3 tenths of sky covered. 

8.00 a. m 5.1 

11.00 a. m 5.7 

Noon 5.9 " 

2.00 p. m 6.0 

3.00 p. m 5.8 

4.30 p. m 5.8 

7.00 p. m 4.8 

8.00 p. m 4.4 

9.00 p. m 4.4 

10.00 p. m 4.3 

11.00 p. m 4.2 

These annual average values have been graphically presented in Fig. 
68. The dotted contour line from midnight to 7 a. m. indicates that 
the curve is based on interpolated values. The form of the curve shows 
a steady increase in cloudiness from early morning to a maximum at 
2 p. m., and then a somewhat more rapid decrease to midnight, with a 
probable minimum sometime in the early morning hours. The average 
daily cloudiness based upon the 8 a. m. and 8 p. m. observations is 
somewhat too low. Any of the series of three daily observations em- 
ployed in past years by the U. S. Weather Bureau, the Smithsonian 
Institution or the Army Medical Department, will yield a daily average 
very closely agreeing with the daily mean based on the twelve daily obser- 
vations distributed as noted in the preceding paragraph. 

Clear, Partly Cloudy ak'd Cloudy Days. 

It has been the custom for many 3^ears to designate the character of 
the day as clear, partly cloudy or fair, and cloudy, the classification 
being based upon the average amount of cloudiness at two or more 
stated hours of the day, or upon prevailing conditions for the day. The 
sky is designated as clear when it is entirely free from clouds, or when 
less than one-third is covered by clouds; it is regarded as fair or parUij 



MARYLAND WEATHER SERVICE 



241 



cloudy when covered to the extent of four to seven tenths; and cloudy 
when it is from eight to ten tenths overcast. There is no instrumental 
method for measuring the exact amount of cloudiness; hence the classi- 
fication must be left to the judgment of the individual observer. How- 
ever, it is a convenient method of designating the character of the day 
as regards the extent of sky covered by clouds and is a fair index of the 
amount of sunshine received at the observing station. The frequency 




Fig. 69. — Relative Frequency of Clear, Partly Cloudy and Cloudy Days. 

of occurrence of days of each of the classes at a given locality is a matter 
of the highest importance to health and personal comfort, and a vital 
factor in plant growth. 

All days since 18T1 have been grouped into the three classes described 
above. The variation in tlie frequency of occurrence of clear, partly 
cloudy and cloudy days from month to month and from year to year is 
shown in Tables LXVII, LXVIII and LXIX, and in Fig. 69. The 
mean monthly frequency of each class is shown in the following table : 

MKAN MONTHLY CLOUDINESS. 
(1871-1902.) 



1 

1 


.Tan. 


Feb. Mar. 


April 


May 


June July 


Aug. 


Sept. 


Oct. 


Nov. 1 Dec. 


Year 


Clear days 

Partly cloudy 
days 

Cloudy (lavs — 


8.3 
12.1 
10..; 


8.4 8.8 

10.9 11.5 

H.!t 10.7 


9.2 
11.8 
9.1 


9.6 
11.6 
9.9 


9.0 10.0 
14.0 13..3 
7.3 1 7.4 


10.7 
12.9 
7.4 


11.9 
10.5 
7.5 


12.4 
10.3 

8.T 


10.2 9.7 
10.2 1 11.5 
9.6 9.4 


118.1 
140.6 
lOfi.ii 



248 



THE CLIMATE OF BALTIMORE 



Frequency of Clear Days. 

The variation in the number of clear days from month to month and 
year to year is shown in Table LXVII. Variations in the annual fre- 



TABLE LXVII.-NUMBER OF CLEAR DAYS. 
(Less than 4 tenths of sky covered.) 



Year. 



1871 

1872 

1873 

1874 

1875 

1876 

1877 

1878 

1879 

1880 

1881 

1883 

1883 . 

1884 

1885 

1886 

1887 

1888 , 

1889 

1890 

1891 

1893 

1893 

1894 

1895 

1896 

1897 

1898 

1899 

1900 

1901 

1903 

1903 

Average 

1871-1880 

1881-1S90 

1831-190 J 

1871-1903 .... 



Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec 



7.9 
6.8 
10.3 

8.3 



7.9 
7.4 
9.1 

8.4 



9.1 
7.3 
9.8 

8.8 



13 



7.9 
8.8 
11.4 



11.1 

7.9 
9.9 

9.5 



7.9 
8.3 
10.7 

9.0 



8.8 
10.3 
11.7 

10.0 



9.3 
10.6 
13.5 

10.7 



11.0 



16 



11.8 



9.3 10.3 
18.0 ! 14.5 



9.4 
10.5 
10.9 



11.9 12.4 10.3 



10 



7.8 
9.5 
11.5 



Ann'l 



110 
133 
113 
115 
99 

86 
113 

lis 

133 
10] 

88 
93 
135 
133 

84 

111 

104 
114 
100 
137 

147 
143 
155 
166 
154 

97 
126 
141 
147 
107 

122 
107 
113 



109.9 
106.9 
138.3 

118.1 



quency are also graphically shown in Fig. 69 in connection with the 
partly cloudy and cloudy days. During the course of a year we may 
count on about 118 clear days, or days with a cloud covering of three- 
tenths or less. The annual frequency has varied greatly from 1871 to 
1903. In 1885 but 84 were recorded, while in 1894 there were 166, or 
about double the number. The table shows a rather remarkable increase 



MARYLAND WEATHER SERVICE 



249 



in the ten-year averac^e for 1891 to 1900 (138) over that of the decades 
from 1871-1880 (110) and 1881-1890 (107), a variation which is diffi- 
cult to account for, considering the close approximation of the values 
for the two preceding ten-year periods. 



TABLE LXVIII.— NUMBER OF PARTLY CLOUDY DAYS. 
(From 4 to 7 tenths of sky covered.) 



Year. 



1871. 
1872. 
1873. 
1874. 

1875. 



1876. 
1877. 
1878. 
1879. 
1880. 

1881. 
1883. 
1883. 
1884. 
1885. 

1886. 
1S87. 
1888. 
1889. 
1890. 

1891. 
1893. 
1893. 
1894. 
1895. 

1896. 
1897. 
1898. 
1899. 
1900. 

1901. 
1903. 
1903. 



Average. 



Jan. 



1871-1880 13..") 

1881-1890 13.4 

1891-1900 10.8 

1871-1903 1 12.1 



Feb, 



Mar. 



12.3 11.4 
11.8 13.3 



9.3 
10.9 



9.8 
11.5 



Apr. 



12.1 
13.5 
10.3 

11.8 



May 



12.0 
12.4 
10.8 



11.6 



June 



14.7 
14.9 
12.6 

14.0 



July 



14.0 
13.3 
12.6 

13.3 



Aug. 



11.5 
14.1 
13.6 

12.9 



Sept, 



10 
10 
14 
8 
13 

9 

10 
10 
13 
10 

14 
13 
13 
11 
16 

19 

15 

8 

8 

14 

11 
6 
9 

7 
8 

10 
6 
4 
8 

10 

12 

9 
10 



10.7 
13.0 

7.9 

10.6 



Oct. 



11.9 

11.3 

7.9 



Nov. 



11.4 

9.8 
9.9 



10.2 10.2 



Dec, 



Li 



13.0 
13.5 
10.0 



Ann'l 



148 
160 
157 
143 
102 

164 
130 
131 
142 
148 

164 
173 
154 
140 
183 

157 
160 
131 
1.30 
131 

112 
140 
112 
118 
141 

136 
122 
116 
111 

137 

119 
126 
114 



US. 5 
1.52.2 
124.5 



140.5 



September and October have the highest percentage of clear days, 
followed closely by August, November and July. The minimum fre- 
quency occurs in January. There is an almost uniform increase in the 
number from January to a ma.ximum in October, followed by a steady 
decrease to a inininuiiii in January. The only interruption in the 



250 



THE CLIMATE OF BALTIMORE 



regularity of the increase is a slight falling off in frequency in the 
month of June. In the months of September and October the number 
of clear days has occasionally reached 20 out of the total of 30 or 31 
days; the number sometimes falls as low as 6 or 7; the average fre- 
quency for September is 11.9, and for October 12.4. 



TABLE LXIX.-NUMBEK OF CLOUDY DAYS. 

(Over 7 tenths of sky covered.) 



Year. 

1871 

1873 

1873 

1874 

1875 

1876 

1877 

1878 

1879 

1880 

1881 

1882 

1883 

1884 

1885 

1886 

1887 

1888 

1889 

1890 

1891 

1892 

1893 

1894 

1895 

1896. 

1897 

1898 

1899 

1900 

1901 

1902 

1903 

Average. 

1871-1880 

1881-1890 

1891-1900 

1871-1903 



Jan. 
13 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


6 


10 


6 


8 


6 


7 


13 


3 


7 


11 


6 


5 


9 


11 


4 


9 


6 


8 


11 


11 


8 


6 


13 


10 


10 


7 


16 


10 


5 


10 


6 


13 


3 


11 


9 


5 


8 


10 


17 


5 


n 


10 


10 


10 


9 


6 


8 


11 


7 


1.T 


13 


6 


11 


10 


10 


10 


13 


9 


11 


13 


8 


5 


10 


10 


14 


8 


13 


7 


6 


6 


7 


12 


5 


16 


6 


5 


5 


11 


16 


16 


8 


14 


10 


8 


14 


5 


3 


13 


7 


11 


10 


15 


1 


5 


8 


16 


5 


8 


8 


6 


5 


3 


4 


10 


12 


12 


9 


7 


7 


in 


6 


9 


6 


8 


8 


17 


5 


6 


8 


11 


4 


9 


8 


13 


8 


9 


6 


/** 


13 


13 


7 


6 


7 


9 


8 


13 


13 


10 


9 


16 


5 


7 


7 


13 


13 


10 


9 


8 


13 


14 


6 


13 


12 


10 


6 


11 


4 


8 


7 


8 


10 


15 


6 


13 


8 


10 


11 


11 


13 


11 


11 


6 


3 


3 


4 


8 


9 


9 


10 


9 


10 


4 


6 


6 


8 


7 


8 


11 


o 


3 


3 


8 


2 


6 


8 


4 


6 


8 


2 


13 


11 


12 


9 


14 


13 


13 


5 


13 


16 


13 


7 


11 


7 


10 


7 


11 


8 


14 


9 


13 


2 


7 


6 


11 


U 


14 


5 


10 


6 


6 


10 


11 


11 


15 


10 


13 


10 


3 


5 


13 


4 


9 


18 


16 


6 


10 


9 


14 


13 


10 


13 


13 


10 


13 


6 


11 


12 


13 


14 


10 


16 


6 


16 


9.6 


8.1 


10.6 


10.0 


7.9 


7.4 


8.3 


10.3 


11.8 


9.0 


10.5 


7.7 


10.7 


6.8 


7.5 


6.3 


9.9 


9.8 


11.4 


8.3 


10.3 


7.7 


5.7 


6.9 


10.6 


8.9 


10.7 


9.1 


9.9 


7.3 


7.4 


7.4 



Sept, 



8.3 
7.7 
6.1 

7.5 



Oct. 



15 



7.3 
10.5 



8.7 



Nov. 


Dec. 


15 


8 


6 


8 


4 


8 


6 


12 


10 


5 


13 


10 


8 


11 


11 


12 


4 


18 


15 


10 


11 


9 


7 


5 


8 


5 


9 


13 


13 


8 


8 


11 


4 


11 


16 


7 


15 


11 


6 


10 


6 


8 


7 


10 


9 


6 


fi 


6 


10 


14 


11 


13 


10 


10 


13 


10 


n 


7 


10 


11 


13 


13 


13 


13 


8 


10 


9.2 


10.3 


9.7 


8.0 


9.3 


9.6 


9.6 


9.4 



Ann'l 



107 

83 
96 
107 
104 

116 
133 
116 
101 

io- 
ns 

100 

86 

103 



101 
121 
135 

107 

106 
83 
98 
81 
70 

133 
117 
108 
107 
121 

124 
132 
138 



106.9 
106.1 
103.4 

106.6 



Frequency of Partly Cloudy Days. 
The details concerning the monthly and annual distribution of partly 
cloudy days may be learned by consulting Table LXVIII. The average 
annual frequency is 140 days, with a maximum occurrence of 182 in 



MARYLAND WEATHER SERVICE 251 

1885 and a minimum of 111 in 1899. The partly cloudy days are most 
frequent in June and least frequent in October and ^NTovember. There 
is a fairly uniform distribution throughout the year, the monthly aver- 
ages varying only between a minimum of 10.2 and a maximum of 14.0, 
as shown in Table LXYIII. The annual variations are shown graphic- 
ally in Fig. 69. 

Cloudy Days. 

.The frequency of occurrence of cloudy days during each month and 
year since 1871 is shown in Table LXIX. The average annual number 
for the entire period of 33 years has been about 107, with a maximum 
frequency of 138 in 1903 and a minimum of 70 in 1895. Cloudy days 
have been most frequent in the months of March (10.7) and January 
(10.6) and least frequent in the month of June (7.3). The average 
annual variation is shown graphically in Fig. 69. 

THE WINDS. 

INTRODUCTION. 

A Kobinson anemometer with a continuous recording attachment has 
been in operation since the establishment of the oflQce of the U. S. 
Weather Bureau in January, 1871. Hence we have an excellent and 
complete record of the hourly changes in the velocity of the wind for a 
period of 34 years. While it is a matter of great importance to have a 
permanent observatory for meteorological observations, it is a difficult 
problem for the National Weather Service to secure such permanence in 
large and rapidly growing cities where changes in neighboring buildings 
so alter the conditions of exposure of instruments as to make a change 
in the location of the observatory a necessity. Since 1871 the successive 
changes in the elevation of the anemometer were as follows : 

CHANGES IN THE ELEyATION OW THE ANEMOMETER. 

Above Ground. Above Sea-leveL 

1873 to Oct. 12, 1878 75 feet. 90 feet. 

1878 to Jan. 1, 1889 SO " 100 

1889 to May. 1891 100 " 120 

1891 to Sept. 7. lSn.-> 100 " 208 

1895 to Auk. 1. 189G 136 " 173 

1896 to Apr. 30, 1902 82 " 185 

1902 to Dec. 1903 117 " 220 



'i.yi THE CLIMATE OF BALTIMORE 

The exposure of the anemometer was very satisfactory during the 
entire period, excepting from 1896 to 1902, when neighboring buildings 
obstructed the free movement of the atmosphere over the station. The 
elevation of the anemometer above sea level was approximately the same 
from 1871 to 1889, namely, between 90 feet and 100 feet; from 1891 
to 1904, with the exception of September, 1895, to July, 1896, the sea- 
level elevation was increased by approximately 100 feet. Changes in 
elevation above sea level affect the velocity of movement of the atmosphere 
no less than changes in elevation above ground. The abrupt increase in 
the velocity shown from 1890 to 1891 is doubtless due to the change in the 
sea-level elevation of the anemometer. 

Since 1893 a continuous record of wind direction has been maintained 
without interruption excepting for a few hours at a time when difficulty 
was experienced with the recording instrument. The hourly changes in 
wind direction discussed in the following pages are based upon the ten- 
years' record from 1893 to 1902, unless otherwise stated. 

Average Hourly Wind Movement. 

The recorded hourly velocities for the twenty-year period from 1881 
to 1900 have been reduced to average hourly values in order to determine 
the periodic variations in velocity during the day. The results are 
shown in Table LXX, and graphically in Figs. 70 and 71. In Fig. 70 
the hourly changes in velocity are given for the months of January, 
April, July and October, and the average for the entire year. The curves 
for all months are similar in form. There is a minimum velocity in all 
months just before sunrise. The velocity rises rapidly to a maximum 
between two or three in the afternoon, which it maintains approximately 
for two or three hours, then decreases rapidly to 8 p. m. or 9 p. m., and 
more slowly to the minimum for the day just before sunrise. The same 
hourly variation is shown for all months of the year in another manner 
in Fig. 71. The influence of the diurnal variations in temperature upon 
the coincident variation in wind velocity is strikingly exhibited in the 
table and diagrams; the increase in velocity accompanies the increase in 
temperature throughout the course. The time of maximum rate of 



MARYLAND WEATHER SERVICE 



increase and decrease in velocity is coincident with the time of maximum 
rate of change in temperature, the most rapid increase occurring between 



1234.56789 10 II tn 




ftlttS 


- 




/ 


\ 




















/« 


/ 






\ 




























































\ 




^ 






I 


1 






; 


1 




__^_ 


J 



Ju Fu Ken lor May June Juljf Au^. Stu Oct No Dec 

Annual Vabiations or Wind Velocity 



Fig. 70. — Hourly and Annual Variations of Wind Velocity. 

Expressed In miles and tenths of miles per hour for the months of January, April, 
.Tuly and October, and for the entire year. 

8 a. m. and 10 a. m.. and the most rapid decrease between 6 p. m. and 
8 p. m. 

In the annual fluctuation in velocity, however, a similar relationship 
does not exist. On tlie contrary, there is almost a direct inversion of 



354 



THE CLIMATE OF BALTIMORE 



TABLE LXX.-AVERAGE HOURLY WIXD MOVEMEXT. 
(In miles and tenths.] 







4 


p 


'u 

< 


^ 

S 


2 

3 


>. 

■^ 


bib 

3 
<5 


a. 

X 


O 


o 
2; 


® 


3 

<5 


Midn't to 1 a. m. . . 


5.5 


5.9 


6.0 


5.3 


4.5 


4.2 


4.0 


3.5 


4.0 


4.6 


4.8 


5.1 


4.8 


•J •■ 


5.0 


5.8 


5.9 


5.2 


5.0 


4.1 


4.0 


3.5 


3.9 


4.5 


4.8 


5.0 


4.8 


3 " .... 


5.3 


5.7 


5.8 


5.1 


4.3 


3.6 


3.9 


3.5 


3.9 


4.4 


4.8 


5.0 


4.6 


i " .... 


5.3 


5.8 


5.8 


5.0 


4.1 


3.8 


3.8 


3.6 


3.9 


4.6 


4.9 


5.0 


4.6 


5 " .... 


5.2 


5.8 


6.0 


4.8 


4.3 


3.9 


3.7 


3.7 


3.9 


4.6 


4.8 


4.9 


4.6 


6 " .... 


5.1 


5.b 


5.9 


4.8 


4.3 


4.0 


3.8 


3.7 


4.0 


4.6 


4.7 


4.9 


4.6 


7 " .... 


5.4 


5.9 


6.0 


5.3 


5.0 


4.8 


4.3 


3.9 


4.3 


4.7 


4.8 


5.0 


4.9 


8 " . . 


5.6 


6.2 


7.0 


6.4 


5.7 


5.7 


5.3 


4.7 


4.9 


5.4 


5.2 


5.2 


.'1.6 


9 " .... 


6.1 


7.0 


■ 8.5 


7.6 


6.6 


6.3 


6.0 


5.5 


5.8 


6.2 


6.0 


5.7 


6.4 


10 " .... 


6.9 


7.8 


9.0 


8.2 


7.1 


6.9 


6.5 


6.0 


6.5 


7.3 


7.1 


6.6 


7.3 


11 " .... 


7.5 


8.3 


9.3 


8.7 


7.7 


7.4 


6.8 


6.5 


6.8 


7.7 


7.7 


7.3 


7.6 


Noon 


7.8 


8.7 


9.6 


9.0 


8.2 


7.7 


7.2 


6.8 


7.2 


7.9 


8.1 


7.7 


8.0 


1 p. m — 


8.1 


9.1 


9.9 


9.4 


8.6 


8.1 


7.8 


7.1 


7.5 


8.2 


8.4 


7.S 


8.3 


O " 


8.3 


9.1 


9.9 


9.7 


8.8 


8.2 


8.0 


7.3 


7.6 


8.2 


8.4 


8.0 


8.4 


3 " .. . 


8.2 


9.1 


10.0 


9.7 


8.9 


8.3 


8.2 


7.5 


7.6 


8.2 


8.3 


7.9 


8..'. 


4 " .... 


7.7 


8.8 


9.9 


9.5 


8.6 


8.3 


8.0 


7.6 


7.6 


8.0 


7.9 


V.5 


8.3 


5 " .... 


7.1 


8.3 


9.6 


9.1 


8.4 


8.1 


7.6 


7.2 


7.0 


7.1 


6.9 


6.6 


V.8 


6 '• .... 


6.2 


7.3 


8.4 


8.3 


7.6 


7.3 


7.1 


6.4 


6.0 


5.7 


6.9 


6.8 


6.8 


7 " .... 


5.8 


6.6 


7.3 


6.8 


6.4 


6.1 


6.0 


5.2 


4.8 


4.9 


5.4 


6.6 


5.9 


8 " . 


5.6 


6.3 


6.6 


5.8 


6.5 


5.1 


4.7 


4.2 


4.4 


4.9 


6.3 


5.4 


5.3 


9 " 


5.5 


6.0 


6.5 


5.7 


5.2 


4.6 


4.4 


3.9 


4.4 


4.8 


5.1 


5.3 


5. J 


10 " .... 


5.5 


5.9 


6.3 


5.5 


4.8 


4.4 


4.2 


3.7 


4.3 


4.7 


5.0 


5.2 


5.0 


11 " ... 


5.4 


6.0 


6.3 


5.4 


4.8 


4.3 


4.0 


3.7 


4.3 


4.7 


6.0 


5.2 


4.9 


Midn't. ... 


5.5 


5.9 


6.1 


5.3 


4.8 


4.2 


4.0 


3.6 


4.1 


4.6 


4.9 


5.1 


4.8 


Means 


6.3 


7.0 


7.6 


6.9 


6.2 


5.8 


5.6 


5.1 


5.4 


5.8 


6.0 


5.9 


6.1 



Table LXX is based on the continuous record of a self-registering anemom- 
eter during the 20 years from 1881 to 1900. 



t 2 3 4 5 6 7 8 9 




Fig. 71. — Average Hourly Variations in Wind Velocity. 

The heaviest shading shows the time of occurrence of the highest average wind veloci- 
ties for the day. The curved lines mark intervals of half a mile in the average velocity. 
The dotted lines marked S.R. and S.S. show the time of sunrise and sunset, respectively. 
The diagram is based on hourly values for a period of 20 years. 



MARYLAXD WEATHER SERVICE 



255 



the relation existing between temperature and wind velocitA'. The light- 
est winds occur in the months of greatest heat, while the highest veloci- 
ties occur in March, with a slight secondary increase in October and 
November (see Fig. 71). The annual fluctuations are due to the varia- 
tions in cyclonic activity at different seasons of the 3'ear, The highest 
average hourly wind velocities occur between 2 p. m. and 3 p. m. in the 
month of March, when they attain an average velocity of 10 miles per 
hour. The lowest velocities occur in the early morning hours of June, 
July and August, when the average falls to about 3.5 miles per 
hour. This law of increase and decrease is remarkably constant through- 
out the year and is recognizable at any time when not interrupted by 
the presence of a well-developed cyclonic or anti-cyclonic disturbance. 



Average Daily, and Total Monthly Wixd Movement. 

In Table LXXI the total monthly wind movement for each month of 
the year from 1873 to 1903 is shown, together with the average daily 
movement for each year during the same period. As the elevation of 
the anemometer was changed several times during this period, it is essen- 
tial to bear in mind the fact in discussing the variations in wind veloci- 
ties as shown in Table LXXI. Xo attempt has been made to reduce the 
records to a single elevation; the changes in elevation are distinctly 
traceable in the monthly and daily values for the wind movement. In- 
ferences as to fluctuations in the annual velocity should be made with 
caution. The average daily wind movement is approximately 145 miles 
for the entire year. The velocity varies from a minimum of 122 miles 
in August to a maximum of 175 miles per day in March. The following 
figures represent the average daily wind movement for each month, as 
derived from hourly observations from 1873 to 1902, a period of 30 
years : 

AVERAGE DAILY WIND MOVEMENT. 





Jan. 


Feb. 


Mar. 

175 


Apr. 


May June July Aug. 
149 142 134 122 


Sept. 
129 


Oct. 
137 


Nov. 
143 


Dec. 


Year 


Miles 


146 


162 


160 


142 1 14S 















256 



THE CLIMATE OF BALTIMORE 



TABLE LXXL— TOTAL MONTHLY AND AVERAGE DAILY WIND MOVEMENT. 



Year. 




© 

PR 


1 


< 




e 
>-> 


3 

1-5 


bo 

3 
< 


® 











Oi 

Q 


c 5 




S i 

m 


1873 


3665 

4848 
3571 

4510 

3764 
5335 
5.33t> 


3R.3S 
4180 
4136 

4636 
3671 
4021 


633S 
5S11 
3977 

5766 
4854 
5124 
4158 
6295 

6519 

4466 
4753 
4615 
4876 

5847 
5893 
5495 
5.336 
4964 

6033 
7326 
6904 

5872 
7070 

8038 
4.33S 
4226 
5040 
4340 

E007 
4594 
4938 

5330 
5.514 
5543 

5435 
175 


513S 
5565 
4891 

4718 
4065 
4870 
5149 
6398 

5045 
4423 
4583 
5139 
4.3a3 

4085 
4963 
4736 
5455 
3997 

44.30 
64.33 

6604 
6351 

6308 

6056 
3974 
4S.39 
4077 
3574 

5-387 
4963 
6449 

4936 
4815 
5303 

4983 
166 


4389 
4670 
4990 

4495 
4016 
4344 
4350 
4764 

3841 
4310 
4993 

4483 
3917 

4335 

3783 
4041 
419.S 
3970 

4838 
5987 
65S8 
5S80 
5435 

5669 
4196 
4235 
36.50 
4101 

4359 
5933 
6336 

4397 
4444 
5004 

4616 
149 


4136 


3966 3.f;73 


3680 
4032 
3353 

4764 
3775 
3937 
3610 
3629 

3253 
3774 
4110 
3576 
3531 

3443 
3184 
3570 
5140 
3386 

4008 
4867 
.5063 
4731 

4857 

3453 
3395 
3425 
3362 
3176 

3383 
4754 
4549 

3780 

3881 
3940 

3867 
139 


4103 
3802 
3687 

41^0 
4317 
4600 
.3461 
3939 

.3456 
33.33 
4213 
3905 
3926 

3796 
4093 
4626 
4669 
3975 

6125 

5484 
5360 

5,S0S 
5645 

3589 
41.36 
4080 
.3415 
3585 

3074 
6197 
6339 

3878 
4481 
4389 

4349 
137 


4256 
3665 
3267 

4079 
4715 
4617 
4715 
3640 

4135 
3542 
4061 
3934 
4199 

4708 
38.33 

3,S31 
3342 

5937 
6567 
5422 
6213 
5373 

3133 

3463 
3559 
3197 
3732 

4299 
4781 
4619 

4062 
4474 
4316 

4384 
143 


4181 
45S6 
3959 

4014 
43S5 
5765 
3905 
4661 

3795 
4439 
3634 
3928 
5047 

4038 
4176 
4376 
3714 
4440 

5766 
6567 
6798 
5054 
6033 

3343 
3526 
4180 
4036 
3450 

3699 
5940 
6330 

4359 
4458 
4505 

4407 
143 


4254 
4543 
3780 

4474 
3994 
4575 
4355 
4318 

4067 
4a36 
4166 
4119 
4294 

4338 
4331 
4205 
4333 
3863 

5033 
5796 
5837 
5.364 
5643 

5031 
3769 
■3913 
3780 
3729 

3931 
6014 

5483 

4240 
4425 
4598 

4421 


140 


1874 


4529 4646 41,SS 
3793 J 34:55 , .3310 

4433 4341 3769 
4479 1 4083 , 3907 
4479 4189 3T17 
4326 4518 4026 
4644 3949 3876 

.3906 3,867 ' 3073 
4733 4213 3.523 
4076 4184 3861 
3655 4074 3237 
4354 , 3740 4073 

3803 i 3.508 3680 
4133 4073 1 3653 
3949 3.SO6 3690 
3900 3948 3945 
3281 3506 3631 

5112 5502 i 4399 
5635 1 4451 : 4536 
4867 4957 5233 
4654 4960 3880 
4561 ; 4.541 4890 

4955 ! 5397 3133 
3440 3551 3908 
3754 3765 3130 
34S9 3.3.55 3918 


149 


1876 


124 


1876 

1877 


147 
131 


1878 

1879 


150 
143 


1880 


3443 ■IR.HR 


143 


1881 


3654 
4.335 
3471 
4609 
5090 

4789 
4600 
4551 
3938 
4191 

3740 
6393 
6510 
5413 
6030 

6399 
4559 
4097 
3840 
3933 

3793 
3874 
6410 

4135 
4536 
4834 

4498 
145 


4362 
3567 

4054 
4273 
4439 

4831 
4407 

:^iH8 

3.S08 
3661 

4385 
6410 
6628 
5553 
7090 

7297 
3840 
3668 
3946 
4547 

4036 
4839 
6073 

4070 
4365 
5143 

4636 
163 


134 


1883 

1883 

1884 

1885 


133 
137 
135 
141 


1886 


139 


1887 


139 


1888 

1889 


138 
143 


1890 


137 


1891 ... 

1893 


165 
191 


1893 


193 


1894 


176 


1895..... 

1896 


186 
165 


1897 


134 


1898 


139 


1899 


134 


1900 

1901 


3715 3397 
3363 3689 


3304 
3163 


123 
139 


1903 


5698 1 fiOQK 


4606 
4731 

3696 
3870 
3807 

3791 
133 


165 


1903 


5115 

4334 
4189 
4230 

4351 
143 


5007 

4131 
4079 
4361 

4154 
134 


180 


Average 

1873-1883 

1883-1893. . . . 
1893-1903 

1873-1903 

Average ) 

daily V . . 
1873-1903 \ 


139 
145 
151 

145 



Table LXVIII shows the total monthly and average daily wind movement 
for 31 years, from 1873 to 1903; also the average daily movement for the 
entire 30 years ending 1902. The figures are based on the continuous record 
of a self-registering Robinson anemometer. 



The average daily movement for an entire year has been as low as 
124 miles, as in 1875, and as high as 150 miles, as in 1878, confining 
our choice of limiting values to the period from 1873 to 1890, during 
which the elevation of the anemometer remained practically unchanged. 



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358 



THE CLIilATE OF BALTIMORE 



Maximum Wind Velocities. 
In the preceding paragraphs the total wind movement over the station 
for an entire month, and the average hourly and average daily move- 
ment were alone considered. In Table LXXII a record will be found 
of the highest velocity of the wind attained in any 5-minnte period 
during each month and year from 1875 to 1903, together with the 
accompanying direction of the wind and the date of occurrence. In 
determining the maximum wind velocity for the day, the sheet contain- 





































60 










































■ 
















- 


- 














40 






















































- 




- 


- 


- 




- 






- 




- 


- 


- 






- 


- 


























- 


20 






































































































Fig. 72. — The Frequency of Storm Winds. 

The diagram shows the variations in the annual frequency of winds exceeding 25 
miles per hour. 



ino^ the continuous record of the anemometer is examined and the five- 
minute interval selected during which the velocity is greatest. The 
number of miles or fractions of a mile registered during this 5-minute 
period is then multiplied by 12 in order to obtain the rate of movement 
per hour, or what is usually termed the hourly velocity of the wind. All 
of the daily records since 1875 have been carefully examined and the 
highest velocity recorded during each month selected and entered in 
Table LXXII, at the same time noting the date of occurrence and the 
direction of the Avind during the selected 5-minute period. An examina- 
tion of the table shows that high winds are not confined to any particular 
season of the year, but have occurred in all months. The high winds of 



MARYLAXD WEATHER SERVICE 259 

the winter months occur in connection with the well-defined cyclonic 
disturbances, while the high velocities of the summer months accom- 
pany the thunderstorms, or the tornado, in the rare instances of its 
occurrence in this vicinity. The annual variations of the maximum 
velocity are shown in Fig. T2. 

The highest velocity of the wind recorded at the Baltimore Station 
of the U. S. Weather Bureau since 18T5 occurred during the storm of 
July 20, 1902, when the wind blew at the rate of TO miles per hour for 
five minutes. Further particulars of this storm, which was one of the 
most destructive ever visiting this vicinity, will be found in a later 
section of this report. Selecting the highest recorded velocities, in miles 
per hour, for each month of the year, we have the following comparative 
figures : 

HIGHEST MONTHLY VELOCITIES. 



> 


6 


o 


« 


Z. 


Q 


48 


5-t 


S 


E 


1891 


1898 


28 


4 


28 


30 



Highest vel.... 48 45 50 , fiO 4.S i-2 70 45 38 45 48 , 54 70 

Direction W NW S NW | W SW W SW NW SW S E W 

Year 1894 1893 1896 1879 1893 i 1893 1902 ' 1888 I 1892 1878 \ 1891 | 1898 1902 

Day .30 19 19 3 23 I 27 20 8 ■ 26 | 

Av.vel.of ma.\.- 29 , 30 30 i 30 26 1 26 28 24 24 27 28 30 28 



The average of all maximum velocities during the 28 years from 1875 
to 1902, as shown in the last line of the above table, indicates a remark- 
ably uniform value for this factor, throughout the year. The highest 
monthly average velocity (30) differs from the lowest (24) by only 
6 miles. The lowest velocities occur in August and September, and the 
highest in February, March, April and December. The fact that the 
September records show the lowest average velocities for storm winds 
is significant in view of the popular association of the so-called " Equi- 
noctial " storms with this month. 

As already stated, wind velocities are generally expressed in terms of 
the rate per hour based upon the actual velocity during a five-minute 
period. By basing the rate per hour upon the duration of the mile 
made in the shortest time, we obtain what is officially designated as the 
extreme velocity. By this method we are more liable to obtain the 
18 



260 



THE CLIMATE OF BALTIMORE 



velocity in brief gusts of wind, velocities which are lost when the hourly 
rate is based upon the movement during a period of five minutes. As 
much of the destruction due to high winds is wrought during these brief 
gusts, or squalls, the extreme velocity is a factor of great importance. It 
is, in nearly all cases, higher than the maximum; it cannot be lower. 
There is no fixed relation between the two velocities ; it may be of inter- 
est, however, to show to what extent they have differed from one another. 
Basing our inquiry upon the official record of the monthly maximum and 
extreme velocities during the period from 1888 to 1903, we have the 
following comparative figures: 

RELATION BETWEEN MAXIMUM AND EXTREME VELOCITIES. 
(In miles per hour.) 





Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 


Nov. 


Deo. 


Maximum 


48 
13 


45 
55 


50 
60 


42 
50 


4.3 

48 


42 

50 


70 
75 


45 
52 


38 
50 

12 


43 
50 

8 


48 
60 

12 


54 




60 








10 


10 


8 


5 


8 


5 


7 


6 







This relationship may be expressed b}' another method. In place of 
selecting the highest maximum and highest extreme velocities for each 
month, we may examine all cases of high winds occurring in a stated 
time and note the difference between the maximum and extreme veloci- 
ties. This has been done for a period of three years with the following 
result : 

DIFFERENCES BETWEEN MAXIMUM AND EXTREME VELOCITIES. 
(In miles per hour.) 



Jan. Feb. Mar. Apr. May .lune July Aug. Sept. Oct. Nov. Dec. Year 



Average diff 4.5 



Greatest " 
Least " 
No. of cases. 



4.5 

10 

1 

29 



4.3 



4.3 3.4 

5 

3 

11 



3.6 
12 

14 



7.8 3.9 

17 I 10 

I 

1 1 

15 7 



4 8 

16 

1 

13 



3.6 

10 

1 

17 



5.0 



4.2 



4.5 
17 

195 



The highest wind velocities generally occur in connection with a 
northwest wind in all months of the year. These winds usually accom- 
pany a rising barometer and occur a short time after the shift in tlie 



MARYLAND AVEATHER SERVICE 



261 



wind which follows the turn in the barometer. While northwest is the 
usual direction of the storm wind, all directions of the compass are rep- 
resented. In Table LXXII there are 348 records of high winds covering 
a period of 29 years; placing these in the order of frequency of the 
directions from which they came, we have the following relative positions 
for the entire year : 

RELATIVE FREQUENCY OF HIGH WINDS. 



Direction of wind 


NW 


W 


SW 


NE 


N 


SE 


E 


S 


Percentage of frequency 


41 


20 


12 


8 


1 


4 


4 


4 



The same order of frequency obtains practically in all months of the 
year. In nearly three-fourths of all instances of storm winds, the direc- 
tion is from some point between southwest and northwest. In only 12 
per cent of instances is the direction from some point between east and 
south. High winds from the north or from the east are of compara- 
tive] v rare occurrence at Baltimore. 



Frequency and Duration of Stated Wind Velocities. 

The hourly wind velocities during a period of five years (namely, 
from 1893-96 and 1903) were tabulated into groups in order to deter- 
mine the relative frequency of stated velocities. The result is shown in 
the following table, in which the frequencies are expressed in terms of 
percentages of the total number of hours in each month : 
frequency of stated wind velocities. 

(In percentage of possible number of hours per month.) 



Miles per hour.. 


Jan. 


Feb. 


Mar. Apr. 


May 


June 


July 


Aug. 


Sept. 


Oct. 

41.8 


Nov. 


Dec. 


An'l 


5 


37.6 31.2 


33.7 28.8 


35.3 


41.5 


44.3 


49.0 


45.3 


39.3 


40.7 


39.1 


ti-10 


35.7 .35.2 


3(i.4 40.7 


4.3.0 


44.4 43.1 39.7 38.5 


.37.6 


35.6 


.35.3 


38.8 


11-15 


15.8 15.7 


16.2 19.9 


15.6 


12.2 10.8 9.1 13.2 


13.8 


16.1 


15.7 


14.5 


16-20 


6.4 , 9.9 


8.3 . 8.2 


4.9 


1.8 


1.3 1.3 2.5 


4.6 


6.5 


6.3 


5.2 


21-25 


2.8 1 4.9 


3.5 1 1.9 


O.T 


0.1 


0.1 0.3 0.4 


1.7 


2 2 


1.5 


1.6 


26-30 


1.2 2.1 


1.4 


0.5 


0.4 




0.1 


0.2 


0.2 


0.3 


0.2 


0.4 


0.6 


.31-40 


0.3 : 0.9 


0.3 




0.06 






0.1 


0.06 


0.1 


0.2 


0.1« 


0.1 


41-50 


0.03' .... 


0.03 




















0.0 



262 



THE CLIMATE OF BALTIMORE 



Winds of 10 miles per hour and under prevail during about 78 per 
cent of the total number of hours of the year; winds of 11 miles to 20 
miles during less than 20 per cent. Hence the total duration of veloci- 
ties exceeding 20 miles per hour is only about 2.3 per cent of the entire 
year, or about eight and a third days. Storm winds, or winds exceeding 
25 miles per hour, prevail during about 62 hours in an average year. 



Average Duration of Storm Winds. 

It will be seen from the statements in the preceding paragraph that 
winds having a velocity exceeding 25 miles per hour are of brief duration. 
The duration decreases rapidly with increase in wind velocity. The rate 
of decrease may be readily judged from the figures in the above table 
representing the annual relative frequency of stated velocities. Basing 
our calculations upon the same period of five years employed in deter- 
mining the relative frequency of stated velocities, v^e obtain some inter- 
esting figures defining the average duration of storm winds. 







AVERAGE 


DURATION OF 


STORM WINDS. 














(In hours and minutes.) 








1-5 


i 


03 


b 
< 


>> 

03 


0) 

c 

3 

1-5 


>> 


be 
< 


P. 


o 
O 


> 
o 
I? 


6 


OS 

>> 


Total annual 




























duration . . . 


23.36 


43.35 


28.25 


12.25 


5.35 


0.26 


0.60 


4.10 


3.40 


11.10 


9.50 


13.06 


1.56.40 


Averag-e 




























frequency.. 


6.2 


9.2 


7.6 


5.0 


4.4 


2.0 


2.6 


1.4 


2.6 


4.2 


4.8 


4.4 


54.4 


Duration per 




























storm 


3.50 


4.40 


3.40 


2.30 


1.20 


0.12 


0.20 


3.00 


1.25 


2.40 


2.00 


3.00 


2.50 


Greatest 




























duration — 


19.00 


46.00 


23.00 


16.00 7.00 


.36 


.30 


18.30 


7.00 


9.10 


6.00 


19.00 


46.00 



In the winter and early spring months a storm wind usually continues 
from three to four hours. The duration rapidly diminishes on the 
approach of summer, reaching a minimum in June and July, when the 
average duration is only a few minutes. The high winds of summer 
usually occur in connection with thunder squalls of brief duration, 
while those of winter, spring and fall accompany the passage of well- 
defined cyclonic storms. The comparatively long duration of August 
storm winds in the above table is due entirely to the severe gulf storm of 
August 28-29, 1893, during which the wind blew a gale for many hours. 



MARYLAND WEATHER SERVICE 263 

a duration which would be considered long even for a winter gale of the 
severest type. Xeglecting this storm, the August average duration for 
the remaining four years is about 35 minutes. 

During the passage of the Gulf storm of February 7-10, 1895, the 
wind blew at Baltimore with a velocity exceeding 25 miles per hour for 
about 46 consecutive hours. It then fell below the storm velocity for 
about 12 hours and again went above 25 miles per hour for another 
period of 12 hours. It is one of the longest storm periods on record at 
Baltimore. The storm originated in the Gulf of Mexico on the 6th, 
and moved rapidly eastward and northward along the Atlantic coast 
from Florida to the Gulf of St. Lawrence, the center passing just east- 
ward of Baltimore on the 8th, with a maximum velocity of 42 miles per 
hour from the west. The barometric gradient between the center of the 
storm and the center of the area of high pressure to the west and north- 
west was very great throughout its course, amounting at one time to 
about two and a half inches. The extreme velocity of 50 miles per 
hour was reached on the 8th at about noon. 

As the summer high winds occur mostly in connection with thunder- 
storms, their time of greatest frequency, and hence greatest probability, 
is from 3 p. m. to 4 p. m. The winter, spring and fall storm winds, ' 
accompanying cyclonic disturbances which occur at any hour of the day, 
also have a well-marked tendency to fall within the early afternoon 
hours. This may easily be explained by supposing that the cyclonic 
winds are augmented at these hours to a maximum extent by the diurnal 
wind movement. 

Gales. 

A gale is technically defined by a wind velocity of 40 miles, or over, 
per hour. Such winds have been recorded on 42 occasions at the Balti- 
more office of the U. S. Weather Bureau since 1873. They have been of 
comparatively great frequency in some years, notably in 1893, which is 
credited with nine; there were seven in 1903. In half the years since 
1873, none were recorded. The highest velocity registered in the years 
from 1880 to 1887 was 39 miles. As is the case with most high winds, 



364 



THE CLIMATE OF BALTIMORE 



TABLE LXXIII.— SUMMARY OF WIND VELOCITIES. 
(1873-1902.) 



January . . 
February. 

March 

April 

May 

June 



July 

Aug'ust — 
September. 

October 

November. 
December . 



Year 6.1 8.0 



Means. 



6.0 
6.7 
7.3 
6.9 

6.3 

6.9 

5.6 

5.1 
5.4 
5.7 
6.0 
5.9 



10.8 
10.8 
9.3 

8.9 

7.8 

7.4 

7.0 
7.1 
8.2 
9.1 
8.1 



1893 



1891 



1891 
1893 
1895 



3.7 
3.9 
5.3 
5.0 

4.9 

4.5 

4.5 

3.9 
4.4 
4.1 
4.4 
4.0 



1877 
1877 
1875 
1900 



1901 

1899 

1897 
1900 
1901 
1896 
1875 



5.1 1900 28 



Maxima. 



c3 sj 



1879 
1893 
1893 

1903 

1888 
1893 
1878 
1891 



70 1903 14 



1881 
1883 
1901 
1885 
1891 
1900 

1881 

1884 
1886 

1890 
1900 

1883 

1881 

1883 

1898 
1880 



Storm winds.* 



as 

2-5 

sa 




4.4 


10 


5.5 


12 


6.7 


13 


5.3 


13 


3.9 


9 


3.3 


5 


1.8 


4 


1.2 


4 


1.4 


5 


3.0 


7 


3.7 


9 


3.9 


8 


42.0 


70 



1878 

1895 

1881 

1880 

(1878 
'11893 
11877 
11879 
11878 
-^1896 
(1901 

1887 
(1889 
11896 

1894 

1886 
j 1885 
11887 



* Winds exceeding 25 miles per hour. 



gales blow mostly from the northwest or west. Of the 42 instances 
referred to above, the relative frequency of the points of the compass 
from which they blew is as follows : 



DIRECTION OP THE WIND IN GALES. 





NW 


W 


S 


SW 


SE 


NE 


N 


E 


Total 


Number of gales. . . 


15 


13 


4 


3 


2 


3 


3 


1 


42 



The distribution of gales by months shows that they have been most 
frequent in February. The month of " equinoctial storms " is the only 
month without a gale to its credit in 30 years. 







FREQUENCY OF GALES IN 30 YEARS. 






Jan. 


Feb. 


Mar. 
3 


Apr. ; May June July 


Aug. 


Sept. 


Oct. j Nov. Dec. 


Year 


2 


10 


2 


2 


3 


4 


2 





3 5 


6 


43 



MAKYLAN'D WEATHER SERVICE 



265 



Prevailing Hourly Wind Directions. 

We have seen in preceding paragraphs that there is a well-defined 
diurnal fluctuation in the velocity of the wind. Without a close obser- 
vation of diurnal changes of direction in the locality of Baltimore, a well- 
marked periodicity would scarcely be suspected. Such is the fact, how- 
ever, as demonstrated by a reduction of the hourly observations for a period 
of ten years. The results are shown statistically in Table LXXIY and 
graphically in Plate XI, Fig. 73. A well-defined diurnal period was 

TABLE LXXIV.-PREVAILING HOURLY WIND DIRECTION. 



Hours 


a 

S3 

1-5 


i 


i 


a. 


>> 

03 


c 


>. 


sib 

3 




*> 

u 


o 




1 




pc 


^ 


< 


s 


►-5 


i-s 


< 


CQ 


O 


(z; 


P 


"^ 


lA.M 


NVT 


w 


NW 


W 


w 


SW 


sw 


sw 


sw 


N 


w 


W 


W 


o 


sw 


w 


NW 


w 


w 


SW 


sw 


sw 


NW 


N 


w 


w 


SW 


3 


sw 


w 


NAV 


NW 


SW 


sw 


sw 


N 


NW 


NW 


w 


NW 


NW 


4 


w 


NW 


W 


NW 


NW 


sw 


sw 


NW" 


N 


NAV 


NW 


w 


NAV 


5 


w 


W 


W 


W 


NAV 


sw 


sw 


NW 


N 


N 


NW 


w 


AV 


6 


w 


W 


W 


W 


N 


sw 


sw 


NW 


N 


NW 


N 


W 


W 




NW 


NW 


W 


W 


NE 


sw 


sw 


N 


N 


N 


NW 


AV 


NW 


8 


W 


W 


W 


E 


N 


sw 


sw 


N 


N 


JN 


W 


W 


AV 


9 


W 
W 


w 


SAV 
NAV 


N 

N 


SW 

SW 


sw 

N 


sw 
sw 


SW 

SW 


NE 
E 


NW 
E 


N 
N 


W 

SAV 


SW 


10 


SW 


11 


w 


w 


E 


SE 


SR 


w 


sw 


N 


N 


E 


SW 


SW 


SW 




w 
w 


w 
w 


SE 
SR 


SE 
SE 


SE 
SE 


SE 
SE 


sw 

SE 


SE 

SE 


E 

SE 


E 

SE 


w 
sw 


NAV 
NAV 


SE 


1 P.M 


SE 


2 


w 


NW 


SE 


SE 


SR 


SE 


sw 


SE 


SR 


SE 


w 


A\" 


SE 


3 ... 


w 
w 


W 
NW 


SE 
SE 


SE 
SK 


SE 
SR 


SE 
SR 


SE 
SR 


SE 
SE 


SE 
SR 


SE 
SE 


sw 

SE 


SW 
AV 


SE 


4 


SE 


6 


w 


W 


SE 


SR 


SE 


SE 


SE 


SE 


SR 


SR 


SE 


AV 


SE 


fi 


w 


W 


SE 


SE 


SE 


SE 


SW 


SE 


SR 


SK 


SR 


AV 


SE 


"r 


NW 
W 


W 
W 


E 
W 


SE 
SE 


SE 
SR 


SE 
SE 


SW 

sw 


SE 
SR 


SE 
SE 


SE 
SE 


SE 

w 


NAV 
AV 


SE 


S 


SE 


9 


w 


NW 


NW 


SE 


SR 


S 


sw 


SW 


SW 


E 


w 


NW 


SW 


10 


\w 


W 


NW 


SE 


SR 


SW 


sw 


SW 


SW 


K 


w 


AV 


sw 


11 


NW 


W 


NW 


SR 


SR 


sw 


sw 


SW 


SW 


NW 


w 


NW 


sw 


Midnig-ht 


SW 


W 


NW 


W 


SW 


sw 


sw 


SW 


SW 


N 


N 


W 


sw 


Prevail, direction. . 


W 


W 


NW 


SE 


SE 


sw 


sw 


SE 


SE 


SE 


w 


W 


SE 



Table LXXIV, showing the prevailing direction of the wind at the hours 
stated in the first column, is based upon a record of ten years, extending 
from 1893 to 1902. 



not at first expected to be revealed by the average hourly values which 
included all the observations of the year, or e\'en all of any particular 
month. Hence the first attempt to detect a periodic movement was 
made by selecting days in the months of January, April, July and Octo- 
ber, during which the skies were prevailingly clear, and the wind move- 
ment was light. This was done with a view to eliminating the influence 



266 THE CLIMATE OF BALTIMORE 

of neighboring .cyclonic disturbances. The result of such classification 
is shown in Fig, 74 for January, the diagram being based on ten selected 
days during which the sunshine exceeded 90 per cent of the possible 
amount, and the wind movement was less than 100 miles. The wind 
direction observations were classified into morning and afternoon winds, 
the former class including the hours from midnight to noon, the latter 
from noon to midnight. A prevailing westerly wind during the morn- 
ing hours and an easterly wind during the afternoon hours was so 
clearly revealed in all months in these diagrams that the hourly obser- 
vations for each hour and for the entire period of ten years were tabu- 
lated and charted, with the result shown in Table LXXIV and Fig. 73. 
These tables and diagrams reveal some interesting and probably unsus- 
pected facts concerning the daily fluctuations in the wind direction at 
Baltimore. A well-defined diurnal periodicity appears in all seasons 
of the year when the local conditions are not influenced by the presence of 
cyclonic disturbances. This is quite as well marked on cold winter days as 
in the summer time. Even by employing all observations, the average 
of all conditions of the weather, this periodic movement is conspicuous 
excepting in the winter months of December, January and February, 
when the cyclonic winds almost completely mask the periodic movement. 
An examination of Fig. 73 shows a prevailing wind from some quarter 
between northwest and southwest at all hours between midnight and 
11 a. m., with a very few exceptions when they are from the north. This 
is true for all months of the year. In January, February and December 
these westerly winds continue throughout the day. In all other months 
there is an abrupt change in the direction to the southeast about noon; 
a little earlier in March, April and October and a little later in July 
and November. The southeast wind then continues without interruption 
to an early evening or a night hour, when the direction returns quite 
as abruptly to the southwest or west. The hour of return to the morn- 
ing direction varies more than the change from the morning to the 
afternoon direction. The southeast returns to southwest in July as 
early as 6 p. m. ; in April and May as late as 11 p. m. The southeast or 
afternoon direction is maintained, accordingly, for a minimum period 



MARYLAND WEATHER SERVICE. 



VOLUME 2, PLATE XI. 



o z o 



y- 



"^ 1 "l >l < 1 >|r 



vl^^^-Sr^^^^r^ 



HH^ 



^'-n^'-^- 



^^^^r 



v?^ 



-?^^^^^^ 












i^ 



)(}()( k k k A A \ >{ 






^r 



\ < 1 - 



-4-^^- 

^s^ 



v^^ 



-^^ 



% Y "" ' ^ ""^ 



V*^ 



v'^ 



V'^ 



^^ 



ri^-A- 



^^ 









v*-v^ 



^^ 



V- 
V- 









s r? 



.2 M 

a 
>5 ■- 



>» .a bij 

S a; ■— 



2 -^ 



71 OJ 



2 3 



MARYLAND WEATHER SERVICE 



267 



of 5 hours, as in July, to a maximum of 13 hours, as in April, May and 
October. For the year as a whole, the southwest wind changes to a 
southeast at noon, maintains this direction until 8 p. m., and then 
returns to the southwest. The southwest becomes a west or northwest 
wind from 1 a. m. to 8 a. m., and then southwest again from 9 a. m. to 
11 a. m. These hourly changes are surprisingly uniform throughout 
the year when prevailing directions for a long period are considered, or 
on quiet days, for short periods of only a few days. 




Fig. 74. — Prevailing Morning and Afternoon Wind Directions in January. 

The heavy black lines Indicate the prevailing: winds during the hours from midnight 
to noon ; the light lines show the prevailing winds during the hours from noon to mid- 
night on selected days in January with a light wind and bright sunshine. 

In the figure based on the rougher grouping into morning and after- 
noon directions, the percentage of frequency of occurrence of the wind 
from each quarter is also shown. Fig. 74 indicates that even in mid- 
winter, represented by the month of January, the morning winds are 
distinctly west of the north and soutli line, and the afternoon winds 
mostly to the east. In Fig. 73, which is based on all observations during 
a period of ten years, the winds are westerly in January, February and 
December, both morning and afternoon, as stated above. A feature 
especially worthy of note is the abrupt change from southwest to south- 



368 THE CLIMATE OF BALTIMORE 

east about midday. The change from northwest or west to southeast, 
and in the reverse order, is made without lingering in the south. A 
prevailing south wind is not revealed in the diagrams or table for even 
an hour in any month of the year. 

It is difficult to assign a satisfactory cause for this daily periodic 
movement in the vicinity of Baltimore. The first explanation which is 
suggested is that it is a land and sea breeze effect. The station is, how- 
ever, too far removed from a body of water sufficiently large to produce 
the effect, even at the season of the year when contrasts in temperature 
between land and water are strongest. The harbor presents a compara- 
tively small water area in Patapsco Eiver, which is in turn twelve to 
fifteen miles from Chesapeake Bay, while the station is fully a mile 
distant from the harbor. These facts of local conditions make it ex- 
tremely improbable that the winds are the effect of an interchange of 
air between land and water areas. The suggestion arises whether the 
fluctuations are an integral part of the diurnal cyclone described in the 
preceding section on pressure changes. To demonstrate this would 
require a similar discussion of the hourly changes in direction at many 
widely scattered stations, especially at points somewhat nearer the path 
of the center of the diurnal cyclone, and on both sides of the equator. 

Prevailing Monthly and Annual Directions. 
In view of what has been presented in the foregoing paragraphs con- 
cerning the hourly changes in the direction of the wind, it becomes 
obvious that the choice of hours of observation is an important matter 
in determining the prevailing monthly and annual direction of the wind 
at any given locality. Most systems of observations, before the days of 
continuously recording instruments, provided for three eye observa- 
tions : one about 7 in the morning, another about 3 in the afternoon, 
and the third at about 9 in the evening. This combination yields a very 
fair value for the average direction for the day. The prevailing direc- 
tions based on two daily observations from 1893 to 1903 are placed 
alongside of the prevailing directions computed from three daily obser- 
vations and from hourly observations covering the same period. The 




Fig. 75. — Relative Frequency of Prevailing "Wind Directions. 

The diagram shows the relative froquency of the prevailing directions of the wind in 
the months of January, April, July and October, and in the year. For example, for the 
month of July, the prevailinK directions during a period of ten years were confined to 
southwest and southeast winds ; in January, the prevailing winds were always westerly 
during the same period, etc. 



270 



THE CLIMATE OF BALTIMORE 



WARM Year. 


NORMAL YEAR. 


Colo Yt'- 


1893-4 


DEC JAN FEB 


1903-4 

\ 
\ 
\ 




1903 

\ 
\ 

\ 




1893 

\ 


1900 


JULV y'V AUG. 

JUNE y^ 


1903 


1900 


Kr., 


\_ 


1890 

D 


N , D 


1893 

J 



Fig. 76. — Prevailing Monthly Directions of the Wind in Warm, in Normal 
and in Cold Seasons and Years. 



differences are marked only in August, September and October. By 
the system of two daily eye observations we obtain a prevailing north 



MARYLAND WEATHER SERVICE 



271 



wind in September and northwest in Octol^er, whereas the hourly obser- 
vations show a prevailing direction from the southeast during both 
months. The resultant prevailing directions based on three daily 
observations agree somewhat more closely with those derived from hourly 
observations, the chief divergence occurring in August and October. 
The annual path pursued is best represented by the diagram in Fig. 76, 
which is based on 24 hourly observations. 

The prevailing monthly directions derived from the three series of 
observations are as follows: 

PREVAILING DIRECTIONS. 





Jan. 


Feb. 


Mar. 


Apr 


May 


June 
SW 

sw 

SW 


July 

SW 
SW 
SW 


Aug 


Sept. 


Oct. 


Nov. 


Dec. 


Year 


7 a. 3 }). 

8 a. 8 p. . 


3P 


W 
W 
W 


W 
W 
W 


W 
NW 


SB 
SB 


SB 
SB 
SB 


SB 
SW 
SW 


SB 

N 
SB 


N 

NW 
SB 


W 

NW 
W 


W 

NW 
W 


W 

NW 


Hourly ob 


ser%'t'ns 


NW 


SB 


W.SE 



There is a fair degree of uniformity from 3'ear to year in the prevail- 
ing directions of the wind for the same months. The extent of the 
departure from the average direction is indicated in Fig. 76, in which 
the prevailing directions are shown for seasons and a year with a normal 
temperature, a well-marked temperature below the normal and for those 
well above the normal in temperature. In each case these seasons and 
years have been selected from the period from 1893 to 1904, and hence 
the prevailing directions are based on hourly observations. An inspec- 
tion of the figure will show that in nearly all cases there is an unusual 
percentage of northwest winds in the cold seasons, and a predominance 
of southerly winds (southwest to southeast) in the warmer seasons. 

This is in harmony with the results obtained by determining the 
average temperature of winds from each quarter. Selection was made 
of a number of days in each of the months of January, April, July and 
October, during which the wind blew from the same quarter all or most 
of the day. This was repeated for each of the eight points of the com- 
pass. The average temperature of these days was when computed from 
the hourly observations. It was not always possible to find days during 
which the wind blew from the same quarter more than half the day; in 



272 



THE CLIMATE OF BALTIMORE 



Buch cases it was necessary to admit days with a direction 45° on either 
side of the desired point of the compass. 

In the winter months, the southeast winds are the warmest; in the 



TABLE LXXV.- 


PREVAILING MONTHLY AND ANNUAL DIRECTION OF WIND. 


Year. 


a 

03 
1-5 




OS 


a 
< 


83 


d 

3 


D 

1-5 


si 

< 


D. 
0) 


o 

O 






c 
a 
< 


1871 


NE 

NW 
NW 

NW 

SW 
NW 
NW 
NW 
NW" 

W^ 
NW 
NW 

W" 
NW 

N 
NW 
NW 
NW 
SW" 

NW 

NW 

NW 

NW 

W 

w 
w^ 
w 

SW" 
SW 

W" 

w 

W" 

NW" 

NW 

w 

NW 


NE 
NW 
NW" 
NW" 
NW 

NW 

NW" 

W" 

W 

NW 

NW 
NW 
NW 
NW" 
NW 

NW 
NW" 
NW^ 
NW 
NE 

NW" 
NE 
NW 

N 

NW^ 

W 

w 

W" 
W" 
W" 

W" 

w^ 
w 

NAV 

NW 

W 

NW 


NW 
NW 

NW 

NW 
SE 

NW 

NW 
NW 
NW 

NW^ 

W 

NW^ 
NW" 
NW" 
NW 

NW 
NW" 
NW 
NW^ 

NW" 

NE 
NW 
NW 

NW 
NW 

NW 
E 
E 
E 
W 

W" 
W 

SE 

NW 

NW 
NW" 

NW 


W 

W" 

NW 

NE 
NW 

NW 

NE 
NW^ 
NW 

NW 

W 
NW 

N 
NW^ 
NW 

NE 
NW 
NW 

NE 

NE 

NW 
NW 

SE 
NW 

SE 

SE 
W 

w 

SE 
NW 

N 

W 

NW 

NW 
NW" 
NW 
NW 


NE 
NW^ 
NE 
SE 
SE 

SE 
NW" 
NW" 

SE 

S 

SE 

s 

SE 
NW 

NE 

NW 

SE 
SE 

SE 

S 

NE 

NW" 

W" 

SE 
SE 

SW 
NW^ 

SE 
SE 
W" 

E 
SW" 

SE 

SE 
SE 

SE 
SE 


SW 
SW 

SE 
SW 

s 

SE 
SE 
W" 
SW 
W" 

w 

s 
s 

SE 
NW^ 

SE 

S 

NW 

SW 

s 

NW 

SW" 

E 
SW^ 

E 

SW 
NW 
SW 
N 
SW^ 

SB 
NW^ 
SE 

SW 

s 

SW 
SW 


NW 

SW 
SW 
SW 

S 

NW 
SW 
SW 

s 
s 

W" 

s 

SW 

NW" 

SE 

SW 

s 
s 

SW 

s 

SW 
SW 
SW 
SW 
NW 

SW 

w 

SR 
SW 
SW" 

SW 
SW" 

w 

SW 

s 

SW 
SW" 


SW 

w 

NE 

N 
NE 

SE 

SE 

NW 

W 

S 

N 
S 
N 
N 
SW 

NW 

N 

SW" 
SW 

s 

NW 
NW 

SE 
SE 
SW 

N 
NW 
SW 
NE 
AV 

S 
SW 

NE 

SE 

N 

NW^ 

SW 


N 

N 

N 
NE 
SW" 

NE 

SE 

E 

SE 

NW 

SE 

N 
NE 

S 
N 

S 

N 

N 
NE 
NE 

NW" 
SE 
W 

NE 

N 

SW 

N 
SW 

SE 
SE 

N 
NW" 
NW^ 

N 

N 
SE 
N 


NW^ 

NW 
NW^ 

NW 
NW" 

NW^ 
NW 
NW" 

SE 
SE 

S 
NE 

N 
N 

N 

NW 

NW 
NW" 

NE 
NW 

NW 
NW 
NW 
NW 

N 

N 

N 

SE 

E 

E 

SE 
NW 
NW 

NW 
NW" 
NW 
NAV 


NW 

NAV 
NAA" 

NAV 
N 

NAV 
NAV 
W^ 
NW 
NW 

NW 

NE 

N 

NW 

NW 

NAV 
NAV 
NAV 
NAV 
NAV 

NAV 

NAV 

N 
NW 

N 

SW 

AV 

AV 

NAV 
NAV 

W 

N 
NW 

NAV 

NW 
NAV 
NAV 


SAV 
AA" 
NAV 
NAV 
NE 

NW 
SAV 
W^ 
E 

NAV 

AV 
NAV 
NAV 

N 
NW^ 

NAV 
NW 
NW 

NE 
NAV 

NAV 
NW 
SAV 

NW 
N 

W 
NW 
AV 
AV 
AV 

NAV 
NAV 
NW 

NAV 
NAV 
NAV 
NAV 


NW 


1873 


NW 


1873 

1874 


NAV 
NAV 


1875 


NW 


1876 


NAV 


1877 

1878 

1879 


NAV 
NAV 
NAV 


1880 


NAA" 


1881 


AV 


1883 


NAV 


1883 


NAV 


1884 


NAV 


1885 


NW 


1886 

1887 


NAV 

NAV 


1888 

1889 


NAV 
NW 


1890 


NW 


1891 


NAV 


1893 


NAV 


1893 


NAV 


1894 


NAV 


1895 


N 


1896 


SAV 


1897 


AV 


1898 


AV 


1899 

1900 


SE 
AV 


1901 


AV 


1903 


AV 


1903 

1871-1880 


NAV 
NAV 


1881-1890 


NW 


1891 1900 


NAV 


1871-1903 


NAV 



January, 1871-Oct6ber, 1879, from eye observations at 7.30 a. m., 4.30 p. m., and 11.00 p. m. 
November, 1879-December, 1886, " " " " 7.00 " 3.00 " " 11.00 " 

January, 1887-June, 1888, " " " " 7.00 " 3.00 " " 10.00 " 

July, 1888-November, 1893, " " " " 8.00 " and 8.00 p. m. 

December, 1893-December, 1903, " hourly record. 

spring, the south winds; in the summer, the southwest; in autumn, the 
winds from any quarter between east and southwest have about the same 
temperature. The relative position of the winds, arranged according to 
temperature, Avith the Avarmest first, is indicated below: 



MARTLAXD WEATHER SERVICE 
RELATIVE TEMPERATURE OF THE WINDS. 



273 





Warmest. 




Coldest 


January 


SE 


E 


sw 


NE 


W 


s 


NW 


N 


April 


S 


SW 


SE 


w 


N 


E 


NE 


NW 


July 


sw 


S 


w 


SE 


NW 


NE 


E 


N 


October 


SE 


S 


sw 


E 


NE 


NW 


W 


N 


Tear 


SE 


sw 


s 


E 


W 


NE 


NW 


N 







COMPARATIVE PREVALENCE OF STATED DIRECTIONS. 
(In average number of hours and minutes per day.) 



NE 



January 3.54 

April 1.24 

July 2-3« 

October 4.18 



Average '2.54 1.24 3 24 



1.42 
1.54 
0.43 
1.42 



E 


SE 


S 


SW 


W 


3.06 
2.24 
1.13 
2.36 


2.36 
7.24 
3.18 
3.24 


0.30 
0.30 
3.12 
1.24 


3.06 
3.24 
7.12 
3.06 


5.30 
3.24 
3.24 
2.54 


3 24 


4.06 


1.13 


4.06 


3.48 



NW 



4.36 
3.36 
8.24 
4.36 



4.06 



In the above table the figures show the number of hours and minutes 
during which the stated winds prevailed during the five years from 
1893 to 1897. For example, in January a north wind prevailed on the 
average for the five years, during 12 per cent of each day, or a little less 
than three hours. This is equivalent to about 3.7 days for the entire 
month. A south wind is in all months of the 3'-ear of decidedly shortest 
duration and of least frequency. 



Monthly Frequency of Stated Directions. 

A four years' record of hourly wind directions was examined and the 
observations tabulated in such manner as to show the number of days 
per month upon which the wind blew from each quarter. The monthly 
number of days for the entire year is as follows : 



.\ viTHgc no. of days . . 
Highest number 



NE j E 

I 



SE 



19.0 16.0 15.9 I 16.4 
21.8 19.5 19.2 22.0 



IX)weBt number 16.2 14.0 14.0 i 9.5 

1 I 



s 


SW 


W 


15.8 


18.7 


19.7 


19.5 


23.8 


22.8 


9.3 


11.0 


16.5 



NW 

20.5 
15.8 



274 THE CLIMATE OF BALTIMORE 

The above figures indicate that the wind blows from nearly every 
quarter once in about two days. Take for example a northwest wind; 
on the average it blows on 20.5 days per month the year round; in Janu- 
ary, February, March and July it has an average frequency of 22.2 days, 
and in September 15.8 days. We may also learn from these figures that 
the wind blows from four to five different directions every day, on the 
average. The exact figures, based on the four years' record, are as fol- 
lows for each month of the year : 

AVERAGE DAILY NUMBER OF WIND DIRECTIONS. 



Jan. 


Feb. 


Mar. 


Apr. 


May 


June 


July Aug-. 


Sept. Oct. 


Nov. 


Dec. 


Year 


4.6 


4.3 


4.4 


4.8 


4.8 


5.2 


4.9 5.3 


4.6 4.1 


4.4 

1 


4.6 


4.7 



These figures are in harmony with the facts recorded in the discussion 
of the diurnal periodicity of wind direction. It was there shown that 
the wind backed daily from a westerly direction in the morning to south- 
east or east in the afternoon, and then returned again at night to the 
west or northwest ; in other words, that the wind shifted through four or 
five points by noon and returned to its original position at night. 

The Direction of Upper and Lower Clouds. 

Table LXXVI is inserted at this point simply to show the prevailing 
direction of the wind at the level of the upper and lower clouds. The 
observations cover a period of five years and indicate the directions at 
7 a. m. and 3 p. m. The upper clouds include all cirrus forms, the alto- 
stratus and alto-cumulus; the lower forms include the cumulus and 
stratus forms. 

The upper clouds move from the west throughout the year, both in 
the morning and afternoon, with an occasional exception in the way of a 
northwest or southwest direction, especially at the afternoon observation. 
The lower clouds are also mostly from the west at 7 a. m. from May to 
December; from January to April they are generally northwest. In the 



^klARYLAXD WEATHER SERVICE 



275 



TABLEILXXVU.— PREVAILING DIRECTION OF LOWER AND UPPER CLOUDS. 





1 


^ 


1 


'C 


1 


2 


-. 


bl 


X 


1 


z 




-< 


7 o. m.... 














tr 


ir 


jr 


ir 


ir 


tr 


ir 


i««3if^:m:::: 










.... 




tr 
w 


tr 




.sir 
w 


ir 
sw 


tr 

X 


ir 
w 


. 3 p. m.... 














\\" 


\\ 


\\ 


w 


w 


X 


w 




7 a. m.... 






xir 


Air 


tr 


.vir 


tr 


tr 


ir 


ir 


A' 


ir 


ir 




3 p. HI.... 


fr 


s 


ir 


xtr 


tr 


E 


tr 


If 


ir 


ir 


ir 
Air 


ir 


ir 


1884 - 


7 a- m.... 


N 


NE 


X w 


NW 


w 


NK 
SH 


\\ 


w 


w 


w 


NW 


w 


w 




3 p.m.... 


>> 


s 
s\\ 


w 


xw 


xw 


E 


w 


w 


X w 


xw 


xw 


xw 


xw 


7«. HI.... 


fr 


.VK 


ir 


ir 


.sir 
Air 


tr 


ir 
Air 


.sir 
tr 


ir 


.s'lr 


ir 


Air 


ir 


3p. m.... 
1885-1 

Ta. m.... 


}r 


(r 


ir 


xtr 


sir 
tr 

E 


tr 


tr 


II' 


tr 


ir 


Air 


ir 


ir 


sw 
w 


N 


\\ 


\\ 


xw 


\\ 


w 


\\ 


sw 


w 


xw 


w 


[sp.m.... 


w 


NW 


w 


sw 
xw 


xw 


xw 


xw 


sw 


\\ 


sw 


NW 


xw 


xw 




7a. HI.... 


»r 


STF 




Nir 


tr 


ir 


A-ir 


Air 


tr 


.s' tr 
xtr 


ir 


ir 


»r 




$p. in.. . 


Tr 


Sir 


Nir 


NW 


xtr 


tr 


Air 


tr 


.sir 
Air 

XE 


tr 


ir 


ir 


ir 


1886 


7a. m.... 


w 

NW 


NW 


NW 


NE 
NW 


NE 

w 


w 


w 


w 


X 


w 


sw 
xw 


w 




3 p.m.... 


sw 
w 


xw 


w 


XW 


NW 


X \v 


xw 


sw 
w 


w 


xw 


xw 


\v 


NW 




7a. HI.... 


ir 


IC 


ir 


tr 


ir 


ir 


tr 


tr 


tr 


ir 


ir 


ir 


ir 




3 p. HI.... 


w 


ir 


ir 


X 

tr 


ir 


ir 


tr 


tr 


tr 


ir 


ir 


tr 


ir 


1887 ^ 


7 a. m.... 


sw 


SE 

sw 


xw 


XE 
NW 


s 


XE 


sw 


NE 


sw 


w 


w 
sw 


w 


sw 




3 p.m.... 


NW 


NE 
NW 


NW 


SE 
NW 


E 


w 


sw 

NW 


N 


SE 


xw 


NW 


xw 


xw 


j 7 a. »!.... 


w 


ir 


Air 


)(' 


tr 


tr 














IK 


1888 -{ 3 p. m.... 
1 7 a. m.... 


NW 


w 


ir 
w 


IC 

w 


tr 
sw 


tr 
sw 






.... 








W 
W 


13 p.m.... 


w 


w 


NW 


X \v 


sw 


\v 














W 


1 7a. HI,... 


w 


ir 


((• 


tr 


tr 


tr 


tr 


tr 


tr 


tr 


ir 


ir 


ir 


1 3 IJ. III.... 


jr 


tr 


tr 


xtr 


tr 


ir 


tr 


ir 


tr 


tr 


tr 


ir 


ir 


Prevail, i 7 a. m.... 


sw 

NW 


N E 


xw 


NW 


\\ 


w 


w 


w 


w 


w 


w 


w 


w 




3 p.m.... 


w 


NW 


w 


NW 


xw 


w 

NW 


N w 


\v 


w 


xw 


X w 


NW 


NW 



a. ] 

a. s. 1 

Vpiiff chpiiih. ■{ Ci. Cit. y in Italic. 
I .1. Ci(. 1 
L A. S. J 



Lower clouds. •{ |- ^'"• 



f Cu. 

! S. ■ 

I s 

L N. 



in Roman. 



afternoon, a northwest direction prevails during eight months of the 
year in the lower cloud layer, with a direction from the west in January, 
March, August and September. 



19 



276 



THE CLIMATE OF BALTIMORE 



ELECTEICAL PHENOMENA. 

Thunderstorms. 

The intimate relation existing between thunderstorm formation and 
temperature is demonstrated by an inspection of Table LXXVII and 
Fig. 77. The maximum frequency occurs in the month of greatest heat 



TABLE LXXVII.-HOURLY FREQUENCY OF THUNDERSTORMS. 



Midn't to 1 a. m. 

1- 2 

2-3 

3-4 

4- 5 

5- 6 

6- 7 

7-8 

&- 9 

9-10 

10-11 

11-Noon.. 
Noon- 1 p. m 

1-2 

2-3 

3-4 

4- 5 

5- 6 

6- 7 

7-8 

8- 9 

9-10 

10 11 

ll-Mldn't. 

Totals 



24 



97 



141 



164 



100 



38 



61 
66 
69 
38 
34 
31 
16 
13 



610 



Table LXXVII shows the total number of thunderstorms recorded as begin- 
ning within the stated hours in the 27 years from 1876 to 1903, during each 
month and during the entire year. 

and at the hour of the daily maximum temperature. In tabulating all 
thunderstorms which passed over Baltimore during a period of 28 years, 
a total of 678, we find the following distribution by months: 



Jan. 


Feb. 


Mar. 


Apr. 


Maj' June July 


Aug. 


Sept. Oct. 


Nov. 


Dec. 


Year 


13 


11 


20 


33 


107 1.56 179 


Ill 


43 7 


6 


2 

1 


678 



\ 



IMARYLAXD WEATHER SERVICE 



277 







- 










i ' 


1 


1 


Na 


ON 1 


2 


3 


4 


5 


6 


7 


8 


9 


1 


) 1 


Mdt 

1 1 














' — ' 






























s — i 
















































• 
































































t 


. 








^>-. 

N 


^^^-^/T\: 


•i^ 




x 


> Ir 


























4 


^^ 


s 






, 






• 


• 


• 




" 










u 


V 


^ 




^ 


\! 


— 


LV 


'. 


• 
















— 1 


• 


( 








^fe—V^y 






























^\ 






— 


-^ 
































• "^ 














■ 

































































































Fig. 77. — The Frequency and Distribution of Thunderstorms. 

The diagram represents over 650 thunderstorms which passed over Baltimore in the 
30 years from 1871 to 1900. The density of the shading increases with the frequency 
of the storms, showing a maximum frequency between 3 p. m. and 5 p. m. in the month 
of July. For the hours of the day and month during which less than five storms were 
recorded in 30 years, the actual number is indicated by the small dots. The figures 
attached to the curved lines represent the total frequency in 30 years. 























































































































































1 










1 










1 


1 


,ll 








1, 




1 


1 1 


1 



Fio. 78. — The Average Monthly Frequency of Occurrence of Thunderstorms. 



278 



THE CLIMATE OF BALTIMORE 



About five-sixths of the total annual number occur in the months of 
May, June, July and August. In the winter months they occur only at 
rare intervals, generally in connection with cyclonic storms which ex- 
hibit strong contrasts in temperature. The summer storms occur mostly 
in connection with shallow and not very well defined cyclonic depres- 













1880 








1885 








1890 








1895 








1900 




































































































































































































30 


























































































































20 


























































































































10 


























































































































































































Fig. 79. — The Annual Frequency of Occurrence of Thunderstorms from 
1871 to 1904. 

sions. That there is a strongly marked diurnal periodicity in the for- 
mation of the summer thunderstorms is shown bv the following figures: 



HOURLY FREQUENCY OF THUNDERSTORMS. 



Hours ending 

Morning' 

Afternoon 1 22 



3 

1 I 3 
45 76 78 



5 fi 7 8 9 10 n 12 

I 

3 6 fi 6 8 9 1.5 

61 .55 69 3d , 31 , 31 . 16 13 



:XIARYLAXD WEATHER SERVICE 



279 



The hourly distribution for all months, expressed in terms of the 
total frequency in 30 years, is shown in Fig. 77. The monthly and 
annual distribution by years, from 1876 to 1903, is shown in Table 
LXXVIII. The annual changes in frequency are also shown graphically 



TABLE LXXVIII.— NCMBER OF THUNDERSTORMS PER MONTH. 



Year. 



1876 .. 

187T 

1878 

1879 

1880 1 

1881 

1882 

1883 

1884 

1885 

1886 

1887 

1888 

1889 

1890 

I89I 

1892 

1893 

1894 

1895 

1896 

1897 

1898 1 

1899 1 

1900 

1901 .. 

1902 

1903 

Totals, 1876-1903... 3 
Averatre 0.1 



11 

0.4 



•? — 



30 
0.7 



33 
1.2 



107 
3.8 



166 
5.6 



6 


15 


3 


6 


5 


5 


2 


5 


8 


7 


5 


1 


« 


4 


3 


3 


8 


5 


11 


7 


7 


6 


8 


7 


8 


6 


6 


7 



179 
6.4 



111 

4.0 



4H 
1.5 



26 
16 
14 
8 
20 

21 

18 
36 
20 

37 

19 
24 
17 
16 

18 

25 
31 
21 
43 

24 



25 



0.2 0.1 



678 
24.2 



in Fig. 79. The average annual number is appro.\iinately '■^4, with a 
maximum frequency of 42 in 189-4, and a minimum of 8 in 1879. The 
following figures express more exactly the average monthly and annual 
frequency (See also Fig. 78) : 



.lull. 


Fob. 


Mar. 
0.7 


A,,r. 


•May 


.June 


July 


Aug. 


Sept.! Oct. 
1.6 0.2 


Nov. 
0.3 


Dec. 


Yiar 


0.1 


0.4 


1.2 


3.8 


6.6 


6.4 


4.0 


0.1. 


24.2 



280 the climate of baltimore 

Thunderstorm Probability. 

The probability of the occurrence of a thunderstorm upon any desig- 
nated day may be expressed in a very rough way by finding how many 
times a storm occurred on that day in the past. By examining all 
records of thunderstorms for 27 years and arranging them according to 
the day of the month upon which they occurred, we may roughly obtain 
a percentage of probable occurrence. 

Not much reliance should be placed upon such a method of forecast- 
ing, but some interesting relative values are brought out. In the past 
27 years one or more thunderstorms have occurred on every day of May, 
June and July, and on all but one day in August (the 20th) ; no thun- 
derstorm has occurred on September 6, 13, 21 to 23, 27, 28 or 30. In 
April there is no record of a storm on the following days : 1, 3, 6, 7, 13 
to 15, 21 to 23, 25, 30. The highest number occurring on any stated 
day in May is 7, on the 21st; in June, 11 on the 21st; in July, 11 on 
the 5th; in August, 7 on the 12th. Hence the highest probability of 
occurrence upon any day in the year is only eleven twenty-sevenths, or 
about 41 per cent. The average probability for a day in May is 14 per 
cent; in June, 20 per cent; in Jnlj, 23 per cent; in August, 15 per cent. 
The probability for the Fourth of July is only 17 per cent, or 5 per cent 
less than the average for July days. According to the Baltimore records 
a thunderstorm has passed over the city on July 4 only five times in 29 
years. One has occurred 11 times on July 5, in the same period, making 
the maximum probability 41 per cent of the total number of such days 
in 29 years. 

Consecutive Days with Thunderstorms. 

Thunderstorms generally occur as isolated storms in the vicinity of 
Baltimore. In over 80 per cent of all instances, a second storm does 
not occur on the following day. In only 14 per cent of all cases have 
there been thunderstorms recorded on two successive days, and in only 
a little over three per cent have storms occurred on three successive 
days. Only on one occasion have as many as 5 occurred on 5 successive 
days. These percentages vary in different months but they are not 



MARYLAND WEATHER SERVICE 



281 



large in any month. The following table shows the figures for each 
month and for the year: 

CONSECUTIVE DAYS WITH THUNDERSTORMS. 
(Total number in 28 years.) 





s 

1-5 




si 


p. 
< 




o 
a 

3 




si 
< 


OQ 


O 


> 

o 




03 




3 


11 


16 

i 


26 

2 




65 
13 
2 


87 
18 
4 
1 
1 


94 

21 

6 

3 


6T 

12 

4 


26 
6 

1 


T 


6 


2 


410 

72 
17 

4 

1 


On 2 consecutive days 

"3 " " 

"4 " " 

"5 " " 




s. w. 
Fic;. 80. — The Direction of Movement of Thunderstorms. 

The diagram shows the actual and relative frequency of thunderstorms from each 
direction of the compass. The total number of storms represented is nearly 400. 

Direction of Thunderstorms. 

In the vicinity of Baltimore, thunderstorms usually come into view 
from some point between northwest and southwest. Out of a total of 



282 



THE CLIMATE OF BALTIMORE 



about 400 storms, nearly 90 per cent moved from some one of tliese 
points. (See Fig. 80.) The order of frequency of direction is as 
follows : 

THUNDERSTORM DIRECTIONS. 
(1876-1902.) 





Jan. 


Feb. 


Mar. 


Apr. 

8 

3 

I 


May 


June 


July 


Aug. 

21 
18 
21 

3 

2 

2 

'3 


Sept. 

18 
6 

'i 
i 


Oct. 

1 
2 

2 


Nov. 

"i 


Dec. Year 


NW to SE 


1 
i 


3 


3 
3 
4 

i 


26 

19 

13 

2 

2 

2 


20 
29 
31 

'3 

1 

1 


33 

32 

20 

3 

6 

'5 
3 


.. 126 


\V •* E 


.. 115 


SW " NE 

S •• N 


.. 104 

s 


SE " XW 

E " W 

NE " SW 


1 14 
4 


N " S 


13 









Pressure Changes Duking Thunderstorms. 

A thunderstorm usually occurs with a falling barometer; the bar- 
ometer rises during the first few minutes after the storm has begun, 
falls slightly before the close of the first hour, and then maintains a 
steady pressure for several hours, eventually rising slowly (see Pig. 81). 
In other words, the storm usually breaks in the trough of a cyclonic dis- 
turbance. The following table shows the average hourly barograpli 

PRESSURE BEFORE AND AFTER THUNDERSTORMS. 
(Station readings; not reduced to sea-level.) 





No. 


Hours Preceding. 


Rise. 


Hours following. 


Month. 


5th 


4th 


3rd 


3nd 


1st 


Begin- 


P. 


'H 


1st 


2nd 


3rd 


4th 


5th 






29.45 










ning. 


H 


ss 




.30 




.20 




Feb.... 


(3) 


.43 


.40 


.36 


.34 


29.31 


0.25 


29.36 


.34 


.26 


.18 


March. 


(3) 


.67 


.63 


.60 


.56 


.51 


.49 


0.25 


.53 


.51 


.51 


.51 ! .53 


..>t 


April.. 


(2) 


.38 


.38 


.38 


.38 


.38 


.38 


MS 


.47 


.44 


.44 


.43 


.44 


.44 


May... 


118) 


.78 


.77 


.76 


.76 


.74 


.74 


0.42 


.79 


.77 


.77 


.78 


.79 


.80 


June... 


(16) 


.79 


.78 


.77 


.76 


.75 


.74 


0.43 


.77 


.75 


.74 


.74 


.75 


.75 


July... 


(19) 


.77 


.77 


.75 


.74 


.73 


.72 


0.51 


.78 


.74 


.74 


.74 


.74 1 


.76 


Aug... 


(11) 


.80 


.80 


.79 


.79 


.79 


.79 


0-40 


.84 


.81 


.80 


.79 


.79 


.80 


Sept. . . 


(6) 


.76 


.75 


.74 


.73 


.72 


.70 


0.53 

• 


.76 


.74 


.75 


.77 


.78 


.80 


Aver... 


(76) 


.68 


.66 


.65 


.63 


.62 


.61 


.45 


.66 


.64 


.63 


.63 


.63 


.6?$ 



* Time from beginning of the rise to its maximum, in hours and minutes. 

readings before and after about 75 thunderstorms selected from the 
records of the past three or four years. The readings of the barograph 




3»ia ' 



M«.rck 3o, 130I 



M».rt.lv 2}, ?9e3. 




flpr. 2,6, I9»2.. 



Moy i, <»o2.. 



Muy iS. 1902. 




F«.l).a8, (>«3 



/Mav, i*. f9»3. 



iM»3 (1, I9» + . 




Ju N 3, 1901. 



7u^y 20, '^oi. 



7u/y ', 'J't. 



flgg fc, i9o2 



/?uj. i 7, ( j«z 



ffw) 2 9 j9oS. 



Fio. 81. — Some Typical Barograms During Thunderstorms and Squalls. 



284 THE CLIMATE OF BALTIMORE 

are given for the five hours preceding and following the breaking out of 
the storm. The minimum reading is also given, just before the begin- 
ning of the " hump/' which constitutes the characteristic feature of a 
barograph curve during the passage of a thunderstorm. The duration 
of the rise in pressure, from the minimum to the maximum point attained 
in the " hump " is given in hours and minutes. 

Of the 76 thunderstorms examined in the above table, about one-third 
began with a value between 29.60 inches and 29.69 inches for the bar- 
ometer reading, assuming the beginning of the rise in tlie barometer to 
be the beginning of the storm. Tabulating the barometer readings ac- 
cording to the pressure at the breaking out of the storm, we have the 
following comparative frequency of stated values : 

FREQUENCY OF STATED READINGS OF THE BAROMETER AT THE BEGINNING 

OF THE STORM. 



Barometer Re 

29.30-39 in 
4049 


ading. 

;hes 


Actua 

3 

5 


Frequency 
I. Pel 


•centage. 
4 

7 


50-59 






5 


7 


60-69 
70-79 
80-89 
90-99 




24 

14 

17 

7 


32 

18 

22 

9 


30.00-09 




1 


1 



7G 100 

The thunderstorms in the above table were confined almost entirely to 
the months of May to August. We see that the storm broke most fre- 
quently when the pressure registered some value between 29.60 inches 
and 29.69 inches; this was true of 32 per cent of all cases tabulated; 
in 72 per cent of all cases the barometer reading was between 29.60 
inches and 29.89 inches. In only one instance was the pressure above 
30.00 inches, namely, in July, 1900. The lowest pressure recorded in 
any case was 29.30 inches, in February, 1903. See Fig. 81 for the 
character of the rise in pressure during thunderstorms. 

Hail. 

The phenomenon of hail formation is so intimately associated with the 
dynamics of thunderstorms that the treatment of the subject is taken 



MARYLAND WEATHER SERVICE 



285 



up in connection with these storms rather than with the subject of pre- 
cipitation. Hail is not of frequent occurrence in the vicinity of Balti- 
more. During a period of 28 years it has been recorded but 49 times, 
or less than two times per year. The annual number has varied between 
and a maximum of 6. The monthly and annual distribution is shown 
in Table LXXIX, and the hourly distribution by months in Fig. 82. 



TABLE LXXIX.-FREQUBNCY OF OCCURRENCE OF HAIL. 



Year. 



1876. 
1877. 
1878. 
1879. 
1880. 

1881 

1883. 
1883. 
1884. 
1885. 



188fi. 

1887.. 
1888.. 
1889.. 
1890.. 



1891. 
1892. 
189:5. 
1894. 
1896. 

1896. 



1899. 
1900. 

1901. 

1903. 



Total in 28 years... 



9 I 6 



66 



Tlie liourly distribution for the entire year is as follows: 



HOURLY FREQUENCY OF HAIL. 
(Total number iu 28 years.) 



Time A. M. 1 9 


10 


11 


Noon. 


1 


Z 


8 4 6 


6 


7 


8 


9 


10 


11 


P.M. 


Frequency, i 2 


1 


1 


2 


8 


< 1 i 
4 6 13 


7 


4 


8 


2 





1 





286 



THE CLIMATE OF BALTIMORE 



5 6 7 8 9 10 II Noon 12 3 4 5 



7 8 9 10 tl MO 







































■ 














1 — 






























































































































































' 






....i. 






































.it' 
















































































(1 - 




1 ■ 








































• 


•[ 



























































































































































Fig. 82. — The Frequency of Occurrence and the Hourly and Seasonal 
Distribution of Hailstorms. 

The diagram show.s all of the hailstorms recorded as occurring in Baltimore during 
a period of 28 years. Each black dot represents a storm. 



Mdt. Noon Mot Noon Mot. Noon Mdt 



Inches 


1 




2 




3 




30.00 




^ 








-^ 






.50 
29.00 
30 00 








s ~--— 






4 




5 




6 




29.50 






— ■ 


-..^ — 





















Fig. 83. — Barograms During Hailstorms. 

Each barogram represents a period of 24 hours, from midnight to midnight. The time 
of occurrence of the hail-storm is indicated by the sharp temporary rise and fall in 
the curve. 

Dates of the storms represented : 1. .July 7, lOiH. 2. February 28, 1002. .3. August 
27. 1902. 4. June 8. 190.3. ."). May It). 1904. 6. .July 5. 1004. 



MARYLAND AVEATHEU SERVICE 



287 



The hourly frequency rises to a maximum between 4 p. m. and 5 p. m. 
The dates of all recorded occurrences of hail in the vicinity of the Balti- 
more station of the Weather Bureau are siven in Table LXXX. 



TABLE LXXX.— DATE AND HOUR OF OCCURRENCE OF HAIL. 



Date 


Time 


- 


First 
precip.* 


Date 


Time 


First 
precip. 


1876, Mar. 2» 


7.1.5 p. ra. 


1.S95, July 5 


12.25 p. m.-12.30 p. m. 




" May 12 


2.1.5 p. m.- 2.30 p. 


ra. 




" 16 


+ 


4.'36p 


" 21 






9.35P 


" Aug. 11 
" " 31 


4. .50 p. m.- 4.52 p. m. 
4..30p. m.- 4.35 p.m. 




1879, June 11 


2.00 p. m.- 3.03 p. 


m. 










., 28 


+ 




4.'29p 


" Sept. 19 


+ .... 


3.10p 


1880, Apr. 17 


Early a. m. 






1S96, July 27 


8 13 p. ra.- 8.17 p.m. 




" July 20 


t 




4.40p 


1897, Mav 21 
" Aug. 23 


1.45 p.m.- 1.55 p.m. 
11.47 a. m.-11.52a. m 




1881, June 8 

1882, •• 19 


t 




3.15p 
Noon. 


1898, May 16 


\ 4.10 p.m.- 4.13 p.m. 
1 5.14 p. m.- 5.17 p. m. 




1884, July 11 


4.00 p. m.- 4. .50 p. 


ra. 




1899, Mar. 12 


J 8.27 p. m.- 8 35 p. m. 




1887, Feb. 18 


5.02 p. m.- 5.05 p. 


ra. 




•' 28 


8.07 a. m.- 8.09 a. ra. 




" May 26 


t 




5.26p 


'• Apr. 16 


jl0.25a.ra.- .... 
1 12.20 p. m.- .... 




" June 18 


+ 




4.35p 








" July 18 






5.10p 


" May 16 


t 


6.50p 


1888, June 16 


2.30 pVin.- 2.35 p. 


m. 




" June 6 


t 


7.22p 


" 23 


t 




ll.'o.^a 


" Aug. 21 


7. '5:3 p. m.- 7.3S p. m. 




" Aug. 8 


5.45 p. m.- 5. .50 p. 


m. 




1901, May 25 


t 


10 15p 


1890, Apr. 27 


3.45 p. m.- 4.00 p 


m. 




" July 7 


7.00 p. m 




" May U 


t 




e'.srlp 


1902, Feb. 2.s 


9..50a.m 




" June 12 






3.55p 


" Aug. 27 


4.43 p. m.- 5.05 p. ra. 




1892, Mav 2:! 


i 4.08 p. m.- 4.]6p 


m. 




19C3, May 24 


3.24 p. ra.- 3.27 p. m. 




" June 30 


12. 02 p. m.-12.10p 


m. 




" June 8 


3.27 p. m.- 3.33 p. m. 




1893, July 3 


5.25 p. m.- 6.35 p 


m. 










J, 


t 




4."l6p 








1894, May 6 


fi..57 p. m.- 7.03 p 


m. 










" June 12 


4.37 p. m- 4.40 p 


m. 










" 24 


t 




4.'l6p 









* 1 n the absence of the exact time of occurrence of hail the time of beginning ot precipi- 
tation is given. 
+ Hail in city or suburbs ; none at station. 
i Not accompanied by a thunderstorm. 

In Fig. 83 a few typical barograms are reproduced showing the char- 
acteristically sharp rise and fall of the atmospheric pressure during the 
passage of a hailstorm. In the thunderstorm curve the summit of the 
" hump '' is usually more rounded, as shown in Fig. 81. 



288 



THE CLIMATE OF BALTIMORE 



Auroras. 

The following brief list contains all occurrences of the aurora borealis 
reported in the records of the U. S. Weather Bureau at Baltimore since 
the establishment of the station in 1871 : 



Date. 


Duration. 


Date. 


Duration. 


1872, Feb. 3 


8 p. m. to 9 p. m. 


1892, Feb. 13 


6.30 p. m. to 9 p. m. 


Apr. 11 


8 p. m. " 10 p. m. 


May 18 


8 p. m. " 11 p. m. 


Auf?. 3 


8.40 p. m. " 10 p. m. 


July 16 


10.30 p. m. " 11.30 p. m. 


Aug. 4 


About 9 p. m. 


1893, Feb. 4 


9 p. m. " 12 md't. 


Aug. 8-9 


9 p. m. to 3 a. ra. 


1894, Feb. 23 


9 p. m. " 10 p. m. 


Oct. 14 


6.30 p. m. " 7 p. m. 


Mar. 30 


7.20 p. ra. " early a. m. 


Nov. 1 


10 p. m. ." 11 p. m. 


1897, Jan. 23 


Evening. 


1873, June 36 


About 10 p. m. 


1898, Sept. 2 


About 10 p. ra. 


1882, Apr. 16-17 


10 p. m. to 3 a. m. 


1903. Oct. 12 


7 p. m. to 7.30 p. m. 


Apr. 20 


12.30 a. m. to 3 a. m. 







SuxspoTS AND Weather. 

The effort to extend the period covered by weather forecasts has ever 
been one of the chief aims of the practical meteorologist. The limit of 
time for which forecasts are now issued by American and European 
official weather services is about three days. The forecasts made from 
day to day generally cover from 24 to 48 hours ; under favorable conditions 
the time is occasionally extended to three, or even four days, but this 
is only done in exceptional cases. The three or four day limit is probably 
the utmost that will be realized from present methods, and with the 
material now at our disposal. The only hope of extending the period 
lies in the discovery of some new laws of weather sequences. 

The search for periodical recurrences of similar weather conditions has 
long been one of the most interesting, and, at the same time, one of the 
most elusive problems in cosmical ph3^sics. The investigations have 
usually been along two lines : A series of observations has been subjected 
to close examination and critical analysis in order to discover any periodic 
change which may be hidden in the constant fluctuation of values; or a 
periodic movement has been assumed and the weather observations 
examined for synchronous changes. 

There is but one undisputed source of terrestrial weather changes — ; 
namely, the sun. While no one doubts the influence of the sun upon 



MARYLAND WEATHER SERVICE 389 

the earth^s atmosphere many claims have been, and are still being made 
in favor of attributing to other heavenly bodies, such as the moon or the 
planets, a considerable effect. The champions of the moon's influence 
are legion, and they never grow less; but the arguments of several 
centuries, including much serious and intelligent effort, have not suc- 
ceeded in securing for lunar or planetary forecasts a position more 
exalted than the pages of the perennial almanac. 

It is now approximately 100 years since a definite period was dis- 
covered in the increase and decrease of sunspot frequency, and less 
than 50 years since the flames emanating from the surface of the 
Sim, or the solar prominences, were first observed. The first definite 
relation between sunspot frequency and terrestrial changes was the 
discovery of the synchronous activity of the magnetic needle. There 
is now no question about the coincidence of these phenomena whatever 
may be the true relation existing between them. 

In attributing terrestrial changes of the weather to " sunshine," we have 
until comparatively recent times assumed a constant output of radiant 
energy from the sun. In view of the fact that our present knowledge 
concerning the physical condition of the sun indicates a surrounding 
atmosphere composed of incandescent metallic vapors, is it not more 
rational to suppose that the temperature of these highly heated gases 
is var\ang constantly, than it is to think of them as at a constant 
temperature? If the temperature does vary, the fluctuations must 
necessarily affect to a greater or less degree the physical condition of 
our own atmosphere. The question then becomes one of degree of 
influence. There are many observed facts which point to a varying 
output of solar radiant energy, and quantitative measurements will not 
long remain unknown. Just what the nature of this influence is has 
certainly not yet been demonstrated. One obstacle in the way of more 
rapid progress toward a solution of these problems may be found in the 
crudeness of much of our observational data, and the lack of uniformity 
in the methods and hours of observation. Moreover, in the middle lati- 
tudes where most of our best observations, and the longest scries which 
we possess, have been made, the non-periodic fluctuations are so much 



390 THE CLIZilATE OF BALTIMORE 

greater than the periodic changes sought that the latter are separated 
out from the former only with the greatest difficulty and care. The 
most favorable regions of investigation for periodicities based on solar 
changes are the tropics. Here the daily, seasonal, and incidental changes 
in weather conditions are more uniform and less pronounced, permitting 
of more ready detection of the periodic changes of longer duration. 

It seems highly probable that changes in terrestrial temperature, in 
rainfall, storm frequency, etc., may be due to changes in the physical 
constitution of the sun's surface. It may also be that these solar changes 
are not reflected directly in the conditions above mentioned. Similar 
weather conditions should not be expected in all parts of the earth at 
the same time. The results of efforts thus far to find a direct connection 
between the sunspots and weather changes have apparently failed largely 
as a consequence of dissimilar weather conditions found in different 
localities during similar phases of the solar period. These contra- 
dictions may be only apparent, not real. Let us suppose, for instance, 
that the normal distribution of pressure over large areas is disturbed 
as a result of changes in the quantity of heat received from the sun 
from year to year. We would then have excessive heat in some places 
and at the same time abnormal cold at others; or we would have 
excessive rains here and droughts there; or an increase in storm fre- 
quency in one place and a decrease in another, when compared with 
average conditions. Such variations cannot be looked upon as contra- 
dictory; they are the natural results of changes in the distribution of 
pressure, changes such as we see upon our weather charts every day. 

The present status of the problems concerning the relation between 
the varying physical conditions of the sun and synchronous changes in 
our terrestrial atmosphere is well stated in the following extract from 
a recent paper by Professor Bigelow,* who is one of the most active and 
able investigators in this most promising field of cosmical physics. 

" The numerous studies during the past fifty years into the apparent 

*Bigelow, F. H. Synchronism of the Variations of the Solar Prominences 
with the Terrestrial Barometric Pressures and the Temperatures. Monthly 
Weather Review, Washington, D. C. November, 1903. 



MAETLAXD WEATHER SERVICE 291 

synchronism between the solar variations of energy and the terrestrial 
effects, as shown in the magnetic field and the meteorological elements, 
have been on the whole unsatisfactory, if not disappointing. Just enough 
simultaneous variation has been detected in the atmospheres of the sun 
and the earth to fascinate the attentive student, if not to justify a large 
expenditure of labor, in view of the great practical advantages to be 
obtained in the future as the result of a complete understanding of this 
cosmical pulsation. The attack upon the problems has really consisted in 
rather blindly groping for the most sensitive pulse in the entire cosmical 
circulation, and in disentangling the several interacting t}-pes of impulses. 
It is e^-ident that the partial failures hitherto attending this work have 
been due to two principal causes: (1) The comparison was made 
between the changes in the spotted areas of the sun and the terrestrial 
variations, but these solar changes were not sensitive enough to register 
a complete account of the action of the solar output. Discussions of 
the spots are being replaced by others upon the solar prominences and 
faculse, which respond much more exactly to the working of the sun's 
internal circulation: (2) The magnetic and meteorological observa- 
tions have not been handled with sufficient precision to do justice to the 
terrestrial side of the comparison. It is evident that all these physi- 
cal data at the sun and at the earth must be computed with an 
exactness comparable to that of astronomical observations of position, 
if meteorology is to be raised to the rank of a cosmical science. When 
one considers the crudeness of the meteorological data, taken the world 
over, due to the character of the instruments employed, the different 
local hours of observation, and the divergent methods of reduction, 
it as no wonder that small solar variations have been swallowed up 
in the bad workmanship of meteorologists. The prevailing methods 
have been sufficient for forecasting and for climatological purposes, but 
they are entirely inadequate for the cosmical problems whose solution 
will form the basis of scientific long-range forecasts over large areas 
of the earth — that is, for forecasting the seasonal changes of the 
weather from year to year. It is perfectly evident that if secular varia- 
tions f)f any kind, such as the annual changes in terrestrial pressure, 
20 



292 THE CLIMATE OF BALTIMORE 

temperature, or magnetic field, are to be attributed to solar action, the 
original observations must be finally reduced to a homogeneous system. 
The local peculiarities of each station must be carefully eliminated, 
and the data of numerous stations must be concentrated before anything 
like quantitative cosmical residuals can be obtained. When we consider 
that .there have been numerous changes in the elevations of barometers, 
various methods of reducing the readings, and many groups of selected 
hours of observations entering into the series at the same station, how 
could it be expected that anything better than negative results in solar 
problems would be obtained? The skeptical attitude of conservative 
students, who declare that the many indecisive results already obtained 
mean that there is no true and causal solar-terrestrial synchronism, is, 
of course, quite fallacious until it has been demonstrated by the use of 
first-class homogeneous data that the suspected physical connection is 
imaginary. There is but little question that the existing uncertainty is 
in ^f act based upon the use of the ver}^ imperfect methods of observation 
and reduction which have prevailed in meteorological offices, rather than 
upon the unreality of the phenomena in nature." 

The results of a comparison of Baltimore weather observations Avith 
the sunspot and solar prominence frequency curves have not differed 
from those arrived at in similar investigations elsewhere — they neither 
prove nor disprove an intimate relationship. As pointed out in preced- 
ing paragraphs there are synchronous changes here and there in the 
constantly fluctuating terrestrial conditions, but on the whole the evi- 
dence is negative. In view of the complicated character of the weather 
conditions, especially in our middle latitudes, a close agreement in phase 
of any periodic changes need scarcely be looked for, but the length of 
the period of the terrestrial and solar changes should harmonize. 

In Fig. 84, the sunspot and solar prominence curves, constructed from 
Wolf's tables as printed in the Monthly Weather Eeview* are shown 
in connection with curves representing the actual annual changes at 
Baltimore in: (a) the mean pressure, (&) the mean temperature, (c) 

♦Monthly Weather Review of the U. S. Weather Bureau for April, 1902. 



MARYLAND WEATHER SERVICE 293 

the total rainfall, (d) the frequency of thunderstorms, and (e) the 
fiequency of storm winds (exceeding 25 miles per hour). In Plate XII, 
these facts have been presented again in a modified form, the annual 
values for the climatic conditions having been smoothed, eliminating 
some of the irregular fluctuations in order to show more clearly any 
periodic occurrences of longer period. The values employed in the con- 
struction of the curves of Plate XII were computed by means of the 

following formula, ^^^^ TL^ in which a, h, and c represent actual 

4 

values for three successive years. In this manner a smoothed value was 
computed for each year of the entire series. 

Plate XII contains in addition a record of all excessive rains at Balti- 
more from 1836 to 1904, an excessive rain being defined as a fall of 2.50 
inches or more in 24 consecutive hours. These excessive rainfalls were 
taken from two distinct records; those occurring from 1871 to 190-4 are a 
part of the official record of the U. S. Weather Bureau; those of the 
period from 1836 to 1870 are from the record of the Army Medical 
Department at Fort McHenry, with very few exceptions. The rainfalls 
of the earlier period are apparently too frequent, owing to the fact that 
there was more uncertainty in noting beginnings and endings of precipi- 
tation than in the later series. The earlier record doubtless contains 
excessive rains in which the time limit was extended to 30 or 36 hours. 
However, the grouping and relative frequency of these excessive falls are 
the features to which especial attention is called ; the actual frequency 
is of less importance. 

In a preceding paragraph reference was made to the fact that the 
periods of excessive frequency of heav}^ rains coincided very closely with 
the periods of minimum sunspot frequency from 1871 to 1901. The 
earlier observations were not then at hand. On extending tlie series of 
observations back to 1836, the same nice agreement does not hold good; 
there is a gradual change to a reversal in the phase of the sunspot period. 
However, the grouping is very striking, and the average length of the 
periods from maximum to maximum, or minimum to mininnun. agrees 
very well witli the average length of the sunspot and solar prominence 
periods. 




(Smoothed annual values for weather condifo"'' • excessive rainfalls, which are observed values.) 



MARTLAXD WEATHER SERVICE 395 

These selected Baltimore observations are reproduced in Fig. 84 and 
Plate XII, not so much to call attention to any particular cycle of changes 
as to place tliein into convenient form for a critical study by some who 
may later find a more suitable clue to the solution of the difficult problem 
of the relationship between solar activity and terrestrial weather changes. 

General Character of the Seasons. 

The average values of climatic conditions and the departures from 
these values have been discussed in considerable detail in the text and 
tables of the preceding pages. To all but the expert in the study of 
statistical tables it is a difficult matter to derive, from a table of figures, 
no matter how perfect the arrangement, a satisfactory conception of the 
general character of a selected period, be it a day, a month, a season, or 
a year. The graphic method of presenting results appeals to a greater 
number because it enables the eye to take in at a glance relations between 
groups of values which would be more or less obscure to the casual reader 
when presented in tabular form. While recognizing the limitations of 
the graphic method for representing such a complex conception as the 
general character of the season, it has yet seemed profitable to resort to 
the use of a diagram for grouping such factors as are deemed most 
important in characterizing the weather conditions of a season. 

The results arc siiown in Plates XIII to XVII. in which eight selected 
factors, expressed as departures from the normal climatic conditions at 
Baltimore, are presented for each season and year from 1871 to 1904. 
The choice of factors, as well as their arrangement, was a purely arbitrary 
matter, hut they, as a iiiaitcr of course, remain the same for each season 
and year. Tlir method of cliartiug may be briefly described here, 
although the legends on the plates will be found sufficiently clear for this 
purpose. The normal value of each of the climatic factors selected is 
represented by a point in the circumference of a circle, at the inter- 
section of a radius representing one of the eight points of the compass. 
Taking for example the mean seasonal temperature: A departure above 
the normal would be represented by placing the point a given number 
of units beyond the circle along the extension of the radius representing 



296 THE CLIMATE OF BALTIMORE 

temperature ; for a departure below the normal, the point would be placed 
within the circle along the same radius. By applying a similar method 
for each of the factors and joining the points thus located, we have a char- 
acteristic octagonal figure. A season having all its points located in the 
circiimference of the circle would be represented by a regular octagon. 
The degree of departure from the regular octagon shows at a glance the 
amount and the character of the departure from normal climatic con- 
ditions of the season inspected. The unit adopted for measuring the 
amount of departure is the same in the discussion of all seasons and 
years, namely, the average variability of the factor. The average varia- 
bility was obtained by adding up the individual departures for each 
season or year and dividing by the number of years employed. 

The normal value for each factor is shown by the figures given below 
the designation of the factor in the key accompanying each set of 
diagrams. To bring the form representing the character of the season 
into further relief and separate it from the scale, the former is tinted. 
The comparison of the seasons at Baltimore from 1871 to 1904 will be 
found to be greatly facilitated by the use of these diagrams. 

OBSERVATIONS AND INSTEUMENTAL EQUIPMENT. 

Historical Notes. 

Volume I of the Eeports of the Maryland Weather Service (1899) 
contains a report upon the progress of meteorology in Maryland. Eefer- 
ences are there made to the early records of the weather which came to 
the notice of the writer, and to the development of systematic instrumental 
observations. While temperature records were regularly kept as early as 
1753 in Prince George's County, we find no evidence of any instrumental 
observations within the limits of Baltimore City, or in the immediate 
vicinity, until the series made by Capt. Lewis Brantz, referred to in the 
preceding pages in connection with the discussion of temperature and 
rainfall. To the best of the writer's knowledge the very complete and 
accurate observations of Capt. Brantz were the earliest made within 
Baltimore City. 



VOLUME 2, PLATE XIII. 




PRECIPITATION 
10 INCHES 



corner of the plate. A departure above ( + ) or below ( — ) the 
f departure (indicated by the figures 1, 3, 5 along the radii) is the 



MARYLAND WEATHER SERVICE 




General CHAnACTEn of the Seasons.— Winter. 
average seasonal variability of the factor. °"' ■ '■'=^P«'="^«'y. =■"• ^'o^g a radius or its extension. Tlie unit of departure (indicated by the figures 1. 3, 6 along the radii) is the 



VOLUME 2, PLATE XIV. 




DAYS WITH 
ECIPITATPON 



PRECIPITATION 
10 9 INCHES 



lor of tho plate. A dpparture above (4-) or below ( — ) the 
jparturc (indirated Ijy the figures 1, 3, 5 along the radii) is the 




General Character i 



Seasons. — Sprin 



Pointe in the oircumferencp of the circle, at the Interaection of the railii (0), represent the average value of factors enumerated In the key In the lower right-hand corner of the plate. A departure above ( + ) or below (— ) the 
average (or normal) value of the factor is shown by the position of points .beyond or within the circle, respectively, and along a radius or Its extension. The unit of departure (indicated by the figures 1, 3, 5 along the radii) is Ihe 
average seasonal variability of the factor. 



VOLUME 2, PLATE XV. 






PRECIPITATION 
'»« INCHES 



coriipr of Iho plate. A departure above ( + ) or below ( — ) the 
of departure (indicated by the figures 1, 3, 5 along the radii) is the 




General Chakactee of the Seasons. — Sujimer. 

Points In the circumference of the circle, at the intersection of the radii (0). represent the average value of factors enumerated in the key in the lower right-hand corner of the plate. A departure ahove ( + ) or below (— ) the 
average (or normal) vahie of the factor Is shown by the position of points beyond or within the circle, respectively, and along a radius or its extension. The unit of departure (indicated by the figures 1. 3, 5 along the radii) is the 
average seasonal variability of the factor. 



VOLUME 2, PLATE XVI. 




DAYS WITH 
ECIPITATION 



PRECIPITATION 
98 INCHES 



cornpr of tlio platp. A doparturp ahovp (-f ) or 1)p1ow ( — ) thp 
of rlepartiirp ( indicated by the fiKiires 1. 3, 5 along the radii) is the 




Seasons. — Autumn. 



Points in thp rircunitcrenrp of the rircle. at the inlorspotion of the railii (( 
veragp (or normal) value of tlie factor is .shown by the position of points tje 
verage seasonal variability of the factor. 



below I — ) the 



represent thp average value of factors enumerated in the key in the lower right-hanil corner of the plate, A rleparture above ( + ) 

ntl or within the circle, respectively, and along a radius or its extension. The unit of departure (indicated by the figures 1. 3. 6 along the radii) is the 



VOLUME 2, PLATE XVII. 



cornor of tlio platp. A departure above (-|-) or below ( — ) the 
if departure (indicated by the figures 1. 3. 5 along the radii) is the 



PRECIPITATION 
« INCHES 





Point 
average ( 
average seasonal 



I the circumferpnoe of the circle, at the intersection of the radii (0), represent the average value of factors enumerated in the key in the lower right-hand corner of the plate. A departure above ( + ) or below ( — ) the 
normal) value of the factor is shown by the position of points beyond or within the circle, respectively, and along a radius or its extension. The unit of departure (indicated by the figures 1, 3, 5 along the radii) is the 
annual variability of the factor. 



MARYLAND WEATHER SERVICE 297 

Some years later, in 1831, a station of the U. S. Army Medical Depart- 
ment was established at Fort McHenry, where observations were main- 
tained with but little interruption until 1892. Between 1830 and 1840, 
two or three individual Baltimore observers sent occasional reports to 
the Maryland Academy of Sciences, or to the Franklin Institute in 
Philadelphia. 

The Smithsonian Institution was established in 1847, and very soon 
after numerous voluntary observers reported weather conditions regularly 
from different parts of the State. The Baltimoreans who cooperated with 
the institution between 1850 and 1860 were Dr. A. Zumbrock (1850-52), 
Dr. Lewis H. Steiner (1853), and Prof. Alfred W. Mayer (1857-59). 

In the year 1870, the U. S. "Weather Bureau (then known as the IT. S. 
Signal Service) was established by act of Congress. The Baltimore 
station was among the first to be opened, and observations were main- 
tained without the interruption of a single day until the present time. 
The instrumental equipment was always of the first order and was 
steadily increased with the growth of the Bureau. During the past ten 
years, continuous records of air pressure, temperature, wind velocity and 
direction, sunshine, and rainfall, by means of self-recording instruments, 
have been maintained. In 1902, a Richard hygrograph was added, the 
property of the Maryland State Weather Service. Details concerning 
the history and equipment of the U. S. Weather Bureau station are given 
in the following pages in tabular and statistical form for ready reference, 
including changes in the location of the observing station, in the elevation 
of instruments and in tlie personnel of the station. 

In 1891, the Maryland State Weather Service was organized; the 
])urpose and method of organization are fully described by the director 
in Volume I, of the Reports of the Maryland Weather Service, issued 
in 1899. From the beginning there has always been an intimate and 
harmonious cooperation between the National Service, the State Service, 
and the Johns Hopkins University. The offices of the U. S. Weather 
Bureau and of the Maryland Weather Service have been in the buildings 
of the University since the establishment of the State Service. The 
Board of Control of the Maryland Service comprises a representative of 



298 



CLIMATE OF BALTIMORE 



MARYLAND WEATHER SERVICE 



299 



METEOROLOGICAL OBSERVERS AND OBSERVATIONS IN BALTIMORE, MD. 



West part of city 1 Private 



Fort McHenry 



Observer 



Capt. Lewis Brantz 



Post Surgeons, L'. S. A. 



Dr. G. S. Sproston, U. S. N. 



Md. Academy of Science 
and Literature 



Dr. T. Edmondson, .Jr. 1 West part of city 

Dr. A. Zumbroclt 

Dr. Lewis H. Steiner Baltimore Medical 

Institute 

Prof. Alfred M. Mayer 

Wiu. Lutber Woods ! .Johns Hopkins 

Hospital 



Auspices 



North Longitude Elevation 
West of 



Latitude 



Md. Acad. Sci. 
and Lit. 



Johns Hopkins 



*U. 8. Weather Bureau; 

{U. S. Signal Service un-j University 
til July 1, ISiil) 



Smithsonian 
Institution 



Smithsonian 
Institution 



Smithsonian 
Institution 

Maryland S. W. 

S. and U. S. AV. 

Bureau 

United States 
Government 



16 ' 76 

17 76 

I 
17 I 76 



feet 



Period. 



50 
113 

+123 



1817-1834 

Jan. to Aug., 1839 

Nov., 1S36 to June, 

1837 



1S31 to June, 1859 

Apr., 1«61 to Jan., 1863 

1864 to Feb., 1893 



B6, 21 and 33, June, 
Sept. and Dec. 



1835 to 1837 
1846 to Oct. 1853 



Apr., 1850 to July, 1853 
Jan. to Oct., 1853 



Hours of Observation 



Items Observed 



8 a. m., 3 p. m. and \ Temperature, rainfall, winds, ' Published in pamph- 
10 p.m. clouds; Barometer and hygro. i let form from 1818 

meter added in 1836. i to 1825 ; reprinted in 

I the American Alma- 
nac, 1834, and in 
I other publications. 

7 a. m., 3 p. m. and Temperature, humidity, wind 
9 p. m. direction and weather; rainfall 

added in May, 1836. 



I., 2 p. m. and 
9 p. m. 



Hourly 



Sunrise, 10 a. m., 
3 p. m. and 7 p. m. 



■ a. m., 3 p. m. and 
y p. m. 



a. m., 3 p. m. and 
9 p. m. 



Temperature, 
and rainfall. 



Nov.,1857toAug., 1859| 7 a. m.,3p. m. and 
9 p. m. 

^o^; 1894 to dat 



■'«"■. 1871 to date 



Self-registering maxi- 
mum and minimum 
thermometer 

See page 303 for houri 
of observation 



' For earlier locations and elevations see page 304. 



ometer elevatioi" 



Occasional reports 

to Franklin Institute 

of Philadelphia. 

Pressure, temperature, wind | 

direction and velocity, hygro- | Published in Trans. 

meter, cloud movement and Md. Acad. Sci., Vol. I, 

state of weather. 1837 

Pressure, temperature, wind Printed copies of re- 
direction and force, dewpoint, ports for Jan., Mar., 
rainfall and weather. July and Aug. pre- 

sented to Maryland 
Academy of Science. 

Temperature, wind direction 
and raiufall. 

Pressure, temperature, winds, 
clouds, rainfall, relative humid- 
ity. 

Temperature, wind direction 
and rainfall. 

Temperature, wind direction, 
weather and rainfall. 



Daily observations at stated 
hours, since Jan. 1, 1&71 of 
pressure, temperature, wind 
direction and velocity, humid- 
ity, clouds and rainfall; tem- 
perature of water in harbor, 
Sept. 1881 to March, 1887; at- 
mospheric electricity, 1882 to 
1887. 

Continuous automatic record 
of wind velocity since Jan., 
1871 ; wind direction since 1874; 
rainfall since Nov., 1893; pres- 
sure and temperature since Jan., 
1893 ; sunshine since July, 1893; 
humidity since Feb., 1903. 



300 THE CLIMATE OF BALTIMORE 

each of the three institutions. Since 1896, the system of voluntary 
observing stations established in 1891, and later, by the State Service, and 
the publication of weekly and monthly reports, have been under the con- 
trol of the National Service. The special appropriation by the State has 
since been devoted to the investigation of special climatic problems 
and the publication of the results. 

The local office of the U. S. Weather Bureau is intimately associated 
with the commercial organizations of the city. The results of observa- 
tions and specially prepared reports are quickly put into the possession of 
those most benefited thereby through the instrumentality of the public 
press, by special bulletins, by telephone, or otherwise. Special efforts 
have always been made to place the daily telegraphic reports of weather 
conditions from all parts of the countrv' before the commercial interests 
at the earliest possible hour each day. These reports, as soon as received, 
are placed upon a large glass map upon the floor of the Chamber of 
Commerce, by means of symbols and lines; the Baltimore station was 
among the earliest to adopt this method of publishing the daily weather 
conditions. 

One of the most important duties of the local office of the Weather 
Bureau is the prompt distribution of the daily forecasts to the public, 
and of special warnings of the approach of storms, or cold waves, or the 
occurrence of frost. This is accomplished by a liberal use of the 
telegraph and the telephone, and by the hearty cooperation of the public 
press and the U. S. Postal authorities, especially through the instru- 
mentality of the recently established system of rural free delivery, 
by means of which the daily weather forecasts are being placed regularly 
each day in possession of all who dwell along the established routes, even 
those in remote farming communities. 

The shipping interests are informed directly by telephone of the 
approach of a severe storm; the owners of the smaller craft in the 
neighboring waters depend mostly for their information upon the dis- 
play of storm warnings at a conspicuous point along the harbor by means 
of special storm flags by day and lights by night. For many years, these 
. special warnings were displayed from the flag staff on Federal Hill ; they 



MARYLAND WEATHER SERVICE. 



VOLUME 2, PLATE XVIII. 




OFFICE OF THE U. S. WEATHER BUREAU, 
JOHNS HOPKINS UNIVERSITY, No. 532 NORTH HOWARD STREET. 

MARYLAND STATE WEATHER SERVICE. 



MAETLAXD WEATHER SERVICE 301 

are now displayed from the top of a steel tower, 50 feet in height, erected 
upon the roof of the seamen's home, known as " The Anchorage," at the 
intersection of South Broadway and Thames Streets in East Baltimore. 
Eeports on the weather and crop conditions throughout the States of 
Marjdand and Delaware have been printed and widely distributed from 
the local oflSce each week during the season of crop growth from 1891 to 
the present time. Monthly and annual reports have also been issued 
during the same period, showing the daily weather conditions and the 
monthly and annual average values, together with the progress of crop 
growth and farm operations from month to month. A printed daily 
weather map (single sheets 11 in. by 16 in. in size) was issued from the 
local oflBce from October, 1896, to N'ovember, 1898. These received a free 
and wide distribution in the business portions of Baltimore and among 
educational institutions. The maps showed the weather conditions over 
the entire country, based on over a hundred telegraphic reports of obser- 
vations made each morning at 8 a. m., 75th meridian time. They 
reached the public about 1 p. m. each day. In November of 1898, the 
printing office connected with the Baltimore station was closed and the 
daily weather map was replaced by the larger and more complete litho- 
graphic map issued from the Central Office at Washington, D. C. 

Observers and Observations. 
The table on pages 298 and 299 contains a list of the individuals and 
institutions responsible for systematic instrumental observations of the 
weather within the city limits of Baltimore. The geographical location 
of the station, the period, and the character of the observations are also 
stated as far as known. There were doubtless other observers but their 
contributions to the observational literature of meteorologv' in Baltimore 
have not come under the notice of the writer. 

Instrumental Equipment. 
The instruments in use at the local station of the U. S. Weather Bureau 
since the organization of the Service in 1871 are enumerated in the 
following list; the dates of installation and discontinuance of tlie instru- 
ments are also jrivcn: 



302 



THE CLIMATE OF BALTIMORE 



Instruments 



In Use Since 



Anemometer, Robinson Jan. 1, 1871 

Anemoscope Jan. 1, 1871 

Barograph, Richard I Dec. 30, 1892 

Barometer (Mercurial) Jan. 1, 1871 

*Hygrograph, Richard Feb., 1902 

Hygrometer, stationary Jan. 1, 1871 

Psychrometer (whirling) July 11, 1886 

Rain Gage (ordinary) Jan. 1, 1871 

Rain Gage (self-registeriug, float) May 30, 1891 

Rain Gage (self- registering, tipping bucket) , June 13, 1897 

* Rain recorder, continuous Jan., 1903 

Register, single (wind velocity) i Jan. 1,1871 

Register, double (wind velocity and direction) Feb. 10, 1874 

Register, triple (wind velocity, direction and rain). . Nov. 10, 1892 

Snow Gage Jan. 1, 1871 

Sunshine Recorder (added to triple register) June 30, 1893 

Shelter, standard window ■ Jan. 1, 1871 

Shelter, standard roof I Oct. 1, 1885 

Thermograph, Richard j Dec. 30, 1892 

Thermometer, dry bulb Jan. 1, 1871 

Thermometer, maximum July 22, 1872 

Thermometer, minimum July 22, 1872 

Thermometer, water i Sept. 1, 1881 

* Property of the Maryland State Weather Service. 



Use Discontinued 



July 11, 1886 



June 20, 1897 



Feb. 10, 
Nov. 10, 



1874 
1892 



Sept. 30, 1885 



Mar. 31, 1887 



Hours of Observation. 

The prescribed hours of observation, as well as the kind of time em- 
ployed in the National Service, have been changed from time to time 
since the organization of the Bureau in 1870. Local time was in use at 
all stations from January 1, 1871, to July 31, 1881 ; on August 1, 1881, 
Washington time was adopted, making a difference, of two minutes 
between the local times of observation in Washington and Baltimore. 
Since January 1, 1885, observations have been made on 75th meridian 
time. The difference between Baltimore local time and 75th meridian 
time is six minutes, the former being slower than the latter by this 
amount. 

The combination of selected hours for observation has varied con- 
siderably in the thirty-four years since 1871. From 1871 to June 30, 
1888, at least three direct observations were made daily, one at an early 
morning hour, from 7 a. m. to 7 :30 a. m., another in the middle of the 
afternoon, at 2 p. m., 3 p. m., or 4 :30 p. m., a third at night between 9 



MARYLAND WEATHER SERVICE 



303 



p. 111. and 11 :3U p. m. Five observations were made daily for some years. 
Since July, 1888, there have been but two observations, one at 8 a. m., 
another at 8 p. m. The exact hours of observation and the duration of 
their employment are indicated in the following tabular statement : 



HOURS OF OBSERVATION. 
(U. S. Weather Bureau, 1871-1904.) 



Time of Observation. 




11:87 p. u). (1. t, 
11:02 p. m. (1. t 



7:00 a. m. (1. t.) 
2:00 p. m. (1. t.) 
9:00 p. m. (1. t.) 



12:02 p. m. (1. t.) (special) Oct. 22, 

12:02 p. m. (I. t.) (daily) Feb. 23, 



.Telegraphic 



11:02 a. m. (1. t.). 

7:02 p. m. (1. t.) 

7:02 a. m. (1. t.) 

7:00 a. m.(w. t.) 

3:02 p. m. (1. t.) 

3:00 p. m. (w. t.) 

11:02 p. m. (1. t.) 

11:00 p. m.(w.t.) 

2:00 p.m. (w. t.) 

2:00 p. m. (7.5th m. t.) 

7:(I0 a. m. (7.5th m. t.)^ 

3:00 p. m. (75th m. t.) 

11:00 p. m. (75th m. t.) 

10:00 p. m. (7.5tli m. t.)J 



Water temperatures. . 



Tehii^raphic 



11:00 a. m. (75th m. t.). 
7:00 p. m. (75th m. t.). 



8:00 a. 
K:00 p. 



(75th m. 
(75th m, 



!:;} 



Telefj^raphic 



Local lime (1. t.) 6 min. slower than 75tli m. . . . 
Wasliin^^ton (w. t.) 8 iiiin. slower than 75th ni. 
7.5th meridian time 



Ended. 



Oct. 31, 1879 

Oct. 31, 1879 

Aug. 24, 1872 

Dec. 31, 1884 



June 30, 1881 

Feb., 1872 

Dec. 31, 1879 

Dec. 31, 1884 
Dec. 31, 1884 

July 31, 1881 
Dec. 31, 1884 

July 31, 1881 
Dec. 31, 1884 

July 31, 1884 
Dec. 31, 1884 

Dec. 31, 1884 
Mar. 31, 1887 

June oO, 1888 
June .30, 1888 

Dec. 31, 1880 
June 30, 1888 

Aug. 3, 1886 
Aug. 3, 1886 

Cnrrent 
Current 

July 31, 1881 

Dec. 31, 1884 

Current 



304 



THE CLIMATE OF BALTIMORE 



Changes iisr the Location of the Station. 

Changes in the location of the observing station of the U. S. Weather 
Bureau have necessitated changes in the elevation of instruments. During 
a period of 34 years^ five different stations have been occupied. The 
office has always been in the thickly settled portion of the city; from 
1871 to 1889 in the heart of the business section, later in the buildings 
of the Johns Hopkins University, No. 532 North Howard Street. The 
details of the changes experienced in station and instruments are indi- 
cated in the tabular statement below. With unimportant exceptions, 
the instruments have always had a fairly free exposure, as good, perhaps, 
as can be had in the midst of a large city. 



location of u. s. weather bureau stations and elevation 
of instruments. 







ci 








Above Ground (Feet). 




o 
o 


Observations begu 


3 

o 
12; 


a) 

a 
-c 

3 

'3) 

a 

a 


^•» 

— V 

< 




Location. 


a 

2 

03 
P5 


• 

a 

o 

a 

n 
J3 

33 

76 


9 

6c 
08 

a 
"3 
P3 

69 


© 

a 

o 

a 
§ 

<! 

75 
86 


® 

a 

as 
■d 

c 


Fireman's Insurance 
Building, S. W. Cor. 
South* Water Sts 


3d 


Jan. 1, 1871 
Oct. 12, 1878 
Oct. 1, 1885 


39° 18' 


76° 37' 


45 


33 


85 
95 


Neal Office Building, 
S. W. Cor. HoUiday & 


4th 

Upper 

floor 

of 

tower 


Jan. 1, 1889 

June 1, 1891 
1893 


39° 18' 
390 18' 


76° 37' 
76° 37' 


76 
179 


56 
71 


86 

87 


78 

78 
80 


100 
100 


110 


Johns Hopkins Univer- 
sity. Physical Labora- 
tory, N. W. Cor. Linden 
Ave. & Monument St. . . 


110 


Equitable Building, 
S. W. Cor. Calvert & 
Fayette Sts 


9th 


Sept. 7,1895 


39° 18' 


76° 37' 


143 


105 


120 


116 


136 


146 


Johns Hopkins Univer- 
sity, Treasurer's Build- 
ing, No. 533 N. Howard 
St 


2d 


Aug. l,189fi 
Apr. 30,1902 
Oct. 1, 1903 


39° 18' 


76° 37' 


123 


20 


68 
69 


73 


83 
117 


94 
115 







♦Thermometers in louvered window shelter on north side of building from Jan., 1871 to 
Sept. 30, 1885 ; later in standard shelter on the roof of the station building. On Oct. 1, 1902, 
the standard shelter was mounted within the 60 ft. steel tower supporting the wind vane 
and the anemometer, the base of the shelter being nine feet above the roof. 



MARYLAND WEATHER SERVICE. 



VOLUME 2, PLATE XIX. 




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MAEYLAXD WEATHER SERVICE 



305 



VARIATIONS IN THE ELEVATION OF THE BAROMETER AND CORRECTIONS 
TO THE EPOCH, JANUARY 1, 1900.* 

Barometer above reference plarte 25.3 feet. 

Reference plane, top of iron pipe underneath sidewalk on west side of Howard 
street opposite Center street, transverse station No. 482 of the City of Balti- 
more Topographical Survey, above mean sea level 98.0 " 

Station elevation of barometer, Jan. 1, 1900 123.3 " 



Date of 
change. 



Building's Occupied. 



1870, Dec. 23 
1889, Jan. 1 ! 

1891, June 1 • 

I 

1895, Sept. 7 

1896, Aug. 1 

1899, Jan. 1 



Southwest corner of South & Water 

streets 

Southwest corner of Baltimore and 

Hollidaj- streets 

Johns Hopkins University (Physical 

Laboratory I 178.8 

Southwest corner of Calvert and 

Fayette streets U1.5 

Johns Hopkins University (No. 532 

North Howard street) 



45.2 
75.9 



123.3 



^ 3fe 



+30.7 
+102.9 
-37.3 

-18.2 



la 
111 

;g|| Sgc 



+78.1 
+47.4 -.054 



-55.5 
-18.2 



+ .063 
+ .020 






-.015 
-.015 
-.015 
-.015 
-.015 



-.103 

-.069 

+ .048 

+ .005 

-.015 
.000 



* For further information regarding the reduction of barometric observations see : Bige- 
low, F. H. The Reduction of Barometric Pressure Observations at Station of the United 
States Weather Bureau. Vol. II of the Report of the Chief of the L'. S. Weather Bureau for 1900. 



OFFICIALS IN CHARGE, U. S. WEATHER BUREAU OFFICE, BALTIMORE. 

(1871-1904) 



Name. 



Official title. 



Cowan, J. E. . . 
Penrod, II. J. . 
Boyd, W. T. . . 
Kabernagle, J. 

Bell, R. J 

McCiann, E. W. 

Black, W 

Seyboth, Robt. 
Felfrer, G. W. . 
Cronk, C. P. . . 
Marburv, J. B. 
Hunt, G. E. ... 
Walz, F. J. . . . 
Fassig, O. L. . . 



Sergeant 



Local Forecast 
Official 

Section Director 



Date of 
assignment. 



Jan. 

May 

Sept. 

April 

Dec. 

May 

June 

Oct. 

Sept. 

Oct. 

July 

July 

May 

July 



1871 

1871* 

1874 

1875 

187.5 

1877* 

1879* 

1879* 

1882 

1888 

189.5 

1896 

1897 

1900* 



Date of relief. 



Mar. 27, 1871 
Sept. 23, 1874 
April 24, 1875 
Dec. 20, 1875 
May 21, 1877 
June 6, 1879 
Aug. 29, 1879 
Sept. 6, 1882 
Oct. 26, 1888 
July 6 
July 1 
May 22 
May 29, 1900 
In charge 



1895 
1896 
1897 



• During intervening intervals the station was in charge of the first assistant. 



306 



THE CLIMATE OF IJALTIMOKE 






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MARYLAND WEATHER SERVICE 



309 



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MARYLAND WEATHER SERVICE. 



VOLUME 2, PLATE XXL 




to 



-r O 

I- -- UJ 

< . E 

Oil ^ 

oo ^ 

§? = 

uj2 > 

Oz I 

I- — °- 



THE WEATHER OF BALTIMORE 



INTEODUCTION 



Leaving now the discussion of climate^, or the average and extreme 
values of the principal factors which constitute the sum total of atmos- 
pheric conditions, we come to a consideration of some of the more im- 
portant types of weather characteristic of the geographical horizon of 
Baltimore. As stated in the introduction to this report, the term weather 
is restricted in its use to the actual state of the atmosphere as regards 
temperature, humidity, wind movement, etc., at any given instant, or 
short period of time. The method employed in the discussion of cli- 
matic conditions is not applicable to descriptions of weather; the various 
factors cannot be considered separately, but must be studied in their 
relations to one another at a given instant of time, in order to afford a 
proper mental picture of actual conditions in nature. 

The past fifty or sixty years have witnessed a gradual but radical 
change in our views of weather conditions and sequences. Before the 
days of the telegraph, observers were isolated and independent of one 
another; we had but vague ideas as to synchronous weather conditions 
prevailing at distant points. Here and there in the eighteenth century 
we find a suggestion of the importance of co-operation in the methods 
and time of making observations ; but intercommunication was slow and 
the important discoveries of Franklin and Jefferson in America, of Bran- 
des in Germany, and others, met with rather tardy recognition, or were 
entirely overlooked and had to be rediscovered when times were more 
propitious for utilizing the results of new discoveries. 

The rich collection of weather proverbs, based upon natural signs — 
changes in the wind, forms of clouds, the habits of animals — are based 
upon the accumulated experience of individual effort. For centuries 
21 



312 THE CLIMATE OF BALTIMORE 

weather changes were minutely observed and carefully recorded. Espec- 
ially was this true of changes in wind direction, as this factor was almost 
universally regarded as the underlying cause of variations in the other 
elements. Not until the use of the telegraph became general, making it 
possible to gather reports from an area covering thousands of square 
miles, and to obtain a picture of actual weather conditions at the same 
instant of time over this area, was the true meaning of weather changes 
gradually revealed to the student of meteorology. Change in the direc- 
tion of the wind, while it still holds a conspicuous place in weather 
prognostics, is no longer regarded as the fundamental factor in the 
weather situation. The weather map, so familiar to us to-day, shows us 
that atmospheric pressure, or the height of the barometer, is the key to 
the problem of coming weather — not the actual height of the barometer 
at a single station, or at a number of stations, but the relative heights 
over a large area. Having given the relative distribution of pressure 
over a given area, the remaining weather elements can be supplied with 
a fair degree of accuracy by the expert student in weather forecasting. 

The Synoptic Weather Chart. 
The development of the synoptic weather chart, a chart showing the 
actual physical condition of the atmosphere at the same hour over an 
area of thousands of square miles, forms one of the most interesting 
chapters in the history of modern meteorology. Conceived before the 
close of the first quarter of the nineteenth century, the middle of the 
century witnessed a remarkably rapid development and application of 
the idea. This was brought about by the rapid spread of the electric 
telegraph and the recognition of the vast commercial importance of such 
a chart. The successive steps of its progressive development in this 
country and in Europe have been carefully traced by Abbe ^ and Hell- 
mann," to whom we are indebted for much of our accurate history of 
meteorology. 

^ Abbe, Cleveland. The Development of the Daily Weather Map. Vol. I, 
Maryland Weather Service, Baltimore, 1899, pp. 225 et seq. 

^ Hellmann, G. Neudrucke von Schriften und Karten iiber Meteorologie 
und Erdmagnetismus. No. 8, Berlin, 1897. 



MARYLAND WEATHER SERVICE 313 

To-day the great majority of the nations of the world support a 
national weather service and issue such charts daily. We now have 
daily charts of synchronous observations for most of the land area of 
the northern hemisphere excepting Asiatic Eussia and China; and of 
the North Atlantic. In the southern hemisphere, there are charts for 
Australia, the Indian Ocean, South Africa, and the Argentine Eepublic. 
The time will soon come for the realization of one of the fondest hopes 
of the meteorologist, when we shall be able to construct such maps for 
practically the entire globe. The importance of the constant endeavor 
to extend the area of observations becomes apparent when we realize 
that there are no definite boundaries in the atmosphere of the globe, and 
that no extensive disturbance can take place in any portion of its vast 
extent without affecting, sooner or later, every other portion. We proba- 
bly do not yet realize fully the nice adjiistment of atmospheric forces, 
and we are still ignorant of many of the important laws underlying the 
larger atmospheric movements. 

Cyclones and Anti-cyclones. 
Before the advent of the synoptic weather chart, the constant changes 
in the direction of the Avind, the increase and decrease in cloudiness, the 
occurrence of rain with a certain wind and clear skies with another, 
were very little understood. Certain sequences in weather changes had 
long been accurately noted, but why these changes should follow a defi- 
nite order was incomprehensible. The weather map, however, revealed 
the clue to the interpretation of the changes in the relative distribution 
of atmospheric pressure over extended areas. It was soon learned iliat 
differences of pressure, or of the height of the barometer in neighboring 
localities, set the air in motion, causing it to flow from the area of high 
barometric pressure to the areas of lower barometric pressure, much as 
water is transferred from a higher to a lower level. This flow of the air 
from place to place in an effort to restore a disturbed equilibrium is 
what is termed the wind. Changes in wind direction in turn bring about 
changes in temperature, the wind blowing warmer with a change from a 
northerly to a southerly direction in the northern hemisphere, with inter- 



314 THE CLIMATE OF BALTIMORE 

mediate changes in temperature wlien blowing from tlie east or west. A 
study of the weather map soon led to the formulation of a new set of 
rules of weather changes, more general in their application and more in- 
telligible than those based on the study of observations at a single sta- 
tion. Two distinct types of weather were soon recognized. The most 
conspicuous of these, and the first to be investigated was the storm area, 
an area in which the readings of the barometer decrease rapidly from all 
sides to a minimum in the center of the area. Such areas were observed 
to be accompanied by cloudy skies, and more or less rain, by winds in- 
creasing in force as the center was approached, and blowing approxi- 
mately toward the point where the barometric pressure was lowest. As 
the types of weather first investigated were naturally well developed 
storm areas, the winds high and blowing in paths nearly circular, or at 
least spirally inward, the term cyclone was applied to them; a term first 
used about the middle of the nineteenth century by Captain Henry 
Piddington to describe revolving storms in the Indian seas. Later, the 
term cyclone was given to all atmospheric disturbances of wide area in 
which the winds blow toward a central point or line of low pressure. 

Another weather type which later claimed the attention of students 
of meteorology was the fine weather type, in which the barometric pres- 
sure is highest in the central area, decreasing in all directions from the 
center outward. In these areas the winds were observed to blow in 
general away from the center of highest pressure. As the character- 
istics of these areas were in many respects the opposite of those observed 
in cyclones, they were given the name anti- cyclones. 

Between these two well defined types, there are innumerable forms par- 
taking more or less of the characteristics of one or flie other of the two 
principal types. 

In the northern and southern hemispheres, from latitude 30° to 70°, 
the upper portions of the atmosphere apparently flow in a constant 
stream in a direction approximately from west to east around the globe. 
The winds within the lower layers of the atmosphere in the middle lati- 
tudes do not always follow the course of the upper currents; the atmos- 
phere from the earth's surface to the level of the highest clouds is broken 



MARYLAND WEATHER SERVICE 315 

up into areas in which the barometer is alternately low and high, into 
cyclones and anti-cyclones, as they are generally designated. These 
alternate areas of unsettled weather and tine weather are carried along 
as a whole in an easterly direction with the general drift of the upper 
atmosphere, constantly changing in form and intensity as they move, but 
retaining, in the main, their chief characteristics for thousands of miles ; 
the areas of high pressure are accompanied by comparatively little 
cloudiness, and temperatures below the seasonal average; while the areas 
of low pressure are attended by clouds and rain and a temperature above 
the seasonal average. These cyclonic and anti-cyclonic areas of the middle 
latitudes have a diameter varying from a few .hundred to a thousand, or 
even two thousand, miles and move eastward, as a whole, with a velocity 
averaging about 600 miles per day, across tlie United States, and hence 
occupy two or three days in passing a fixed point. Their passage east- 
ward explains the constant shifting in the direction of the winds of a 
given locality, the direction of the change in wind, as will be explained 
later, depending upon the position of the center of the anti-cyclones and 
cyclones with reference to the given locality. For instance, when a 
" low," or cyclonic area, approaches Baltimore from the west, if the 
center is north of the latitude of Baltimore, the wind becomes easterly ; as 
the center passes Baltimore, the wind shifts to the south, and then to the 
west or northwest. If the center of the storm passes to the south of the 
city, the changes in the wind direction are successively, east, north, and 
west, the reverse of those in the first case cited. If the center passes 
over Baltimore, the easterly wind is followed by a calm, or light variable 
wind, after which the wind will spring up abruptly from the west. 

These rules of weather sequences are applicable only to the well devel- 
oped and definitely formed areas of high and low pressure, and but im- 
perfectly apply to the more numerous moderately developed " highs " and 
" lows " whose eastward drift gives us our daily routine of weather 
changes. 

A clear conception of the character of cyclones and anti-cj'clones, as 
described above — of the distribution of pressure, the system of winds, 
the distribution of temperature, tlie state of the weather, and the move- 



316 THE CLIMATE OF BALTi:\IORE 

ments of these areas, as a whole — is essential to a proper understanding 
of our daily weather changes, and especially of the more conspicuous 
weather types known as storms and cold waves. The essential features 
of cyclones and anti-cyclones may be most readily understood by a study 
of actual examples of the simpler well developed types. As an illustra- 
tion of a typical storm or cyclone, the weather chart of the morning of 
December 27, 1904, is reproduced in Figs. 85, 86. To those unfamiliar 
with the weather majD — and indeed to all excepting those who have de- 
voted much time to their study — the usual weather map is a confused 
tangle of lines and symbols requiring careful explanation and analysis 
before even the essential features of the weather conditions are under- 
stood. Some of these difficulties may be obviated by the use of a series 
of charts portraying the separate factors which go to make up the com- 
plex weather conditions, retaining in each chart, however, the controlling 
factor, namely, the system of lines representing the distribution of atmos- 
pheric pressure, or isobars, as they are called. 

AREAS OP UNSETTLED WEATHER (CYCLONES). 

It may be well at this point to call attention to the very frequent 
misuse of the terms cyclone and tornado. In scientific literature there 
is a clear distinction between the two, while in the popular mind, they 
are often synonymous. The confusion in the use of these terms is natural, 
and is largely due to a change in the meaning of the word " cyclone " 
as used in technical literature. A cyclone has at all times, since the days 
of Piddington (about 1850), been regarded as a severe and destructive 
storm. Later when the natiire of storms and weather changes was 
better understood, the meaning of the term was enlarged by the student 
of meteorology to include all atmospheric disturbances, whether large 
or small, intense or barely perceptible, in which the winds blow inward 
toward a central point, or area of low barometric pressure. Tlie general 
public has not yei adopted this amended definition, and all intense 
storms continue to be called cyclones, when the particular storm in mind 
may be a tornado, squall, an intense thunderstorm, or a hurricane. While 
the tornado, as a revolving storm, must be classed with cyclones, the 



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LEGEND. 

The following explanation applies to U. S. daily weather charts reproduced 
in this report. The charts are based on simultaneous observations made at 
8 a. m., 75th meridian time. 

Black lines are lines of equal temperature (in degrees Fahr.). 

Red lines are lines of equal atmospheric pressure (in inches). 

Shaded areas mark the limits of overcast skies. 

The arrows fly with the wind. 

R indicates rain. 

S indicates snow. 

T indicates the occurrence of a thunderstorm during the preceding 12 

hours. 

} clone ^' 
-,..,.: the '^ — 

■1 rlcc;ff^,. 



' ^enerfil 



MARYLAND WEATHER SERVICE 



317 




Fig. 85.— Typical Cyclone of December 27, 1904. 




Fio. 86.— Typical Cyclone of December 27, 1904. 



318 THE CLIMATE OF BxVLTIMORE 

term is restricted to the intense and very destructive local storms of very 
small area occurring within certain limited portions of a larger storm; 
it is a cyclone within a cyclone. 

The accompanying charts, Figs. 85 and 86, show the distribution of 
pressure, wind direction, temperature, cloudiness, and precipitation, 
within the area of the typical cyclone which passed over the United 
States during December 27, 1904. 

Pressure and Winds. — The series of red curved lines show the dis- 
tribution of atmospheric pressure. The inner, nearly circular, line en- 
closing the word " low " in the chart for December 27 is drawn through 
localities in which the barometer read 29.40 inches, the lowest reported 
pressure. The remaining curves connect localities in which the barome- 
ter stood higher by successive intervals of two-tenths of an inch, until 
around the outer limits, at approximately a thousand miles from the 
center, the barometer read 30.40 inches, showing a difference in pressure 
of one inch. 

The atmosjihere is very responsive to local differences of pressure. With 
very slight differences, a small fraction of an inch, for example, there 
will be set up a movement of air from the locality having the higher 
toward that having the lower reading of the barometer until equilibrium 
is restored, for the same reason that water always has a tendency to 
flow from a higher to a lower level. Bearing in mind this general physical 
law of the flow of gases, we may understand the general drift of the 
atmosphere toward the central area of low pressure in a cyclone. This 
is clearly shown by the arrows which indicate the direction of the wind 
at so many stations of observation; the arrows fly with the wind and 
point in a general way toward the area of lowest pressure. As will be 
observed they only occasionally point directly toward the center; in 
most instances the direction taken by a particle of the atmosphere in its 
journey from the outer portion of a storm area toward the center is 
along a spiral course. This is due to the effect of the rotation of the 
earth about its axis, which always tends to urge a freely mo'ving particle 
toward the right of its initial direction, in the northern hemisphere. 
The topography of the region over which the storm passes, and more 



]MARYLAND WEATHER SERVICE 319 

important still, the distribution of the pressure about the central area as 
shown by the curved lines, or isobars, also greatly influence the direction 
of the wind. In general, and especially over land areas, the isobars vary 
widely from the circular form, and the actual wind directions fall roughly 
into two classes, easterly and westerly. Drawing a north and south line 
through the center of the cyclone, the winds to the east are observed to 
flow mostly from some point between northeast and southeast, while 
those to the west of the line blow mostly from some point between south- 
west and northwest. The winds blowdng directly from the south or from 
the north are not so frequent, or of as long duration as those from other 
directions. The effect of pressure distribution on the direction and force 
of the \\-'w.d will be brought out very clearly in later discussions of 
weather types. In general, it may be stated that the force of the wind 
is greater the greater the difference of pressure between two neighboring 
points ; that is, it is proportional to what is called " the gradient,'^ which 
is equivalent to difference of level in the flow of streams; the steeper the 
bed of the stream, the more rapid is the flow of water. 

Temperature and Wind Direction. — Especial attention is directed to 
the relation existing between wind direction and temperature in a 
cyclone. Localities reporting the same temperature at 8 a. m. are- joined 
by means of lines, or isotherms, as they are styled. The lines are drawn 
at intervals of 10° or 20° Fahr. It will be observed that the temperature 
of 70° above zero in lower Florida gradually diminishes, as we proceed 
northward, to 30° below zero in the extreme northern limits. This is 
the usual direction of decrease in temperature at all seasons of the year, 
though tlie rate of decrease is here much more rapid than under normal 
conditions. In the absence of a well defined atmospheric disturbance 
the isotherms run nearly parallel with the lines of latitude; a steady and 
fairly uniform decrease in temperature from south to north is the normal 
condition all over the northern hemisphere. The relative temperature 
of winds from different quarters may vary greatly in different localities, 
but in the main, a southerly wind is warmest and a northerly wind 
coldest, with intermediate degrees for the east and the west wind. Hence 
we may readily understand how a change in the direction of the wind 



320 THE CLIMATE OF BALTIMORE 

may affect the temperature of a locality. On the approach of a cyclone, 
in most cases from the west, the winds to the east of the center become 
easterly, veering to southerly; the temperature rises in the eastern half 
of the storm, and particularly in the southeast quadrant where the 
winds are from southeast or south. In the southwest quadrant of the 
storm there is usualh- an abrupt change from the warm south or south- 
east wind to a much colder west or northwest wind. This condition of 
temperature distribution is strikingly exhibited in the charts. The 
isotherms are seen to bend northward far beyond their seasonal values 
in the southeast quadrant; on the other hand, in the southwest quadrant, 
the cold northwest winds are carried far beyond their normal limits to 
the southward. The result is a stronger contrast in temperature from 
the center of the storm westward than from south to north ; the isotherms 
run north and south, in the particular case cited, and not east and west 
as in normal weather conditions. These shifts in the direction of the 
wind during the passage of a cyclone are sufficient cause for the great 
majority of the temperature changes experienced in our latitudes. 

Distribution of Clouds and Precipitation. — The proportion of the area 
covered by clouds at 8 a. m. is indicated by the extent and intensity of 
the shading. It will be observed that in practically all of the region 
within the influence of the system of closed isobars, the skies were en- 
tirely overcast, and that over an area extending 500 to 600 miles in 
nearly all directions from the central point of lowest pressure, precipi- 
tation occurred at the hour of observation — rain to the south of the 
isotherm of 30° and snow north of this line. The area of precipitation 
in this particular storm was exceptional, but it serves to illustrate the 
general law of the distribution in well developed storms of large extent. 
In most cases the area of precipitation is more limited in extent and is 
surrounded by a band of overcast skies, beyond which is a partly clouded 
band merging into regions of clear skies. Not all cyclones, even when 
well developed, show the symmetry in the distribution of weather con- 
ditions indicated in the type selected; the variations are infinite, but 
there is a general conformity to the type when the storm is well developed. 
The center of the rain or snow area is usually to the east and south of 



MARYLAND WEATHER SERVICE 321 

the center of low pressure. The details of the distribution and the 
character of the preciiDitation will be brought out more clearly in the 
later discussions of weather types of the season. 

The elements described separately in the preceding pages are finally 
brought together upon a single chart, the usual form of presenting in our 
daily weather charts the actual condition of the weather at a stated hour. 
Such charts, showing the actual state of the weather at 8 a. ra. throughout 
the United States and the British Provinces to the north, are issued daily 
about 11 a. m. by the United States Weather Bureau, based on telegraphic 
reports from about 175 stations. 

AREAS OF FAIR WEATHER (aNTI-CYCLONES) . 

As already stated in a preceding paragraph, the term anti-cyclone was 
first used to describe a weather type which shows characteristics just 
the opposite of those of the cyclones. The pressure is highest in the 
centre of the area, the winds blow in a general direction away from the 
center, the skies are mostly clear to partly clouded, with little or no 
precipitation, the temperatures are, in general, loiver at the center than 
on the eastern or western sides of the area ; that is, their isotherms 
curve southward toward the center of high pressure while those in the 
cyclone bend northward toward the central area of low pressure. 

A typical example of an anti-cyclonic system is shown in the weather' 
map of April 4, 1904, reproduced in Figs. 87, 88. It occupied approxi- 
mately the same position and covered the same area as did the cyclone 
of December 27, 1904, shown in Figs. 85, 86. 

Isobars and Winds. — The inner circle, or isobar of 30. GO inches, marks 
the central area of the anti-cyclonic system, from which there is a 
steady and uniform decrease in the height of the barometer outward in 
all directions, the successive isobars marking intervals of two-tenths of 
an inch in the height of the l)aromoter, as in the case of the cyclonic 
system described above. It will be observed that the gradient, or 
steepness of the successive steps between isobars, is less in the anti- 
cyclone than in the cyclone : the area covered by each system is approxi- 
mately the same, wliilo tlio total difference in pressure between tlie center 



322 



THE CLIMATE OF BALTIMORE 




Fig. 87. — Typical Anti-cyclone of April 4, 1904. 



LOW 




Fig. 88. — Typical Anti-cyclone of April 4, 1904. 



LEGEND. 

The following explanation applies to U. S. daily weather charts reproduced 
in this report. The charts are based on simultaneous observations made at 
8 a. m., 75th meridian time. 

Black lines are lines of equal temperature (in degrees Fahr.). 

Red lines are lines of equal atmospheric pressure (in inches). 

Shaded areas mark the limits of overcast skies. 

The arrows fly with the wind. 

R Indicates rain. 

S indicates snow. 

T indicates the occurrence of a thunderstorm during the preceding 12 
hours. 



IMORE 



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.STUOfl 



Fig. 85.— Typi. clone of April 4, 1904. 



MARYLAND WEATHER SERVICE 333 

and outer circumference in the case of the anti-cyclone is only half 
that in the cyclone, namely, one-half an inch. This is also shown by 
the number of the isobars, the cyclone having double the number shown 
in the anti-cyclone, while the successive steps of increase or decrease in 
pressure, and the entire areas covered by the two systems are the same. 
This is characteristic of the two types, though the proportions may 
vary greatly. The winds are observed to blow in a general direction 
away from the center. As the winds in an anti-cyclone are in general 
much lighter in force than in a cyclone, the actual directions recorded 
near the surface are influenced to a greater extent by local topography. 
This is especially marked near the center of the area where the winds 
are very light. 

The law of deviation of a particle of air to the right of the initial 
direction flowing from the center of the high area outward holds good, 
as noted in the discussion of the inward flow in cyclones; hence the 
arrows, indicating the direction of the wind in the anti-cyclone, are 
observed to point, not directly from the center but to the right of the 
radial path, the angular deviation depending upon the configuration of 
the isobars, the topography and other factors. 

The Winds and Distribution of Temperature. Especial attention is 
directed to the relation between wind direction and temperature. Here 
again, as in the case of the cyclone, the temperature is seen to be directly 
dependent upon wind direction. In those portions of the area where 
the winds are mostly from a warm southerly direction, particularly 
noticeable in the northwest quadrant in the illustration shown, the 
isotherms are bent far northward of their normal position for the season. 
In the northeast quadrant of the area, where the winds are mostly from 
the colder north, the isotherm of 30° is seen to dip far to the south. 
Along the Atlantic coast the cold of the northerly wind is considerably 
tempered by the presence of the ocean, over which the rate of change in 
temperature is much less marked than on land along a north and 
south line. 

In the center of a " high," another factor enters to lower the tem])era- 
ture below the normal for the season. Here the skies are clear and 



324 THE CLIMATE OF BALTIMORE 

radiation from the surface of the earth is rapid. This is especially true 
during the night hours and^ in consequence, the effect is quite marked 
on a chart based on observations made at 8 a. m., before the heating 
effect of the rising sun becomes marked. As a result of the conditions 
described, the lowest temperatures in a well developed anti-cyclone are 
generally observed to be within the central isobar of a system, if meas- 
ured by departures from the normal seasonal values for a given locality. 

Distribution of Clouds. — The chart selected as an example of an anti- 
cyclone shows almost too well one of the characteristic features of this 
type of weather, iiamely, the absence of clouds. While freedom from 
precipitation and clouds is the most striking difference between a cyclone 
and an anti-cyclone, it is not usual to see a weather chart with a high 
area of so great an extent with skies practically free from clouds, as 
is here shown. Over an area embracing all the states and the Canadian 
Provinces, from the Mississippi Valley eastward to the Atlantic coast, 
an overcast sky was reported at 8 a. m. from only four or five observing 
stations. Usually, even in the well defined and well developed areas of 
high pressure there is a fair percentage of cloudiness, excepting within 
a radius of two hundrea ^r three hundred miles from the center. 

The presumption is that this freedom from clouds and precipitation 
in an anti-cyclone is due to a descending atmosphere, with attendant 
increase of temperature, due to compression and consequent decrease 
in the relative humidity; the opposite process, namel}^, a rising atmos- 
phere cooled by expansion, accompanied by an increasing relative humid- 
ity and by cloud formation, and later by rain or snow, marks the cyclone. 

THE EASTWARD DRIFT OF CYCLONES AND ANTI-CYCLONES. 

In describing the distribution of presfeure, winds, temperature, and 
clouds in typical c3^clones and anti-cyclones in the preceding paragraphs 
no reference was made to their movement as a whole. The systems are 
not stationary for any length of time. In addition to the internal 
circulation of the winds described, the entire systems are carried east- 
ward by the general drift of the upper atmosphere in the middle lati- 
tudes, retaining at the same time their chief characteristics as cyclones 



MARYLAND WEATHER SERVICE 



325 



and anti-eycloues for many hundreds, and sometimes thousands of miles. 
The small whirls formed in a rapidly flowing river and carried down 
stream with the general current, while at the same time maintaining their 
own gyratory motion, are often cited as illustrations of the drift of 
cyclones and anti-cyclones in the general eastward flow of the atmos- 
phere between the parallels of 30° and 70° north and south latitude. 
These " highs " and " lows " do not extend to a great altitude, but are 




Fig. 89. — Typical Cyclone and Anti-cyclone of March 3, 1904. 

formed apparently only in the lower portions of the atmosphere, the 
great majority of them being confined to the atmospheric strata below 
the highest mountain ranges. They are carried in an easterly direction 
in the middle latitudes with a varying velocity but averaging about 600 
miles per day. wliile moving across the United States. There is a con- 
stant and rapid succession of these atmospheric whirls, or waves of high 
and low pressure, in the winter and spring season. In summer and 
early fall they arc loss frequent and not so well developed. There is 



326 THE CLIMATE OF BALTIMORE 

an excellent example of a well developed " low " or cyclone, followed by 
a "high." or anti-cyclone in the chart for March 3, 1904. (Fig. 89.) 

The shifts in the direction of the wind experienced during the passage 
of these " lows " and "highs " may be illustrated upon this chart by 
noting the directions of the wind at points along a given parallel of 
latitude passing from east to west. The nature of the change of wind 
depends upon the position of the center of the cyclone or anti-cyclone 
with reference to the parallel selected. Taking the latitude of 40 '^ for 
example in the chart for March 3, 1904, we have, as we pass westward 
from the Atlantic seaboard, first a southeast wind followed by a small 
area of south wind, in AVest Virginia, followed by a northwest wind to 
the center of the succeeding " high " in western Nebraska. To the west of 
the center of the anti-cyclone we have again a southerly wind in Colorado 
and Utah. Selecting a parallel of latitude north of the center of the 
" low " and " high," we have a reversed order in the shift of the wind. 
In the " low " the change is from easterly to westerly by way of the 
north ; and in the " high " from westerly to easterly by way of the south. 

Similar shifts in the wind are experienced in a fixed locality, such as 
Baltimore, for example, as these cyclones and anti-cyclones approach 
and pass beyond the observing station. An easterly wind at Baltimore 
heralds the approach from the west or southwest of a more or less 
developed cyclone, or storm area, followed by increasing cloudiness and 
rain or snow, as the center of the storm approaches. After the wind 
veers to the south and then to the southwest the precipitation soon 
ceases, the solid cloud mass begins to break into patches of cloud and, 
as the wind gets into the west, the proportion of clear sky increases until 
the cyclonic system passes beyond the horizon in the east. This is the 
usual order of change. With the path of the storm center to the south 
of Baltimore, the wind backs from east to west by way of the north. 

The weather types described above are of unusual symmetry. The forms 
met with in our daily routine of weather conditions are infinite in variety. 
No two are exactly alike in all their details of pressure, temperature and 
cloud distribution, or in the paths pursued, but there are many easily 
recognizable types with marked family resemblances which are of great 



MARYLAND WEATHER SERVICE 



327 



assistance to the practical meteorologist engaged in weather forecasting. 
Some of these types will be discussed in detail in the following pages. 

WEATHER CHARTS OF THE NORTHERN HEMISPHERE. 

A remarkable series of daily weather charts covering a period of ten 
years was prepared and the results published under the auspices of the 




Fig. 90. — Pressure Distribution over the Northern Hemisphere, Dec. 4, 1886. 



United States Weather Bureau, then known as the Signal Service, from 
1878 to 1887. The charts were based on reports received from co-operat- 
ing national weather services and covered the whole of the northern 
hemisphere between the latitudes of about 20° to 65°, excepting the 
22 



328 THE CLIMATE OF BALTIMORE 

Pacific Ocean. One of these cliarts, sliovving tlie actual distribution of 
pressure at noon, Greenwich time, for December 4, 1886, is reproduced 
in Fig. 90. The chart shows clearly the manner in which the lower 
atmosphere of the middle latitudes is segregated into successive areas 
of low and high pressure, or cyclones and anti-cyclones at a given hour. 

Weather of the Prixcipal Climatic Zones. 

It has been customary for convenience to divide the surface of the 
globe into three climatic zones, the tropical, the temperate, and the polar, 
separated by fixed parallels of latitude and based upon the altitude of 
the sun above the horizon. The climatic conditions experienced within 
these zones have no such definite boundaries. When we come to the 
question of daily weather conditions, it is even more difficult to assign 
any fixed limits to areas of characteristic weather types. Still, it is 
possible to designate a number of zones, in the central portions of which 
the weather conditions are sharply marked off from conditions in neigh- 
boring zones. 

the tropical zoxe. 

The climatic belt designated as the tropical zone has several sub-zones 
of characteristic types of weather. The entire zone is marked by a 
uniformly high temperature, but the moisture conditions and atmos- 
pheric movements vary greatly in neighboring regions. The temperature 
changes from day to day, or from season to season being very small, the 
seasons are marked by a varying frequency or quantity of rainfall, or by 
a change in the direction of the wind. One day is very much like another 
the year round, and the weather cycle is the daily cycle, offering a strong 
contrast with the rapid fluctuations experienced in more northern 
latitudes. 

The doldrums, or equatorial calms, are characterized by high tempera- 
ture and humidity, light winds or calms, much cloudiness and frequent 
and heavy rains, and almost daily thunderstorms — a combination caus- 
ing an oppressive and debilitating atmosphere. 



MARYLAXD WEATHEK SERVICE 329 

To the north and south of tlie doldrums are the trade wind belts. Here 
the skies are mostly clear, while a fresh, dry, northeast or south^\"est wind, 
strongest over tlie ocean, l)lows steadily toward the equatorial belt of 
highest mean temperature. 

Beyond the northeast and southeast trades, there is another belt of 
light winds or calms, the so-called "horse" latitudes; these are areas of 
permanently high pressure, clear skies and warm dry air, resembling in 
many respects the summer anti-cyclone of the middle latitudes. Within 
this belt most of the great desert areas of the earth are formed, in the 
southern as well as the northern hemisphere. 

The moving cyclones and anti-cyclones, described in preceding pages 
as characteristic of temperate zone weather, are conspicuous by their ab- 
sence in most portions of the hot zone. In some portions, notably in the 
West Indies, the Philippines, and the Indian seas, cyclones of great inten- 
sity occur during the late summer and early fall, the well-known hurri- 
canes and typhoons, which are carried in a westerly direction by the 
general drift of the atmosphere in the equatorial regions ; but they are of 
infrequent occurrence when compared with the constant succession of 
temperate region cyclones. 

THE TEMPERATE ZOXES. 

In the middle latitudes, north and south of the equator and extending 
beyond the Arctic and Antarctic circles, the weather is completely domi- 
nated by the moving cyclones and anti-cyclones described in preceding 
paragraphs. Here the daily monotony of tropical weather is replaced by 
great variability of temperature conditions which mark the seasons, and 
by the more rapid fluctuations which accompany the passing of cyclones 
and anti-cyclones. Tropical heat succeeds polar cold and all the weathers 
of the globe are brought to our doors during the course of a year. These 
contrasts become more and more marked as we approach the central por- 
tions of the great continental areas of the northern hemisphere. In the 
extreme nortliwest of our own country and in the British northwest 
territory, the breeding ground of cyclones and anti-cyclones, the contrasts 
in temperature experienced at a single station within a few hours, or 



330 THE CLIMATE OF BALTIMORE 

within very limited areas at the same hour, are sometimes marvelous. 
On the 10th of February in the year 1899, an anti-cyclone developed over 
Montana and the British territory just beyond. It spread rapidly over 
the United States as one of the most intense cold waves ever experienced 
in this country,, lowering the record of intense cold in many states in its 
progress southeastward to the Gulf of Mexico and the Atlantic coast. 
On the morning of the 10th a minimum temperature of 65° below zero 
was registered in the western part of Montana. Just west of the moun- 
tains in the neighboring state of Washington, the temperature at the 
same hour was 63° above zero, a difference of 128° between two points 
along the same parallel of latitude (50° north) less than 300 miles 
apart. 

The southeastward progress across the United States of some of the 
more marked cold waves is frequently attended by strong inversions in 
the normal distribution of temperature along the Atlantic coast. The 
front of the cold wave, with its cold northwest winds, may reach Florida 
from 12 to 24 hours before it is felt in New England and the British 
maritime provinces, which may be in the center of the well developed 
cyclone which frequently precedes the cold wave. Under such conditions, 
a strong southerly wind will blow along the north Atlantic coast and raise 
the temperature high above its normal seasonal value for these coasts, 
while the Gulf states are dominated by the intensely cold northwest wind 
of the anti-cyclone. In such cases, temperatures of 20° above zero or 
less are experienced in northern Florida while the warm southerly winds 
blowing over Newfoundland raise the temperature to 40° or 50° above, 
although the latter region is nearly 25° of latitude farther north than 
Florida. 

THE POLAR ZONES. 

While we are less familiar with the weather conditions of the extreme 
north and south portions of the globe than with those of the hot and 
temperate zones, we have abundant evidence that within the Arctic and 
Antarctic the passing cyclone controls conditions to the highest latitudes 
attained. The fluctuations in temperature are very great, with changes 



MARYLAND WEATHER SERVICE 331 

in the direction of the wind, while the cold is usually intense. While 
there are apparently bright, clear and exhilarating days, the weather is 
mostly gloomy with a high humidity and frequent fogs, sleet and snow. 
The intense cold weakens the powers of resistance. With these disa- 
greeable weather conditions predominating there is the added gloom of 
long-continued darkness, the physiological effects of which are exceed- 
ingly distressing. 

THE SEASONS. 

In the middle latitudes, and particularly over the continental areas, 
the most conspicuous feature of the advance and retreat of the seasons 
is the marked rise or fall in the mean temperature from month to month. 
Take, for example, the annual rise and fall of the thermometer at Balti- 
more as shown by Fig. 25 on page 111 ; the curves b, c, and d are based on 
the mean monthly maximum, the normal monthly average and the mean 
monthly minimum temperatures respectively. The lowest temperatures 
occur in the months of January and February; from this portion of the 
curve there is a steady rise at a fairly uniform rate to the month of July, 
followed by an uninterrupted fall to midwinter. The smooth, simple 
curve represents a uniform increase and decrease in the power of the solar 
rays as the sun increases and decreases in altitude in the annual revolu- 
tion of the earth about the sun. This uniform increase and decrease 
throughout the year is still shown by constructing the annual curve from 
the mean temperatures of successive five-day periods. (See Table 
XVIII, page 89.) If, however, we represent the advance and retreat of 
the seasons by curves based on mean daily temperatures in place of mean 
monthly temperatures, we find a striking difference in the character of 
the two sets of curves, as is clearly shown by consulting Plate III. There 
is no such uniformity in the daily progress of temperature; the serrated 
appearance of the curve indicates clearly that the progress is marked by 
successive temperature waves having an average period of three or four 
days. The annual curve is broken up into a series of subsidiary curves 
of short but irregular periods, due to the constant succession of cyclones 
and anti-cyclones with their accompanying large fluctuations in tempera- 



332 THE CLIMATE OF BALTIMORE 

ture. By substituting the actual temperatures experienced upon each 
day of an}'- given year, in j^lace of the meari daily temperatures for a 
long series of years, the irregularities of the annual curve become enor- 
mously increased. The extent to which the temperatures experienced 
upon a given day of the year have varied in past years is shown in curves 
A, C, and D in Plate IV. For example, on the 11th day of February, 
1899, the luinimum temperature at Baltimore was 6° below zero; on the 
11th of February in 1887, the maximum was 72° above zero, an extreme 
range of 78° for the 11th of February. Even in the summer months, 
when the variability in temperature conditions is least marked, the ex- 
treme ranges are about -10°. 

While differences in temperature constitute the most conspicuous fea- 
ture of the weather of successive seasons in most portions of the middle 
latitudes, the character and amount of precipitation, and the duration 
and the force of the wind are factors of great importance, and indeed 
these sometimes overshadow the temperature changes. 

The departures from the normal conditions of temperature, precipi- 
tation and wind experienced in a given season, must be referred back to 
the prevailing type of pressure distribution upon which they depend. 
A clear knowledge of the relative distribution of pressure over a widely 
extended area is essential to a proper understanding of the weather 
changes in any given locality; this knowledge should extend, not only to 
the rapidly moving cyclones and anti-cyclones, but also to the larger areas 
of high and low pressure known as permanent cyclones and anti-cyclones, 
which are a direct result of the general atmospheric circulation.^ 

As a result of the increasing cold of the winter months there is formed 
over the Xorth American continent a vast anti-cyclonic area. AVith the 
passing of the winter, this gradually disappears to give place to a baro- 
metric depression, or cyclone. The process is reversed over the neighbor- 
ing oceans; here, while the contrasts between the winter and sumiuer 
pressure distribution are not so marked, the pressure is lower in winter 

^ See: Teisserence de Bort; Etude sur I'hiver de 1879-80. Ann. du Bureau 
Centr. Met'L. Paris, Vol. IV, 1881. 



MARYLAND WEATHER SERVICE 333 

than in summer. The changes in the intensity and in the position of 
these great atmospheric systems have a direct influence upon the char- 
acter of the seasons over the eastern portions of the United States, and 
especially in the Atlantic coast states." The best developed and most 
conspicuous instance of this semi-annual transfer of vast quantities of 
air from the continent to ocean during the winter and from ocean to con- 
tinent during the summer, is seen in the winter and summer monsoons 
over India and the Indian Ocean. 

Bearing in mind what has been said concerning the influence of pres- 
sure distribution on the direction of winds, and hence on temperature 
and precipitation, we may realize how variations from the normal type 
of pressure distribution for a given month or season will affect the 
general character of the weather of the period in question. This influ- 
ence may be graphically shown by calculating the mean monthly distri- 
bution of atmospheric pressure over the North American Continent and 
adjacent oceans during an abnormally cold month and an abnormally 
warm month, and charting the results in connection with a map showing 
the normal distribution of pressure based on a long series of years of 
observation. This has been done in succeeding pages for the months of 
January, April, June, and October as types for the winter, spring, sum- 
mer, and autumn seasons respectively. 

In the succeeding pages some of the more conspicuous types of cy- 
clonic and anti-cyclonic control of the weather of the Middle Atlantic 
states will be considered in connection with a discussion of the seasons 
in which they most frequently occur. 

WINTER WEATHER. 

The winter season presents the most variable weather conditions of 
the year. Practically every type of weather may be experienced at one 
time or another during the course of the three months, and sometimes a 
great variety of types may be crowded into the short period of 24 to 48 

* See: O. L. Fassig. Types of March Weather in the United States. Amer. 
Journ. Sci., New Haven, November, 1899, Vol. III. 



334 THE CLIMATE OF BALTIMORE 

hours. A description of winter weather conditions which prevail in the 
vicinity of Baltimore would include all the types of the year, though 
some of them attain their greatest development in other seasons. 

An account of weather conditions from day to day in our latitudes is 
mostly confined to a consideration of the eastwardly moving procession 
of cyclonic and anti-cyclonic systems across the United States. While 
for purposes of convenience and clearness our descriptions are confined 
to well developed types, the fact must not be overlooked that the faintly 
developed systems are the most frequent and consequently in the long 
run determine the general character of the weather of a given locality. 

All of our weather types may be roughly separated into two fairly 
distinct classes — (a) areas of unsettled weather accompanying the pas- 
sage of cyclones, or areas of low pressure — (b) areas of fair weather asso- 
ciated with passing anti-cyclones, or areas of high pressure. While it 
is often difficult to distinguish these types clearly it will be found of 
great convenience to adhere to the classification in the following pages. 

Winter Cyclones. 

As stated in preceding paragraphs, the weather of our middle latitudes 
is characterized by an irregular succession of atmospheric waves passing 
from west to east; the areas in which the barometer reads high corres- 
ponding with the crest of the waves, while the areas of low barometric 
pressure may be compared with the troughs. When these crests and 
troughs are well developed and sharply defined the latter are known as 
cyclones, or simply as storms, while the crests are called anti-cyclones; 
in the winter season when these anti-cyclones develop to unusual inten- 
sity they constitute our cold waves. When they are well developed and 
move with average speed across the country these cylonic disturbances 
usually cover a period of two to three days in passing a given meridian. 
As they pass over a region they bring to it a fairly regular sequence of 
weather changes. The character of these successive changes is modified 
by various conditions. First in importance is the position of the region 
with reference to the center of the barometric depression, or storm. The 
path traversed by the center of the storm with reference to Baltimore 
depends largely upon the place of its origin. 



MARYLAND WEATHER SERVICE 335 

In selecting a series of storms for illustration to show the different 
varieties of weather experienced in the vicinity of Baltimore during the 
course of the year, it will be found convenient to classify them, basing 
the classification upon the place of origin of the depression, or, perhaps 
better, the position of the center of the depression a day or two before 
its arrival over the region about Baltimore. 

Four types will be described in the order of their percentage of 
frequency across the horizon of Baltimore — the Lake storm, the South- 
west storm, the Gulf storm, and the Coast storm. These types imper- 
ceptibly merge into one another at times, but they have sufficient 
individuality to permit of ready separation into the classes named. 

All of these classes show their most intense development in the winter 
season, with perhaps the exception of the Coast storm; the latter is 
likely to be the northward extension of a West Indian hurricane, and 
hence shows a maximum frequency in the early autumn, or late summer. 

THE LAKE STORM. 

The Storm of December 24-26, 1902. 
The daily weather map of the United States Weather Bureau for 8 
a. m., December 23, 1902, shows a distribution of pressure which caused 
a fairly normal condition of winter temperatures. A barometric pres- 
sure above the seasonal average prevailed over the eastern half of the 
country with a maximum over the Great Lakes, giving rise to northerly 
winds east of the Mississippi River. A depression, first shown on the 
map of the 22d over Puget Sound, had made its way eastward to Montana 
and North Dakota. West of the Mississippi this depression had already 
shown its influence in a drift of southerly winds towards the center of 
depression, but the isotherms had not as yet been greatly bent from 
their normal trend. Twenty-four hours later, at 8 a. m. of the 24th, 
the center of the depression had moved eastward a distance of about 
600 miles, its center being over Lake Superior. The effect of 24 hours 
of southerly winds in advance of the center of the storm, coupled with 
the southward flow of winds from the colder northwest quadrant behind 
the storm center, changed the isotherms from their normal east-west 



336 



THE CLIMATE OF BALTIMORE 




Fig. 91.— The Lake Storm of December 24, 1902. 



29 e 




Fig 92.— The Lake Storm of December 25, 1902. 



MARYLAND WEATHER SERVICE 



337 



trend to north-south lines. Temperatures in ach ance of the center were 
raised 15° to 20° in the southeast quadrant, while there was a fall of 
equal amount in the southwest quadrant. On its way eastward the area 
of precipitation grew in extent. The temperatures on all sides of the 
storm center being below freezing point the precij)itation was practically 
all in the form of snow. As is most frequently the case, the storm 
area was oval in shape, with its long axis extending north and south ; 




Fig. 93.— The Lake Storm of December 26, 1902. 



as a result the winds about the center blew from the southeast in advance 
of the long axis, and from the northwest in the rear of the advancing 
central line, on both sides l)lowing towards the trough of tlie lowest pres- 
sure. 

By Christmas morning the storm center had moved eastward to the 
Lower Lake region, a distance of about 500 miles, and before the close 
of the day the trough of lowest pressure had crossed the meridian of 
Baltimore. The eastern edge of the area of snowfall had reached the 



338 THE CLIMATE OF BALTIMORE 

Atlantic coast by 8 a. m., from Massachusetts to Xorth Carolina. Dur- 
ing the preceding 12 hours snow had fallen in varying amounts over the 
entire area from Chicago eastward to the Atlantic coast, and from the 
Lakes southward to North Carolina. The area in advance of the storm 
over which the temperatures rose 15° to 20° now extended to the coast, 
while a cold wave (an area of high pressure) followed close behind the 
storm center, attended by northwesterly winds and clear skies. 

By 8 a. m. of the 26th of December, the center of the storm had 
reached the New England coast in its due eastward progress, covering 
another 600 miles in the preceding 24 hours. Here it remained nearly 
stationary for 24 hours before continuing its eastward course over the 
North Atlantic Ocean, By this time the area of high barometric pres- 
sure following the storm had spread over the entire region from the 
Eocky Mountains eastward to the Atlantic coast and from the Lakes to 
the Gulf coast, carrying freezing temperatures southward into Middle 
Florida. 

The weather map of December 27th shows a condition which frequently 
occurs in the winter months — a striking inversion of temperatures 
between Florida and Nova Scotia. Jacksonville, Fla., had a tempera- 
ture of 24° at 8 a. m., while Sydney, N. S., reported a temperature 
of 40° at the same hour. The reason for this apparent anomaly is 
readily found on examining the weather maps of the preceding days. 
The cold northwest winds flowing out of the area of high pressure in the 
rear of the advancing storm had reached the Gulf states while the warm 
southerly winds were still blowing over Nova Scotia in the southeast 
quadrant of the storm area. 

The path of this storm of December 24-26, 1902, from the Lake region 
eastward is the approximate path of nearly three-fourths of the baro- 
metric depressions which exert a direct influence upon the weather con- 
ditions in the vicinity of Baltimore. The path of the center 
lies well to the north of Baltimore. The successive changes in the 
elements of the weather experienced during the passage of this type of 
storm across the meridian of Baltimore are graphically illustrated in the 
accompanying diagram, a brief description of which will suffice to call 
attention to the most important factors. (See Figs. 91-94.) 



MARYLAND "WEATHER SERVICE 



339 



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340 THE CLIMATE OF BALTIMORE 

Within the horizon of Baltimore the ajjproach of the storm from the 
west was announced by a steady fall in the height of the mercury in 
the barometer after 10 o'clock in the morning of the 24th of December. 
The day began with clear skies; soon after sunrise the clouds began to 
form, increasing in amount until the sky became overcast by 10 a. m. 
The winds blew from the north in the early morning. About 10 a. m. 
the direction changed to northeast, continuing to veer to east and then 
to soutlieast and south with the continued fall of the barometer. Co- 
incident with the changes in the wind from north to east and southeast 
the temperature rose steadily until nearly midnight; the usual diurnal 
fall in temperature after 3 p. m. being eliminated by the cyclonic rise. 
The barometer continued to fall until 8 a. m. of Christmas day, the 
time at which the trough of lowest pressure of the storm area crossed the 
meridian of Baltimore. At about the same hour the wind veered from 
southeast to southwest. The temperature continued to rise until 10 a. m. 
and then fell steadily with the persistent blowing of west to noi-thwest, 
winds. The humidity increased from 40 per cent at 10 a. m. of the 24th, 
when the winds changed to an easterly direction, to a maximum of 90 
per cent at 8 a. m. of the 28th. With the change of wind from southerly 
to westerly the humidity fell rapidly to 45 per cent within a period of 
about two hours. 

A light dry snow began to fall betw^een 10 and 11 p. m. of the 24th, 
with a falling barometer and a southerly wind. The snow continued 
until 2 a. m. of the 25th, the total amount lieing about three inches. 
The sky remained overcast until 10 a. m. of the 25th, when the clouds 
began to break away soon after the shift of the wind from southeast to 
southwest. By 8 p. m. the sky was clear. The usual sharp rise in pres- 
sure after the storm was not experienced in the passage of this depres- 
sion, the barometer remaining comparatively low through the 25th and 
26th. As a result there were no high winds in Baltimore during the 
progress of the storm : there was a slight increase in velocity, however, 
following the turn in the barometer and the change in direction of the 
wind from west to northwest. 

The series of maps showdng the progress of this storm across the 
United States well illustrates the normal winter conditions of a succes- 



MARYLAND WEATHER SERVICE 341 

sion of areas of high and low pressure moving from west to east across 
the continent. On the map of December 2-ith we see an area of high 
pressure over Xew England, a depression over the Upper Lake region, 
another high area over Montana and the Canadian Xorthwest, and 
another depression appearing on the Pacific coast over Oregon and 
Washington. These systems move eastward at an average rate of about 
GOO miles per day with their centers mostly between the 40th and 50th 
parallels of latitude, bringing to localities over which they pass their 
characteristic changes in temperature, in wind direction and force, in 
clouds and sunshine, and in rainfall or snowfall. 

The Storm of January 7-S, 1903. 

The daily charts of January 6, 7, and 8, 1903, issued by the United 
States Weather Bureau, show the progress of another storm of the Lake 
region type. On the 5th a depression appeared upon the field of the 
map in the extreme northwest of the Canadian Provinces. By 8 a. m. 
of the 6th the depression had crossed the boundary line into Xorth Dakota 
as a well developed storm, its influence being felt over most of the area 
between the Eocky Mountains and the Mississippi Valley. 

In the succeeding 24 hours the center of the storm had traversed a 
distance of nearly a thousand miles, from Quaj^elle, Manitoba, to Central 
Michigan. The depression developed no precipitation area until the 
night of the 6th. By 8 a. m. of the ith snow had fallen over a sym- 
metrical oval area about the center, extending about a thousand miles 
from east to west and about six hundred miles from north to south. 
Passing eastward witli the same rapid rate of progress the center moved 
over Nova Scotia by 8 a. m. of the following day. Wliilc the area of 
snowfall attending this storm reached as far soutli as Tennessee and 
North Carolina, the precipitation was extremely light in Maryland and 
Virginia, and was of short duration. The rainfall or snowfall in Balti- 
more and vicinity is generally light and falls in tlic form of brief 
showers or snow flurries with storms of this type, unless their centers 
pass the meridian of Baltimore within a hundred miles, or less, to the 
north, when the precipitation may be heavy and of considerable duration. 



343 



THE CLIMATE OF BALTIMORE 




Fig. 95. — The Lake Storm of January 7, 1903. 




30.0 



Fig. 96.— The Lake Storm of January 8, 1903. 



MARYLAND WEATHER SERVICE 



343 




23 



344 THE CLIMATE OF BALTIMORE 

In this storm the normal trend of the isotherms was not disturbed to the 
same extent as in the case of the storm of December 24-26, 1902, described 
above. The usual decided fall in temperature (20 degrees or more) 
followed in the path of the storm, but in this instance the cold wave 
R-as nearly 24 hours behind the center of the depression and did not 
reach the Atlantic coast. The local changes during the progress of the 
storm were not ver}' pronounced but they were representative of the 
type following a similar path. Early in the morning of the 7th Balti- 
more came within the area of influence of the Lake storm described 
above. The barometer began to fall about midnight of the 6th-7th, 
the wind changed at the same time from northwest to west and soon 
after to the south^^•est ; by 6 a. m. the wind had backed to the south and 
this direction prevailed until 4 p. m. Between 4 and 5 p. m. there 
was an abrupt change of the wind from south to west, accompanied by 
a rise in the barometer. The pressure rose slowly though steadily 
throughout January 8th as the center of the depression moved over the 
Atlantic oif the New England coast. The wind did not materially 
increase in force until 2 a. m,, about 10 hours after the beginning of 
the rise in the barometer, attaining a maximum velocity of 28 miles per 
hour before noon. 

The passage of a coast storm on the 5th-6th left a raw blustery Mdnd 
blowing from the west and northwest in the afternoon of the 6th, witfi 
clearing skies. Cloudiness increased during the morning of the 7th 
upon the approach of the Lake depression and the sky soon became 
overcast. There was a brief breaking away of the clouds between three 
and four p. m. Light snow fell between 8.30 and 10.30 a. m. of the 
7th, between 11.50 a. m. and 1.45 p. m., between 4 p. m. and 5.30 p. m., 
and again between 9.15 and 9.40 p. m. The total fall of snow was not 
much over half an inch. During the 8th the sky was overcast with only 
occasional brief intervals of sunshine. There was a slow and steady 
fall in temperature and a steady rise in the barometer, with brisk westerly 
winds in the forenoon. Traces of snow fell between 11.20 and 11.51 
a. m., 12.05 and 12.25 p. m., and between 7.55 and 8.25 p. m. 

The rise in temperature in advance of the storm was not well marked. 



MAKYLAXD WEATHER SERVICE 345 

This may be readily accounted for by the brief duration of tlie south- 
erly winds in advance of the center, covering a period of less than 10 
hours. (See Figs. 95-97.) 

The Storm of February 27 -March 1, 1903. 

These Lake storms sometimes develop into disturbances of great extent 
and intensity. The weather chart of 8 a. in., February 27, shows a depres- 
sion centered over Eastern Nebraska, formed apparently by the union 
of two distinct depressions; one of these had its origin in the Canadian 
Northwest Provinces, the other in the extreme southwest, over Arizona. 
By 8 a. m. of the 27th a very considerable rain area had already developed 
over the Central and Southern states, aided largely by the presence of a 
well developed area of high barometric pressure over the Atlantic Ocean 
oft the Middle Atlantic states. During the succeeding 24 hours the 
storm area grew to unusual proportions, while it moved eastward across 
the Lake region at a rate slightly above the normal rate of progress for 
such storms. By 8 a. m. of the 28th the precipitation area of the pre- 
ceding 12 hours embraced all of the country east of the Mississippi Elver. 
To the South and east of the storm center the areas in which southerly 
winds prevailed, temperatures rose from 15° to 40°, and the precipitation 
was in the form of rain; M^est of the center of the storm, in the area 
of northwest winds, there was a fall of 20° to 30° in 24 liours. The 
rain area was not only of unusual extent, but the eastward movement 
of the storm was marked by very heavy rains, measuring an inch and a 
half to two and a half inches in 24 hours at many stations in the South 
Atlantic and Gulf states. The passage of the trough of low pressure 
was also the occasion for the production of severe squalls and local 
storms. By the morning of March 1 the storm center had moved east- 
ward to the Gulf of St. Lawrence followed closely by a fall of 20° to 30° 
over a large area, embracing a dozen or more states. 

The local changes during the passage of this wide-spread storm across 
the meridian of Baltimore were exceptionally well marked and character- 
istic of the well developed storm of the type with a path across the 
Lake region and down the St. Lawrence Valley. (See Figs. 98 101.) 



THE CLIMATE OF BALTIMOItE 




Fig. 98.— The Lake Storm of February 27, 1903. 




Fig. 99.— The Lake Storm of February 28, 1903. 



MARYLAND WEATHER SERVICE 



34.7 



On the morning of the 26th an area of high barometric pressure rested 
over the Atlantic states, with its center over Maryland and Virginia. 
The winds were light and variahle in direction. The skies were clear, 
resulting in heavy frosts during the preceding night and in the early 
morning hours, throughout the state. 

With clear skies and light winds the temperature rose rapidly during 
the day, and the air became balmy and spring-like. Tliere were a few 




Fig. 100.— The Lake Storm of March 1, 1903. 



cirro-stratus clouds in the early morning, but they soon disappeared. 
At 10.40 a. m. the local Weather Bureau Office received the following 
telegram from the Central Olhce in Washington : " Southeast storm 
warnings ordered hoisted along the Atlantic coast from ]\Iiami, Fla., 
to Charleston, S. C. Storm over Texas is moving northeastward. Brisk 
to high easterly winds arc indicated this evening and tonight on the 
South Atlantic coast.*' 



348 



THE CLIMATE OF BALTIMORE 




MARYLAND WEATHER SERVICE 349 

Clouds gathered during tJie night, and at dawn of the 37th the sk}^ 
was entirely overcast with a thin veil of stratus clouds. The atmosphere 
was humid and the clouds began to thicken. A solar corona was observed 
in the forenoon. The barometer fell steadily and rapidly throughout 
the day, the wind changed to southeast and east, while the temperature 
rose rapidly from a minimum of 33° at & a. m. to 52° at 4 p. m. A 
light rain fell from 2 p. m. to 2.10 p. m., began again at 3.35 p. m., 
continuing through the night. The amount of rainfall at midnight was 
0.68 inch. At 11.45 a. m. southeast storm warnings were ordered up 
along the Atlantic coast as far north as Ft. Monroe, and later, southwest 
storm warnings were ordered from Baltimore to New York. 

The following day, February 28, was cloudy and warmer. The 
atmosphere was humid and oppressive in the forenoon, but became more 
pleasant in the afternoon. Light fog formed during the night; at dawn 
it was dense, but soon became lighter, disappearing by 11 a. m. About 
1 p. m. there was a temporary break in the clouds, but in about an hour 
a heavy stratus mass arose and rapidly covered the sky. At 10.30 a. m. 
storm warnings were ordered changed to northwest from South Carolina 
to Virginia. The rain which began on the preceding day continued 
to 7 a. m., began again about 8.30 a. m. ; it was heavy for a few minutes 
after 10 a. m. and continued wdth brief interruptions until 3.15 p. m. 
The total fall from midnight was 0.46 inch. The winds were fresh to 
brisk between 10 and 11 a. m., increasing in the afternoon and evening 
to high ; the maximum velocity was 38 miles from the west at 2.45 p. m. 

The barometer continued to fall rapidly to a minimum of 29.27 inches 
at 2 p. m., while the temperature rose steadily from 52° at midnight to 
71° at 2 p. m. At this hour the wind veered from south to southwest 
and then to the west, accompanied by a rapid rise in the barometer and a 
sharp fall in the temperature. The atmosphere became crisp and 
invigorating throughout the balance of the day, and the day following 
(March 1) the barometer rose and the temperature fell rapidly and 
steadily, while the wind continued from the west and northwest. 

During the passage of this storm the temperature rose 38° in advance 
of the center, from a minimum of 33° at 6 a. m. of February 27 to a 



350 THE CLIMATE OF BALTIMORE 

maximum of 71° at 2 p. m. of the 28tli. Tlie barometer fell an inch 
during the same period. After the passing of the center of the storm 
across the meridian of Baltimore the temperature fell 38° in 30 hours, 
while the barometer rose over an inch during the same period. 

THE SOUTHWEST STORM. 

A much frequented path for storms has its origin in the Southwest 
and trends northeastward across the Lake region and down the St. 
Lawrence Valley, or across the New England states. Storms of this 
type may have their origin in the extreme northwest, or they may enter the 
United States from the Pacific Ocean off the coast of California, but 
they dip far to the south, their centers passing over Oklahoma or Texas, 
b.efore proceeding on their way eastward by way of the Lake region. In 
their journey southeastward these storms gather energy and moisture 
with increase in temperature. They are characterized by a sharp rise 
in temperature in advance of the center of the depression, as the warm 
moisture laden southerly winds from the Gulf and South Atlantic are 
drawn into the circulation for a relatively long period. As they move 
northward the temperature is not only lowered by rising currents in 
advance of the storm, but also by reason of their entrance into cooler 
latitudes. As a result of the lowering of temperature and their prox- 
imity to the main sources of water supply- — the Gulf and flie Atlantic 
Ocean — clouds and rain form rapidly over a very large area about their 
centers. While the paths of such storms may not pass in closer proximity 
to Baltimore than do the Northwest Lake storms, their rain areas extend 
farther southward and eastward from their centers and hence bring to 
Baltimore a longer period of unsettled weather and a heavier rainfall. 

The Storm of Fchniary 3-5, 1903. 

This storm entered the United States from the Pacific Ocean on 
February 1. At 8 a. m. of the 2d its center was over Arizona, and on 
the 3d over Texas. During the succeeding 24 hours the storm turned 
sharply to the northeast, increasing in energy and area, reaching Lake 
Michigan by 8 a. m. of the -ith. By this time the rain area had already 



MARYLAND WEATHER SERVICE 



351 



reached the Atlantic coast from Florida to Maine, while it extended 
westward to ^Tebraska. 

In its southeast quadrant the temperature rose 20° or more in 24 
hours, while a marked cold wave closely followed the center of the depres- 
sion to the southwest, with a fall of 20° to 40° in 24 hours. In advance 
of the storm the precipitation occurred as rain, excepting in the north- 
east where the temperature fell below 32°. Here, and to the west of the 




Fig. 102.— The Southwest Storm of February 3, 1903. 



storm center, snow fell over a large area, in many places to a great 
depth. The barometric gi'adients in this storm were very steep, the 
difference between tlic pressure at the center and the outer edge of the 
storm being an inch or more. (See Figs. 102-10.").) 

Tiie following description of the conditions at Baltimore during the 
passage of this storm is taken from the records of the local ullice of 
tlie United States Weather lUireau : 



352 



THE CLIMATE OF BALTIMORE 




Fig. 103.— The Southwest Storm of February 4, 190:; 




Fig. 104.— The Southwest Storm of February 5, 1903. 



MARYLAND WEATHER SERVICE 



353 



February 3, 1903. A warm cloudy day. Cirrus clouds formed rapidly after 
7 a. m., the sky becoming overcast by 10 a. m. The clouds increased in 
density. Light rain began at 10.55 p. m. and continued into the night. At 
midnight the amount of precipitation was 0.05 inch. The atmosphere was 
balmy and springlike. 




Fig. 105.— The Southwest Storm of February 3-6, 1903. 



February //, 1903. The day continued cloudy and warm with a sultry 
atmosphere in the morning. The temperature rose to 66° at 4 p. m., then fell 
sharply 7° just before 5 p. m., followed by a steady fall. The sky was over- 
cast until 1.50 p. ra. The strato-cumulus clouds changed to cumulus by 2.30 
p. m.; these in turn disappearing by 4 p. m. Dense fog prevailed during the 



354 THE CLIMATE OF BALTIMORE 

preceding night, became light in the early morning, and disappeared by 10.30 
a. m. The light rain of the night before continued into the morning, be- 
coming heavy about 6.30 a. m., 0.20 inch falling in 5 minutes. This brief 
downpour was preceded by a single flash of lightning. The rain continued at 
intervals until 8.30 a. m. The total fall from midnight was 0.86 inch. A 
slight peal of thunder was heard at 10.21 a. m. At 4.50 p. m. there appeared 
an inky-black, closely compacted mass of strato-cumulus clouds, driven from 
the northwest, though the cloud mass showed a distinct northeastward move- 
ment. By 5 p. m. the entire mass had risen above the western horizon, 
covering about six-tenths of the sky. On the northern edge of the cloud 
mass several cumulo-nimbus of the " anvil " variety were seen. Rising above 
the western horizon were cumuli, small in size, and extending north and 
south for about 25°, with an overlying cirro-stratus layer. There were three 
air currents: The upper current was moving from the west; the middle 
current from the southwest; the surface wind was from the northwest. 
Though the cloud mass moved eastward in a body, the northeast end seemed 
fixed, and a general commotion was noticed in the base of the cloud strata 
in this portion; mammo-cumulus clouds appeared and disappeared for about 
ten minutes. At 5.15 p. m. breaks occurred in the mass, exposing snow-white 
cumulus peaks with the crowns growing in size, indicating ascending air 
currents. At 5.30 p. m. the mass was steadily being pushed southeastward 
and an alto-stratus layer set in from the northwest. The western edge of the 
cloud mass passed over the station at 5.40 p. m. From this time until 6.50 
p. m. the mass was very dark in color, except on the extreme northeast edge, 
where several snow-white mountainous cumulo-nimbus prevailed. From and 
among these cumulo-nimbus broad flashes of lightning were seen from 5.50 
p. m. until 6.50 p. m. Two successive peals of thunder were faintly heard 
in the eastern suburbs of the city at 5.30 p. m. Southwest storm warnings 
were ordered up along the coast from "Wilmington, N. C, to New York. The 
winds became brisk to high after 7 p. m., with a maximum velocity of 38 
miles from the west at 7.15 p. m. 

Fehruary 5, J903. The day was partly cloudy and colder. The sky was 
overcast during the forenoon; clouds began to break away about noon, and 
by 3.30 p. m. they had disappeared. Brisk to high westerly winds con- 
tinued throughout the night and during the day, decreasing to fresh in the 
evening; the maximum velocities exceeded 40 miles an hour. Considerable 
damage was done by the wind to signs and awnings, and a few houses were 
partially unroofed. Snow flurries occurred between 8 a. m. and 11 a. m., 
but no snow remained on the ground; the total fall was less than a tenth of 
an inch. 



The Storm of December 26-28, 190Jf. 
This disturbance, like the storm described in the preceding paragraphs, 
had its origin over the Pacific Ocean. Its center appeared off the coast 
of Oregon on the 24th inst. Moving rapidly southeastward across the 



MARYLAND WEATHER ■ SERVICE 355 

Eocky Mountains the storm reached Texas on the morning of the 26th; 
recurving sharply northeastward the center was over Central Illinois 24 
hours later and over Toronto on the morning of the 28th. Following 
the usual course down the St. Lawrence Valley the storm passed eastward 
over Labrador to the Atlantic, crossing the continent from ocean to ocean 
in just five days along a path about -iOOO miles in length. Assuming 
a uniform rate of speed the average daily movement was 800 miles. 
The rate varied from 1000 miles in 24 hours from the Pacific Ocean 
across the Eocky Mountains, to 500 miles in crossing the Lake region. 

The storm was characterized by a precipitation area of unusual extent, 
and by heavy local rains and snows. The isotherms were bent from their 
normal east-west direction to a north-south trend near the center by the 
warm southerly winds in advance of, and the cold northwest winds in 
the rear of, the center of the storm. The rise in temperature in the 
southeast quadrant was 20° to 30°, while the subsequent fall in the 
southwest quadrant varied from 20° to 50° in 24 hours. 

The local changes in Baltimore during the progress of this storm 
were well marked and characteristic. While the center of the storm 
was over Texas, on the morning of the 26th, an area of high barometric 
pressure rested over the Xew England states. Tliis distribution of 
pressure caused north to northeast winds in the Middle Atlantic states. 
As the storm moved eastward and northward toward the Lake region the 
wind at Baltimore veered to east and southeast, and by noon of the 
27th it had become south. During the night of the 27th-28th, while 
the center of the storm was over the Lake region, a secondary depression 
developed in the southeast quadrant of the main storm, over eastern 
Pennsylvania and Xew York, causing a sudden change of Avind to north 
at Baltimore; as the storm center moved eastward the winds settled to 
northwest witli rapidly increasing velocity. 

The barometer fell from 30.24 inches at 8 a. m. of the 26th to 29.10 
inches at 4 a. m. of the 28th, and rose again to 30.00 inches by 10 a. 
m. of the 29tii. Co-incident witli tlie fall in pressure and the changes 
in the direction of the wind to the south, noted above, the temperature 
rose from 26° at 6 a. m. of the 26th to 55° at 4 a. m. of the 28th, then 



356 



THE CLIMATE OF BALTIMORE 




Fig. 106.— The Southwest Storm of December 26, 1904. 




Fig. 107.— The Southwest Storm of December 27, 1904. 



MARYLAND WEATHER SERVICE 



357 



fell with change of wind to the north and northwest, to 20° at 6 a. m. 
of the 29th. This cyclonic rise and fall in temperature totally obliterated 
the diurnal fluctuation usually noted in the daily temperature curve. 
The maximum temperature occurred at 4 a. m. of the 28th. There was 
a steady rise during the 27th from midnight to midnight, and a steady 
and regular fall throughout the following day. 

The rain was continuous but light. Beginning at 9 a. m. of the 26th 
as a light misting rain it continued as such without interruption until 




Fig. 108.— The Southwest Storm of December 28, 1904. 



10.25 p. m., when it became heavier. About 6 a. m. of the 27th it again 
changed to a light mist which continued to the end of the precipitation 
period between 4 and 5 p. m. The total amount of rainfall (including 
some sleet) fdr the 32 hours was only 0.44 inch. (See Figs. 106-109.) 

The daily journal of the local Weather Bureau Office contains the 
following remarks concerning conditions on the 27th and 28th: 

December 27, 190 Jf. A cloudy day. Continuous fog. Light rain, continu- 
ing from midnight yesterday, turned to misting rain at 6.05 a. m.. and ended 



358 



THE CLIMATE OF BALTIMORE 




MARYLAND WEATHER SERVICE 359 

at 4.30 p. m. Southwest storm warnings were ordered up by the Chief of 
Bureau at 9.45 a. m. from Jacksonville, Fla., to Fort Monroe, and southeast 
warnings at 11.15 a. m. from Baltimore to New York. A cold wave warning 
was received at 9.55 p. m. 

Dece7nber 2S, 190.'i. Partly cloudy until 9 a. m., followed by cloudy; clear 
after 1.30 p. m. A slow steady rise in temperature since Christmas morning 
culminated in a sharp rise to 55° at 4 a. m. From this hour the temperature 
fell steadily to 22° at midnight. Light rain fell between 1.15 a. m. and 2.10 
a. m., amount 0.01 inch. The wind became brisk at 9.15 a. m. and continued 
so until nearly midnight. The velocity rose to a maximum of 40 miles per 
hour from the west at 12.45 p. m. 

The Storm of December 12-13, 1903. 

This storm first appeared within the field of view ou the lUth of 
December in the extreme Nortliwest. It crossed the Eoclcy Mountain 
range in Montana in a southeast course during the night of the lOth-llth, 
and its center was over Missouri and Arkansas at 8 a. m. of the 12th. 
Here it recurved to the uortlieast, taking the usual course across the 
Lake region, down the St. Lawrence Vallev and over Labrador, where it 
disappeared beyond the field of the weather map on the 14th inst. 
This storm resembled the southwest storm described above in most 
respects. There was an important dift'erence, however, in the form of 
the system of isobars surrounding the center as the storm crossed the 
meridian of Baltimore on December 13. The (jval shape of isobars, with 
the long axis extending approximately north and south is characteristic 
of many of this class of storms. The change from easterly to westerly 
winds, as the trough of low l)aro meter moves eastward, is very abrupt, 
and is frequently attended by severe squalls or thunderstorms. The 
isotherms extend nearly north and south and are close together in the 
vicinity of the trough of low pressure, or, in other words, the tempera- 
ture gradient is very steep and contrasts are great. In the case of this 
particular storm of the 13th there was a difference of 50° at 8 a. m. 
between Baltimore and Tiidinimpolis. on the same ])arallol of latitude. 
or a difference of 50° between the southerly winds prevailing in advance 
of the center and the northwest winds which blew out of the w^ell 
developed area of high pressure in the rear of the storm. 

The trough of low pressure passed over Baltimore almost at the 
exact time of the 8 a. m. observaiions of the Unilcd States Weather 

24 



360 



THE CLIMATE OF BALTIMORE 



LOW 



LOW 




Fig. 110.— The Southwest Storm of December 12, 1903 



LOW\ !^o\> 




Fig. 111.— The Southwest Storm of December 13, 1903. 



MARYLAND WEATHER SERVICE 



361 




362 



THE CLIMATE OF BALTIMORE 



Bureau. A detailed presentation of the successive local changes during 
the 13th, as this storm traversed the horizon of Baltimore will Ije found 
in the accompanying diagram. It is also possible in this case to show 
the hourly changes in the relative humidity. At 8 a. m., with change 
of wind from south to southwest and then to northwest, there was a 
remarkably rapid change in the humidity, the decrease amounting to 
about 55 per cent in four hours. (See Figs. 110-112.) 




Fig. 113. — Paths and Rain Areas of Southwest Storms of January, 1898. 



In the reports of the local office of the United States Weather Bureau 
the loth is described as cloudy in the forenoon and clear in the afternoon. 
The temperature was very high in the morning, with a maximum of 52° 
about 8.30 o'clock. With a sudden change of wind at this hour from 
southwest, through the west, to northwest, a rapid fall in temperature 
took place (10° in the first hour), and it continued to grow colder to a 
minimum of 30° at midnight. The atmosphere was crisp and invigorat- 



MARYLAND WEATHER SERVICE 363 

ing in the afternoon. A blustering wind prevailed in the forenoon. 
Light rain began during the night or early morning, and ended at 9.30 
a. m. The total amount was 0.30 inch. The wind became brisk shortly 
after 9 a. m., changing to high westerly winds, and then to northAvest, 
with a maximum velocity of 41 miles per hour at 9.30 a. m. A cold 
wave warning was received at noon, announcing a probable change of 
20° to 30° before the close of the following day. Southwest storm 
warnings had been ordered up along the coast from Savannah to New 
York on the 12th; tlVose were changed to northwest on the morning of 
the 13th. 

These southwest storms are usually accompanied by large rain areas 
and heavy local rains. At times a series of these storms will follow one 
another in close succession, all taking approximately the same path, 
from Texas across the Lake region and Xew England, or the St. Lawrence 
Valley out into the Atlantic. A remarkable series of tliis kind was 
experienced during the month of January, 1898. The .accompanying 
chart (Fig. 113) shows the paths of six storms of this type all occurring 
between the 8th and 26th of January, 1898, together with the total 
amount and distribution of precipitation recorded along the various paths. 
The rate of movement of the storms is shown by the circles along the 
lines illustrating the storm paths, the intervals representing periods of 
twelve hours. 

THE GULF STORM. 

Many of the storms which have their origin in the southwest or over 
the Pacific, and cross tlie country along the southwest path, continue 
their southeast course to the Gulf before recurving to the northeast. 
Some have their origin over the Gulf of Mexico and move northeastward 
to tlie Gulf of St. Lawrence. The path taken by these storms brings 
tlicir centers very close to Baltimore. Sometimes the center of the 
barometric depression passes just to the west of Baltimore, sometimes 
to the east, and occasionally immediately over the city. They are usually 
accompanied by heavy precipitation, and by high winds along the coast. 
Very frequently these storms develop over the Gulf of ]\Iexico while 



364 THE CLIMATE OF BALTIMORE 

an area of high pressure prevails over the New England states. Under 
the influence of this distribution of pressure, northeast to east winds 
set in over the Middle Atlantic states and southeast winds over the 
South Atlantic states. The rain area spreads rapidly northward and 
eastward under these conditions and reaches Baltimore while the center 
of the depression is still in the Gulf states. 

The average winter temperature of Baltimore is close to the freezing 
point; hence slight changes in temperature will change the form of 
precipitation from rain to snow or from snow to rain, or to the disagree- 
able intermediate stage of sleet. As these Gulf storms are nearly always 
preceded by comparatively high temperatures and followed by tempera- 
tures below the freezing point, they are apt to cause much personal dis- 
comfort, with their rain, sleet, and snow, resulting in slushy or icy 
streets in the cities. Farther north the precipitation is mostly in the 
form of snow, and a short distance to the south it is all rain. The high 
winds which frequently accompany this type of storm not only increase 
the discomfort but add an element of danger. 

The Storm of February 1-3, 1902. 
{Center passes west of Baltimore.) 

The weather map of S a. m., February 1, 1902, shows the prevalence 
of two well developed areas of high barometric pressure, one in the north- 
east, with its center over the Gulf of St. Lawrence, the other in the 
extreme northwest, centered over Idaho and Montana. In the Gulf 
states and in the Southwest the barometer was low, and unsettled weather 
prevailed from the Lake region to the Gulf, and from the Atlantic coast 
westward nearly to the Rocky Mountains. In the Atlantic coast states 
the barometric depression was already well developed, and rain was fall- 
ing at 8 a. m. throughout the South Atlantic states, in Virginia, Mary- 
land, and Pennsylvania, and snow in New York and the New England 
states. 

As the storm moved rapidly northeastward it developed in intensity 
and in definiteness of outline, the rains became heavier and the area 
of precipitation increased. The high area over the Gulf of St. Lawrence 



MARYLAND WEATHER SERVICE 



365 



.30.4 




Fio. 114— The Gulf Storm of February 1, 1902. 




Fig. 115.— The Gulf Storm of February 2, 1902. 



366 



THE CLIMATE OF BALTIMORE 



remained stationary while that in tiie extreme Northwest moved rapidly 
southeastward accompanied by a decided fall in temperature in the 
southwest quadrant of the storm area. At 8 a. m. of the 2d of February 
the area of lowest barometer was over Pennsylvania, the center of the 
storm having passed just to the west of Maryland during the preceding 
nisrht. The center of the western hi2:h area was over Kansas and Okla- 
homa. During the preceding 24 hours a fall of 15° to 30^ in tempera- 




FiG. 116.— The Gulf Storm of February 3, 1902. 



ture was experienced over a wide area from Iowa and Nebraska southward 
to the Gulf coast. High easterly winds prevailed during the night and 
early morning along the coast from the South Atlantic to the New 
England states. (See Figs. 114-117.) 

By the morning of the 3d the storm center had moved to tlie New 
England states, the cold wave had reached the Atlantic coast from 
Florida to North Carolina, and had overspread most of Virginia, Mary- 
land, and Pennsylvania. The local changes at Baltimore during the 



:makylaxd weather service 



367 




368 THE CLIMATE OF BALTIMORE 

passage of this storm are indicated in the accompanying diagram, and in 
the following extracts from the daily journal of the Weather Bureau : 

February 1, 1902. On February 1, while the center of the storm was over 
the Gulf States, the day was cloudy, the sky being continuously overcast. 
At 7.45 a. m. precipitation began in the form of sleet, turning in 10 minutes 
to a light misting rain. The winds were from northeast to north from noon 
to midnight, and very light, averaging but 3 to 4 miles per hour. Light rains 
continued at intervals until 8.40 p. m., the entire amount for the day being 
but 0.07 inch. The day was disagreeable; the sidewalks were icy. The 
maximum temperature of the day was 37° at 6 p. m. The barometer fell 
steadily throughout the day from 30.03 inches at 4 a. m. to 29.77 inches at 
midnight. The relative humidity was approximately 100 per cent all day. 

February 2, 1902. The day continued cloudy during the forenoon. The 
temperature rose slowly to 39° at 2 p. m., while the barometer fell to 29.25 
inches. The wind changed from north to east at 8 a. m. and to west at 10 
a. m., and continued light in force. Early in the afternoon the wind began 
to increase in force, reaching a maximum of 33 miles per hour from the west 
between 4 and 5 p. m., shortly after the barometer began to rise. From 2 
p. m. the temperature fell steadily to 15° at midnight of the following day. 
After an interval of several hours light rain began again between 2 a. m. and 
3 a. m. and continued without interruption until about noon, becoming heavy 
at 'times. From noon to 1.40 p. m. the precipitation was a mixture of rain 
and snow, the snow melting as it fell. The total fall of rain and snow com- 
bined was 0.44 inch. 

The day as a whole was extremely disagreeable. The sidewalks were icy 
and dangerous to pedestrians. In the forenoon the gutters and streets were 
filled with slush, which, as night approached, became frozen solid. Some 
damage was done to awnings, signboards, and chimneys by high winds. The 
wind, however, cleared the harbor of floating ice. Light fog prevailed during 
the preceding night and lifted at about 11 a. m. Northwest storm warnings 
were ordered up at 10 a. m. from Florida to Baltimore. A cold wave warning 
was received at 2.50 p. m., forecasting a fall to 15°, or below, in the interior 
of the State, and to 20° along the coast. The clouds disappeared rapidly after 
3 p. m. and by 5 p. m. the sky was clear. 

February 3, 1902. The day was clear and much colder than the 2d. The 
temperature fell to 15° at 9 a. m. There "were no clouds excepting a few 
small cumuli in the afternoon. The wind continued brisk during the night, 
but diminished towards noon. Navigation was free on the western side of 
the Bay, but along the eastern shore the ice was piled up by the winds. All 
tributaries of the Chesapeake were frozen solid. 

The Storm of January 5-1 , 1905. 
(Center passes over Baltimore.) 
On the morning of January 5, 1905, a somewhat similar distribution 
of pressure obtained to tliat of February 1. 1902, described above. An 



MARYLAND WEATHER SERVICE 



369 




Fig. 118.— The Gulf Storm of Jauuary 5, 1905. 




Fio. 119.— The Gulf Storm of .lanuary 6, 1005. 



370 



THE CLIMATE OF BALTIMORE 



area of liigh barometer prevailed over the Atlantic coast states, and 
another over the Rock}- ]\[ountain Plateau. The pressure was low over 
the Mississippi Valley with a tendency to deepen over the Gulf states. 
By 8 a. m. of the following day the Atlantic coast area of high pressure 
had concentrated over the Xew England states, while the Eocky ^lountain 
high area had changed but little in intensity or outline. The center 
of the barometric depression had been transferred to Xorthern Floi'ida 




^. 



% 



-36 



Fig. 120.— The Gulf Storm of January 7, 1905. 



and Southern Georgia. This combination of pressure along the Atlantic 
coast always gives rise to northeasterly winds with a steady rain or 
snow. At 8 a. m. of the Gth rain was falling in the South Atlantic 
states and snow in the Middle Atlantic and Xew England states. The 
snow area also reached westward to the Ohio Valley and the Lake region 
in connection with the development of a secondary depression over Lake 
Michigan. (See Figs. 118-121.) 



MARYLAND WEATHER SERVICE 



371 




372 THE CLIMATE OF BALTIMORE 

After reaching the coast the storm took a sharp turn northward, 
increasing in intensity as it followed the coast line. The center passed 
directly over Baltimore at about 4 a. m. of the 7th with an abrupt change 
in the direction of the wind from south to northwest, and a fall in 
temperature. The winds were light to fresh during the progress of the 
storm over Baltimore, only exceeding 20 miles per hour for a short 
time between 3 p. m. and 4 p. m. By the morning of the 8th the center 
had passed northward to the Lower St. Lawrence River. 

The temperature rose rapidly 20° to 40° along the Atlantic coast 
in advance of the center of the storm, but fell more slowly after the 
center had passed, a^? the high area of the Eocky Mountain region was 
advancing but slowly eastward behind the storm. Heavy rains marked 
the spread of the storm in its eastern half all along the Atlantic coast; 
rains of one to two inches in 24 hours were reported from many of the 
Weather Bureau stations. The precipitation at Baltimore amounted 
to 2.34 inches during the 12 hours from noon to midnight of the 6th. 

Some details of the local conditions at Baltimore are shown in the 
following extracts from the daily journal of the local office of the United 
States Weather Bureau : 

January 5, 1905. A cold cloudy day. Light snow began at 8.30 a. m. and 
ended at 10 a. m. The winds were westerly in the forenoon and easterly in 
the afternoon. 

January 6, 1905. The day was cloudy and somewhat warmer than yester- 
day. Light rain began at 8.55 a. m., ended at 9.10 a. m. ; began again at 
12.10 p. m. and continued to midnight. The rain was heavy from 7.40 p. m. to 
7.49 p. m., 0.22 inch falling within the 9 minutes. The total precipitation for' 
the day was 2.34 inches. 

January 7, 1905. A cloudy day until 6.30 p. m.; the clouds broke away soon 
after and by 8 p. m. the sky was clear and remained so until midnight. The 
rain of the preceding night continued until 12.10 a. m. Rain began again at 
6.40 a. m. and ended at 7.35 a. m.; began again at 8.50 a. m. and ended at 
8.55 a. m. From 12.30 p. m. to 12.50 p. m. snow was mixed with rain. The 
total precipitation for the day was 0.02 inch. 

A continuous record of changes in the meteorological elements at Balti- 
more is shown in the accompanying diagram. (See Fig. 121.) 



MARYLAND WEATHER SERVICE 



373 



The storm of February 20-22, 1902. 

{Center passes east of Baltimore.) 

February, 1902, was remarkable for the number of Gulf storms 
experienced. In fact, these storms were a conspicuous feature of the 
entire winter of 1901-02. While the great majorit}' of our storms 
follow the northern route across the Lake region in a normal winter, 




Fig. 122.— The Gulf Storm of February 20, 1902. 



the storms of February, 1902, without exception, followed the southern 
path and crossed the horizon of Baltimore with remarkable regularity 
by way of the Gulf of Mexico. Occasionally there will occur a series 
of three or four storms in regular succession following this track. The 
area of cloudiness and rain accompanying a Gulf storm passes over a 
given locality in about two or three days; this is followed by four or live 
days of fair weather before the approach of another storm. During 
the winter of 1901-02 there was a remarkablv regular succession of these 



374 



THE CLIMATE OF BALTIMORE 




Fig. 123.— The Gulf Storm of February 21, 1902. 




Fig. 124.— The Gulf Storm of February 22, 1902. 




25 



376 



THE CLIMATE OF BALTIMORE 



storms, the period of rain and succeeding fair weather covering seven 
days and causing the unusually long continued series of rainy Sundays 
so generally commented upon at the time. This is not an uncommon 
occurrence but the regularity of the succession was unusually well 
marked. (See Figs. 126 and 127; also Fig. 113.) 

Why storms take this southern course with such unusual frequency 
at times it is difficult to say. Perhaps all that can be said in explana- 




FiG. 126. — Normal Paths of Storms for February in Black. Average Path of 
Storms for February, 1902, in Red. 



tion is that it is due to a departure from the normal conditions in the 
general circulation of the atmosphere — some unusual movement of the 
large persistent areas of high and low pressure referred to in an earlier 
paragraph. 

One of the most notable of the series of Gulf storms referred to above 
passed over Baltimore on February 21 and 22, 1902 — a storm which will 
long be remembered by Baltimoreans on account of the intensely disagree- 



MARYLAND WEATHER SERVICE 



377 



able combination of rain, sleet, snow, and high winds experienced. The 
storm originated off the North Pacific coast on the 17th, moved rapidly 
southeastward, reaching the Western Gulf coast on the morning of the 
19th. Here it lingered for a day, increasing in intensity and enlarging 
its rain area. Moving eastward along the Gulf coast to the Atlantic, 
the center followed the coast northward, passing to the east of Baltimore 
during the day of the 22d, then out to sea. The presence of an area 
of high pressure to the northeast of the storm assisted in producing a 
steady north to northeast wind during the 21st. The official records of 
the Weather Bureau describe the local conditions as follows: 

February 20, 1902. The day dawned clear and cold. A thin veil of cloud 
soon appeared, however, increasing in thickness as the day advanced, at 





SEPT. OCT 


NOV 


DEC. 


JAN. 


FEB. 


MCH 1 




7 14 21 28 5 12 19 26 2 9 16 23 30 7 14 21 28 4 H 18 25 


8 15 22 


8 15 22 1 


MOT. 
6 P 
































1 












1 


























































1 


1 
























































1 
1 


















6 A. 




1 


I 


































1 








1 










j 
i 


1 ' 
























■ 




























..,' . . . 1 



Fig. 127.— Diagram of Rainy Sundays of the Winter of 1901-2. 



times becoming dark and threatening. The winds throughout the day were 
light and varying in direction between west and north, and changing to 
south in the evening. The temperature rose with the advance of the day, 
but barely reached the melting point of ice even at mid-day. Sleet began to 
fall at 8 p. m., turning to rain during the night. The barometer fell slowly 
but steadily throughout the day and night. 

February .11, l'.i0.i. The rain of the preceding night froze as it fell, cover- 
ing everything with a thick coating of ice. On trees, telegraph wires, and 
all exposed objects, the ice collected to a thickness of an inch or more, the 
heavy weight causing considerable damage. The rain continued with scarcely 
any interruption until 7.15 p. m.; at times it fell in torrents. Travel upon 
the streets became difficult and dangerous. The heavy rains of the afternoon 
converted the ice upon the streets into a heavy slush. The temperature re- 
mained nearly constant, varying but little from the freezing point of water. 
The winds were northeast to north all day, and increasing in force, not 
attaining a storm velocity, however. The barometer continued to fall 



378 THE CLIMATE OF BALTIMORE 

steadily and more rapidly toward night. The total precipitation for the day 
was 2.13 inches. Thq center of the depression was off the coast of North 
Carolina, the storm moving east of north and increasing in intensity. 

February 22, 1902. A rainy day, with very little range in temperature, 
the maximum being 36° and the minimum 34°. The barometer reached its 
lowest reading at 6 a. m., rising slowly but steadily from this hour. Fresh 
northerly winds prevailed. The heavy rain of the preceding day turned to a 
mist at 11 a. m. and was accompanied by light flurries of snow between 3 p. m. 
and 6 p. m. At 7.20 p. m. a light moist snow began to fall, continuing at 
8 p. m. The total precipitation of the day was 0.40 inch. The ice remained 
upon the streets most of the day in spite of the heavy rains and was a source 
of great discomfort. The heavy accumulation of ice caused much damage 
to trees and to telegraph and electric wires. 

February 23, 1002. The day was clear and somewhat warmer than yester- 
day. The snow of the preceding night ended about midnight. The ice on 
the streets rapidly disappeared with the increased warmth. The day was 
pleasant and the atmosphere balmy. 

Altogether this storm was one of the most disagreeable experienced in 

Baltimore. (See Figs. 122-125.) 

THE BLIZZARD. 

When storms such as have been described in preceding paragraphs 
are accompanied by heavy snow, high winds, and a temperature well 
below the freezing point, they are popularly known as blizzards. This 
type of storm is fortunately of infrequent occurrence in the Middle 
Atlantic states. When they have occurred it has been in connection 
with a Gulf or Southwest storm. An invariable accompaniment of the 
blizzard is the presence of an excessively developed area of high baro- 
metric pressure following in the wake of the depression, causing a steep 
barometric gradient and feeding into the storm center with great energy 
the cold westerly winds of the anti-cyclone. 

Two storms of this type are especially worthy of consideration at 
some length owing to their exceptional severity all along the Atlantic 
coast — one is known as the blizzard of March, 1888, the other as the 
blizzard of February, 1899. The former, while occurring in March, 
is a marked instance of " winter lingering in the lap of spring." 

The Blizzard of March 11-13, 1888. 
The daily weather charts of the Weather Bureau for March 11, 1888, 
show the existence, in the morning, of an area of high pressure (anti- 



MARYLAND WEATHER SERVICE 



379 




Fig. 128.— The Blizzard of March 11, 1888. 



LOW 



^<i \ HIGH 

I / \ " 




Fig. 120.— The Blizzard of March 12, 1888. 



380 



THE CLIMATE OF BALTIMORE 



cyclone) centered over New England, and another of unusual extent and 
energy west of the Mississippi Eiver with its center over the Southern 
Eocky Mountain slope. Between these two anti-cyclones there was a 
pronounced trough of low pressure extending from Lake Huron to 
Florida. Strong east to southeast winds prevailed in the Atlantic coast 
states with heavy rains, excessive in the South Atlantic states, and with 
temperatures varying from 30° in New England to 60° in South Caro- 







Fig. 130.— The Blizzard of March 13, 

Una. To the westward of the trough of low pressure, the winds were 
from the west or northwest, with snow in the Lake region and Ohio 
Valle}^, and rain farther south. The barometric gradients were steep, 
and the temperatures fell rapidly toward the northwest, ranging from 
50° above zero in Georgia to 20° below zero in Northern Minnesota. 
As the storm moved eastward the trough of low pressure changed to a 
well developed elliptical depression, with its center off the coast of Hat- 
teras by 10 p. m. of the 11th; at the same time the storm was increasing 



MARYLAND WEATHER SERVICE 



381 




382 THE CLIMATE OF BALTIMORE 

in intensity, causing high and destructive winds along the Middle Atlan- 
tic coast. The center of the storm moved northward near the coast, 
the high easterly winds of the 11th giving way to high off-shore winds by 
7 a. m. of the 12th. The precipitation continued heavy in the form of 
snow in the Middle Atlantic and New England states. The cold wave 
had reached the Atlantic coast from New England to Virginia by the 
morning of the 13th. The center of the storm remained off the coast of 
Massachusetts for 24 hours and then moved eastward over the Atlantic. 
The snowfall over the southern portion of the New England states was 
unprecedented in the annals of that section. The heavy snows and high 
winds attending this storm caused serious interruption to telegraphic 
and railway communication in the Middle Atlantic and New England 
states from the 11th to the loth of the month. (See Eigs. 128-131.) 

The storm passed over Baltimore on the 11th and the early morning 
of the 12th. The following paragraph is copied from the local official 
records : 

Light rain fell at intervals until noon of the 11th, then heavy rain until 
6.50 p. m., when it changed to snow, accompanied by high northwest winds. 
In a short time telegraphic communication was cut off with nearly all points. 
The snow storm ended during the night of the llth-12th and was followed 
by cold weather. The wind continued from the northwest throughout the 
12th, attaining a maximum velocity of 40 miles per hour, and causing the 
lowest tide in many years, the bottom of the harbor being exposed in many 
places. This severe storm caused an almost entire suspension of business 
on the 12th. Reports from the surrounding country and from the Chesapeake 
Bay show the storm to have been very severe, and many vessels arriving on 
the 14th and 15th reported having experienced remarkably rough weather. 
The tide in Baltimore harbor did not resume its normal height until the 16th. 

The Blizzard of February 12-14, 1899. 

This storm was probably the most remarkable in the history of Balti- 
more. The amount of snow on the ground at the close of the storm 
was the greatest noted in the oflBcial records of the local Weather Bureau 
Ofl&ce while the intense cold just preceding the snow storm lowered all 
existing oflBcial records. The winds maintained a storm velocity for 
more than 48 hours. 

The cold wave which preceded the blizzard was one of the most wide- 



MARYLAND AYEATHER SERVICE 383 

spread as well as one of the most severe experienced in the United States, 
covering the entire country east of the Eocky Mountains to the Atlantic 
coast, and from the Lake region to the Gulf of Mexico, from the 9th to 
the 12th. 

The distribution of atmospheric pressure over the northern hemisphere 
during this period, with accompanying weather conditions, was of peculiar 
interest and great significance. On the 10th of February practically 
the entire North American continent was covered by an area of high 
barometric pressure (an anti-cyclone) with a pressure of over 31 inches 
at the center, just north of Montana, a pressure only occasionally rec- 
orded. The degree of cold experienced near the center of this area (65° 
below zero) was exceeded but once in the official records of the ^Yeather 
Bureau. Upon the same day a barometric depression (a cyclone) of great 
intensity covered the North Atlantic Ocean, with its center along the 
same parallel of latitude (50° north) and just west of the British 
Isles. This situation gave to Southern England a strong southerly wind 
which raised the temperature in London to 65° above zero, a degree of 
heat not experienced in February in a hundred years of recorded observa- 
tions. Thus upon the same day and along the same parallel of latitude 
there was a difference of 130° Fahrenheit. The contrast was even more 
marked in the United States. ^Yhile the minimum of 65° below zero 
was being experienced in Western Montana, the temperature in Western 
Washington (just across the Eocky Mountains, a distance of less than 
300 miles) was 63° above zero, a difference of 128°. 

This anti-cyclone, or cold wave, which overspread the United States 
from the 9th to the 11th of February, caused heavy snows to the south- 
east, along the line of advance. By the morning of the 12th a baro- 
metric depression began to develop over Northern Florida, and heavy 
snow was falling in the Gulf states, the South Atlantic, ]\Iiddle Atlantic, 
and Southern New England states, with fresh to brisk north to northeast 
winds and falling temperature. At the same time the anti-cyclone in 
the west, maintaining its severity, was moving southward and eastward. 
By the morning of the 13th the center of the depression, which formed 
over Florida on the preceding day, moved northward along the coast, 



384 



THE CLIMATE OF BALTIMORE 



s 




Fig. 132.— The Blizzard of February 9, 1899. 




Fig. 133.— The Blizzard of February 10, 1899. 



MARTLAXD WEATHER SERVICE 



385 



tow 




Fig. 134.— The Blizzard of February 11, 1899. 




Fig. 135.— The Blizzard of February 12, 1899. 



386 



THE CLIMATE OF BALTIMORE 




Fig. 136.— The Blizzard of February 13, 1899. 




Fig. 137.— The Blizzard of February 14, 1899. 



MAKYLAXD WEATHER SERVICE 



387 



increasing in intensity and causing high northwest winds and heavy 
snowfall. The center of the storm crossed the latitude of Baltimore 
during the day of the 13th (Monday), just off the coast. The fall of 
snow during this day was the heaviest recorded in Baltimore in a 24 
hour period. The temperature during the entire day did not exceed 
10° above zero, while the northwest wind blew a gale. During the fol- 
lowing day the storm continued its course northeastward along the coast 




Fig. 138. — Snow on the Ground after the Blizzard of February, 1899. 



and out over the Atlantic Ocean by way of the Grand Banks of New- 
foundland. (See Figs. 132-138.) 

Tlie local conditions of the weather during the passage of the cold 
wave and blizzard described above are indicated in the following extracts 
from the daily journal of the office of the Weather Bureau, and in the 
accompanying diagram based upon the records of the self-registering 
instruments : 

February 9. ]89!>. The day was rloar and much colder than that of the 
8th. The maximum temperature of the day was the lowest maximum re- 



388 THE CLIMATE OF BALTIMORE 

corded in Baltimore, namely 7°. There was a light fog in the morning. The 
winds were brisk to high, reaching a maximum velocity of 25 miles per 
hour. At 8 p. m. snow covered the ground to a depth of 11.3 inches. The 
ice in the harbor has increased in thickness to two inches. 

February 10, 1899. A clear day. Severe, cold weather. The maximum 
temperature was 3°, the minimum 7° below zero, the mean 2° below zero, 
the lowest in the official records for the maximum, minimum, and mean for 
a day. Much suffering resulted from the intense cold. Several mortormen 
were overcome, and were revived with difficulty. A number of persons were 
picked up out of the snow drifts benumbed and unconscious. The suffering 
among the poor was very great. A series of accidents followed the sudden 
thawing of water in the water pipes when fires were started in the morning. 
Ten inches of snow covered the ground at 8 p. m., and the ice in the harbor 
increased to six and eight inches in thickness. The two ice boats were busy 
all day in their attempts to keep the channel clear, but the ice formed almost 
as fast as it was broken. 

February 11, 1899. A clear day. The minimum temperature was 6° be- 
low zero. A light fog prevailed in the morning. Light snow began to fall 
at 5.35 p. m. and continued into the night. The snow on the ground at 8 
p. m. was 9.7 inches in depth. There continues to be much suffering from 
cold, and one death from exposure is reported. 

February 12, 1899. A cloudy day, with slowly rising temperature. North- 
east storm warnings were ordered up in the forenoon. The following tele- 
gram was received from the Central Office in Washington: "Heavy snow is 
indicated this afternoon and to-night. Notify railroads and transportation 
companies." The snow which began yesterday at 5.35 p. m. became heavy at 
8.15 p. m., then changed again to light snow during the night. It continued 
throughout the day. About five inches of snow fell during the day; the 
depth of snow on the ground at 8 p. m. was 14.5 inches. The weight of the 
snow had crushed a number of small sheds and a few wooden structures. 
To-day the President Street freight sheds gave way, owing to the accumula- 
tion of snow on the roof, and about 300 feet of the building fell; the damage 
amounted to about $20,000. The ice in the harbor is 6 to 8 inches thick. 
Navigation is practically suspended. Only heavy steel steamships are able 
to move. Trains are late and irregular. Much suffering continues among 
the poor. 

February 13, 1899. A cloudy day. Heavy snow fell all day; the 24-hour 
fall was 15.5 inches. The depth of snow on the ground at 8 p. m. was 30 
inches, the greatest recorded in the official records. Brisk to high north to 
northwest winds attained a maximum velocity of 28 miles per hour. The 
continued high blustery winds and the increasing snowfall combined to pro- 
duce a typical blizzard. Railroad traffic was interrupted at an early hour. 
The street railways struggled to continue service, but the lines closed one 
by one, and none were in operation by nightfall. Much suffering continues. 
At least a score of people were overcome by the cold during the day. Birds 
are reported perishing in large numbers from cold and lack of food. 



VOLUME 2, PLATE XX. 



NOON 



MD" 



NOON 



MDT. 




MARVLANO WEATHER SERVICE. 



VOLUME 2, PLATE ) 




/-^\-^ ./- 



I l\\\l\\\l-\ ^W 



" ■* « 5 « " " '2 1° 5 7 6 6 5 9 s 10 10 s 10 e 1 3 5 3 4 t ■• 4 2 5 5 3 6 6 5 7 7 II 10 II (I l( II 2 10 12 1J 13 15 17 21 20 /a re 1( (( (0 II '6 



,8 13 979*«86222 




lloimLY Observations at BAr/rrMORE Purinq the Buzzard and Cold Wave op Febboary 9-14, 1899. 



MARYLAND WEATHER SERVICE 389 

February 14, 1899. A cold day with bright sunshine. Snow ended at 11.10 
p. m. yesterday; one inch of snow was recorded this morning. Total snow 
depth at 8 p. m., 28 inches. The ice in the harbor is 10 inches thick. The 
city is practically snowbound. There was no mail delivery, no railroad move- 
ment, no street car service. Some vessels forced their way out of the harbor, 
but they were few in number. Much work is being done by the city and 
railroad authorities on the streets and lines of travel, but traflac was only 
partially restored. Much suffering continues. One man was found frozen 
to death this morning within six doors of his home. A milk famine is 
threatened. There has been a genei'al rise in the price of commodities, 
especially of country produce. The Merchants and Miners Transportation 
Company lost the steamship Texas this morning. The vessel was run ashore 
in an ineffectual attempt to force a way through the ice and sank. 

February 15, 1899. A clear day. Light fog in the morning. Street car 
service was resumed to-day in part. Trains are beginning to run on time. 
The ice in the harbor is one foot thick. The ice boats have succeeded in 
keeping a clear channel of 50 feet width. Four arrivals of vessels and one 
departure are reported for to-day. Snow on ground at 8 p. m., 26 inches. 

Areas of Fair Weather (Axti-cycloxes), 

In the preceding pages the cyclonic type — or unsettled weather — has 
been described in considerable detail. The characteristic conditions of 
this type are cloudiness, rainfall or snowfall, brisk to high winds, a 
relatively high temperature, and a low barometric pressure, with winds 
converging toward the central area of low pressure. 

In the Middle Atlantic states the cyclonic type dominates the weather 
conditions somewhat less than half the time, basing the calculation upon 
the number of days in the year during which some rain or snow falls. 
The annual number of days with precipitation at Baltimore has varied 
from 114 to 224 with a mean of 170. This implies that during some- 
what over half the year the anti-cyclonic, or fair weather type, prevails. 
The chief characteristics of anti-cyclonic areas are : Barometric pres- 
sure higher than that over surrounding areas; a system of comparatively 
light winds, diverging from the central portion of the area : comparatively 
clear skies; and relatively low temperature. (See Figs. 85 to 89.) 

High areas, or anti-cyclones, have already been described incidentally in 
the preceding discussion of storms. They are most numerous and more 
intensely developed during the winter season, when they move in rapid 
succession from the central continental areas, in the extreme Northwest, 



390 THE CLIMATE OF BALTIMORE 

along the eastern slope of the Kocky Mountains, eastward or southeast- 
ward across the United States. 

When these areas grow to unusual proportions and develop a baro- 
metric pressure greatly in excess of the pressure in areas along their line 
of eastward progress, they constitute our " cold waves." There is no 
sharp line of separation, however, between the cold w-ave and the winter 
anti-cyclone — no more than there is between the storm and the barometric 
depression technically known as the cyclone. The anti-cyclone attains 
its greatest severity when a barometric depression develops in advance 
of it, causing an energetic inflow of cold northwest winds into the western 
portion of the depression. In area and in rate of movement the cyclone 
and anti-C3'clone resemble one another ; in the character of attendant 
weather conditions they are in most respects the exact opposite. 

The difference in the character of the weather prevailing over cyclonic 
and anti-cyclonic areas is strikingly exhibited in the weather chart for 
the 2d of February, 1902, reproduced in Fig. 115, showing the actual 
condition of the weather at 8 a. m. as reported by the observers of the 
United States Weather Bureau in all parts of the United States and 
Southern Canada. (See also Fig. 89, page 325.) 

A storm, or cyclone, of great extent and energy prevailed over the 
eastern portion of the country, with its central area of low barometric 
pressure over Pennsylvania and Maryland. The area of clouds extended 
from the Atlantic Ocean westward to the Mississippi Valley, and from 
the Great Lakes southward to the Gulf coast. The region over which 
rain or snow was falling at the time of observation, 8 a. m., w^hile more 
limited in extent still covered a considerable area, comprising practically 
all of the Kew England and Middle Atlantic states, Ohio, Kentucky, and 
the eastern half of the Lake region. In the Eastern Gulf states and the 
Atlantic coast states as far north as Virginia the rains of the preceding 
24 hours were very heavy, in some localities exceeding an inch and a half. 
It will be observed also that the winds within the area just outlined 
blew in the main toward the central area of low pressure, and that 
the temperatures were markedly higher within the cyclonic area than 
in the anti-cyclonic area immediately to westward of the storm 



MARYLAND WEATHER SERVICE 391 

area. The isotherms, or lines of equal temperature, bent far northward 
in advance of the storm center, where easterly to southerly winds pre- 
vailed ; to the west of the center the cold northwest winds reached far 
to the south. 

High atmospheric pressure prevailed over the area west of the Missis- 
sippi River, the highest barometer being over Kansas and Oklahoma. 
The skies were mostly free from clouds, the winds blew, in general, away 
from the central region of high pressure, the temperatures were compara- 
tively low, while the isotherms were bent southward, with a maximum 
dip near the center of the area. 

The intimate relationship existing between wind direction and the 
trend of the isotherms, as explained in preceding pages, is strikingly 
exhibited in this chart. The difference in temperature between the 
centers of cj'clone and anti-cyclone along the 40th parallel of latitude 
at 8 a. m. was fully 50°. 

The successive changes in weather conditions at Baltimore as these 
two systems — the cyclone and anti-cyclone — moved eastward are shown 
in the accompanying diagram. (See Fig. 117.) 

The weather conditions over the United States do not always exhibit 
these cyclonic and anti-cyclonic systems so well defined, but during the 
winter season their outlines are nearly always easily recognizable. In 
place of a definite succession of " highs " and " lows " such as are shown 
by this chart of February 2, 1902, there may be a number of ill-defined 
and scattered centers of high and low pressure, causing a period of 
unsettled weather conditions, 

COLD WAVES. 

When anti-cyclonic waves are accompanied by a fall of 20° or more 
(exclusive of the diurnal fluctuation) to a stated minimum, within a 
period of 24 hours, they are technically known as cold waves. Sudden 
changes in temperature such as are here described are of comparatively 
frequent occurrence in the northern tier of states, but do not reach as far 
south as Baltimore in any great numbers. In their progress eastward 
and southward these cold waves lose much of the severity shown when 

20 



393 THE CLIMATE OF BALTIMORE 

they first enter this country from the Canadian Northwest Territory. 
By the time they reach the Atlantic coast many of them have lost the dis- 
tinguishing marks of a genuine cold wave. In the official records of the 
United States Weather Bureau we find that out of a dozen or more anti- 
cyclones which enter this country every winter in the extreme Northwest 
as cold waves, but three, on an average, retain sufficient of their severity to 
be classed as cold waves as they pass over Baltimore. 

In some winters Baltimore has been entirely free from them. This 
was the case in the winters of 1873-4, 1885-6, and 1889-90. Some times 
as many as six have been experienced in one season, as in the winters of 
1881-2, 1884-5, and 1903-4. From 1870 to 1904 the monthly distribu- 
tion of cold waves * in Baltimore has been as follows : 



November. 


December. 


January. 


February. 


March. 


Total. 


8 


19 


23 


25 


17 


93 



The Cold Wave of December 13-15, 1901. 

The eastward and southward progress of cold waves from the north- 
west is well exemplified in the weather charts of the United States 
Weather Bureau for the 13th, 14th, and 15th of December, 1901. 

At 8 a. m. of the 13th there were two well defined and extensive areas 
of high pressure, or anti-cyclones, shown upon the chart: One covered 
the eastern section of the country, comprising all of the Atlantic coast 
states, with the maximum barometer over Nova Scotia; the other spread 
over most of the country west of the Mississippi Eiver, with the center 
to the north of Montana. Between these two vast anti-cyclones, there 
was a narrow trough of relatively low pressure, extending from the Upper 
Lake region to the Gulf of Mexico. In advance of this trough of low 
pressure, or elongated cyclonic depression, the temperatures rose rapidly 
under the influence of strong southerly winds. To westward of the 
trough the cold northwest winds blowing out of the well developed anti- 
cyclone brought freezing weather far down into the southern states. The 
western anti-cyclone moved southeastward as a great wave of cold air, 
closely following the cyclonic depression, causing a very steep gradient 

^ See page 128 for details of cold waves. 



MARYLAND WEATHER SERVICE 



393 




Fig. 139.— Cold "Wave of December 13, 1901. 




/ HIGH 



Fig. 140.— Cold Wave of December 14, 1901. 



394 



THE CLIMATE OF BALTIMORE 



in temperature from west to east along the wave front. In a straight 
line from Central Alabama northwestward to the Dakotas, from the 
center of the cyclone to the center of the anti-cyclone, there was a fall 
of 100° at 8 a. m. of the 14th; Montgomery, Ala., reported a temperature 
of 70° above zero, and Bismarck, N. Dak., a temperature, at the same 
hour, of 30° below zero. By 8 a. m. of the following day, the 15th, the 
isotherm of 20° extended along the West Gulf coast. The cold wave did 




Fig. 141.— Cold Wave of December 15, 1901. 



not reach Baltimore until the forenoon of the 15th, and the Atlantic 
coast on the morning of the 16th. 

When the cold wave takes a southeastward path, as it did in this in- 
stance, the freezing temperatures very frequently reach the Gulf coast 
states from 12 to 24 hours in advance of their occurrence in the Middle 
Atlantic and ISTew England states. (See Figs. 139-141.) 



MARYLAND WEATHER SERVICE 395 

The Cold Wave of Fehruanj 10-13, 1S99. 

This, the most intense and wide-spread anti-c^'clone experienced in 
many years in this country, has already been referred to in preceding 
pages in the discussion of the '' Blizzard of Februar}^, 1899 " with which 
it was associated. (See Figs. 132 to 138, and PL XX.) 

Eecords of minimum temperatures long undisturbed were lowered in 
many states east of the Eocky Mountains during the eastward and south- 
ward progress of this cold wave. It extended even to the West India 
Islands. At Havana, Cuba, the temperature fell to 54° on the 13th. At 
Washington, D. C, a minimum of 15° below zero was reported on the 
11th. The lowest temperature in the official record at Baltimore was 7° 
below zero, but temperatures considerably lower were reported from the 
suburban districts. In passing across the Gulf states zero temperatures 
were experienced on the 13th as far south as the coast. 

Cold waves probably differ from anti-cyclones in general only in the 
intensity of their development and in the circumstance of being preceded 
by a cyclonic depression. The conditions most favorable for the develop- 
ment of anti-cyclones are the clear skies and dry quiet atmosphere of the 
extreme Xorthwest — conditions which favor rapid terrestial radiation. 
The general eastward drift of the air in latitudes between 30° and 60° 
north latitude carries the anti-cyclones, when once formed, along in the 
general current. With the eastward movement there is a southward ten- 
dency of these anti-cyclones, due probably to the centrifugal force of the 
earth's axial revolution. 

These anti-cyclones, like the cyclones, move across the continent at 
irregular intervals, occupying four to five days in travelling from the 
Pacific to tlie Atlantic coast, according to their extent and path. In 
most cases the center of the anti-cyclone, or high area, passes eastward 
to the north of Baltimore, but frequently the center passes directly over 
Maryland, and occasionally to the south. Those passing along the 
northern route are most likely to bring very low temperatures to our lati- 
tudes, but this is not always true. In fact, in the case of the cold wave 
of December 13th to 15th, 1901, described above, the center passed 
directly over Baltimore. At such times the temperatures duo in the cold 



396 THE CLIMATE OF BALTIMORE 

winds from the north or northwest, which accompany all cold waves, 
are further lowered by the rapid terrestial radiation which takes place 
in the calm clear centers of the anti-cycloues, especially during the night 
hours. 

The low temperatures associated with cold waves do not as a rule con- 
tinue more than three or four days. By the fourth or fifth day the normal 
minimum for the month is again reached. (See pages 117 to 133 for 
additional details of cold days.) 

The Origin of Cold Waves. 

The cold waves of the United States have been explained in various 
ways by those who have studied their history. One hypothesis accounts 
for them by attributing them to the upper westerly winds which are 
drawn across the Eocky Mountain range into the cyclonic depressions to 
the east of the mountains. The clear dry air which descends along the 
eastern slope cools by radiation into space during the long winter nights 
to such an extent as to greatly overbalance the warming effect due to 
compression and insolation as the air descends from the higher to the 
lower levels. 

Another theory attril)utes them to the southward movement of detached 
masses of the dry cold atmosphere which form rapidly during tJie winter 
months in the region to the west of Hudson's Bay. 

A third hypothesis refers them to slowly descending currents of the 
anti-trades which flow from the equator toward the Arctic region in 
the higher levels of the atmosphere, the loss of heat during the long 
journey over the continent being sufficient, by the time they reach the 
surface in latitudes between 60° and 70°, to account for the low 
temperatures observed in our cold waves. 

These explanations appear plausible, and it may be that the excessively 
low temperatures observed in our severest cold waves — temperatures 
of 50° to 60° below zero — must be attributed to a combination of two 
or all of the causes mentioned. 

Eecent investigations into the conditions at very great elevations over 
the equator have shown the existence of low temperatures never experi- 



MARYLAND WEATHER SERVICE 397 

enced at the earth's surface, e\en within the Arctic Circle. In the sum- 
mer of 1906 M. Teisserenc de Bort and Mr. Rotch by means of pilot bal- 
loons succeeded in obtaining records of a temperature of 123° below zero 
from an elevation of about nine miles above the earth within the Tropics. 

THE COLD VS^INTER OF 1903-4. 

One of the coldest winters on record in the history of Baltimore was 
that of 1903-4. In some respects it was more severe than the winter of 
1855-6, generally regarded as the hardest winter experienced in the 
Middle Atlantic states. There were very few excessively cold days, the 
low average temperature for the three winter months being due to long 
continued moderate cold. The average temperature for the entire season 
was 6.3° per day below the normal. The winter most nearly approach- 
ing this in continued cold was that of 1892-3, with a daily departure of 
5.2° below the normal for the season. Next in order comes the famous 
winter of 1855-6 with an average daily departure of 4.6° below the sea- 
sonal average. 

With a minimum of 2° above zero on January 5, the winter was not 
at all remarkable for low temperatures. The trying combination of 
intense cold, high wind, and snow, occasionally experienced in Baltimore 
winters, was entirely lacking. The season was characterized by an 
almost unbroken period of moderately cold weather, and an unusual 
frequency of snowfall, rather than an excessive quantity. The ice was 
very heavy; an abundant crop was cut before the first of January, an 
unusual event for the vicinity of Baltimore. The ice in the Bay 
and harbor impeded navigation to a greater extent than for many years. 
For a time during the second decade of January, and again in the 
middle of February, ice covered the entire Bay from the Susquehanna to 
the Patuxent, and even the larger steamers were obliged to remain in 
port. 

There was but a single " cold wave " in the technical sense of the 
term, that is, a fall of 20° within 24 hours to a minimum of 20°, neglect- 
ing the usual diurnal variation in temperature ; this occurred on the 
26th of December with a fall of 22° and a minimum of 11°. 



398 THE CLIMATE OF BALTIMORE 

There was less than the average amount of precipitation for the winter 
season, including snow and rain. The deficiency for the three montlis 
aggregated over four inches. The frequency of days witli snow was 
particularly significant. The normal number of sno^vs in the winter 
season at Baltimore is 12; in 1903-4 there were 22. It is a well recog- 
nized fact that a snow cover lowers the temperature materially. Hourly 
observations of temperature at Baltimore during a period of ten years 
show that on days when the ground was covered with snow the mean 
temperature was 10° lower than on the normal winter day. The tem- 
perature is lowered during the night by reason of the intense radiation 
from the snow surface; during the day much of the heat which would 
otherwise go to W' arm the atmosphere is employed in melting and vaporiz- 
ing the snow. 

Some of the most significant departures from normal winter condi- 
tions are indicated in the following comparison : 

THE WINTERS OF 1903-4 AND 1889-90 CONTRASTED. 

TO , Cold Warm 

w-^Tf Winter Winter 
W inter. i9n3_4. 1889-90. 

Number of days with a mean temperature below 32°. . 34 66 11 

Number of days with a mean temperature above 40°. .20 8 63 

Number of days with a min. temperature below 32°. . 59 78 28 

Number of days with a max. temperature below 32°. .14 21 5 

Number of days with a temperature of 20° or less. ... 18 39 6 

Number of days with a measurable amount of snow. .12 22 7 

Total depth of snowfall in inches 24 26 5 

Total precipitation (rain and melted snow) in inches. 10 6 7 

Number of days with snow on ground 22 40 — ' 

First killing frost occurred Nov. 7 Nov. 7 Nov. 6 

Last killing frost occurred Apr. 4 Apr. 17 Apr. 2 

^ No record. 

The following notes on ice conditions during the winter of 1903-4 are 
taken from the daily journal of the United States Weather Bureau : 

1903. 
Nov. 7. First ice. 
Dec. 16. Ice 4 to 5 inches thick. 

1904. 
Jan. 4. Ice 6 to 7 inches on Druid Hill Lake. Bay frozen over from 
Sharp's Island to Ft. Carroll. Ice off Sharp's Island 8 inches 
thick. 



MAKYLAXD WEATHER SERVICE 399 



Ice on Druid Hill Lake, S inches. 

Heavy ice 50 to 60 miles down the Bay. All sailing vessels and 

small steamers tied up. 
Heavy drift ice in Bay — flows 5 feet thick, making navigation 

dangerous. 
From Sandy Point up the harbor ice jams delay strongest steamers. 
Ice on Druid Hill Lake, 12 inches. 
Ice fields from Susquehanna to Patuxent. Passable only by means 

of ice boats. 
Whole Bay solidly frozen over down to Cove Point. Ice 3 to 18 

inches. Navigation suspended. 
Fields of heavy drift ice reported as far south as the Potomac. 

Conditions the worst of the winter. 
Some ice floes have an area of 400 to 500 acres. Navigation 

hazardous. 
Navigation practically suspended. 
Ice in harbor again becoming a serious obstacle to navigation in 

spite of the good work of the ice boats. 
Ice conditions nearly as bad as at any time of the season. 
The steamer Alabama from Old Point ran into drift ice extending 

from shore to shore only 30 miles above Old Point. 
The largest steamers are remaining in port on account of ice. 
The ice in the Bay is causing no more serious trouble. 

THE WARM WINTER OF 1889-90, 

The winter of 1889-90 was quite as remarkable for its mildness as that 
of 1903-4 was for its severity. The excess in temperature was even 
greater than the deficiency of the winter in 1903-1, being nearly 8° above 
normal per day, as compared with 6° below in 1903-1. The winter 
passed without a single cold wave. The seasonal snowfall (5 inches) 
was the lightest since 1871, or since the establishment of the local office 
of the Weather Bureau. The number of days upon which snow fell was 
but 7 as compared with an average number of 12 and as compared with 
22 in 1903-4. There were but six days of the season on which the 
temperature fell as low as 20° ; in 1903-4 there were 39 days with a 
temperature of 20° or below. 

On 12 days of the winter of 1889-90 the tempernture rose to points 
never reached upon those days in 30 years, and upon two days the highest 
temperature recorded in December and January occurred, namely, 73° 
on December 26, 1889 and January 13, 1890. 



1904. 


Jan. 


6. 


Jan. 


8. 


Jan. 


12. 


Jan. 


13. 


Jan. 


18. 


Jan. 


19. 


Jan. 


20. 


Jan. 


29. 


Jan. 


31. 


Feb. 


2. 


Feb. 


15. 


Feb. 


18. 


Feb. 


19. 


Feb. 


21. 


Mar. 


2. 



400 THE CLIMATE OF BALTIMORE 

THE DISTRIBUTION OF ATMOSPHERIC PRESSURE DURING THE COLD 
WINTER OF 1903-4 AND THE WARM WINTER OF 

1889-90. (PI. XXIY.) 
The character of the weather ol a given locality depends, so far as 
temperature and winds are concerned, upon the general distribution of 
the atmospheric pressure over a large area surrounding the locality. 
The pressure determines the wind direction directly, and the winds, in 
turn, modify the temperature. If we examine carefully a chart showing the 
normal winter atmospheric pressure over the North American continent, 
we find that the barometer is relatively low over Labrador and surround- 
ing regions, and high over the interior of the continent and over the 
central and southern states. Such a distribution of pressure, as shown 
in the discussion of cyclones and anti-cyclones in preceding pages, gives 
to Baltimore and vicinity a prevailing west to northwest wind. If this 
normal winter distribution of pressure is disturbed, a change in the 
prevailing wind directions will result, with attendant changes in tempera- 
ture, and other factors. If we examine the distribution of the mean 
seasonal pressure over the country during the winter of 1903-i, we find 
a distribution differing greatly from the normal. The centers of the 
areas of high pressure during December, January, and February were to 
the west and northwest of, or over Baltimore, but the areas were much 
more extensive and intensely developed, causing a steady and strong 
flow of cold northwest winds during the entire season. A winter season 
may be severe by reason of a considerable percentage of exceptionally 
cold days, or by reason of long continued moderate cold. The season of 
1903-4 belonged to the latter class. If we examine, on the other hand, 
the distribution of seasonal pressure during the winter of 1889-90, we 
find a totally different condition — a wide departure from the normal 
type. The barometer was persistently high over the southeastern por- 
tion of the country — an extension westward of the permanent area of high 
pressure over the Atlantic Ocean. This distribution of pressure gave 
to Baltimore and vicinity a prevailing wind direction from the south 
and southwest during December and January, and an easterly direction 
during February. As opposed to the prevailing northwest winds of an 
average winter, these directions all give a relatively high temperature. 



MAEYLAND WEATHER SERVICE 401 

In addition, the storm tracks of the winter 1889-90 were, without excep- 
tion, far to the north of Baltimore, diifering widely from the usual dis- 
tribution, and causing an unusual percentage of southerly winds. 

The influence of the mean distribution of atmospheric pressure upon 
the general character of the weather will be further considered in con- 
nection with the discussion of weather conditions in other seasons. 

THE VARIABILITY OF WINTER WEATHER. 

In the middle latitudes winter is a season of great contrasts. This is 
particularly characteristic of regions lying near the paths of the storm 
centers. It is not difficult to find a reason for the great and sudden 
fluctuations experienced when we bear in mind the conditions described 
in the preceding pages. In our latitudes there is a continuous succes- 
sion of atmospheric waves moving from west to east across the continent. 
Within the troughs of these waves as they move eastward we find, in any 
well developed type, warm southerly winds which raise the temperature 
of the localities over which they blow approximately 10° above the 
normal for the time and place. As the crest passes over the locality it 
brings with it a cold northwest wind, carrying the temperature an equal 
amount below the normal. The crests usually follow t!ie troughs within 
24 to 36 hours; they may, if they are associated with a rapidly moving 
storm, follow one another at intervals of 13 hours, or even less. 

In the winter season conditions are favorable in the United States for 
bringing about great fluctuations in temperature in short periods of 
time. Just to the south we have the tropical regions where temperatures 
are high throughout the year, and atmospheric moisture abundant. 
Beyond our nortliern boundary line are the regions of long winter nights 
and a clear dry atmospliere, factors favoring a rapid lowering of tempera- 
ture by radiation. Warm and cold air are alternately brought to the 
regions midway between these reservoirs of heat and cold, as areas of 
low and high atmospheric pressure, or cyclones and anti-cyclones, follow 
one another in rapid succession from west to east across tlie continent. 
This material transfer of warm air by southerly winds and cold air by 
northwest winds is responsible for fluctuations of great magnitude. The 



402 



THE CLI]\rATE OF BALTIMORE 




Fig. 142.— Cold February 11, 1899. 




Fig. 143. — Warm February 11, 1887. 



MARYLAND WEATHER SERVICE 403 

contrast is, in all well developed cold waves, intensified by conditions 
attending all cyclones and anti-cyclones. The warm, moist southerly 
winds are attended with cloudy skies which cut off rapid terrestrial radia- 
tion, thus preventing loss of heat. The cold northwest winds are dry, 
while the sky is generally clear, permitting rapid loss of heat by terres- 
trial radiation. These processes, attending all storms, are sufficient to 
account for the greatest fluctuations observed in terrestrial temperatures, 
making it entirely unnecessary to call in the aid of exti-a-terrestrial forces 
to explain any unusually high or low temperatures observed. 

The great fluctuations in temperature experienced in past years in 
Baltimore are discussed at considerable length in the preceding pages of 
this report (see especially Plate lY, following page 82) ; the conditions 
under which they occurred, however, are not described. Curve D, Plate 
IV, shows the extreme range of temperature for each day of the year 
in a period of 30 years. The great ranges are shown to be most frequent 
in the winter, months, although the month of March is not far behind 
in this respect. February 11 shows the greatest range — on the 11th of 
February, 1887, a maximum of 72° was recorded in Baltimore; on 
February 11, 1899, a minimum of 6° below zero, making a total range for 
the 11th of February of 78°. Even in the month of least variability, 
August, the difference between the observed maximum and minimum 
is 31°. 

The general weather conditions which prevailed upon the two days 
of February, showing such great contrasts in temperature are represented 
in the accompanying charts. (See Figs. 142 and 143.) 

On the 11th of February, 1887, a well developed cyclone was passing 
eastward with its center almost over Baltimore. Warm southerly winds 
had been blowiEg over the city for a considerable period, and with some 
force. By the afternoon of the 11th the temperature had risen to 50°. 
The depression was followed closely by an anti-cyclone, and on the fol- 
lowing days the temperature fell to a minimum of 23° as the center of 
the anti-cyclone approached. 

On the 11th of February, 1899, Baltimore was within one of tlie most 
extensive and intense anti-cyclonic areas recorded in local weather 



404 THE CLIiLA.TE OF BALTIMORE 

chronolog}', two days before the great blizzard of this month. The cold 
northwest winds, aided by the intense terrestial radiation permitted by 
the clear skies and dry atmosphere, lowered the early morning tempera- 
ture to 6° below zero, within a degree of the greatest cold recorded in 
Baltimore in 30 years. 

Even more striking are the fluctuations in temperature attending the 
passage of single storms. On the 24th of February, 1900, Baltimore 
was within the area of influence of a cyclone, followed closely by an 
energetic anti-cyclone. By 7 p. m., with strong southerly winds, the 
temperature rose to 55°. About 8 p. m. the wind suddenly changed to 
a strong northwest wind and the temperature fell rapidly to 8° by mid- 
night, a fall of 47° in five hours. 

HOURLY CHANGES ON FEBRUARY 24, 1900. 

P. M. Mid- 

Xoon. 1 2 3 4 5 6 7 8 9 10 11 night. 

Pressure Unches) 29.43 .36 .31 .23 .19 .19 .18 .19 .18 .18 .19 .20 29.21 

Temperature (F.) 43 45 46 46 49 61 62 55 37 33 28 18 8 

Wind Direction SE E SE SE SE 8 SW SW NW W NW W NW 

Wind Velocity (miles per hour) 8 8 7 12 8 7 10 12 12 6 8 7 8 

On the 31st of December, 1898, as the center of a depression passed 
over Baltimore and was followed by the advancing front of an anti- 
cyclone, the temperature fell from 57° at 7 a. m. to 18° at midnight. 
The usual diurnal rise in temperature to 3 p. m. was totally obliterated. 
The temperature continued to fall steadily to a minimum of 5° by 
8 a. ra. of January 2, 1899. 

HOURLY TEMPERATURE CHANGES OF DECEMBER 31, 1898. 

A. M. P. M. Mid- 

7 8 9 10 n Noon. 1 3 3 4 5 6 7 8 9 10 11 night. 

Temperature (F.).. 57 57 55 47 41 38 38 38 37 35 33 31 29 37 25 22 20 18 

WindDirection SW SW NW NW N N N N N N N N N N N N N N 

The Weather of Christmas Day {December i;i5). 

In order to illustrate the variability of wanter weather in Baltimore, 
special days have been selected and the general conditions on that day 
for a long series of years graphically represented upon a single page. 



MARYLAND WEATHER SERVICE 405 

As the popular interest in the weather conditions upon public holidays 
is always great, one or two days have been selected in each season. For 
the winter season we have chosen Christmas Day and Washington's 
Birthday. Charting the weather conditions in this manner the contrasts 
are more readily perceived than b}' the use of words and figures. 

The factors represented are: The maximum, minimum, and mean 
temperatures for the day, the mean atmospheric pressure, the average 
amount of cloudiness during the course of the day, the prevailing wind 
direction, and the amount of precipitation. 

Take for example the weather conditions experienced upon the 25th 
of December in each year from ISTl, the date of establishment of the 
local office of the United States Weather Bureau in Baltimore, to the 
year 1906. 

The mean temperature for the month of December is 37° ; for the 
25th of December it is 35.5°. The maximum temperature on the 25th 
was on two occasions (in 1889 and in 1893) as high as 67°; in 1872 
the minimum temperature of the day was 8°, a range of 59°. The 
fluctuations from year to year are very irregular; sometimes they are 
abrupt, as the change from 1871 (44°) to 1872 (14°) ; at other periods 
they are gradual, as from 1881 to 1884, a slight and steady fall from 
40° to 28° in the mean temperature of the day. There were 9 clear 
days, 13 fair or partly cloudy days, and 14 cloudy days. The winds 
were prevailingly east to northeast and but once from the south. 

The day has been remarkably free from precipitation of measurable 
quantities, and this has been mostly in the form of rain. In not a single 
year since 1871 has snow fallen to the depth of one inch upon Christmas 
Day. Snow has been on the ground to a greater depth but in such 
instances it fell on the day preceding and remained on the ground. (See 
Fig. 144.) 



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MAKTLAXD WEATHER SERVICE 



407 



THE CHARACTER OF THE WEATHER ON CHRISTMAS DAY IN BALTIMORE 

FROM 1871-1906. 



Year. 


Max. 
Temp. 


Min. 
Temp. 


Mean 
Temp. 


Character 
of Day. 


Wind 
Direction. 


Daily Wind 
Movement. 


Precip- 
itation.i 




(De 


i-rees Fah 


r.) 






(Miles) 


(Inches) 


1871 


48 


40 


44 


Pt. cldy 


NE 


45 




1872 


19 


8 


14 


Cloudy 


NE 


159 




1873 


41 


30 


36 


Pt. cldy 


SE 


98 




1874 


39 


30 


34 


Clear 


W 


245 




1875 


54 


40 


47 


Pt. cldy 


E&S 


52 


0.09 


1876 


27 


18 


22 


Cloudy 


N&NE 


85 


0.06 


1877 


47 


43 


45 


Cloudy 


E 


58 


0.03 


1878 


25 


15 


20 


Clear 


W 


268 




1879 


52 


32 


42 


Cloudy 


N 


95 


0.38 


1880 


38 


25 


32 


Cloudy 


NE 


111 


0.35 


1881 


49 


32 


40 


Clear 


W 


72 




1882 


47 


36 


42 


Pt. cldy 


w 


108 




1883 


40 


32 


36 


Pt. cldy 


NE&SW 


70 


0.39 


1884 


32 


25 


28 


Cloudy 


N 


191 




1885 


39 


30 


34 


Pt. cldy 


NE 


144 




1886 


45 


28 


36 


Clear 


N&NW 


198 




1887 


38 


31 


35 


Cloudy 


N 


106 




1888 


53 


29 


41 


Clear 


Calm 


17 




1889 


67 


44 


56 


Pt. cldy 


SW 


101 


0.04 


1890 


32 


25 


28 


Cloudy 


NE&NW 


114 




1891 


48 


47 


48 


Cloudy 


SE 


127 


0.02 


1892 


29 


14 


22 


Cloudy 


NW 


225 


0.02 


1893 


67 


40 


54 


Clear 


SW 


116 




1894 


48 


38 


43 


Pt. cldy 


NW 


171 


0.15 


1895 


52 


43 


48 


Cloudy 


E 


122 


0.01 


1896 


28 


18 


23 


Clear 


W 


106 




1897 


34 


16 


25 


Clear 


S 


62 




1898 


38 


32 


35 


Pt. cldy 


NE 


76 




1899 


35 


23 


29 


Pt. cldy 


W 


134 




1900 


50 


35 


42 


Pt. cldy 


W 


115 




1901 


48 


36 


42 


Cloudy 


NE 


85 




1902 


32 


19 


26 


Pt. cldy 


W 


165 


0.17 


1903 


43 


34 


38 


Cloudy 


NW 


83 


0.19 


1904 


28 


23 


26 


Cloudy 


NE 


194 


0.14 


1905 


38 


26 


32 


Pt. cldy 


NW 


162 




1906 


33 


13 


22 


Clear 


NW 


299 




Means 


42 


30 


36 


Pt. cldy 




142 


0.14 






• Amounts loss 


than .01 inch 


not considered. 







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MARYLAND WEATHER SERVICE 



409 



The Weather of Washington's Birthday (February 22). 
The weather of the month of February presents more contrasts than 
that of any other portion of the year. The 22d is no exception ; with 
an average of 38° the temperature was as high as 74° in 1874, and as 
low as 13° in 1896, a range of 61°. From 1878 there was a steady fall 
in the mean temperature of the day to the year 1885, though there is 
apparently no regular period discernible in the annual fluctuation. 
The grouping of the days with precipitation is somewhat striking. From 
1871 to 1890 there were but two occasions upon which rain or snow 
fell to any considerable depth ; namel}-, in 1876 with a rainfall of three- 
tenths of an inch, and IS 78 with a heavy rainfall measuring over an inch 
and seven-tenths. Snow fell in 1879, 1882, 1883, and in 1889, but the 
fall was extremely light in all cases. From 1891 to 1907 rain or snow 
fell in 1891, 1893, 1894, 1897, 1900, 1901, 1902, and in 1904, though 
the amounts were small in 1892, 1898, 1899, 1903, and 1907. Fig. 145. 



THE WEATHER OF FEBRUARY 22. 



Year. 


Max. 
Temp 


Min. 
Temp. 


Mean 
Temp. 


Character 
of Daj'. 


Wind 
Direction. 


Daily Wind 
Movement. 


Precip- 
itation. 






(Degrees Fab 


r.) 






(Miles) 


(Inches) 


1871 


35 


20 


28 


Clear 


W 


. . . 


. . . 


1S72 


35 


28 


32 


Clear 


NW 


170 




1873 


35 


24 


29 


Pt. cldy 


• NW 


199 




1874 


74 


51 


62 


Pt. cldy 


SW 


173 




1875 


48 


29 


38 


Pt. cldy 


E 


97 




1876 


46 


36 


41 


Clear 


W 


166 


0.34 


1877 


57 


30 


44 


Pt. cldy 


SE 


66 




1878 


63 


50 


56 


Cloudy 


W 


196 


1.71 


1879 


36 


21 


28 


Cloudy 


S 


167 


0.05 


1880 


50 


31 


40 


Clear 


S 


110 




1881 


47 


32 


40 


Clear 


SE 


143 




1882 


40 


34 


37 


Pt. cldy 


NW 


279 




1883 


43 


31 


37 


Cloudy 


SW 


95 


0.01 


1884 


49 


36 


42 


Pt. cldy 


SE 


90 




1885 


27 


14 


20 


Clear 


NW 


183 




1886 


43 


32 


38 


Clear 


S 


168 




1887 


54 


35 


44 


Pt. cldy 


N 


158 




1888 


56 


35 


46 


Clear 


N 


74 




1889 


36 


30 


33 


Cloudy 


SW 


81 


0.09 


1890 


44 


25 


34 


Clear 


w 


127 





410 



THE CLIMATE OF BALTIMORE 



Year. 

1891 


Max. Min. Mean 

Temp. Temp. Temp. 

(Degrees Fahr.) 

47 38 42 


Character 
of Day. 

Clear 


Wind 
Direction. 

NW 


Daily Wind 
Movement. 
(Miles) 
203 


Precip- 
itation. 
(Inches) 
0.54 


1892 


49 


39 


44 


Pt. cldy 


NE 


260 




1893 


32 


24 


28 


Clear 


W 


524 


0.21 


1894 


35 


27 


31 


Clear 


W 


109 


0.32 


1895 


32 


29 


30 


Pt. cldy 


NW 


300 




1896 


42 


13 


28 


Clear 


sw 


180 




1897 


43 


36 


40 


Cloudy 


sw 


201 


0.71 


1898 


43 


34 


38 


Cloudy 


w 


111 




1899 


60 


42 


51 


Cloudy 


w 


122 




1900 


57 


41 


49 


Pt. cldy 


w 


164 


0.54 


1901 


34 


21 


28 


Pt. cldy 


w 


120 


0.01 


1902 


36 


34 


35 


Cloudy 


N 


224 


0.40 


1903 


34 


25 


30 


Clear 


NW 


227 




1904 


47 


34 


40 


Pt. cldy 


NW 


240 


0.74 


1905 


39 


31 


35 


Cloudy 


NB 


240 




1906 


54 


39 


47 


Clear 


N 


253 




1907 


26 


16 


21 


Clear 


NW 


249 





Means 



45 



31 



Pt. cldy 



179 



0.44 



SPKING WEATHEE. 

The spring season is a transition period between winter and summer 
weather conditions. Normally the season has three months, but May 
is practically a summer month, while March frequently has more of the 
characteristics of winter than spring. The season, so far as injurious 
weather conditions are concerned, is very brief. The cyclones and anti- 
cyclones of early spring belong to the winter type; they are quite as 
energetic, and follow much the same paths. With the steady approach 
of the sun the increased heat becomes more apparent, however, and the 
contrasts in temperature between the winds preceding and following the 
travelling cyclones become more marked. 

One of the first harbingers of spring is the appearance of an area of 
high atmospheric pressure off the coast of the South Atlantic states. 
This high area may have advanced from the northwest and, after crossing 
the continent, settled off the coast, but it is more likely to be an inde- 



MARYLAND WEATHER SERVICE 411 

pendent formation in place, or the westward extension of the permanent 
area of high pressure normally found over the Atlantic Ocean between 
the Azores and the South Atlantic states. The presence of an anti- 
cyclone in the southeast gives to the Middle Atlantic states a steady flow 
of warm southeast to southwest winds. 

March weather is extremely variable. The month has given us some 
of our severest winter weather, such as the blizzard of 1888, described 
in preceding pages; it may also be excessively cold and raw, as in 
1906. On the other hand, there may be an abundance of fine warm 
days forcing all vegetable growth four or five weeks in advance of the 
average season, as in March, 1898. 

The explanation for these striking contrasts may be found in the 
general distribution of atmospheric pressure over the North American 
continent and adjacent oceans during the early spring season. Under 
normal conditions in the month of March there is a well devek)ped area 
of high pressure over the central portion of the continent — the British 
Northwest Territory; another area of high pressure prevails over the 
Atlantic Ocean with its axis along the parallel of 30° north latitude, 
extending westward nearly to the Atlantic coast. These areas are not 
the same as the travelling anti-cyclones described in the preceding pages. 
They remain nearly stationary for long periods of time, but are sub- 
ject to more or less shifting about a central point from time to time. 
The travelling anti-cyclones are probably detached portions of the larger 
areas. The character of the weather within these so-called permanent 
areas of high pressure is the same, however, as is found within the 
smaller travelling anti-cyclones. Normally, the Middle Atlantic states 
lie in a belt between these two high areas, and alternately fall within 
the influence of first one, then the other. One brings us cold weather, 
the other warm. Occasionally the continental high area will extend 
to, or move southward and eastward of, its normal limits and bring 
within its influence the whole of the Middle Atlantic states. Such was 
the case in the years 1883,' 1885, 1888, and 1801. Tlie month of :March 

^8ee: 0. L. Fassig. Types of March Weather in the U. S. Amer. Jour. 
Soi., Nov., 1899. 



412 THE CLIMATE OF BALTIMORE 

in these years was decidedly below the normal in temperature throughout 
the Middle i\.tlantic states. Again there may be a weak development 
of the continental high area, in conjunction with a strong development 
of the Atlantic Ocean high area, or its extension westward beyond its 
usual limits. The Middle Atlantic states would then be brought within 
its influence and produce strong southeast to southwest winds or light 
variable winds and high temperatures. During the month of March 
in 1878, 1882, 1894, and 1898 the distribution of the monthly mean 
pressure was such as is indicated, and in all of these months the tempera- 
ture was well above the normal value. (See Plate XXIV.) 

March Winds and Storms. 
March is proverbally a windy month. The wind movement for this 
month is the largest of the year, exceeding even that of the winter 
months : 

AVERAGE DAILY WIND MOVEMENT AT BALTIMORE. 
(Average of 30 years.) 
Jan. Feb. Mar. Aiir. May. June. July. Aug. Sept. Oct. Nov. Dec. Year. 
Miles 143 163 175 166 119 143 134 122 129 137 143 143 145 

This comparatively high wind velocity is probably due to the great 
contrasts in temperature, characteristic of the month. It is the time of 
the year when the temperature rises most rapidly and the inter-diurnal 
changes in temperature are greatest: 

MEAN DAILY CHANGE IN TEMPERATURE AT BALTIMORE. 

(Based on 30 years of observations.) 

Jan. Feb. Mar. Apr. May. June. July. Aug. Sept. Oct. |Nov. Dec. 
Av. daily 
change± 1.0° 1.0° 1.2° 0.8° 1.0° 0.6° 0.6° 0.4° 0.9° 0.:° 1.0° 0.8° F. 

March follows February in the frequency and duration of storm winds : 

AVERAGE DURATION AND FREQUENCY OP STORM WINDS AT BALTIMORE. 
(Average of 5 years' observations.) 



Average monthly f requencj- ) 

of winds exceeding25 miles > 6.3 9.3 7.6 

per hour. ) 

Average duration in hours ' o i-n . 

and minutes. \ ^■''" * 

Longest periods of continu- I ,„ 

ous storm winds (hoursj. i' 



fe 




< 


a 


>^ 




ic 




o 

O 


o ® 


9.3 


7.6 


5.0 


4.4 


2.0 


2.6 


1.1 


3.6 


4.3 


■4.8 4.4 


1.40 


3.40 


3.30 


1.30 


0.13 


0.30 


3.00 


1.25 


3.40 


2.00 3.00 


46 


23 


16 


7 


0.5 


0.5 


IS 


7 


9 


6 19 



MARYLAND WEATHER SERVICE. 



VOLUME 2, PLATE XXM. 






EFFECTS OF THE ICE-STORM OF MARCH, 1906 IN THE BLUE RIDGE 

MOUNTAINS. 



MARYLAND WEATHER SERVICE 413 

ICE STORilS. 

While the afternoon temijeratures during March are well above the 
freezing point, the early morning temperatures generally dip below the 
frost line. Hence the isotherm of 32° is frequently crossed during the 
passage of the storms of this month. Precipitation, beginning as rain, 
changes to sleet or snow, or the rain as it falls upon the cold ground 
or trees and shrubs freezes, covering all exposed objects with a coating of 
ice. These ice storms are of frequent occurrence in the early spring, 
but generally occur in March. They frequently cause great damage to 
property by overloading trees, telegraph lines, etc., and are a source of 
considerable danger to pedestrians on the city streets. The ice upon trees 
and telegraph lines sometimes collects in great quantity. Under favor- 
able conditions in the presence of a fog or, in the mountain districts 
in a driving low cloud, the frozen particles of fog or cloud collect upon 
the windward side of exposed objects to a thickness of two and even 
three inches. (See Pis. XXI and XXII.) 

THE SQUALL OF MARCH 1, 1907. 

A type of storm which occurs with increasing frequency with the 
advance of spring is shown on the weather map of March 1, 1907. The 
eastward advance of the storm across the country is marked by the occur- 
rence of severe squalls and thunderstorms, accompanied by heavy rains, 
within a restricted area of the general storm, or cyclone. The isobars 
enclosing the storm area form long narrow troughs which are likely to 
be particularly well marked as the cyclone passes across the Mississippi 
Valley. Drawing an approximately north and south line through the 
center of the storm the winds to the east blow from thu southeast, while 
those to the west blow from the northwest. In the vicinity of this line 
of conflict between the southeast and northwest winds, from the center 
of the storm southward, we have numerous squalls, thunderstorms, and 
heavy rains. These local storms occur almost entirely within the south- 
ern quadrant of the general cyclone of the type here described. The 
contrasts in temperature between the southeast winds to the east of the 
"squall line," as the north-south line above described is sometimes 



414 



THE CLIMATE OF BALTIMORE 



calltjd, and the northwest winds to the west, are very pronounced; these 
contrasts are especially well marked in passing from the center of the 
storm in a northwesterly direction. The map of March 1, 1907, shows 
a difference of 50° at 8 a. m. between St. Louis, Mo., and Huron, S. 
Dak., a distance of about 500 miles. The chart also shows the distribu- 
tion of thunderstorms, occurring within the 12-hour period preceding 8 




Fig. 146.— The Squall of March 1, 1907. 



a. m. of this date. The rains of the Lower Mississippi Valley were 
heavy, and in some localities, excessive: Mobile, Ala., reported a fall 
of 6.42 inches in the preceding 24 hours; Montgomery, Ala., 1.32 inches; 
Anniston, Ala., 1.16 inches; Meridian, Miss., 2.64 inches; Little Eock, 
Ark., 1.42 inches; and Memphis, Tenn., 1.30 inches. (See Fig. 146.) 

Later in the season, with the increasing heat of spring, the most intense 
variety of local storm — the tornado — is frequently developed within the 
thunderstorm and squall area of this type of general storm. As the 



MARYLAND WEATHER SERVICE 415 

storm moves farther eastward the squalls and thunderstorms continue 
to develop, but the tornado becomes of less frequent occurrence, disappear- 
ing almost entirely by the time the center of the general storm reaches 
the coast. In Maryland, for example, a real tornado is of very rare 
occurrence. 

The storm described above changed its shape materially during the 
succeeding 24 hours, becoming by 8 a. m. of March 2 more circular in 
form, with its center over the Lower Lake region. In changing to the 
more common cyclonic type the change in wind direction during the 
passage of the center of the storm became less abrupt and the squally 
character of the shift in the wind disappeared to a great extent. 

EQUINOCTIAL STORMS. 

There is a widespread popular belief in the occurrence of severe storms 
at the equinoctial periods in March and September. Just why there 
should be any unusual atmospheric disturbance when the sun " crosses 
the line " has never been made clear. The belief is an old one, and is 
especially firmly fixed in the minds of sailors. Like many of the old 
weather " saws,'' it will not stand the test of rigid comparison with re- 
corded observations. As a rule, people are not very critical in their defini- 
tions of storms, or in verifying the time of their occurrence. In the ab- 
sence of a severe storm a very mild disturbance will satisfy them. Or if the 
storm should occur three or four days preceding or following the 
equinoxial day it is an equinoxial storm ahead of time or a little delayed 
in its arrival. With such elastic restrictions it is not difiicult to realize 
an equinoctial storm in any month of March. But under these condi- 
tions a storm is just as likely to occur upon any other day in the month. 
If a disturbance be required to show a wind velocity exceeding 25 miles 
per hour in order to be classed as a storm, there is, according to the 
Baltimore records, a storm wind every fourth day. If uniformly dis- 
tributed through the month any clay might be selected for a storm and 
an error of more than two days in time could not be made. Moreover, 
the records show that the wind velocities on the 21st and 22d of Marcli 
are not greater than on the days preceding and following. If we con- 



416 THE CLIMATE OF BALTIMORE 

sider the occurrence of gales, or winds exceeding 40 miles per hour, we 
find that during a period of 30 years there were 42, distributed through 
the year as indicated below : 

FREQUENCY OF GALES NEAR BALTIMORE. 

Jan. Feb. Mar. Apr. Maj\ June. July. Au^. Sept. Oct. Nov. Dec. Year. 
3 10 3 2 2 3 4 3 3 5 6 42 

September, the month of the autumnal equinoctial storm, is the only 
month in the year without a gale to its credit in 30 years, while the 
month of March has less than the average monthly frequency. 

If we regard rainfall as one of the essential features of a storm, statis- 
tics are no more favorable to the equinoctial theory than they are in 
the case of winds. On March 21 the rainfall frequency, based on 31 
years of observations, is slightly less than 50 per cent; that is, in 31 
years, rain occurred 15 times; on the 22d the percentage of frequency 
is 40 per cent. On these days rain has occurred less than half the time. 
Eainfall frequency is somewhat greater on the 19th and 20th, having 
occurred 16 times in 31 years. (See Table XLIV, page 181.) 

When considering amount of rainfall instead of frequency, statistics 
are even more unfavorable. The average amount of rainfall recorded 
in Baltimore during 30 years on the 21st of March is 0.21 inch. The 
amounts for March 19 and 20 are decidedly greater, namely, 0.45 inch 
and 0.37 inch respectively. The daily average for the entire month is 
0.31 inch; hence the amount recorded on the 21st is below the average 
for the month. 

Similar statistics may be shown for the September equinoctial day : 
On September 21, the average rainfall is 0.16 inch; for the 19th it is 
0.93 inch, nearly six times as much; for the 20th it is 0.23 inch; for the 
22d, 0.30 inch. The average daily amount for the entire month of 
September is 0.30 inch. For rainfall frequency in September we have: 

September 19 26 per cent. 

20 30 

21 26 

22 32 



MARYLAND WEATHER SERVICE 417 

The daily average for the entire montli is 30 per cent. Eain occurs 
on the 21st day of March on the average in approximately alternate 
years; on the 21st day of September every third year. The equinoctial 
storm, as a destructive storm, or as a storm confined to a single day in 
the vicinity of Baltimore, is a myth. 

Hail Storms. 

True hail is of infrequent occurrence in the winter months. While 
it is often reported in the cold season a careful observer would in nearly 
every instance report sleet. There is a radical difference between sleet 
and hail, both in the manner of formation and in physical character- 
istics. Sleet is an intermediate stage between rain and snow, or a mix- 
ture of the two forms, and occurs during the passage of a storm when 
the temperature crosses the freezing point. Hail, on the other hand, 
occurs almost entirely during the warm season, when the surface tem- 
peratures are far above the freezing point of water. 

THE FREQUENCY OF OCCURRENCE OF HAIL IN BALTIMORE. 

(Total number in 28 years.) 

Jan. Feb. Mar. Apr. May. .Tune. .Tuly. Aug-. Sept. Oct. Nov. Dec. Year 
4 3 3 3 II 13 9 6 1 1 1 55 

It will be noted by the above figures that over half of the hail storms 
reported in Baltimore in 28 years occurred in the months of May, June, 
and July. Further details as to frequency and time of occurrence may 
be found on pages 284-287 of this report. 

The most favorable period of hail formation appears to be the latter 
part of the spring season, and the early summer. The hail storm, like 
the thunderstorm, is intimately associated with the general cyclone. It 
occurs in the southern quadrant of the general storm, along the " squall 
line," described in a preceding paragraph. Hail storms occur almost 
exclusively in connection with thunderstorms. The reverse of this state- 
ment is, however, not true, while but 55 hail storms are recorded in the 
local annals of the Weather Bureau in a period of 28 years, there is a 
record of 678 thunderstorms during the same period. 



418 THE CLIMATE OF BALTIMORE 

THE HAIL STORM OF MAY 19, 1904. 

On Ma}^ 17 and IS, 1904, a condition of unsettled weather prevailed 
over the country east of the Mississippi Eiver. Cloudy and rainy weather 
accompanied the slow eastward movement of a shallow barometric depres- 
sion from the Middle Mississippi Valley to the Middle and South Atlantic 
states. On the morning of the 18th the center of depression was over 
North Carolina. From the morning of the 18th to the morning of 
the 19th the storm became more limited in area and moved rapidly 
northward. At 8 a. m. the center was over Lake Huron, with an exten- 
sion southeastward, forming a secondary depression. There Avas a well 
defined line of separation between southeast and westerly winds, extend- 
ing from the center of the storm southeastward across Central New 
York, Eastern Pennsylvania, and Southern New Jersey. During the 
19th the storm moved slowly eastward, accompanied by severe local 
storms and heavy rains in the Middle Atlantic and New England states. 
In Maryland the thunderstorm was accompanied by hail between two and 
three o'clock in the afternoon. (See Fig. 147.) 

The local conditions prevailing at Baltimore during the progress of 
this storm are shown in detail in the accompanying diagram (Fig. 148). 

THE HAIL STORM OP APRIL 27, 1890. 

A hail storm of unusual severity passed over the city on the 27th of 
April, 1890. A detailed account of local changes appears in the daily 
journal of the office of the Weather Bureau, from which we quote the 
following : 

Dense fog prevailed in the morning, gradually disappearing during the 
forenoon. The temperature rose rapidly from 48° at 8 a. m. to 71° in the 
afternoon. Cautionary southeast storm signals were displayed at 10.45 a. m. 

The most destructive hail storm on record at Baltimore visited the city 
this afternoon between 3.45 and 4 o'clock. It came from a point between 
west and northwest, and travelled in a direction a little south of east. 

A half-hour before the arrival of the storm, a dense black cloud-mass, 
tinged with purple and green, was observed extending from the western 
horizon across the sky to the northeast, and rapidly approaching. There were 
at this time two or three subdued peals of thunder, following some vivid 
flashes of lightning in the west. The front of the bank was in great commo- 
tion and. as it approached, appeared to be preceded by a thin misty veil of 



MARYLAND AVEATHEE SERVICE 



419 



o H,HIGH 



HIGH 



\ 



A< 



LOV 



\ .3 





/^ 


50° 


X. \^^ 


GH 

\ 


// U'f LOVVl 




V l7~^ ^-""^ 




^/J 


^ ->' — — - 


-/: 


- 



^'^ 7 0- 



Fig. 147.— The Hail Storm of May 19, 1904. 



MDT. 




MAY 1 9 1 904 



2 9': 52 WIND DIRECTION 

V ■/// ^W 

10 10 IS fe 4 ;{ II S 8 8 t) 

§^ WIND MOVEMENT 

ta Tr Tr. 



Fin. 148.— The Hail Storm of May 19. 1904. 



420 THE CLIMATE OF BALTIMORE 

cloud. There was a sound like the roll of musketry, and the storm burst sud- 
denly upon the city with an almost deafening roar as the great hail stones 
rained down upon the tin roofs and crashed into the windows, not a building 
in the path of the storm escaping damage. 

Several persons were knocked down by the stones, and many, including a 
number of children, were cut and bruised. Horses, pelted and cut until the 
blood streamed from them, could not be controlled, and many ran away, 
damaging the vehicles and injuring the occupants. 

Rain fell in torrents with the hail (0.80 inch falling between 3.45 p. m. and 
4 p. m.), poured through the shattered skylights and windows, and flooded 
houses. The streets were like rivers, and in many places the street-car tracks 
were covered to a depth of 6 inches by the soil washed down from adjacent 
hills. 

To add to the general disaster the wind blew with great violence, unroofing 
buildings, breaking in the remains of windows, uprooting trees, and giving 
the hail stones a dangerous slant to the eastward. For 15 minutes the city 
was in a state of complete panic, and then the storm passed away almost as 
quickly as it had come. A half-hour after the storm had left the city, the 
rain had nearly ceased, the wind was again light, and a rainbow appeared 
in the east. 

The hail stones M'ere as large as hen's eggs. Several measured more than 
2 inches in diameter. Three weighed together 12 ounces. Some as large 
as a man's fist were reported by reliable parties as having fallen in West 
Baltimore, where the damage by hail was greatest. The hail stones were of 
various formations. One large stone was covered, on the side unbroken by 
its fall, with a number of sharp-pointed prisms, and there were many others 
like it. A large number were oval in form, and these on examination, ex- 
hibited a lamellated structure, being composed of alternate layers of trans- 
parent and opaque ice, commencing at the center with an opaque nucleus. 
Others were spheroidal in form and were similar in structure to the oval 
ones, and like them, very hard. The large prism-covered stones were com- 
posed, in the center, of a mass of sponge ice, and were generally crushed upon 
striking the pavement; no lamellated structure was observed in these. 

Although the rain commenced falling at 3.40 p. m. and ended at 5 p. m., 
it was excessive only between 3.45 p. m. and 4 p. m. when the 0.80 inch 
referred to above fell. 

There was a marked fall in temperature during the progress of the storm. 
The maximum of the day, 71°, occurred some time before the storm's arrival. 
Between 3.30 p. m. and 4 p. m. the temperature fell from 67° to 52°, rising 
again to 60° after the storm had passed. 

The wind, which had been very light all day, upon the approach of the 
storm suddenly veered from the southeast to west-northwest and blew with 
rapidly increasing force, reaching a velocity of 30 miles per hour at 5 minutes 
before 4 o'clock. After 4 o'clock it decreased in force and again became light. 

From information received from suburban points, it is estimated that the 
hail band began about 10 miles west-northwest of the city and terminated 
5 miles east of the city, and extended in breadth from the southern limits to 



MARYLAND WEATHER SERVICE 421 

5 miles to the north. This would make the baud about 25 miles long and 
about 10 miles broad. 

The amount of property destroyed is estimated at from $60,000 to $100,000, 
the damage for the most part being confined to windows of western ex- 
posure — a great many thousands of which were broken — and to skylights 
and greenhouses. A few windows with a northern exposure were also 
broken. The damage from the wind was great, a number of houses losing 
their roofs, while many trees in all parts of the city were blown down. 
There was no loss of life. 

The physical structure of hail stones is well known. There is a 
central nucleus of opaque snow or ice, consisting of snowflakes and ice 
crystals mixed with air bubbles. This central nucleus is surrounded 
by a series of thin alternating layers of clear ice and opaque snowy 
material. There may be as many as 10 or 12 of these layers. The diam- 
eters of the stones vary from a few tenths of an inch to three and even 
four inches. The variation in the shape of the stones is also very great. 

Although the theory of hail formation has received a great deal of 
attention it is still in a very unsatisfactory state. The explanation of 
the method of formation of the successive layers of packed snow and 
clear ice presents great difficulties as we are obliged to rely almost wholly 
upon speculation as to the physical processes which go on within the 
heavy cloud mass which constitutes the laboratory of the hail stone. The 
outward form of the tall '" thunder head " identified with hail storms is 
being carefully studied, and we may hope soon by means of instruments 
carried aloft by kites and balloons to gather some valuable facts as to the 
physical processes going on within, w^hich will eventually lead to a better 
understanding of hail formation. 

Spring Frosts. 

Marked falls in temperature have a special significance in April and 
the early part of May in most sections of the Middle Atlantic states. 
During an average season spring fruits have passed the critical period 
of injurious frosts by the middle of April in the vicinity of Baltimore, 
as the average occurrence of the last killing frost falls within the first 
week of this month. Frequently, however, a killing frost will occur in the 
latter part of April, and on rare occasions in the first decade of May. 



422 



THE CLIMATE OF BALTIMORE 



During April light to heavy frosts are generally looked for when a 
pronounced area of high pressure (an anti-cyclone) passes directly over 
the Middle Atlantic states from the west or northwest. In addition to 
the fall in temperature occasioned by the actual transfer of masses of cold 
air from the northwest or north into the Middle Atlantic states, there is 
a still further reduction in temperature, owing to the rapid loss of heat 




Fig. 149.— The Frost of May 9, 1906. 



during the night within the anti-cyclone ; the dry air and cloudless skies 
accompanying these areas facilitating rapid radiation from the ground. 
When, as frequently happens, the anti-cyclone is preceded by a baro- 
metric depression accompanied by rain, the probability of the occurrence 
of frost is greatly increased, as the atmosphere is then charged with 
moisture on the approach of the fall in temperature within the anti- 
cyclonic area. If at the time of the 8 p. m. observation the temperature 
is between 40° and 50° at Baltimore, and the arrival of a pronounced 



ilAEYLAND WEATHER SERVICE 423 

anti-cjclone is expected during the following night from the west or 
northwest, frosts are very likely to occur in the early morning hours. 
The injury resulting from a frost depends, not only on the fall in tempera- 
ture, but also upon the state of vegetation, the amount of moisture in the 
atmosphere, and upon the wind movement. Clear, quiet nights greatly 
facilitate the production of frost in the lower places, allowing the colder, 
heavier air to settle near the ground. A clouded sky will prevent rapid 
radiation from the ground. A moderate -wind movement, by thoroughly 
mixing the air, will prevent any great difference in temperature between 
the layers near the ground and the air at higher levels. A frost occurring 
shortly after the appearance of tender plants is likely to do more damage 
than a heavier frost later on when the plant has become more vigorous. 

For details concerning the occurrence of spring frosts see pages 129 
to 135. 

The general weather conditions on the morning of May 9, 1906, show 
a situation from which frost may be expected in the Middle Atlantic 
states during the following night. 

Temperatures ranging between 28° and 35° occurred in nearly all 
counties of Maryland on the 10th and 11th. This anti-cyclone was the 
occasion of one of the severest spells of cold weather experienced in 
Maryland so late in the season. All stations in the mountainous portion 
of the state, and many stations elsewhere, experienced temperatures 
below freezing. Even the southern portion of the Eastern Shore Avas not 
exempt, Salisbury reporting 29° and Princess Anne 31°. 

At a number of stations the temperature did not fall below 40°. The 
minimum in Baltimore was 38° on the 10th. While the frost was quite 
severe, fruits and vegetables were too far advanced to suffer any very 
great amount of injury. (See Fig. 149.) 

ICE WITHOUT FROST. 

The weather map of 8 a. m., April 17, 1905, shows freezing conditions 
throughout Maryland, but no frosts were reported. A well developed 
high area with its crest extending from Montana southeastward to 
Florida was associated with a deep cyclone centered over the Lower St. 

28 



424 



THE CLIMATE OF BALTIMORE 



Lawrence V'alley. Strong, steady west winds prevailed as a result of this 
distribution of pressure over the Middle Atlantic states. The dry air 
aided by considerable movement prevented the formation of frost, 
though ice formed in a number of places. (See Fig. 150.) 

Periods of Unsettled Weather. 
In the eastward drift of cyclones in our latitudes, the rain area occupies 
from one to two days in passing a given point. In 76 per cent of all 




Fig. 150. — Ice without Frost, April 17, 1905. 

occurrences of precipitation the rain or snow falls within a forty-eight 
hour limit in Baltimore. In 13 per cent of instances the precipitation 
covers all or a portion of three days. This leaves very little margin for 
extended periods of consecutive days with rain or snow. (See page 213.) 
Long periods of unsettled weather with rain or snow are of most frequent 
occurrence in the spring season. 



MARYLAND WEATHER SERVICE 425 

PERIODS OF UNSETTLED WEATHER. 

(With 6 or more consecutive days of rain or snow.) 

D. J. F. M. A. M. J. J. A. S. O. N. Y. 

Total frequency ill 34 years 13 II 15 20 14 23 13 12 IT 9 7 11 161 

Maximum duration (days) 22 19 IT 22 13 23 18 15 19 12 10 10 23 

Number of intervening daj'S with- 
out rain 644214324001 4 

Seasonal frequency 38 57 42 27 164 

These periods of unsettled weather may be due to a great variety of 
causes. There is not a regular and periodic succession of well developed 
cyclones and anti-cyclones, such as have been described in preceding 
pages. The well developed types have been selected for illustrative pur- 
poses, as they are simple in outline and more readily interpreted. In 
studying the actual weather map from day to day we find there are no 
two weather conditions exactly alike ; there is an infinite variety in the 
outline of isobars, the trend of isotherms, the shape of rain, areas, etc. 
An unusual succession of rainy days may be due to the very slow move- 
ment of a cyclonic area, to a rapid succession of storms, to a recurving 
of the storm upon its path, or to a combination of these causes. It may 
also be due to the persistence of a so-called " flat map," a map without 
well defined cyclones or anti-cyclones. 

THE RAINY PERIOD OF APRIL 19-25, 1901. 

On the morning of April 16, 1901, a depression entered the United 
States in the extreme Southwest; at 8 a. m. its center was over Arizona. 
This depression travelled slowly eastward, causing moderate to heavy 
rains over Texas and the West Gulf states during the ITth and 18th. 
By the morning of the 18th the center was over Alabama. In connec- 
tion with another depression centered over Ohio a long trougli of low 
pressure was formed extending from the Lower Lake region to the Gulf 
of Mexico. By the morning of the 19th the two depressions had merged 
into a single storm with its center over Georgia. Under the influence of 
these two storm centers, aided by an area of high pressure in the extreme 
Northeast, east to northeast winds set in at Baltimore, and rain began to 
fall on tlie 19th. The storm turned sharply up the coast on the 19th, ac- 
companied by a very heavy rainfall. At 8 a. m. of the 20th the center was 



426 THE CLIMATE OF BALTIMORE 

over North Carolina and Southern Virginia. From the morning of the 
20th to the evening of the 21st the storm center had travelled only from 
Southern Virginia to Western Maryland. On the morning of the 22d the 
center was found over the Ohio Valley, having been deflected westward — a 
rare occurrence. The winds at Baltimore continued to blow from an east- 
erly direction. This depression began to fill up and two secondary centers 
of low pressure developed along the coast, one over Eastern Maryland and 
the other over the South Atlantic states. By the morning of the 24th 
these two secondary depressions had merged into a single storm center 
off the coast of Delaware and New Jersey. This storm moved slowly 
northeastward just off the coast, disappearing by the morning of tho 
26th. 

This very slow movement and peculiar path of the original storm, 
and the subsequent formation and sluggish progress of the secondary 
depressions in the neighborhood of Maryland kept Baltimore within 
their rain areas for six successive days. The amounts recorded upon 
each day of this period were not large, but the six days' total exceeded 
two inches. 

DAILY RAINFALL APRIL 19 TO 25, 1901. 

April 19, 1901 0.06 inch. 

20, " 0.56 

21, " 0.54 " 

22, " Trace 

23, " 0.11 " 

24, " 0.71 " 

25, " 0.05 

Total 2.03 inches. 

The rainfall period lasted 162 hours, but the precipitation was not 
continuous, scattered showers occurring on the 21st, 22d, 23d, and 25th. 

THE RAINY PERIOD OF MAY lG-26, 1894. 

There is a general impression that rainy periods of much greater 
length than that recorded above are of frequent occurrence; a close 
inspection of records, however, will reveal the fact that there are inter- 
vening days without a trace of rain. 



MARYLAND WEATHER SERVICE 427 

One of the longest periods noted in the Baltimore records of unsettled 
weather with daily rainfall was that of the 16th to the 26th of May, 
1894. 

This period was connected with the passage of a Lake storm of great 
extent and energy. Here again the path of the storm after leaving the 
Lake region was peculiar, while the progress was very slow — in fact 
the center was nearly stationary in the vicinity of Maryland for the 
greater part of three days. The storm had its origin over the Northern 
Eocky Mountain slope on the 15th. It moved slowly to the Lower 
Lake region until the 18th, then dipped abruptly southward and remained 
over Virginia, Maryland, and West Virginia until the depression gradually 
filled up on the 21st. In the meantime a second depression developed over 
Georgia and Xorth Carolina and moved slowly up the coast, disappearing 
off the coast of New England on the 26th. From the 16th to the 26th 
Baltimore was within the rain areas of these two storms and much of the 
time very near their centers. 

The rainfall recorded during this period was as follows: 

DAILY RAINFALL AT BALTIMORE FROM MAY 16-26, 1894. 
May 16, 1894 0.13 inch May 22, 1894 0.01 inch 



17, " 0.16 

18, " 0.51 

19, " 0.09 

20, " 1.07 

21, " 0.19 



23, " 1.34 

24, " 0.16 

25, " 0.05 

26, " 0.14 

Total 4.45 



While the entire period covered by the rainfall was a little over 10 
days, there were but 63 hours of actual rainfall. (See pages 174 and 
219 et seq. for frequency and duration of wet spells.) 

The most notable instances of a long continued rainfall occurring 
since hourly records were begun by the United States Weather Bureau in 
1893 at Baltimore, was that of April 27 to May 1, 1895. The records 
show that rain fell for 102 consecutive hours. Though the rain was 
reduced to a light mist at times, it never entirely ceased during this 
period. The total amount of rainfall was 3.69 inches. There was no 



428 THE CLIMATE OF BALTIMORE 

well defined storm area in the vicinity of Baltimore. The barometer 

was high over the New England states, and a shallow though ill defined 

depression covered the Gulf of Mexico; the depression moved slowly 

northward and eastward some distance off the coast, causing a steady 

northeast wind at Baltimore — a mild " northeaster." 

There is a myth associated with the occurrence of rainfall on St. 

Swithin's day which seldom fails to receive the attention of the press on 

the 15th of July: 

" St. Swithin's Day, if ye do rain. 
For forty days it will remain; 
St. Swithin's Day, an ye be fair, 
For forty days 'twill rain nae mair." 

The nearest approach to a fulfillment of this prophecy, according to 
the Baltimore rainfall records of the past 36 years, is a period of 9 con- 
secutive days with rain in the month of July. (See page 213.) 

The Variability or Weather in Spring. 

As an illustration of the changeableness of weather conditions in the 
spring of the 3'ear, we may present the irregular fluctuations from day 
to day, or we may show the changes which have taken place upon the 
same calendar day of each year for a long series of years. As regards 
the degi'ee of variability in temperature the month of March ranks with 
the winter months. Throughout the early spring rapid fluctuations and 
strong contrasts in the conditions of successive days are of common 
occurrence. 

Owing to the general interest in the character of the weather on the 
4th of March, at least once in four years, this day is selected as a type 
of March weather. Whatever reason there may be from a historical 
point of view in favor of continuing to inaugurate our presidents on the 
fourth of March there is little to recommend the day in past experience 
of weather conditions and in the small chances for a favorable day. The 
day falls within a period of rapid warming up in the northern hemi- 
sphere, but the advancing sun has not yet carried the day beyond the 
realm of freezing weather. The early morning temperatures are nearly 
always below 32°, while the frequent incursions of cold air from the 



MARYLAND WEATHER SERVICE 429 

north and west bring even the average heat of the day close to the freez- 
ing point on most occasions. 

Added to the discomforts of a raw cold atmosphere we have the pro- 
verbial March wiads, not infrequently combined with sleet or rain or 
snow, or a combination of all of these disagreeable elements. 

The probability for a fine day is so small that it is surprising that the 
efforts to change the date of inauguration to the latter part of the fol- 
lowing month have not yet been successful. 

THE WEATHER OF MARCH 4. 

The condition of the weather on the 4th of March is graphically repre- 
sented in the accompanying diagram for each year since 1871. It might 
have added interest to carry the diagram back to an earlier date but the 
period of 37 years represents practically all the chief combinations likely 
to have occurred in the longer period. While the conditions charted 
represent Baltimore weather, the close proximity of Washington makes 
it improbable that there were at any time any material differences in 
the weather conditions of the two cities. There is most certainly no 
difference in the variability of the elements. 

The average daily temperature on the 4th of March is not far from the 
point of frost formation. With the usual daily range the fluctuations 
will be above and below the frost line. When precipitation takes place 
it is apt to change from rain to snow or from snow to rain, with the 
intermediate stage of sleet. 

In 1873 the temperature fell to 5° above zero in the early morning of 
the 4th; on the following 4th of March, 1874, the afternoon temperature 
registered 68° above. In 1880 the temperature rose as high as 74°. 
Between these widely separated limits the temperature has kept up a 
continual see-saw about the middle point of 38°. 

In the past 37 years rain, snow, or sleet has fallen on 16 occasions, 
just 46 per cent of the days. The average amount of precipitation is 
about half an inch and the average duration about 9 hours. The sky 
has been overcast 10 times, partly clouded 15 times, and clear 11 times. 
The prevailing winds have been from the west and northwest. 





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MARYLAND WEATHER SERVICE 431 

THE WEATHER OF MARCH 4. 



Year. 


Max. 


Min. 


Mean 


Character 


Wind 


Daily Wind 


Precip- 


Temp. 


Temp. 


Temp. 


of Day. 


Direction. 


Movement. 


itation. 




(De 


grees Fah 


r.) 






(Miles) 


(Inches) 


1871 


44 


39 


41 


Pt. cldy 


N 




0.51 


1872 


44 


IS 


31 


Clear 


SW & NW 


106 




1873 


21 


5 


13 


Pt. cldy 


NW 


362 




1874 


68 


42 


55 


Pt. cldy 


NW 


188 


0.21 


1875 


35 


27 


31 


Clear 


NW 


197 




1876 


42 


27 


34 


Pt. cldy 


SE 


138 




1877 


57 


33 


45 


Pt. cldy 


NW 


137 


0.09 


1878 


53 


35 


44 


Pt. cldy 


NW 


206 




1879 


45 


26 


36 


Cloudy 


NE 


76 


0.02 


1880 


74 


48 


62 


Pt. cldy 


W 


223 


0.01 


1881 


38 


32 


35 


Pt. cldy 


w 


331 


1.13 


1882 


57 


40 


48 


Clear 


NW 


141 




1883 


43 


28 


36 


Pt. cldy 


NW 


218 




1884 


32 


18 


25 


Clear 


NW 


214 




1885 


53 


24 


44 


Cloudy 


E 


95 




1886 


42 


29 


36 


Pt. cldy 


NW 


236 




1887 


36 


27 


32 


Cloudy 


NE 


117 


0.45 


1888 


35 


24 


30 


Clear 


NW 


198 




1889 


44 


36 


40 


Cloudy 


NE&NW 


309 


2.71 


1890 


48 


27 


38 


Clear 


NE 


133 




1891 


42 


30 


36 


Pt. cldy 


NW 


206 


6.18 


1892 


55 


35 


45 


Cloudy 


NW 


181 




1893 


31 


24 


28 


Pt. cldy 


NW 


438 


0.18 


1894 


56 


31 


44 


Clear 


SE 


110 




1895 


55 


36 


46 


Pt. cldy 


SW 


306 


0.02 


1896 


42 


23 


32 


Clear 


N 


458 




1897 


47 


35 


41 


Clear 


E 


117 




1898 


40 


35 


38 


Cloudy 


N 


205 


0.13 


1899 


44 


36 


40 


Cloudy 


E 


171 


0.07 


1900 


53 


31 


42 


Cloudy 


SW 


65 




1901 


50 


35 


42 


Cloudy 


NW 


128 


0.21 


1902 


43 


32 


38 


Cloudy 


W 


108 


0.03 


1903 


54 


35 


44 


Pt. cldy 


SE 


84 




1904 


35 


24 


30 


Clear 


NW 


244 




1905 


47 


27 


37 


Pt. cldy 


N 


250 


0.01 


1906 


54 


38 


46 


Pt. cldy 


NW 


340 




1907 


39 


24 


32 


Clear 


NW 


185 




Means 


45 


31 


38 


Pt. cldy 




175 


0.53 



432 THE CLIMATE OF BALTIMORE 

THE WEATHER OF MAY 1. 

The closing days of April and the first days of May are among the 
pleasantest of the year in many respects. The temperature is well above 
the frost line — the mean temperature for the first of May is 59°. The 
early morning temperatures are more constant than in March or April, 
the minimum averaging about 50°. The maximum has gone as high as 
85°, but it has generally been below 70°. The winds are light and 
largely from a southerly or easterly direction. (See Fig. 152.) 

The rainy days are less frequent than earlier in the season; there are 
few days in the year with a smaller rainfall probability (see Plate IX). 
The duration of rainfall is about 7 hours as compared with 9 hours in 
March and 10 to 12 hours in the winter months. The average amount 
of rainfall is also quite small compared with that of other days. 

The chances for fine weather — a moderate temperature and without 
rain — are better for the 30th of April and the 1st of May than for any 
period of the year, excepting the first week in September and the middle 
of October. 

The weather conditions on the 4th of March and on the first of May 
represent with a fair degree of accuracy the conditions of the early and 
late spring respectively. They represent sharp contrasts — the former 
showing all the characteristics of our variable winter climate, while the 
latter resembles closely the more uniform and settled conditions of our 
summers. 

THE WEATHER OF EASTER SUNDAY. 

The weather of this day has been prevailingly cloudy to partly cloudy, 
with a moderate westerly wind. Eain has occurred on 14 of the 37 
anniversaries from 1871 to 1907, and was light in amount on all but 
one occasion. With an average temperature of 52°, the extremes have 
ranged between 84° in 1887 and 28° in 1874. 



434 



THE CLIMATE OF BALTIMORE 



THE WEATHER OF MAY 1. 



Year. 

1871 


Max. Min. 
Temp. Temp. 
(Degrees Fahr.) 
67 58 


Mean 
Temp. 

62 


Character 
of Day. 

Cloudy 


Wind 
Direction. 

E 


Daily AVind 
Movement. 

(Miles) 


Precip- 
itation. 
(Inches) 


1872 


73 


59 


66 


Cloudy 


SE 


175 




1873 


59 


47 


53 


Cloudy 


S 


127 


0.24 


1874 


70 


47 


58 


Cloudy 


W 


184 




1875 


61 


47 


54 


Cloudy 


E 


214 


0.01 


1876 


61 


34 


48 


Clear 


NW 


324 




1877 


53 


44 


48 


Cloudy 


NW 


109 




1878 


80 


52 


66 


Pt. cldy 


SB-SW-W 


82 




1879 


65 


45 


55 


Pt. cldy 


NW 


249 




1880 


61 


38 


50 


Clear 


NW 


245 




1881 


67 


46 


56 


Clear 


SE 


175 




1882 


68 


47 


58 


Clear 


W&NW 


131 




1883 


60 


45 


52 


Pt. cldy 


SE 


125 




1884 


74 


57 


66 


Pt. cldy 


SE&S 


155 




1885 


62 


48 


55 


Cloudy 


NE 


97 


0.23 


1886 


58 


46 


52 


Cloudy 


NE 


163 




1887 


70 


52 


61 


Pt. cldy 


SE 


110 




1888 


76 


54 


65 


Pt. cldy 


NW 


151 


0.04 


1889 


54 


48 


51 


Pt. cldy 


N&SW 


100 


0.23 


1890 


87 


58 


72 


Pt. cldy 


S&NE 


144 


0.30 


1891 


80 


64 


72 


Clear 


W 


161 




1892 


77 


49 


63 


Clear 


SE&S 


293 




1893 


64 , 


49 


56 


Cloudy 


E 


194 


0.02 


1894 


80 


50 


65 


Clear 


SW 


124 




1895 


57 


54 


56 


Pt. cldy 


NE 


353 


0.12 


1896 


54 


50 


52 


Cloudy 


SE 


225 




1897 


69 


57 


63 


Cloudy 


E 


287 


0.94 


1898 


79 


50 


64 


Pt. cldy 


SE&NW 


73 




1899 


84 


58 


71 


Clear 


SE 


101 




1900 


77 . 


53 


65 


Pt. cldy 


W 


81 


... 


1901 


78 


51 


64 


Pt. cldy 


E 


171 




1902 


77 


56 


66 


Clear 


NW 


187 




1903 


69 


44 


56 


Clear 


NW 


306 




1904 


72 


49 


60 


Pt. cldy 


NW 


136 




1905 


62 


48 


55 


Clear 


NW 


198 




1906 


75 


59 


67 


Cloudy 


N 


140 


... 


1907 
Means 


63 
68 


52 
50 


58 
59 


Cloudy 
Pt. cldy 


N 


189 
174 


0.04 
0.22 



MARYLAND WEATHER SERVICE 



435 



THE WEATHER OF EASTER SUNDAYS. 



Year. 


Date. 


Max. 
Temp. 


Min. 
Temp. 


Mean 
Temp. 


Character 
ot Day. 


Wind 
Direc- 
tion. 


Daily Pre- 
Wind cipita- 
Movement. tion. 








(Degrees Fahr.) 






(Miles) (Inches) 


1871 


April 


9 


82 


73 


78 


Pt. cldy 


W 






1872 


March 


31 


64 


44 


54 


Cloudy 


W 


183 


0.32 


1873 


April 


13 


60 


42 


51 


Pt. cldy 


NW 


244 


0.01 


1874 


April 


5 


42 


28 


35 


Cloudy 


SE 


174 


0.10 


1875 


March 


28 


54 


35 


44 


Pt. cldy 


S 


137 




1876 


April 


16 


65 


49 


57 


Cloudy 


SW 


212 




1877 


April 


1 


59 


42 


50 


Cloudy 


SE 


116 


0.02 


1878 


April 


21 


79 


60 


70 


Clear 


NW 


156 




1879 


April 


13 


58 


35 


46 


Pt. cldy 


SE 


157 




1880 


March 


28 


59 


38 


48 


Cloudy 


NW 


99 


0.06 


1881 


April 


17 


64 


44 


54 


Pt. cldy 


W 


227 




1882 


April 


9 


56 


49 


53 


Cloudy 


S 


97 


0.38 


1883 


March 


25 


46 


30 


38 


Cloudy 


SE 


22 




1884 


April 


13 


55 


47 


51 


Pt. cldy 


SW 


89 


0.15 


1885 


April 


5 


60 


35 


48 


Pt. cldy 


SW 


241 




1886 


April 


25 


80 


54 


67 


Pt. cldy 


NE 


142 


0.01 


1887 


April 


10 


84 


46 


Go 


Clear 


NW 


102 




1888 


April 


1 


58 


43 


50 


Pt. cldy 


SE 


160 




1889 


April 


21 


80 


60 


70 


Clear 


NW 


172 




1890 


April 


6 


62 


36 


49 


Clear 


SW 


115 




1891 


March 


29 


50 


38 


44 


Clear 


N 


201 




1892 


April 


17 


58 


42 


50 


Pt. cldy 


NE 


158 




1893 


April 


2 


62 


47 


54 


Clear 


NW 


266 




1894 


March 


25 


45 


39 


42 


Cloudy 


N&NW 


137 


O.Ol 


1895 


April 


14 


59 


43 


51 


Clear 


N 


178 




1896 


April 


5 


55 


33 


44 


Clear 


NW 


209 




1897 


April 


18 


61 


41 


51 


Pt. cldy 


W 


103 




1898 


April 


10 


63 


47 


55 


Pt. cldy 


E 


86 


0.01 


1899 


April 


2 


43 


31 


O 1 


Pt. cldy 


W 


146 




1900 


April 


15 


65 


40 


52 


Clear 


SW 


75 




1901 


April 


7 


56 


46 


51 


Cloudy 


W 


154 


0.02 


1902 


March 


30 


62 


45 


54 


Pt. cldy 


W 


118 


0.24 


1903 


April 


12 


55 


48 


52 


Cloudy 


E 


175 




1904 


April 


3 


46 


33 


40 


Cloudy 


NW 


295 




1905 


April 


23 


64 


45 


54 


Clear 


N 


172 




1906 


April 


15 


67 


51 


59 


Cloudy 


NW 


183 


1.07 


1907 


March 


31 


56 


42 


49 


Cloudy 


N 


168 


0.03 



Means 



60 



43 



52 



Pt. cldy 



157 



0.24 



436 THE CLIMATE OF BALTIMORE 

SUMMER WEATHER. 

As the spring advances, atmospheric movements on a large scale become 
more sluggish. Well defined cyclones and anti-cyclones are of less fre- 
quent occurrence and less intense in their development. This is due, 
doubtless, to the decreasing contrasts in temperature between north and 
south, and between the oceans and the continents. Attention has already 
been called to the relatively great differences in temperature between 
Florida, for instance, and Montana, in the winter months, compared 
with the differences in the summer months, an average difference of 
about 75° in January increasing to 100° at times, as compared with 
about 30° in July. 

With the northward movement of the sun the whole atmosphere of 
the northern hemisphere rapidly rises in temperature during the day. 
At the same time the days become longer, and the nights shorter; the 
loss of heat during the long winter nights over the continental masses 
becomes steadily less. 

With the increasing heat of the summer the mass of the air over the 
continents becomes specifically lighter than that over the oceans. The 
general surface circulation of the air between continents and oceans is 
reversed. In the winter time the general drift at the surface is from 
continents to oceans, in the summer time from the oceans to the conti- 
nents. As the winter area of high pressure over the northern-central 
portion of the JSTorth American continent diminishes in strength, the 
Atlantic high area increases in extent and intensity. With its center 
usually over the Azores, it extends westward across the Atlantic Ocean 
to the South Atlantic states in the summer time. With this change in 
the distribution of atmospheric pressure from winter to summer there 
is a change in the prevailing wind direction. Maryland, in common with 
all of the Middle Atlantic states, has a prevailing west to northwest wind 
in the" winter months, the air blowing out of the continental high area; 
in the summer months the prevailing direction is southeast or southwest, 
coming from the Atlantic high area to the southeast of the Middle 
Atlantic states. 



MARYLAND WEATHER SERVICE 437 

In the summer season the paths of the centers of cyclones are con- 
fined mostly to the northern tier of states, the Lake region and the St. 
Lawrence Valley. Hence marked cyclonic changes in temperature are 
infrequent in the states farther south. The distinguishing feature of 
the temperature changes is the diurnal variation, the difference between 
the early morning and the afternoon readings of the thermometer. In 
the winter and early spring months the irregular cyclonic changes are 
far greater than the diurnal change. This conspicuous prominence of 
the diurnal fluctuation, which is characteristic of tropical climates, is 
not confined to temperature; it also appears in the wind direction, the 
rainfall and in local storm frequency. 

The increasing magnitude of the diurnal period in the summer months 
at Baltimore has already been discussed in considerable detail in the 
first part of the report. Further attention will be directed to character- 
istic weather types of the summer season in the following pages, especially 
to conditions which give rise to local storms, such as thunderstorms, 
squalls, tornadoes, and to hot spells. 

Summer Storms. 
A glance at Fig. 77 and Fig. 78, on page 217, will at once reveal the 
existence of an intimate connection between high temperatures and the 
occurrence of thunderstorms. In our latitudes these turbulent atmos- 
pheric disturbances become more and more frequent with the northward 
movement of the sun, increasing steadily from December to July, and 
then rapidly decreasing to December. The following figures show the 
annual average frequency for tlie vicinity of Baltimore : 

THUNDERSTORMS RECORDED AT BALTIMORE (187G-1904). 

Jan. Feb. Mar. Apr. Maj-. June. July. Aug. Sept. Oct. Nov. Dec. Year. 

13 11 20 33 107 156 17D 111 43 7 R 2 678 

Over 80 per cent of the total annual number of these storms occur dur- 
ing the hot season — May to August — and including September and April, 
the total frequency for the summer half year amounts to 92 per cent, 
leaving but 8 per cent for the winter half year. The same intimate 
connection with change in temperature is shown in the diurnal period 
of thunderstorm occurrence. (See page 276.) 



438 THE CLIMATE OF BALTIMORE 

HOURLY FREQUENCY OF THUNDERSTORMS. 

Hoursonding 1 3 3 4 5 6 7 8 9 10 11 13 

A. M 7 1 3 7 3 6 6 8 9 15 

P. M 3;.' 46 76 78 61 65 69 38 34 31 16 13 Total 610 

More than two-thirds of all thunderstorms recorded at Baltimore in 
28 years began during the seven hours from noon to seven p. m. Many 
of the storms occurring later than 7 p. m. had their origin in the early 
afternoon hours and were carried eastward several hours before being 
dissipated. 

When the thunderstorm does occur in the cold season it is in connec- 
tion with a relatively warm inflow of air towards the center of a cyclone. 

During the summer months many thunderstorms occur in the absence 
of any well defined general cyclonic depression. During a period of 
abnormally high temperatures due to bright sunshine and a sluggish 
wind circulation, the lower layers of the atmosphere are excessively 
heated, resulting in a marked disturbance in the normal rate of decrease 
of heat with elevation. A brisk vertical circulation is set up, which, in 
the presence of a high humidity, results in rapid cooling of the ascending 
air, and formation of cumulus clouds. As this circulation increases in 
energy the cloud soon developes into a " thunder head " with its accom- 
paniment of heavy rain, lightning, and thunder. Hence dynamic cool- 
ing of warm moist convection currents is the chief cause of thunder- 
storms of the summer season. When these storms occur in connection 
with a general cyclone the mass of warm moist air which produces the 
thunder cloud is mechanically forced upward in addition to rising as a 
convection current. The thunderstorms occurring in connection with a 
general cyclone are likely to be of wider extent than those due to con- 
vection currents alone, and the cool air following the storm is apt to be 
more lasting. The fall in temperature following the local heat thunder- 
storm is usually of brief duration. 

THE THUNDERSTORM OF JULY 20, 1902. 

On July 20, 1902, a thunderstorm of unusual severity passed over 
Baltimore. Twelve lives were lost, while several hundred houses were un- 
roofed or otherwise seriously damaged, involving a loss of over $200,000. 



MARYLAND WEATHER SERVICE 439 

The wind attained a velocity seldom equalled in the annals of Baltimore 
weather. 

On tlie morning of the 20th the weather chart of the United States 
Weather Bureau showed an area of moderately low pressure over the 
Lower Lake region, the Middle Atlantic and Southern New England 
states, with the center of the depression over Lake Erie and Southern 
Michigan at 8 a. m. Cloudy and rainy weather prevailed over a wide 
area about the center of the oval depression. A well defined area of 
high pressure covered the country between the Mississippi Eiver and the 
Eocky Mountains. By 8 p. m. the barometric depression had moved 
eastward with its center over Pennsylvania, the isobars meanwhile becom- 
ing nearly circular. The weather from the Mississippi Eiver to the 
Atlantic coast and from the Lake region to the Gulf of Mexico was in an 
unsettled condition, thunderstorms occurring during the day at nearly 
every reporting station of the Weather Bureau in the Middle Atlantic, 
the South Atlantic and Gulf states, and the Lake region, accompanied 
in most cases by light rains. The chart for 8 p. m. shows the general 
weather conditions within an hour or two after the occurrence of many 
of the local storms in the Middle Atlantic states. (See Fig. 153.) 

The progressive changes in the elements as recorded at Baltimore 
during the day were particularly interesting and instructive. The baro- 
meter slowly and steadily fell during the forenoon; the wind blew from 
the southwest with a velocity of only 4 to 8 miles until 8 a. m., then 
increased steadily to 15 or 16 miles per hour by 10 a. m. The tempera- 
ture rose rapidly from a minimum of 72° at 6 a. m. to a maximum of 
94° at 1 p. m. The sky was overcast during the night, but there was 
considerable sunshine from 7 a. m. until 1 p. m., when a sheet of stratus 
clouds appeared in the west, intensely dark and advancing rapidly 
towards the zenith. At 1.25 p. m. small torn cumulus clouds passed 
rapidly southwest to northeast in advance of the cloud of dust. This 
was followed by a stratus layer which by 1.30 p. m. covered the entire 
sky. At 1.45 p. m. the stratus clouds, moving southwest to northeast, 
began to break away. Between the open spaces strato-cunuilus clouds 
were visible above moving from west to east. (See Fig. 154.) 

29 



440 



THE CLIMATE OF BALTIMORE 



The first peals of thunder were heard at 1.23 p. m., coming from the 
southwest. The electrical display was brilliant during the height of the 
storm. The last thunder was heard at 2.25 p. m. Light rain began at 
1.27 p. m., changing to a heavy shower at 1.29 p. m. ; the rain moved in 
dense sheets from southwest to northeast. In the meantime the wind 
was increasing in velocity; at 1 p. m. it registered 17 miles per hour, 




Fig. 153.— The Thunderstorm of July 20, 1902. 



remaining at this velocity until 1.20 p. m. In the next five minutes the 
velocity suddenly increased to 46 miles, and at 1.31 p. m. it blew at the 
excessive rate of 75 miles per hour from the west. Coincident with the 
sudden increase in the wind velocity the pressure rose nearly a tenth of 
an inch in the course of a few minutes, while the temperature fell as 
rapidly from 94° to 69° and the rain fell in torrents for a few minutes. 
The wind rapidly fell and by 2 p. m. had regained its early morning 
velocity of 5 to 6 miles per hour. The pressure regained in half an 



MARYLAND WEATHER SERVICE 



441 



hour the height at which it stood before the sudden rise, and then 
continued to fall slowly until about 8 p. m. The temperature rose 
rapidly after the sudden fall, recording 84° at 3.30 p. m., maintaining 



MDT. 



6 A. 



NOON 



6 P. 



MDT. 




Fig. 154.— The Thunderstorm of July 20, 1902. 

this reading approximately until G p. m., when it fell rapidly to 76° 
shortly after 8 p. m. 

An hour or so before the sudden fluctuations in wind velocity, tem- 
perature, and pressure recorded above, the wind veered from southwest to 



443 



THE CLIMATE OE BALTIMORE 




MARYLAND WEATHER SERVICE 443 

west. During the afternoon and evening the direction alternated fre- 
quently between the west and southwest. 

The storm was of short duration; the interval between the first and 
last thunder heard being about an hour. Its greatest intensity was 
reached within 20 minutes after the first peal of thunder was heard. 
The rainfall began at 1.27 p. m. and ended at 1.55 p. m., with a total 
precipitation of a little over half an inch. 

During the morning, cirrus, cirro-stratus, and alto-cumulus clouds 
were observed passing across the sky from west to east with considerable 
rapidity. A cloud of dust preceded the thunderstorm, carrying leaves, 
paper and other light objects high up into the air. Just preceding and 
during the storm the humidity was fully 40 per cent higher than at 
the 8 a. m. observation, ^o portion of the city was free from damage 
caused by the storm, although north Baltimore seemed to have sufi'ered 
less severely than other sections. 

The storm described above caused considerable damage in all parts of 
Maryland, though most of the loss of life and property occurred in the 
vicinity of Baltimore. A special effort was made at the time to trace the 
path of the storm across the state. Co-operative observers in Maryland, 
Virginia, West Virginia, and Delaware were requested to report accurately 
ihe time of day when the first thunder was heard in their respective 
localities. Eeplies were received from about 150 observers, making it 
possible by charting the recorded times upon a map and joining, by a 
line, localities over which the storm passed at about the same hour, to 
follow the storm from West Virginia eastward to the Atlantic coast. 

The accompanying chart shows the hourly rate at which the storm 
travelled. In Central West Virginia the storm began at 10 a. m. From 
West Virginia it passed into Maryland by way of Washington county 
between noon and 1 p. m. It then advanced with an irregular wave 
front eastward to the Chesapeake Bay by 2 p. m. (See Fig. 155.) 

The storm front then moved in a southeast direction, passing beyond 
the limits of Maryland in Worcester County between 6 p. m. and 7 
p. m. The total distance traversed by the storm from 10 a. m. to 6 
p. m. was about 200 miles, or at the rate of 25 miles per hour. The 



444 



THE CLIMATE OF BALTIMORE 



irregular form of the storm front and its var3dng rate of progress across 
the state are clearh^ shown in the chart (Fig. 153). 

THE THUNDERSTORM OF JULY 3, 1902. 

At 8 p. m. of July 3, 1902, a moderate barometric depression was 
centered over Lake Ontario, while the western edge of an extensive area 
of high pressure covered the South Atlantic states and extended far out 




Fig. 156.— The Thunderstorm of July 3, 1902. 

over the Atlantic Ocean. The depression was attended by moderate 
rains in New England, the Lake region, and the Middle Atlantic states. 
In the vicinity of Baltimore, as shown by official records, the day was 
partly cloudy and oppressive; a film of cirro-stratus covered the sky from 
early morning, through which the sun shone with great intensity. At 
4 p. m. stratus clouds were observed moving rapidly from the north and 
northeast. During the morning and early afternoon a light to fresh 



MARYLAND WEATHER SERVICE 



445 



breeze blew from the southwest; at 3.15 p. m. the direction of the wind 
changed to west, and its force increased to brisk for a brief time. B}- 
4 p. m. the velocity of the wind began to increase rapidW, the clouds 



MDT, 



6 A. 



NOON 



6 P. 



MDT 




Fig. 1.37.— The Thunderstorm of July 3, 1902. 



maintaining their original direction. At 4.25 p. m. the wind shifted 
to the northwest, blowing with increasing velocity, and attaining a maxi- 
mum rate of 34 miles per hour at 4.29 p. m. At the same time great 
numbers of cumulus clouds were rapidly carried across the sky from the 



446 THE CLIMATE OF BALTIMORE 

north-northwest. The storm front moved with great rapidity to the 
southeast, the usual dust cloud marking the advance. The squall-wind car- 
ried light objects high into the air. A number of lives were lost during 
the squall, while considerable property was damaged, and many trees 
were uprooted. 

Eain began at 4.35 p. m. and continued until 4.50 p. m., the amount 
being 0.04 inch. On the arrival of the stormfront, marked changes in 
the barometer and thermometer were noted. (See accompanying diagram.) 

The barometer fell rapidly throughout the day until shortly after 4 
p. m., while the thermometer rose from 66° at 5 a. m. to 96° at 4 p. m. 
With the sudden change of wind at 4.30 p. m. from southwest to west and 
northwest, there was an abrupt rise of nearly a tenth of an inch in the 
barometer. The temperature fell as abruptly from 96° to 74°, while 
the wind rose from 12 miles to 32 miles per hour. The change was 
accompanied by a sharp shower of rain of a few minutes' duration. 

The barometer lost a part of the sudden rise during the following 
hour and then continued to rise slowly during the balance of the day. 
The temperature remained low after the storm. No thunder and light- 
ning were noted in connection with this storm while it passed over Balti- 
more. Electrical displays were, however, reported from many parts of 
the state on this da3^ The progressive changes were all characteristic 
of a well defined thunderstorm. 

THE THUNDERSTORM OF JULY 12, 1904. 

There is a type of pressure distribution which invariably gives rise to 
numerous and severe thunderstorms and squalls. It is represented in the 
accompanying chart showing the general weather conditions at 8 p. m. 
of July 12, 1904. It is the V-shaped depression referred to in a preced- 
ing paragraph in connection with a discussion of storm types. In this 
instance the " squall line " is particularly well defined by a long narrow 
band of thunderstorms and rain at or near 8 p. m., extending from the 
St. Lawrence Valley southward through New York, New Jersey, Eastern 
Pennsylvania, Eastern Maryland, Delaware, and along the coast south- 
ward to Florida. (See Fig. 158.) 



MARYLAND WEATHER SERVICE 



447 




Fig. 158.— The Thunderstorm of July 12, 1904. 



THE TORNADO OF JULY 12, 1903. 

The Middle Atlantic states are rarely visited by tornadoes. There are 
descriptions of such storms on record in the annals of Baltimore weather, 
but the storms were of a mild type of tornado so far as can be judged 
by local descriptions. Tornadoes occur under general conditions similar 
to those which give rise to thunderstorms and squalls. They differ, 
however, from the latter in the character of the atmospheric circulation 
within the storm, in their greater destructiveness, and in the fact that tliey 
are more restricted in the area of their activity. The air within a 
thunderstorm moves about a horizontal axis, while within a tornado the 
circulation is about a vertical axis. The thunderstorm moves eastward 
with the general cyclone with a long wave front many miles in length, 
the tornado moves along with the general storm in the form of a vertical 
column of limited extent, generally less than half a mile in diameter. 



448 



THE CLIMATE OF BALTIMORE 



HIGH 




Fig. 159.— The Tornado of July 12, 1903 (8 a. m.). 




Pig. 160.— The Tornado of July 12. 1903 (8 p. m.). 



MARYLAND WEATHER SERVICE 449 

usually recognizable as a downward extension of the cloud mass which 
generally reaches to the ground, but sometimes dangles in mid-air like 
the loose end of a suspended rope. 

The storm of July 13, 1903, as it passed over Baltimore, had, from the 
best information obtainable from eye witnesses, many of the traits of 
the real tornado, although it is frequently difficult to distinguish between 
a mild type of tornado and an intensely developed thunderstorm. 

The general weather conditions were favorable for the production of 
local storms over a large portion of the Atlantic and Gulf Coast states 
and the Ohio Valley. Cloudy and unsettled weather prevailed in the 
sections named at 8 a. m. of the 12th. Thunderstorms were reported 
from many stations for the preceding twelve hours. There was an area 
of high pressure in the northwest, and a barometric depression over the 
Gulf of St. Lawrence, with a secondary depression forming over the 
Lower Mississippi Valley. The temperature conditions were nearly nor- 
mal, but the humidity was high. The prevailing wind direction at 
stations in the Atlantic Coast states was from the southwest and light 
in force, excepting in the South Atlantic states, where they were fresh 
to brisk. During the succeeding 24 hours the secondary depression had 
developed and moved rapidly northeastward over Maryland, the center 
being over Massachusetts at 8 a. m. of the 13th, accompanied by heavy 
rains and severe local storms in the South Atlantic states, and near the 
coast in the Middle Atlantic and New England states. The following 
heavy rainfalls were reported for the preceding 24 hours at 8 a. m. of 
the 13th: Baltimore, Md., 3.98 inches; Washington, D. C, 3.02 inches; 
Atlantic City. N". J., 1.74 inch. (See Figs. 159 and 160.) 

The records of the local office of the United States Weather Bureau 
contain the following account of the storm as recorded by the official 
and other observers: 

On July 12 thunderstorms and heavy rainfall were general throughout the 
section. At Baltimore the storm at its height developed destructive features 
over a limited area. A funnel-shaped cloud, peculiar to the tornado was 
clearly in evidence a few minutes after noon, and the narrow path pursued 
by this cloud was also the path of devastation. The cloud moved from west 
to east, descending to the house-tops at two points within the city, leaving a 



450 THE CLIMATE OF BALTIMORE 

wide gap of comparatively slight loss between; it evidently struck the ground 
again a short distance beyond the city proper, judging from the local damage 
there, and then disappeared, as far as surface traces were concerned. Re- 
ports from parts of Kent County, however, would indicate that the storm 
crossed the Bay and moved over the Eastern Shore, for in that county a nar- 
row area was visited by destructive winds. 

The following is the special report of Mr. James S. Harris, the co-operative 
observer at Coleman, regarding this visitation: "About 1 p. m. an angry 
black cloud came suddenly over, causing a darkness as of twilight, accom- 
panied by a cyclone and hail. Wheat in shock and stack was blown about, 
trees blown down, and houses wrecked." In his use of the word " cyclone " 
the writer doubtless intended to describe a tornado, a confusion of terms so 
frequently met with in popular accounts of storms of this class. Baltimore 
and Coleman seem to have marked the extreme limits of tornado winds, 
although the thunderstorms were more or less severe at many other points 
on the same day. 

In Baltimore the first area visited embraced much of the 1700 blocks of 
Fulton Avenue, Mount Street, and Calhoun Street; here a funnel-shaped 
cloud was distinctly observed by a number of the residents, but no definite 
account of its manner of formation was obtained beyond this. In the second 
district, which extended from Eager Street and Broadway eastward for six 
blocks, with a width varying from two blocks to less than a block, the damage 
was greater and the information obtained was more explicit. A heavy storm 
cloud approached from the northwest and another from the southwest; they 
apparently merged at Eager Street and Broadway, where the destruction 
abruptly began. The funnel-shaped cloud was seen by many, and a loud 
roaring sound was followed by almost complete darkness as the storm burst. 
The upper cloud mass was distinguishable, however, with its narrowing 
extension downwards, the latter appearing to lag slightly behind the mass 
above in its movement eastward. The whole travelled with almost incredible 
velocity, only a few seconds elapsing between the time the cloud descended 
to the house-tops at Eager Street and Broadway and the time when it rose 
into the air again six blocks to the eastward. 

In both districts the nature of the destruction pointed clearly to the fact 
that the city had been visited by a tornado. In some of the wrecked houses 
the walls were blown outward, as though by sudden expansion of confined 
air within, although fully as many fell inward. In one case the four walls 
had bulged outward, and the roof lay within, about half-way down to the 
fioor of the second story, while not far off roofs had been lifted high into the 
air and carried a block and a half away before being deposited in an alley 
to the rear. In all several hundred houses were unroofed or otherwise badly 
wrecked. The money loss was estimated at $200,000; happily there was no 
loss of life, although one man was seriously hurt by falling walls, and nu- 
merous narrow escapes from injury were reported. 

At the Weather Bureau Office, about a mile and a half away, no damage 
occurred. The self-registering instruments, while presenting interesting 
records, do not adequately portray the conditions as they existed at the 



MARYLAND WEATHER SERVICE 



451 



centers of severe damage. The rainfall at the station was unusually heavy; 
2.87 inches fell in 33 minutes, from 12.04 p. m. to 12.37 p. m. The following 
maximum falls were tabulated: 

Greatest amount 



ount in 5 


minutes, 


O.SO inc 


" 10 




1.35 ' 


" 15 




1.92 ' 


" 20 




2.22 ' 


" 25 




2.46 ' 


" 30 




2.75 ' 


" 35 




2.87 ' 



Further details of rainfall in connection with this storm are given on 
pages 212 and 213. The rate of rainfall in the districts of greatest storm loss 
must have been much heavier. The streets were running streams of water, 
and cellars were entirely filled within a few minutes. 

At the Baltimore station the wind was comparatively high from 12.04 
p. m. to 12.15 p. m., and brisk to light thereafter. The maximum velocity 
was 46 miles per hour at about 12.05 p. m. The wind direction veered 
through nearly all of the points of the compass during the storm, as shown 
by the following record: 

Noon to 12.05 p. m Southwest. 



12.05 • 


' 12.15 


12.15 ' 


' 12.20 


12.20 ' 


' 12.25 


12.25 ' 


' 12.45 


12.45 ' 


' 1.00 



.West (mostly). 
.Northwest. 
.North. 
.Northeast. 
.East (mostly). 



There was a sharp fall of about 15° in temperature at the beginning of 
the storm, but at the office of the Weather Bureau the variation in atmos- 
pheric pressure was very slight. The only noteworthy feature of the pressure 
curve was a small but sudden rise of about 0.05 inch, characteristic of severe 
thunderstorms accompanied by hail. 

The general weather conditions on the day of the storm are recorded 
as follows in the local office of the United States Weather Bureau : 

Cloudy day. Not so warm. Atmosphere very oppressive in forenoon; 
pleasant afterwards. Maximum, 85° at 11 a. m.; temperature then fell to 
70° at noon, rose to 74° at 4 p. m., fell to 69° at 8 p. m., and remained 
stationary until midnight. Sky partly covered at dawn, became overcast by 
9 a. m. with alto-stratus clouds. At 11 a. m. a dark low-lying cloud mass 
appeared on the northern and western horizons, moving slowly. Shortly 
before noon, the movement of the cloud mass increased very rapidly, and the 
sky became covered in a few minutes, continuing so the rest of the day. 
This movement of the clouds was followed by a terrific thunderstorm, thunder 
being first heard at 11.48 a. m., continuing all the afternoon at intervals, 
being last heard at 6.45 p. m., becoming recognizable as a second storm at 



452 THE CLIMATE OF BALTIMORE 

about 6 p. m. The first storm moved from west to east, the second passed 
from south to north. A trace of rain fell in the early morning. The periods 
of rainfall during the day were as follows: 

12 noon to 1.10 p. m. 

1.45 p. m. to 2.10 p. m. 

2.50 p. m. to 3.20 p. m. 

4.45 p. m. to 5.05 p. m. 

5.40 p. m. to 7.50 p. m. 

9.25 p.m. to 9.40 p.m. 

The rainfall was excessive from 12.07 p. m. to 12.42 p. m. (2.87 inches), 
and heavy from 6.20 p. m. to 6.40 p. m. (0.72 inch) ; the total amount for the 
day was 3.90 inches. A light southeast wind before noon shifted suddenly to 
west at noon with increased force, being brisk to high from 12.02 p. m. until 
12.27 p. m., with a maximum velocity, at the station, of 46 miles from the 
west at 12.07 p. m. The winds were light and variable the rest of the day, 
mostly from the north. 

WATERSPOUTS. 

Waterspouts are in their mode of formation and in their external 
appearance similar to tornadoes. In extent, however, they are much more 
restricted, while they do not compare with the tornado in destructive 
power. They are of comparatively infrequent occurrence and it is not 
often that they are observed at close range by an intelligent observer, 
hence the following description is of special interest: 

Early in April, 1902, Captain Fergus Ferguson of the British S. S. Hestia 
left Baltimore for one of the Cuban ports. On April 4, towards sunset, while 
off Hatteras, the Captain observed several waterspouts in process of forma- 
tion at a distance of 300 to 400 yards to windward. The largest of these, 
and the only one completely formed, seemed to be headed directly for the 
Hestia. The Captain at first attempted to change his course sufficient to 
avoid running into it, but soon discovered that this could not be done. 
Giving orders for all on deck to go below, he remained until the spout was 
close upon his ship, and then hastily sought a place of safety. In a moment 
he heard a deafening roar which was quickly followed by strong gusts of 
wind and a sudden shock as the spout struck amidships and passed over the 
deck towards the stern. The Captain reappeared upon deck in time to see 
two tarpaulins, which had covered the hatches, and a plank 8 feet long by 
10 inches wide, high in the air, while his log line with log attached extended 
straight up into the air to a distance of about 40 feet. Beyond the loss of 
the lighter movable objects on deck and a temporary feeling of apprehension 
no harm was done. 



MARYLAND WEATHER SERVICE 453 

When first seen, the waterspout was incomplete. A portion of the cloud 
dipped down from the general cloud level of about 2000 feet, while at the 
same time a column of water was apparently rising from the surface of the 
ocean just below. At an elevation of between 200 and 300 feet the ascending 
water column and the descending cloud column met. The diameter of the 
spout was approximately the width of the Hestia, or between 40 and 50 feet. 
Within the column there was a dark core, almost black, with a diameter of 
about 2 feet. Captain Ferguson did not clearly recall evidences of a 
whirling motion, but a strong upward movement is clearly indicated by the 
facts noted above. No reference was made to any considerable quantity of 
water being shipped as the waterspout passed over the vessel, a fact which 
would indicate that the lower portion of the column was composed mostly of 
spray carried up by the strong wind from the surface of the ocean. 

At the time of occtirrence of the waterspout the Hestia was near the 
center of a shallow but well defined barometric depression just off the 
coast of North Carolina. The general storm was moving slowly up the 
coast. A ridge of high pressure extended from the St. Lawrence Valley 
southwestward to the West Gulf states. The winds along the coast from 
New England to North Carolina were northerly. 

Summer Hot Spells. 

One of the most characteristic features of our summer season is the 
frequent recurrence of a longer or shorter series of excessively warm 
days. No summer season is entirely free from them, although at times 
they are not frequent enough or intense enough to cause comment. These 
periods vary greatly in length and in the frequency of their occurrence. 
When the atmosphere is comparatively dry, high temperatures may be 
endured without great personal discomfort. A high humidity combined 
with even moderately high temperatures is the cause of most of the 
unfavorable comment upon the summer weather of the Middle Atlantic 
coast states. 

In the usual course of summer events cyclones and anti-cyclones, 
though not as frequent as in winter or spring, and not so intense, are 
3'et sufficiently frequent in their passage across the northern portion of 
the country to maintain a fairly well mixed atmosphere and thus prevent 
the accumulation of excessively heated air near the surface of the earth. 
At times, however, we have a comparatively stationary system of cyclones 



454 THE CLIMATE OF BALTIMORE 

and anti-cyclones with a small gradient, which may persist with very 
little change in position for many days. Such a system is of frequent 
occurrence in the summer season in the United States. An area of high 
pressure settles over the South Atlantic states, or over the Atlantic Ocean 
with an extension covering the South Atlantic states, while a barometric 
depression rests over the Missouri Valley or the eastern slope of the 
Rocky Mountains. While this distribution of pressure continues there is 
a steady flow of warm dry southeast to southwest winds over the Middle 
Atlantic states. If in addition the gradient is small, or the South 
Atlantic high area moves over the Middle Atlantic states, the winds 
become very light while the clear skies permit uninterrupted sunshine. 
The sluggish movement of the atmosphere together with the unobstructed 
insolation permits the accumulation of excessively heated layers of atmos- 
phere at the surface of the earth. Sometimes these conditions will per- 
sist for two or three weeks before the cyclonic and anti-cyclonic systems 
begin to move eastward in their accumstomed paths and bring about a 
change. 

THE SUMMER OF 1900. 

The summer of 1900 was probably the warmest in the annals of 
Middle Atlantic states weather. The temperatures at the beginning of 
the season were about normal, June averaging but 0.2° per day below the 
average of 30 years. Beginning with July the average monthly tepera- 
tures remained far above their normal seasonal values until the close 
of November, the departures from the normal increasing steadily from 
July to September and then decreasing slowly to November. 

July -f2.0° 

August +4.7° 

September -}-5.1° 

October +4.5° 

November +3.5° 

July, 1900. During the first two days in July northerly winds pre- 
vailed in Maryland, accompanied by a cool morning temperature of about 
60° in the central and eastern portions of the state. On the Allegany 
plateau the night temperatures were as low as 40°. The maximum' after- 



MARYLAND WEATHER SERVICE 455 

noon temperatures were about 85°. On the whole, these days were several 
degrees cooler than the normal for the season. On the 3d the temperature 
began to rise rapidly. At Baltimore the maximum was 92°, and, with 
the exception of four or five days during which the maximum registered 
in the eighties, the afternoon temperatures remained well above 90° until 
the 21st of the month. From the 22d to the 31st the maximum readings 
ranged between 80° and 91°. The hot spell culminated in temperatures 
of 100° on the 16th and 17th. These temperatures, occurring at Balti- 
more, fairly represent the conditions that prevailed in the central, eastern, 
and southern portions of the state. In the valleys of Washington and 
Allegany counties the figures are somewhat higher. Thus, at Hagers- 
town, a reading of 105° was recorded on the 16th; at Hancock, 105° on 
the 15th, 16th, and 17th; at Green Spring Furnace, 106° on the 17th, the 
highest in the state. Within a very restricted area Mar3dand offers a 
great variety of climatic conditions. On the Allegany plateau, in Garrett 
Count}^ the thermometer did not register above 92° during the entire 
month, and then only on one or two days. 

The temperatures here indicated are all shade temperatures, that is, 
they were registered by thermometers placed in standard shelters which 
protect the instruments from the direct rays of the sun, or reflected rays 
from neighboring objects, but are so constructed as to permit of free circu- 
lation of the air. Thermometers exposed to the direct rays of the sun at 
Chase, in Baltimore County, and at Chewsville, in Washington County, 
gave an average maximum of 104° on 13 days, with an absolute maximum 
of 110°. Such temperatures, are, however, not unusual with thermome- 
ters so exposed. The average number of days with a maximum tempera- 
ture of 90° or above in July at Baltimore, based on the 30 years of care- 
fully kept records of the U. S. Weather Bureau, is 9 days. Their fre- 
quency has varied from a total absence in 1891, to 18 in 1876. During 
July, 1900, there were 15 such days at Baltimore, 17 at Washington, 18 
at Hagerstown, 19 at Laurel, 21 at Taneytown, and 27 at Hancock. 
Frostburg had but 5, Grantsville and Deer Park 2 each, while at Sunn)^- 
side, Garrett County, there was but one day. The average daily maximum 
temperature at Baltimore during these 15 days was 95°; the normal 
30 



456 THE CLIMATE OF BALTniORE 

average for the same period is 86°, showing a daily excess of 9°. These 
excessive temperatures caused the average daily temperatures for the 
entire month in Maryland and Delaware to be 2.5° to 8° above the 
normal value for the season. 

The weather conditions whicli usually accompany hot spells were pres- 
ent in a marked degree during July, 1900. The skies were remarkably 
clear ; the winds were prevailingly southwest, and generally light in force ; 
the rainfall was deficient in quantity and frequency. The records from 
over 50 stations in Maryland, Delaware, and the District of Columbia 
show an average of 17 clear, 11 partly cloudy, and 3 cloudy days. The 
average conditions at Baltimore, derived from 30 years of observations, 
are 10 clear, 13 partly cloudy, and 8 cloudy days. The winds were almost 
constantly from the south or southwest while the high temperatures pre- 
vailed. At Baltimore they were from the southeast, south, or southwest 
during 20 days out of the 31. The average hourly velocity was but 4.6 
miles, approximately the lowest in 25 years, during July, while the high- 
est velocity for the month was only 18 miles, the smallest maximum 
recorded at Baltimore. Scattered showers fell from the 3d to the 9th; 
on the 12th and 30th rainfall was general throughout the states of Mary- 
land and Delaware; during the period from the 17th to 26th local 
showers were frequent. With but few exceptions the total rainfall for 
the month Avas decidedly below the average. Baltimore had but 1.31 inch 
and Washington, D. C, but 1.25 inch, whereas the average rainfall for 
July in this vicinity is about 4.50 inches. The relative humidity during 
the period of intense heat was somewhat below the average for the month, 
a circumstance affording some cause for thankfulness. 

While suffering the discomforts of an intense spell of warm weather, 
we are apt to overestimate its severity as compared with those experienced 
in the past. Statistics, however, support the assertion that this July hot 
spell was one of the most trying on record in our vicinity. It is always 
difficult to make just comparisons in dealing with weather conditions. 
We feel hot and uncomfortable and look for the cause in high tempera- 
tures alone, but do not always find them as high as expected. The ele- 
ment of personal discomfort is due to certain combinations of tempera- 



MARYLAND WEATHER SERVICE 457 

ture, humidity, and air movement, and we have no single set of values 
to express this element, ^^'e can and do measure accurately the tempera- 
ture, the humidity, and the wind direction and velocity, each separately. 
Upon these figures we must base our judgment of the severity of any disa- 
greeable period of weather. Since 1871, the date of the establishment of 
the Weather station at Baltimore, the number of days in July with a max- 
imum temperature of 90° or above has exceeded 15 but twice. In 1878 
there were 16 such days with an absolute maximum of 98° ; the average 
of the maximum temperatures was 92.5° as compared with 95° in 1900. 
The average relative humidity was the same in both instances, namely 63 
per cent. The average daily wind movement was greater in 1878 than in 
1900, 128 miles in the former and 117 miles in the latter period. In 
1876 there were 18 consecutive days with an average maximum of 93°, 
and an absolute maximum of 99°; the average relative humidity during 
this period was 63 per cent; while the average wind movement was 125 
miles per day. As a result of this comparison with the two most con- 
spicuous rivals for notoriety, we find that the hot spell of July, 1900, 
was but little shorter in duration ; that the humidity was as high ; that 
the average temperature was fully 2° higher; and that the wind velocity, 
a powerful element of relief on a muggy da}^, was less. 

August, 1900. According to statistics of the Baltimore Health Officer 
there were 30 deaths during August due directly to sunstroke, and 32 in 
addition due to excessive heat as a secondary cause. When we come to 
examine the record of weather conditions during this period, and compare 
it with the hot spells of the past, we find nothing to equal it in intensity 
since the establishment of the Weather Bureau Station in Baltimore in 
1871. 

Baltimore has on an average five days in August with a temperature of 
90° or above, with a maximum in the past of 98°. In August, 1900, 
there were 17 such days, with a maximum of 100°, while this maximum 
was practically maintained for six consecutive days. Temperatures were 
even higher, and hot days more frequent at other points in Maryland and 
Delaware. Thus, in Washington County there were 20 days with a 
maximum temperature of 90° or above, with an absolute maximum of 



458 THE CLIMATE OF BALTIMORE 

103° at Hancock. The highest temperature recorded within the two 
states was 10-i° at Millsboro, Delaware, on the 14th. 

The hot wave began on the 6th, with a maximum temperature at 
Baltimore of 97°; from the 7th to the 12th inclusive the afternoon heat 
reached 99° or 100° each day; from the 13th to the 19th the daily maxi- 
mum ranged between 90° and 94°. Fortunately the relative humidity 
was comparatively low, averaging but 65 per cent, the normal value being 
70 per cent. A comparatively cool period of four days followed, with 
heavy showers. The temperature rose again on the 24th to 87°, and 
ranged between 88° and 96° to the close of the month. While the tem- 
perature averaged 6° less daily during the latter period than from the 
6t]i to the 19th, the relative humidity rose from 65 per cent to 81 per 
cent. To add to the discomfort of heat and humidity, the air movement 
was extremely light. The total wind movement over Baltimore during 
the month averaged but 108 miles per day; this is equivalent to an aver- 
age of 4.5 miles per hour. Such conditions following closely upon the 
long-continued hot weather of July and the first half of AugTist brought 
intense suffering to man and beast. 

Comparing the hot period of this month with earlier notable hot spells 
since 1871 we have the following: 

Length of Period, Averag-e ^^aximum. 

August, 1872 12 days 93° 

August, 1888 10 days 92° 

August, 1896 10 days 94° 

August, 1900 17 days 95° 

A particularly uncomfortable feature of the hot spell was ihe high 
night temperature. During four successive nights the minimum tem- 
perature ranged from 80° to 82°. At no other time in the preceding 30 
years has the night minimum exceeded 78°. The normal temperature for 
the month of August at Baltimore is 75°. During August, 1900, the mean 
temperature was 80°; this value was equalled but once, namely, in 1872. 

The abnormally warm weather of August was not confined to narrow 
limits. During the first week the temperature was above normal from 
the Eocky Mountains eastward to the Lower Lakes and the Appalachian 



MAKYLAXD WEATHER SERVICE 459 

Mountains. In South Dakota the daily excess was 13° above the normal 
value. During the second week the warm area extended eastward to the 
Atlantic coast, and the areas of maximum excess were transferred east- 
ward to Michigan and to the region including Philadelphia, Baltimore, 
and Washington, D. C. The temperature continued abnormally high dur- 
ing the third and fourth weeks, but the maximum daily excess fell from 
12° to 9°. 

The high temperatures have frequently been attributed in the daily 
press to a greater solar activity as shown by the increasing number of 
spots upon the sun's disk. A less remote and more plausible explanation 
may be found in the unusual distribution of atmospheric pressure during 
the hot spell. There is a type of pressure distribution which always 
brings warm weather to the Middle Atlantic states. When the barometer 
is high over the South Atlantic states, or just off the coast, while it is 
relatively low over an extensive area to westward and northward, the 
winds over the Middle Atlantic states are generally from a southerly 
direction, and light in force, while the skies are clear. Near the center 
of high pressure, moreover, the air descends from higher levels and is 
warmed by compression in descending. These conditions, all favorable 
to the production of high temperatures, were present in a marked degree 
during the period of hot weather in July and August. Clear skies favored 
the rapid warming up of the surface of the earth and the adjacent layers 
of air during the day; and the frequent calms and the prevailing light 
winds — the average for the entire period of the hot spell being but 4.5 
miles per hour at the Baltimore station — prevented the rapid exchange 
of temperatures between adjacent regions, or between upper and lower 
layers of the atmosphere. As a result the air near the surface of the 
earth was excessively heated. At the high level stations of Western 
Maryland the temperatures were comparatively moderate. The maxi- 
mum for the month of August was but 89° at Deer Park, and 91° at 
Orantsville. 

General Weather Conditions During the Hot Spell of 1900. 
The distribution of pressure and general weather conditions at the 
beginning of the August hot spell are shown in the cliart for August 



460 



THE CLIMATE OF BALTIMORE 



6, 1900. An extensive area of high pressure which had drifted slowly 
across the Lake region moved southward, the center being over the Middle 
Atlantic states on the 5th. The center of the high area remained for 
nearly two weeks in approximately the same position. Clear skies and 
light southerly winds were the prevailing conditions in the Middle 
Atlantic states. Occasionallv the center of the high area would be a 




Fig. 161.— Chart of August 6, 1900 (during Hot Spell). 



but the atmosphere was drawn from the same source to the south, and 
little further to the southwest, causing a northwest wind at Baltimore, 
change in local direction would bring about no change in temperature. 
Over the Eastern Eocky Mountain slope the pressure remained compara- 
tively low throughout the heated term. On the 12th of August a 
trough of low pressure developed between the southern high area and an 
area of high pressure over the Canadian Provinces, causing cloudiness 
and thunderstorms in the Lake region; this condition developed a 



MAllTLAXD WEATHER SERVICE 461 

depression over the Lower Lake region on the 13th, attended by showers 
and thunderstorms as far south as Maryland and Xorthern Virginia. 
But the relief brought about by these showers was only temporary. Local 
showers in connection with thunderstorms also afforded some relief in 
the Middle Atlantic states, the Ohio and the Missouri valleys on the 
16th, but the temperatures soon regained their intensity. On the 19th 
an area of high pressure developed to the north of the Lake region while 
the South Atlantic states high area drifted to the southwest and gradually 
dissipated. In the meantime a trough of low pressure developed between 
the two high areas in the Middle Atlantic states, bringing clouds and 
rain and breaking up the general conditions of pressure which caused the 
hot spell. 

The high temperatures of a hot spell are generally first experienced 
in the Missouri and Mississippi valleys; the distribution of pressure as 
shown in the chart for August 5 indicates very favorable conditions for 
a strong drift of warm southerly winds into these valleys. The area of 
excessive temperatures then moves eastward toward the Atlantic sea- 
board. This is clearly shown in the accompanying charts which outline 
the areas over which the temperatures were in excess or deficiency of their 
normal values for each week from July 23, 1900, to September 2-1, 1900. 
(See Plate XXIII.) Beginning with the week ending July 30, we find 
the line of no departure from the normal temperature for the week 
passing through Baltimore, and that over practically the entire central 
portion of the country the temperatures were from 1° to 3° below their 
normal values. In tlie accompanying charts, while the line of zero 
change again passes through Baltimore, the " hot wave "' had already 
been well established in the Upper Missouri Valley where the daily 
average temperatures were 9° to 12° above their seasonal values. By 
the close of the following week the area of greatest excess of temperature 
above the normal was transferred to ^Maryland, Pennsylvania, and Vir- 
ginia with a departure of 12^ per day above the normal for the season, 
while the temperatures had somewhat abated in Missouri and Mississippi 
valleys. The following charts show that the unseasonable temperatures 
continued without interruption over practically all of the country east of 



462 THE CLIMATE OF BALTIMORE 

the Kocky Mountains until the middle of September, the week ending 
September 24 showing the first appearance of temperatures below the 
seasonal averages in the northern half of the country, while they still 
continued high south of the Ohio Eiver. 

It is interesting to note in these charts that the area of the hot wave 
embraced practically all of the country east of the Eocky Mountains, 
and that west of the mountain range the temperatures were below their 
seasonal averages. This is generally true of our hot waves, the Kocky 
Mountains forming a natural boundary between areas of excessive and 
deficient temperature. 

THE SUMMER OF 1901. 

During the latter part of June and the first week of July, 1901, a 
heated term of even greater intensity than that of August of the pre- 
ceding year occurred, although it was fortunately of shorter duration. 
Afternoon temperatures exceeding 90° at Baltimore began on June 
26 with an area of high pressure centered over the Middle Atlantic state? 
and a barometric depression over the Upper Missouri Valley. The 
temperature rose steadily until the first of July, reaching a maximum 
of 103° on the 1st and 2d; from the 3d there was a steady fall in the 
mean temperature of the day to a normal condition on the 7th, when a 
thunderstorm accompanied by heavy rain brought on an abrupt fall of 
30° in the temperature between 4 o'clock and 4.15 p. m. 

The excessive heat began about 10 days earlier in the Central West. 
During the weeic ending June 17 the temperature rose to 6° above the 
normal seasonal value in the Middle Mississippi Valley. In the follow- 
ing week the area of excessive heat embraced all the district between the 
Eocky Mountains and the Alleghanys, with a maximum departure from 
the normal still in the Middle Mississippi Valley. By July 1 the area 
had extended to the Atlantic coast, while the heat was steadily increasing 
in intensity in the Mississippi Valley and the Lake region to a daily maxi- 
mum excess of 12° above the normal temperatures. During the follow- 
ing week the center of the heated area was transferred eastward to the 
Middle Atlantic states with a maximum dailv excess of 12° within the 



VOLUME 2, PLATE XXIII. 



*»-« • / 




Fig. 4.— Week ending August 20, 1900. 



r' 




Fig. 8.— Week ending September 17, 1900. 




i 



FIGURES 1-9 SHOW 

TEMPERATURE DEPARTURES DURING THE HOT 
SPELL OF 1900. 

Black lines show temperature departures below normal during hot spell 

of IQOO. 

Red lines show temperature departures above normal during the hot 
spell of 1900. 



)0. 



I MARYLAND WEATHER SERVICE. 



VOLUME 2, PLATE XXIII. 







Fig. 1.— Week ending July 30, 1900. 



Fio. 2. — Week ending August 6, 1900. 



FiQ. 3. — Week ending August 13, 1900. 



Fio. 4.— Week ending August 20, 1900. 







FiQ. B.— Week ending August 27, 1900. 



Fio. G. — Week ending September 3, 1900. 



Fio. 7. — Week ending September 10, 1900. 





Fio. 8. — Week ending September 17, 1900. 



FIGURES 1-9 SHOW 

TEMPERATURE DEPARTURES DURING THE HOT 
SPELL OF 1900. 

Black lines sliow temperature departures below normal during hot spell 
of 1000. 

Red lines show temperature departures above normal during the hot 
spell of 1900. 



Fio. 9.— Week ending September 24, 1900. 



Fm. in. — Maximum temperatures of .luly, 1901. 



Fro. 11. — Maximum temperatures of August, 1900. 



MAEYLAND WEATHER SERVICE 463 

area embracing Baltimore, Philadelphia, and New York, while the heat 
had somewhat moderated in the Middle Mississippi A'alle}-. In the 2d 
week in July the heat was again on the increase in the Middle Mississippi 
Valley, with moderating temperatures in the Middle Atlantic states and 
the Ohio Valley. The New England states experienced but little of the 
excessive heat of this period. In the Middle West during the second week 
of July maximum temperatures of 102° to 104° were of frequent occur- 
rence, establishing new records for excessive heat in a number of localities. 
In Baltimore the hot spell continued about 10 days, while the highest tem- 
perature, 103° on the 1st and 2d of July, was within 1° of the highest 
ever recorded at Baltimore. 

THE HOT PERIODS OF AUGUST^ 1900, AND JULY, 1901, CO.MPARED. 

The two periods of excessive heat described above were the most intense 
noted in the official records of the Weather Bureau since the establishment 
of the Baltimore office of the National Bureau. While there were many 
characteristics in common, the two periods showed a marked difference 
in their effects upon the residents of Baltimore. The death rate is 
always increased during a well marked hot spell in the large cities of the 
country. It is a difficult matter, however, to determine the immediate 
cause of the increased rate. It can not in general be attributed alone to 
increase in temperature of these hot spells, though this is probably the 
dominant factor. The humidity doubtless plays an important part in 
increasing the number of deaths. Perhaps, also, the weather condi- 
tions of the preceding weeks must be taken into account. The hot spell 
of August, 1900, covered a slightly longer period than that of June- 
July, 1901; the temperatures also averaged somewhat higher-; the wind 
movement was approximately the same. There was an astonishing dif- 
ference, however, in the number of deaths reported by the Baltimore 
Health Department as due directly to heat. 

During the 1900 period there were 32 deaths due to heat prostration; 
in the June and July period of 1901 there were twice as many — namely, 
64. The only marked difl'^rence between weather conditions noted was 
the difference in humidity — in 1901 the average daily relative humidity 



464 



THE CLIMATE OF BALTIMORE 



was 66 per cent of saturation, while in 1900 it was 57 per cent. The 
August, 1900, period was preceded by excessive temperatures in May 
and June, though of short duration, and by an exceptionally long and 



1 AUG 1900 

5 }0 15 20 


JUNE JULY 1 901 
25 30 5 


































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NORMAL. 67- 






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Fig. 1G2.— Temperature during Hot Spells of 1900 and 1901. 



intense, though interrupted, spell in July, which may have increased 
the powers of resistance and enabled the residents of Baltimore to with- 
stand the debilitating effects of an additional period after the interval 
of two weeks of moderate summer weather between the 2 2d of Jidy 



MARYLAND WEATHER SERVICE 



465 



and 6th of August. In the case of the hot period of June 2 6- July 7, 
1901, there were practically no excessively hot days in the months of 
May and the first three weeks of June; the city was overwhelmed with- 
out previous preparation, by one of the most intense heated terms exper- 
ienced in Baltimore. 

A comparison of the chief climatic features of the two periods by 
means of statistics and diagrams will enable the reader to understand 
more fully the points of difference and similarity. 



THE HOT PERIOD OP AUGUST, 1900. 



Date 



Aug. 





Max. 
Temp. 


Min. 
Temp. 


Relative 
Humid- 
ity. 


Wind 
Direc- 
tion. 


Hourly 

Wind 

V^elocity. 


Cloud- 
iness. 


Rain- 
fall. 


Thun- 
der 
Storms 




(Degrees Fahr.) 


(Per cent.) 




(Miles.) 






* 


6 


97 


67 


64 


SW 


2.9 


Clear 






7 


100 


76 


58 


W 


4.9 


Clear 






8 


99 . 


80 


52 


NW 


5.5 


Pt. cldy 






9 


100 


81 


54 


W 


4.0 


Pt. cldy 






10 


100 


80 


50 


Var. 


5.1 


Clear 






11 


100 


82 


44 


W 


4.7 


Clear 






12 


99 


73 


70 


'w 


6.1 


Clear 


0.14 


* 


13 


92 


72 


63 


SW 


3.7 


Pt. cldy 


0.03 




14 


94 


76 


60 


SE 


4.0 


Pt. cldy 






15 


91 


76 


72 


S 


4.3 


Pt. cldy 


T 


* 


16 


92 


71 


74 


w 


4.0 


Pt. cldy 


0.36 


* 


17 


91 


75 


73 


N 


4.5 


Pt. cldy 






18 


92 


72 


75 


S 


3.2 


Pt. cldy 


Total 




ige 


95.0 


75.5 


62 


w 


4.4 


Pt. cldy 


0.53 





THE HOT PERIOD OF JUNE-JULY, 1001. 



June 26 


92 


70 


58 


SW 


3.7 


Pt. cldy 




" 27 


92 


73 


60 


SW 


5.5 


Pt. cldy 




" 28 


93 


73 


70 


SE 


4.0 


Pt. cldy 




" 29 


96 


74 


68 


SW 


5.2 


Clear 




" 30 


99 


77 


58 


W 


3.1 


Clear 




July 1 


103 


80 


60 


SW 


3.6 


Pt. cldy 




2 


103 


80 


65 


Var. 


4.5 


Pt. cldy 




" 3 


97 


74 


60 


SW 


4.9 


Pt. cldy 


T 


4 


96 


77 


66 


w 


4.3 


Pt. cldy 


T 


5 


94 


76 


61 


SW 


5.9 


Pt. cldy 




6 


96 


69 


76 


W 


6.0 


Pt. cldy 


0.65 


7 


90 


66 


83 


SW 


5.8 


Pt. cldy 


0.50 
Total 


Average 


95.9 


74.1 


65 


SW 


4.7 


Pt. cldy 


1.15 



466 



THE CLIMATE OF BALTIMORE 



THE ANNUAL DISTRIBUTION OF DAYS WITH A MAXIMUM TEMPERATURE 



Tear. Apr. May. 



OF 90° OR ABOVE. 

(Baltimore, Md., 1871-1907.) 
June. July. Aug. Sept. Oct. Annual. Absolute Maximum. 



1871 




2 


5 


2 




9 


92, 


July 16. 


1872 




5 


10 


12 


2 


.. 29 


97, 


July 2. 


1873 




2 


15 


4 


2 


.. 23 


96, 


July 3. 


1874 


, . 


9 


7 


3 


1 


20 


98, 


June 9. 


1875 




7 


7 




2 


16 


97, 


June 27. 


1876 




5 


18 


2 




.. 25 


99, 


July 9. 


1877 


2 


4 


8 


3 




17 


95, 


June 26. 


1878 




1 


16 


4 




21 


98, 


July 18. 


1879 


1 


4 


10 


5 




.. 20 


99, 


July 16. 


1880 


7 


9 


10 


2 


3 


.. 31 


99, 


July 13. 


1881 


3 


3 


11 


8 


6 


31 


101, 


Sept. 7. 


1882 




6 


8 


1 




15 


97, 


June 25. 


1883 




1 


7 


2 




10 


96, 


July 22. 


1884 




4 


3 


3 


3 


.. 13 


95, 


July 24. 


1885 




4 


15 


3 




22 


99, 


July 21. 


1886 

1887 




2 


4 
10 


4 
3 


2 


10 
15 


92, 
102, 


rJuly 7, 

jAug. 27 

July 18. 


1888 1 




8 


5 


10 




24 


96, 


Aug. 16. 


1889 




2 


5 


1 




10 


93, 


July 9. 


1890 




4 


8 


2 




14 


98, 


July 8, 


1891 




5 




5 


1 


11 


94, 


^ June 16, 
) Aug. 10, 11 


1892 




6 


10 


o 




19 


99, 


July 26. 


1893 




5 


9 


2 




16 


98, 


June 20. 


1894 




8 


11 


2 


2 


23 


98, 


June 24. 


1895 


2 


5 


5 


10 


7 


29 


97, 


June 1, 3. 


189G 2 


5 


3 


10 


10 


3 


33 


98, 


Aug. 7. 


1897 




2 


4 


1 


4 


1 12 


97, 


Sept. 11. 


1898 


1 


7 


10 


9 


8 


35 


104, 


July 3. 


1899 


1 


8 


8 


8 


2 


27 


98, 


June 6. 


1900 


o 


3 


15 


17 


4 


. . 42 


100, 


(July 16, 17 
/ Aug. 7, 9, 1 


1901 




6 


18 


1 


1 


26 


103 


July 1, 2. 


1902 


1 


4 


10 


1 


1 


17 


99 


July 18. 


1903 1 


1 




10 


2 




14 


94 


Aug. 25. 


1904 




4 


6 






10 


97 


July 19. 


1905 




4 


8 


1 




13 


98 


July 18. 


1906 


2 


5 


3 


4 


2 


16 


96 


Aug. 6. 


1907 




1 


6 


3 


1 


11 


93 


July 8, 11. 


Totals 4 


31 


158 


325 


153 


57 


1 729 






Average . . 


1 


4 


9 


4 


2 


20 







makyland weather service 467 

The Cold Summer of 1816. 

There are numerous records in local annals showing that the summer 
of 1816 was phenomenally cold — in fact the coldest of which we have any 
authentic records. Systematic instrumental observations did not begin 
in Baltimore until the year 1817 (see pages 91-95) ; however, it is not 
a difficult matter to reduce reliable records of a neighboring station to 
contemporary conditions in Baltimore. We fortunately have a very 
complete and trustworthy series of daily records for Philadelphia, which 
go back to the year 1790. 

In the main, weather changes in Philadelphia and Baltimore are 
synchronous, and similar in kind; there is, however, a uniform difference 
of 1° to 2° in the average monthly temperatures of the two stations 
due to difference in latitude. By adding this difference to the average 
monthly Philadelphia temperatures we obtain a reliable value for con- 
temporary Baltimore temperatures. 

An interesting little book published in Philadelphia in 1847 by Mr. 
Charles Peirce and entitled " A meteorological account of the weather in 
Philadelphia from 1790 to 1847,^' contains a valuable record of. the 
general weather conditions for each month during this period of 57 
years. 

The following extracts are made concerning the character of the 
weather conditions in 1816, with special reference to the three summer 
months of June, July, and August : 

The Year. The temperature of the whole year was only 49°; it being the 
coldest year we have on record. Although there was no uncommonly cold 
weather during the three winter months, yet there was ice during every 
month in the year, not excepting June, July, and August. There was scarcely 
a vegetable came to perfection north and east of the Potomac. The cold 
weather during summer, not only extended through America, but throughout 
Europe. It was also the coldest summer ever known in the West Indies and 
in Africa. 

June, 181G. The medium temperature of the month was only 64°, and it 
was the coldest month of June we ever remember; there were not only severe 
frosts on several mornings, but on one morning there was said to be ice. 
Every green herb was killed, and vegetables of every description very much 
injured. All kinds of fruit had been previously destroyed, as not a month 
had passed without producing ice. From 6 to 10 inches of snow fell in 



468 THE CLIMATE OF BALTIMORE 

various parts of Vermont; 3 inches in the interior of New York; and several 
inches in the interior of New Hampshire and Maine. 

July, 1816. The medium or average temperature of this month was only 
68°, and it was a month of melancholy forebodings, as during every previous 
month since the year commenced, there were not only heavy frosts, but ice, 
so that very few vegetables came to perfection. It seemed as if the sun had 
lost his warm and cheering influences. One frosty night was succeeded by 
another, and thin ice formed in many exposed situations in the country. On 
the morning of the 5th there was ice as thick as window glass in Pennsyl- 
vania, New York, and through New England. Indian corn was chilled and 
withered, and the grass was so much killed by repeated frosts, that grazing 
cattle would scarcely eat it. Northerly winds prevailed a great part of the 
month; and when the wind changed to the west, and produced a pleasant 
day, it was a subject of congratulation by all. Very little rain fell during 
the month. 

August, 1816. The medium temperature of this month was only 66°, and 
such a cheerless, desponding, melancholy summer month, the oldest inhabi- 
tant never, perhaps, experienced. This poor month entered upon its duties 
so perfectly chilled, as to be unable to raise a warm, foggy morning, or 
cheerful sunny day. It commenced with a cold northeast rain storm, and 
when it cleared the atmosphere was so chilled as to produce ice in many 
places half an inch thick. It froze the Indian corn, which was in the milk, 
so hard, that it rotted up on the stock, and farmers mowed it down and dried 
it for cattle fodder. Every green thing was destroyed, not only in this 
country, but in Europe. Newspapers received from England said: "It will 
be remembered by the present generation, that the year 1816 was a year in 
which there was no summer." Indian corn, raised in Pennsylvania in 1815, 
sold (for seed to plant in the spring of 1817) for four dollars per bushel in 
many places. 

The departures of the year 1816 from the normal summer temperature 
are compared with the departures for some of the coolest summers on 
record in Baltimore in the following table : 

DEPARTURES FROM THE NORMAL TEMPERATURE. 





June. 


July. 


August. 


Season 


1816 


—8° 


—8° 


—8° 


—8.0° 


1836 


—5° 


—3° 


—5° 


—4.3° 


1846 


—4° 


—3° 


—1° 


—2.7° 


1886 


—3° 


—3° 


—2° 


—2.7° 


1891 


—1° 


—6° 


—2° 


—3.0° 


1903 


—6° 


—1° 


—3° 


—3.3° 



The mean daily temperature for each month of the summer of 1816 
fell decidedly below that of any summer month during the period of 
observations — from 1790 to 1906. 



MARYLAND WEATHER SERVICE 469 

DlSTRIBUTIOX OF PRESSURE DURIXG THE CoOL JUXE OF 1903. 

In the normal distribution of pressure during the summer months 
the western edge of the Atlantic high area extends to the South Atlantic 
states while the barometer over the Central states is comparativeh' low. 
Hence the atmosphere which flows over the ^liddle Atlantic states comes 
from the warm southeast. In June of, 1903 the barometer was com- 
paratively high over the Xorth-Central states, with a maximum in the 
Upper Mississippi and in the Missouri valleys, and relatively low in 
the Lower Lake region and the Atlantic coast states. During the same 
period the western portion of the Atlantic high area was found far north- 
ward toward the Gulf of St. Lawrence, causing cool easterly in place 
of the usual warm southerly winds to blow over the Middle Atlantic 
states ; in addition an unusual flow of cool air was derived from the high 
area to the northwest over the central portion of the North American 
continent. The effect of this abnormal distribution of pressure is 
reflected in the temperature departures recorded in the following table: 

COOL JUNE OF 1903. 
Districts. Normal Temp. Departure. 

New England States 57.8° —5.4° 

Middle Atlantic States 65.7° —5.2° 

South Atlantic States 73.4° —3.2° 

Gulf States 74.5° —4.5° 

Ohio Valley and Tennessee 68.4° — 5.6° 

Lake Region 60.8° —3.8° 

At Baltimore the month was cool, wet, and cloudy. The afternoon 
temperatures were high on a number of days, but the warm periods 
were of brief duration. Light frosts occurred in the mountains at the 
beginning of the month. The rainfall was considerably in excess of the 
normal seasonal amounts. The mean temperature of the month was 
67.0°, 5.8° below the average value for a period of 86 years. This large 
departure from the average June temperature in Baltimore marks the 
month of June, 1903, as the coldest since the beginnipg of systematic 
instriitnciital observations in 1817. (See PL XX1\.) 



470 the climate of baltimore 

Distribution of Pressure During the Normal June of 1902. 

The temperatures during the month of June, 1903, were very near 
the normal for a long series of years; they were remarkably uniform, 
the month being without marked departures from the seasonal average 
either above or below the normal. The distribution of pressure was in 
marked contrast with that of the cool June of 1903, described in the 
preceding paragraphs. The pressure was highest over the South Atlantic 
states and low over the St. Lawrence Valley and the extreme Southwest. 
The pressure distribution was such as to give prevailing southwest winds 
over the Middle Atlantic states. (See PI. XXIV.) 

While the immediate cause of departures from the normal seasonal 
temperatures over wide areas may be traced back to abnormal distribu- 
tion of pressure, it is not so easy to find a cause for these abnormal 
movements in the positions of the so-called permanent areas of high and 
low pressure. This slow shifting about of the large areas of high and 
low barometric pressure is sufficient cause for the greatest observed depar- 
tures from the seasonal temperature of a given locality, without calling 

* 

in the airl of extra-terrestrial influences, such as the moon, the planets, 
or sun-spots. 

The Variability of Summer Temperatures. 

While marked temperature changes from day to day during the summer 
months are not as frequent or as large as they are during the winter and 
spring seasons, there is still considerable variability due to the different 
types of pressure distribution. With a pronounced area of high pres- 
sure over the Upper Mississippi Valley and the Lake region the tem- 
peratures in the Middle Atlantic states fall below the seasonal average; 
with a well developed high area over the South Atlantic states, or just 
off the coast to the southeast, the temperatures over the Middle Atlantic 
and the Central states rise above the normal. These two types of pres- 
sure distribution, with resulting departures from the normal seasonal 
temperatures, are well defined in the weather maps of July 1, 1885, and 
July 1, 1901. On the 1st of July, 1885, an area of high pressure cov- 



VOLUME 2, PLATE XXIV. 



^■ 



V. 



/ 




Fig. 10.— Cold October, 1905 (—4°). 




Fig. 11.— Normal October, 1894 (— 0°.l). 




Fig. 12— Warm October. 1900 (-f 4°.5). 
D States, 

BObars, or lines of equal pressure 



/ 



MARVLAND WEATHER SERVICE 



E 2, PLATE XXIV. 




Fio. 3.— Warm December of 1889 (-f 8°.0). 



Pio. 6.— Warm March of 1898 (+G°.5). 



Fia. 9.— Warm June of 1899 (+2°) 



Pio. 12— Warm October. 1900 (+4''.6). 



DiSTRIBDTION OF PrKSSURE, WiNDS AND TEMPERATURE DURING NORMAL, (^OLD AND WaRM SEASONS IN THE UNITED STATES. 

Black lines are isotherms, or lines of equal temperature. Red lines are isobars, or lines of equal pressur 

Arrows fly with the wind. 



MARYLAND WEATHER SERVICE 



471 




FiG. 1G3.— The Cold July 1, 18S5. 




31 



Fic. 1G4.— The Wiirni July ], 1901. 



472 THE CLIMATE OF BALTIMORE 

ered most of the country east of the Kocky Mountains, excepting the 
New England states, the barometer being highest over the Middle Mis- 
sissippi Yalley. This distribution of pressure caused a steady flow of 
cool northwest winds over the Middle Atlantic states. The tempera- 
tures for the day were abnormally low. The early morning minimum 
at Baltimore was 56°, the lowest minimum recorded on the first day 
of July in a period of 36 years. The distribution of pressure noted above 
is typical for periods of cool weather in all seasons of the year. 

TEMPERATURES ON JULY 1, 1885. 
(A cool day with high barometer in Northwest.) 

7 a.m. o p. m. 11p.m. Max. Min. 

62 76 68 79 56 

Mean 67.5° 

On the other hand the warm weather type is represented by the dis- 
tribution of pressure seen in the weath?r map showin.u- conditions on the 
morning of July 1, 1901. 

In this type the' high area covers the South Atlantic states. With 
such a distribution of pressure the winds in the Middle Atlantic sta'tes 
are light and prevailingly from the south or southwest and abnormally 
warm. The minimum temperature on this day at Baltimore was 80°, 
while the afternoon maximum reached 103°, the liighest recorded in 
Baltimore upon the first day of July. 

TEMPERATURES ON JULY 1, 1901. 
(A warm day with high barometer in the Southeast.) 



Hours. 


2 


4 


6 8 


10 


13 


A. M. 


82 


81 


82 88 


96 


103 Noon 


P.M. 


103 


99 


96 91 
Mean 90.9° 


88 


87 Midnight 



THE WEATHER OF JULY 4. 

The variability of weather conditions may be illustrated in another 
way, by charting the various climatic factors for a given typical summer 
day during a long series of years. Thus in the accompanying diagram 
we have noted the maximum, the mean, and the minimum temperatures, 
the barometric pressure, the amount of cloudiness, the prevailing wind 
direction and the amount of rainfall recorded upon each fourth of July 



MARYLAND WEATHER SERVICE 



473 



TOE WEATHER OF JULY 4. 



Year. 
1871 


Max. 
Temp. 

82 


Miu. 
Temp. 
(Degrees 
71 


Mean 
Temp. 
Fahr.) 
76 


Character 
of Day. 

Pt. cldy 


Wind 
Direction. 

E&SW 


Daily Wind 
Movement. 
(Miles) 
120 


Precip- 
itation. 
(Inches) 
0.03 


1872 


93 


78 


86 


Pt. cldy 


SW 


117 


0.05 


1873 


92 


76 


84 


Cloudy 


W 


168 




1874 


92 


67 


80 


Cloudy 


S&W 


100 


1.14 


1875 


83 


69 


76 


Cloudy 


SE 


137 




1876 


95 


73 


84 


Pt. cldy 


SW 


103 


0.22 


1877 


86 


70 


78 


Pt. cldy 


N 


185 




1878 


92 


73 


82 


Clear 


SE 


127 




1879 


98 


74 


86 


Pt. cldy 


SW&W 


170 




1880 


87 


66 


76 


Clear 


N 


145 




1881 


93 


74 


84 


Cloudy 


NW 


137 




1882 


71 


61 


66 


Cloudy 


NE 


178 


1.09 


1883 


94 


74 


84 


Clear 


SW 


207 




1884 


85 


73 


79 


Cloudy 


E-SE-S 


115 


0.04 


1885 


88 


65 


77 


Clear 


N&NW 


85 




1886 


86 


66 


76 


Pt. cldy 


N-E-S 


70 




1887 


86 


72 


79 


Cloudy 


SE 


212 




1888 


85 


64 


74 


Pt. cldy 


S 


143 . 




1889 


84 


74 


79 


Cloudy 


SW 


143 


0.36 


1890 


91 


71 


81 


Pt. cldy 


SW&NW 


77 




1891 


79 


59 


69 


Pt. cldy 


W&NW 


324 


0.08 


1892 


75 


61 


68 


Pt. cldy 


W&NW 


191 




1893 


84 


64 


74 


Clear 


NW 


155 


0.01 


1894 


86 


72 


79 


Pt. cldy 


W 


243 




1895 


76 


64 


70 


Cloudy 


NW 


169 


0.11 


1896 


88 


70 


79 


Cloudy 


SW 


177 




1897 


86 


72 


79 


Pt. cldy 


E 


138 




1898 


100 


74 


87 


Pt. cldy 


SW 


102 


0.07 


1889 


90 


68 


79 


Pt. cldy 


S 


115 




1900 


97 


77 


87 


Pt. cldy 


SW&W 


114 




1901 


96 


77 


86 


Pt. cldy 


w 


103 




1902 


86 


70 


78 


Cloudy 


s 


117 


0.17 


1903 


87 


73 


80 


Cloudy 


SE 


123 


0.08 


1904 


88 


61 


74 


Clear 


SW 


156 




1905 


83 


69 


76 


Pt. cldy 


SE 


204 




1906 


85 


73 


79 


Pt. cldy 


SW&W 


217 


0.02 


1907 


78 


59 


68 


Clear 


N&E 


121 




Average 


87 


70 


78 






150 


0.25 





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MARYLAND WEATHEE SERVICE 475 

i'rom 1S71 to 1903. We see that the lowest temperature recorded on 
any -ith of July was 58° and that this occurred in 1891; the highest 
temperature, namely 100°, is credited to 1898. Between these extremes 
we have had in the past 36 years all degrees of temperature conditions. 
There appears to be no regularity in the fluctuations in temperature from 
year to year, although there are indications of irregular periods of 
steadily decreasing or increasing temperature. 

The barometric pressure has varied but little above or below the normal 
seasonal values, the entire range being less than half an inch. 

The days with a clear sky have numbered but six in 36 years; with 
partly cloudy sky, 18 ; and with an overcast sky, 12. The prevailing 
wind direction has been from the southwest. Kain fell in amounts vary- 
ing from a light sprinkle to heavy showers on somewhat less than half 
the total number of days, making the rainfall probability for the 4th of 
July less than 50 per cent. Thunderstorms were recorded but five times 
during the period of 36 years on this day. 

West Indian Hurricanes. 

Hurricanes do not differ, in essential features, from the temperate 
region cyclones described in preceding pages. They are more restricted 
in area, but relatively more intense in energy and destructive power. 
These storms have their origin in the vicinity of the Windward Islands; 
they move toward the west or northwest at the rate of 10 to 12 miles 
per hour — less than half the average rate of temperate region cyclones — 
and curve northward and then northeastward approximately in the 
neighborhood of Florida, as a rule, following the Atlantic coast, enlarg- 
ing in area after recurving until they resemble in every detail the storms 
common to the higher latitudes. 

While these, the most disastrous of all storms, have occurred in all 
seasons of the year, they are confined almost entirely to the months of 
August, September, and October. The abrupt increase in their frequency 
in August is phenomenal as shown in the following extract from one of 
the publications of the United States Weather Bureau.' 

^ E. B. Garriott. West Indian Hurricanes. Bull. H., U. S. Weather Bureau. 
4to. Washington, D. C, 1902. 



476 THE CLIMATE OF BALTIMORE 

FREQUENCY OF HURIilCANES (1878-1900). 



Jan. 


Feb. 


Mar. 


Apr. 


May. 


June. 


July. 


Aug. 


Sept. 


Oct. 


Nov. 


Dec. 


Vear. 


3 











1 


3 


3 


25 


25 


32 


3 


3 


98 



Fortunately the path of the hurricane rarely falls within the limits 
of the Middle Atlantic states until it has lost some of its violence. By 
the time it has reached the latitude of Baltimore the center is generally 
well off the coast, and we experience only the ordinary storm winds of the 
western quadrant of the liurricane. 

When there happens to be a well developed area of high barometric 
pressure over the eastern half of the country on the approach of a hurri- 
cane the storm is prevented from recurving near the Florida Peninsula 
and moves slowly westward into the Gulf of Mexico, or even entirely 
across the Gulf, before recurving northward. Under such circumstances 
the hurricane is apt to gather in force in its journey across the Gulf. 
The storm which destroyed Galveston in September, 1900, was of this 
type. 

A typical storin of this class passed over Maryland on the 13th of 
October, 1893 ; it is described in the following paragraphs. 

THE HURRICANE OF OCTOBER 13, 1893. 

The first indication of the approach of this storm from the West 
Indies was contained in a report from Saint Thomas on the 5th of 
October; on the following day additional information was received from 
Antigua. The storm advanced slowly westward.' On the Tth it was 
southeast of Port au Prince, and on the 8th southeast of Santiago de 
Cuba. On the 9th it had reached the Bahama Islands and Southern 
Florida, and storm signals were ordered up along the Florida and east 
Gulf coasts by the Chief of the Weather Bureau. By the evening of 
the 10th the wind had freshened to a gale along the Florida coast. On 
the morning of the 11th the storm center was east of the Bahama Islands 
and the barometer was falling rapidly along the Atlantic coast as far 
north as Xew Jersey. During the 12th severe nortlieast gales and heavy 

'.See.- Lake Storm Bulletin. No. 2, 1893, U. S. Weather Bureau. 



MAKYLAND WKITHER SERVICE 477 

rains prevailed along the coast of the South Atlantic states in connec- 
tion with a rapidly falling barometer. 

On the morning of the 13th the storm center reached the South Caro- 
lina coast, the barometer at Charleston indicating 28.88 inches. From 
this point the storm took an unusual course, moving northward into the 
interior, the center passing over the Carolinas and the Middle Atlantic 
states. Xortheast storm signals were ordered for all stations on the 
Middle Atlantic and Kew England coasts. Special w^arnings were sent 
to all Weather Bureau observers in the Middle Atlantic states and Kew 
England, and observers from Southern New England to Maryland were 
authorized to use tlie telegraph at their discretion in distributing these 
warnings in the most effectual manner possible. 

During the evening of the 13th the center of the storm passed over 
Western Maryland, the barometer falling to 28.88 inches at Baltimore. 
Moving due north it crossed Pennsylvania and Western N'ew York to the 
north of Lake Ontario on the 14th. On the 15th the st>orm disappeared 
in the direction of Labrador. 

The storm was attended by high winds and heavy rains all along its 
path across the United States. Some of the records are quoted below 
from the official report of the United States Weather Bureau. 

niGII WINDS AND HEAVY RAINS DURING THE STORM OF OCTOBER 13 AND 

14, 1893. 

(S a. m. to 8 p. m. October 13.) 

„. .. Velocity. Direc- <i.t<itinn V^elocity. Direc- 

Stiition. (Miles) tion. i^tation. (Miles) tion. 

Jacksonville, Fla 38 SW Harrisburg, Pa 36 E 

Charleston, S. C 42 NW Atlantic City, N. J....38 SE 

Atlanta, Ga 38 W Philadelphia, Pa 48 SE 

Wilmington. N. C 56 SE New York City 30 SE 

Raleigh, N. C 36 E Cleveland, 48 NE 

Southport. N. C SO SE Erie, Pa 30 NE 

Washington, D. C 42 SE Baltimore, Md 38 SE 

BAINFALL. 
Station. luches. Station. Inches. 

Raleigh, N. C 2.08 Baltimore, Md 1.00 

Lynchburg, Va 1.66 Pittsburg, Pa 1.01 

Washington, D. C 1.82 Parkersburg, W. Va 2.48 



478 



THE CLIMATE OF BALTIMORE 




Fig. 166.— The Hurricane of October 13, 1893 (8 a. m.), 




Fig. 167.— The Hurricane of October 13, 1893 (8 p. m.). 



MARYLAXD WEATHER SERVICE 



479 



(8 p. m. Oct. 13 to S a. m. Oct. 14.) 



Velocity. 
(Miles) 

...34 

...42 



Station. 
Charleston, S. C. 
Washington, D. C 

Baltimore, Md 40 

Atlantic City, N. J 44 

Philadelphia, Pa 56 

Sandy Hook, N. J 64 

Boston, Mass 36 

Woods Holl, Mass 44 



Direc- 
tion. 

W 
SE 
SE 
SE 
SE 
SE 

E 
SE 



Station. 
Albany, N. Y. . 
Oswego, N. Y. 
Buffalo, N. Y. . 

Erie, Pa 

Sandusky, O. . 



Velocity Direc- 
(Miles) tion. 



.48 
.60 
.60 
.36 
.36 



Detroit, Mich 46 

Grand Haven, Mich... 36 
Marquette, Mich 34 



SE 
SE 
SW 
SE 

NW 

W 

NW 

NW 




Fig. 168.— The Hurricane of October 14, 1893 (8 a. m.). 



RAINFALL. 



Station. Inches. 

Philadelphia, Pa 1.24 

Cleveland, 1.90 



station. Inches. 

Alpena, Mich 1.08 

Port Huron, Mich 1.50 



The meteorological conditions as recorded at the Baltimore Office of 
the Weather Bureau are indicated in the accompanying diagram and in 
the following extracts from the records of the office : 



480 THE CLIMATE OF BALTIMORE 

The barometer reached its lowest point, 28.88 inches, at 10 p. m. of the 
13th, then slowly rose throughout the night and following day. Light rain 
began to fall with a northeast wind and continued without interruption until 
midnight. The total amount of precipitation was 1.60 inch. The maximum 
wind velocity, 40 miles per hour from the southeast, occurred during the 
night of the 13th-14th just before the barometer began to rise, at the time 
of heaviest rainfall. The temperature rose slowly and steadily from a min- 
imum of 56° at 6 a. m. to 73° at midnight, as the wind gradually veered from 
northeast to southeast, obliterating the usual diurnal fall after 3 p. m. 

The following morning began with a clear sky and a fresh southwest wind, 
the storm having passed northward beyond the horizon of Baltimore. 

AUTUMN WEATHEE. 

Although the months of June, July, and August only are allotted to 
the summer season in the division of our calendar, the weather of the 
month of SejDtember in the Middle Atlantic states is truly summer 
weather. The temperatures continue high ; the mean monthly tempera- 
ture is occasionally higher than the mean monthly value for June, July, 
or August, while tlie greatest heat of the summer has on at least two 
occasions within the past 35 years fallen within the first half of the 
month. As stated in an earlier paragraph (page 78) the temperature 
falls more slowly in autumn than it rises in the spring months. There 
is a striking difference in the mean temperature of the equinoctial days 
of spring and fall, the latter being 23° warmer than the former. The 
wind movement is less than that of any other month of the year, except- 
ing August, in spite of the reputation as a month of equinoctial storms. 

AVERAGE DAILY WIND MOVEMENT AT BALTIMORE (1873-1903). 

Jan. Feb. Mar. Apr. May. June. July. Aug-. Sept. Oct. Nov. Dec. Year. 

145 103 175 166 149 142 134 122 129 13r 143 143 145 mis 

Hence the month of Sejjtember is about as free from atmospheric 
disturbances as any portion of the year. The period of the autumnal 
equinox is quite as free from storm and rain as the days immediately 
preceding and following. The wind movement, the rainfall probability, 
and the amount of rain for the 21st and 22d are all below the average 
for the period from the 15th to the 25th. In view of the figures in the 
following table it is difficult to find any support for the existence of a 
particularly stormy period at the time of the September equinox. 



MARYLAND WEATHER SERAICE 



481 



WINDS AND RAINFALL FROM SEPTEMBER 15-25. 
(Average of 35 jears at Baltimore.) 

Wind. Rainfall. 

Movement. Probability. ^^^Aimmnt^^ 

(Miles) (Per cent.) (Inch; 

September 15 139 45 0.60 

16 138 37 0.78 

17 133 40 0.80 

18 130 30 0.15 

19 121 27 0.90 

20 133 28 0.21 

21 130 27 0.15 

22 129 32 0.28 

23 130 35 0.52 

24 133 25 0.30 

25 134 30 0.43 

Average 132 32.4 0.47 

The months of October and Xovember give us some of the most delight- 
ful da3s of the year — days with soft, balmy atmosphere, light southerly 
winds, cloudless skies with warm sunshine during mid-day, and cool 
crisp nights — the days of the American Indian Summer. 

In Maryland light frosts make their first appearance about the middle 
of Octolier, excejjting in the mountain districts where they are earlier, 
while heavy frosts are usually delayed to the early days of Xovember. 
The first snow arrives about the middle of Xovember. 

In the fall months the process of redistribution of barometric pressure 
over ocean and continent is the reverse of that of the spring. The sum- 
mer condition of high barometric pressure over the Atlantic Ocean and 
low pressure over the central continental area is gradually broken up, 
the pressure rising over the continent and falling over the ocean. In 
this process of redistribution of pressure which is brouglit about by tlie 
retreat of the sun, the sluggisli atmospheric movements of the summer 
months give way to a more active circulation. Well defined areas of 
high and low pressure increase in frequency. The Middle Atlantic 
states are alternately brouglit under tlic influence of the Atlantic high 
area with its southerly winds and clear skies, and the cool dry northwest 
winds of the growing continental hii:!! area, but it is not until late in 



482 THE CLIMATE OF BALTIMORE 

December that the continental high area gains control over the weather 
situation in Maryland, and settled winter conditions may be expected. 

Indian Summer. 

In discussing weather types in the preceding pages we have found a 
natural division into cyclonic and anti-cyclonic weather — or the weather 
are relatively low, and those associated with relatively high barometric 
pressure. The weather conditions attending these moving or shifting 
pressure areas vary with the season, or rather with the annual increase 
and decrease in temperature resulting from changes in the declination 
conditions associated with large areas over which the barometer readings 
of the sun. But in all seasons high areas are attended by comparatively 
clear skies and moderate winds, while low areas bring clouds and rain 
and relatively high winds — they are respectively " fair weather " and 
" foul weather " types. The temperatures of a given locality associated 
with these types depend upon the season of the year and the relative posi- 
tion of the center of the high or low area with respect to the locality 
in question. The relative distribution of pressure determines the wind 
direction while the wind direction determines the temperature. In our 
latitudes, and especially over large continental areas with the broad ocean 
of equable temperatures to the east, a north or northwest wind is a 
relatively cold wind, a south or southeast wind is relatively warm, at all 
seasons of the year, while east winds and west winds bring intermediate 
temperatures. Hence an area of high pressure to the west or northwest 
of the Middle Atlantic states, for instance, with its resulting northwest 
to north winds gives to this section in all seasons a temperature below 
the seasonal average, the amount of departure from the normal depend- 
ing upon the intensity of development of the high area. With a high 
area to the east or southeast the winds blowing out of the high area and 
over the Middle Atlantic states are from the southeast to southwest — 
winds which are at all seasons warmer than the seasonal average. On 
the other hand a low area to the west or north brings warm southerly 
winds; when it is to the south or east the winds are from the colder 
areas of the north and northwest. 



MARYLAND WEATHER SERVICE 483 

Under normal conditions there is a constant succession of these high 
and low areas across the country, approximately from the west towards 
the east, with an irregular cycle of two, three, or four days. Sometimes 
these types are quite persistent, the pressure distribution remammg 
practically unaltered for a week or ten days, or longer in exceptional 
cases. In mid-summer an area of high pressure developing over the 
South Atlantic states, or over the Atlantic just off the coast, is apt to 
persist for many days with only slight changes in outline or intensity, 
resulting in " hot spells " of greater or less degree, depending upon the 
intensity of development and persistence of this high area to the 
southeast. 

A similar type of high area frequently, though not annually, develops 
late in the fall after the summer has passed and after the first frosts have 
announced the approach of winter. The development may occur in the 
latter part of October or in November, sometimes even later in the 
season, and controls the weather conditions in the Central, Middle 
Atlantic and New England states for several days, or at rare intervals, 
for several weeks. 

During the periods in which these southeast anti-cyclonic areas prevail, 
light dry southerly winds blow over the Middle Atlantic states, the New 
England states and tlie Central states, there is an absence of clouds, 
haze increases in amount, as is usual during warm pei'iods with a sluggish 
movement of the air; the mid-day temperatures are high, while the 
nights are cool. Separated from the long season of hot sultry summer 
weather by occasional incursions of the cold crisp air from a northwest 
high area, these periods of Indian Summer, or second summer, are among 
the most delightful days of the year. They constitute a temporary 
halt in the steady seasonal fall of temperature and the approach of real 
winter weather. There are similar periods in European weather but 
there the characteristic charm of the American Indian Summer appears 
to be less pronounced ; of such among others are St. Martin's Summer of 
England ; the Summer of St. Denis in Eranee; and in Cicrmany the " Alt- 
weibersommer," or the " old woman's summer." 



484 



THE CLIMATE OF BALTIMORE 



A most interesting and instrnetive account of the occurrence of the 
term " Indian Summer " in the literature of the early writers on Anierica 
is presented by Mr. Albert Matthews ' of Boston. The author finds after 
an exhaustive search in books on travel in North America that " it is 
not until the year 1794 that the expression ' Indian Summer ' occurs 
at all, and not until the nineteenth century that it. became well 
established." 




Fig. 169.— The Weather of October 29, 1903 (Indian Summer), 



The earliest use of the term found by Mr. Matthews is in the following 

journal entry by Major Ebenezer Denny " while at Le Boeuf, a few miles 

from the present city of Erie, Pa., on October 13, 1794 : 

" Pleasant weather. The Indian Summer here. Frosty nights." 

There is very little agreement among writers who used the term as to 

the time of occurrence of the Indian Summer, or as to the length of the 

^ Albert Matthews. The term Indian Summer. Monthly Weather Review 
for January and February, 1902 (Washington, D. C). 
= Military Journal. 1859, p, 198. 



MAUYLAND WEATHER SERVICE 485 

period. This is. however, not surprising in view of the fact that the 
type of pressure distribution which causes the characteristic weather 
may develop at any time of the year. 

The meteorological conditions prevailing over the country on the 
morning of the 29th of October, 1903, at the beginning of a brief period 
of Indian Summer in the Middle Atlantic states are shown in the accom- 
panying weather chart. An area of high barometric pressure, centered 
over the Upper Mississippi Valley on the 26th of October, moved slowly 
southeastward during the 26th, 27th, and 28th, and remained with 
slight changes of configuration and position over the Middle Atlantic 
and South Atlantic states until November 4, when it gave way to an area 
of low pres.sure over the Lake region, bringing general rains to tlie 
Atlantic coast states. 

On the morning of October 29 the center of the high area was over 
Virginia. The skies were clear throughout the Middle Atlantic states, 
the winds were prevailingly from the southwest and light. (See Fig. 
169.) 

The following extracts from the records of the Baltimore Office of the 
United States Weather Bureau indicate the general character of the 
weather during the brief period of Indian Summer weather from October 
28 to November 4, 1903: 

October 28. 1903. Clear day. Warmer and pleasant; maximum 61°,. min- 
imum 40°. Light fog in morning. Wind was brisk at times during the day. 
Wind west and southwest. Average velocity 9 miles per hour; maximum 
velocity 22 miles from the west at 9.10 a. m. 

October 29, 1903. Clear day. Somewhat warmer; maximum 68°, min 
imum 45°. Pleasant. Light fog at 8 a. m. Wind from southwest to north- 
west; average velocity 8 miles. 

October 30, 1903. Clear day. Some cirrus clouds all day. Somewhat 
warmer; maximum 75°, minimum 52°. Pleasant. Light haze at 8 a. m. 
Wind from southwest to northwest; average velocity 5 miles. 

October 31, 1903. Partly cloudy day. Sky a little more than half clouded 
all day. Somewhat cooler; maximum 68°, minimum 44°. Mild and delight- 
ful weather continues. Light fog in morning; light fog and smoke at 8 p. m. 
Wind variable, from southeast to northwest; average velocity 3 miles. 

November 1, 1903. Clear day. Some cirrus present nearly all day. Some- 
what warmer; maximum 74°, minimum 46°. The mild pleasant Indian Sum- 



486 THE CLIMATE OF BALTIMORE 

mer weather continues. Light fog in morning. Wind from west to north- 
west; average velocity 4 miles. 

November 2, 1903. Partly cloudy day. Morning clear; mid-day partly 
cloudy, and sky overcast by late afternoon; evening clear. Warm in morning, 
cooler in afternoon; maximum 68°. Mild and pleasant. Light fog in morning 
and light smoke in evening. Winds variable; average velority 4 miles. 

Noveinber 3, 1903. Clear day. Warmer in afternoon; maximum 75°, min- 
imum 48°. Continued mild and pleasant weather. Light fog in morning; 
light smoke at 8 p. m.; and light haze all evening. Wind west to northwest; 
average velocity 5 miles. 

November 4, 1903. Partly cloudy day. Morning clear; sky became nearly 
overcast with light clouds just after noon, and so continued. Warm and 
pleasant weather continues; rather humid in afternoon. Light rain began 
11.30 p. m. and continued at midnight; amount to midnight, 0.02 inch. Light 
fog in morning, light haze in evening. Lunar corona seen at 8 p. m., and a 
beautiful lunar halo of 22^2° radius seen at 9.15 p. m. and at 10 p. m. Wind 
variable; average velocity 3 miles. 

The Variability of Autumn Temperatures. 

The extreme ranges of the temperature conditions during the fall 
months are shown in detail in the discussion of climatic conditions in 
Part I of this report; these statistical tables are supplemented by the 
records of weather conditions on two selected typical fall days, namely, 
October 1 and Thanksgiving Day, from 1871 to 1907, and on a State 
holiday, September 12, the anniversary of the battle of North Point. The 
variability of the weather of the fall does not differ materially from that 
of the spring — both seasons are transitional periods connecting the 
extreme conditions of winter and summer. A close study of these tables 
based on Baltimore observations for 37 3'ears will give a fair idea of the 
variability of fall weather over the Coastal Plain of the Middle Atlantic 
states. 

The Weather of September 12. 
The 12th day of September, or Defenders' Day, is a state holiday in 
Maryland commemorating the battle of North Point in 1814. In the 
past 37 years the weather on this day has been mostly clear to partly 
cloudy, with an average temperature of about 72°, and extremes rang- 
ing between 93° and 51°. The winds have been light easterly, while 
rain has occurred on an average once in three years. The distribution 
of rainfall has been peculiar, having occurred 12 tim-es during the first 
20 years and but once in the succeeding 17 years. 



MARYLAND WEATHER SEKVICE 



487 



THE WEATHER OF SEPTEMBER 12. 



Year. 
1871 


Max, Min. 
Temp. Temp. 
(Degrees Fahr 
69 64 


Mean 
Temp. 

) 
66 


Character 
of Day. 

Cloudy 


Wind 
Direction. 

SE 


Daily Wind 
Movement. 
(Miles) 
70 


Precip 
itation 
(Inch) 
0.01 


1872 


81 


72 


76 


Cloudy 


S 


171 


0.24 


1873 


78 


58 


68 


Clear 


N&SE 


100 




1874 


90 


70 


80 


Ft. cldy 


SE 


61 




1875 


70 


52 


61 


Cloudy 


E 


143 




1876 


67 


57 


62 


Cloudy 


NE 


140 


0.01 


1877 


74 


69 


72 


Cloudy 


E 


122 


0.89 


1878 


80 


69 


74 


Cloudy 


E 


174 




1879 


72 


51 


62 


Clear 


S 


81 




18S0 


77 


56 


66 


Clear 


SE 


77 




1881 


81 


69 


75 


Clear 


N&, NW 


107 


0.01 


1882 


72 


61 


66 


Clear 


N 


183 


0.03 


1883 


63 


56 


60 


Cloudy 


NE 


281 


1.13 


1884 


78 


67 


72 


Pt. cldy 


NB 


117 


. . . 


1885 


72 


62 


67 


Pt. cldy 


NE-SE-S 


95 




1886 


86 


61 


74 


Pt. cldy 


NE-SW-W 


129 


0.53 


1887 


77 


63 


70 


Cloudy 


SW 


118 


0.64 


1888 


84 


62 


73 


Clear 


SW 


158 


0.01 


1889 


69 


64 


66 


Cloudy 


NE 


368 


0.55 


1890 


82 


71 


76 


Cloudy 


S 


171 


1.06 


1891 


76 


60 


68 


Clear 


NE&S 


128 




1892 


79 


61 


70 


Pt. cldy 


SB 


200 




1893 


70 


60 


65 


Cloudy 


B 


239 




1894 


73 


55 


64 


Clear 


E 


130 




1895 


93 


73 


83 


Clear 


W 


126 




1896 


80 


67 


74 


Cloudy 


B&SW 


62 




1897 


79 


67 


73 


Cloudy 


E 


173 




1898 


74 


53 


64 


Clear 


N&NW 


155 




1899 


82 


57 


70 


Clear 


SW&NW 


105 




1900 


88 


70 


79 


Clear 


SW&NW 


165 




1901 


87 


69 


78 


Pt. cldy 


SW 


119 


0.01 


1902 


75 


57 


66 


Clear 


s 


153 




1903 


84 


G9 


76 


Clear 


E 


154 




1904 


87 


62 


74 


Clear 


N 


123 




1905 


75 


63 


69 


Cloudy 


NW 


138 




1906 


88 


72 


80 


Pt. cldy 


S&SW 


151 


... 


Average 


78 


63 


72 






144 


0.39 


32 



















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