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THE CLIMATE
NEAR THE GROUND

PROF. DR. RUDOLF GEIGER

Professor of Meteorology, University of Munich
and Director of the Meteorological Institute

A TRANSLATION
BY

MILROY N. STEWART
AND OTHERS

OF THE SECOND GERMAN EDITION OF

DAS KLIMA DER BODENNAHEN LUFTSCHICHT

WITH REVISIONS AND ENLARGEMENTS

BY THE AUTHOR

PUBLISHED FOR

BLUE HILL METEOROLOGICAL OBSERVATORY
r^TfHKVARP UNIVERSITY 1

I BY " -

PKESS

CAMBRIDGE, MASSACHUSETTS
1950

PRINTED BY THE HARVARD UNIVERSITY PRINTING OFFICE
CAMBRIDGE, MASSACHUSETTS, U.S.A.

CONTENTS

PREFACE TO THE FIRST EDITION xi

FROM THE PREFACE TO THE SECOND EDITION xiii

PREFACE TO THE TRANSLATION OF THE SECOND EDITION . . . xv

INTRODUCTORY CHAPTER: The Microclimate and Microclimatic
Research xvii

The zone of disturbance near the ground. Climate on a large scale,
or macroclimate. The climate near the ground. Plant climate in
contrast to human climate. The microclimate. Definitions. His-
tory of microclimatology. Relation to allied sciences.

PART ONE

Concerning the microclimate existing near the ground by virtue of
its proximity to the ground surface i

SECTION I
HEAT EXCHANGE NEAR THE GROUND

CHAPTER i. Midday Heat Exchange at the Ground Surface.
The Incoming Radiational Type 2

Evaluation of solar radiation. Significance of the ground surface.
Total radiation and horizontal radiation. Mountain radiation.
Heat exchange at midday. The incoming radiational type. Large
scale temperature gradients and formation of dust whirls.

CHAPTER 2. Nocturnal Heat Exchange at the Ground Surface.
The Outgoing Radiational Type 13

Outward radiation, counter-radiation, effective outward radiation.
Influence of cloudiness. Radiation in different directions. Outward
radiation in basins, furrows and valleys. Outward radiation in the
mountains. The nocturnal heat exchange. The outgoing radia-
tional type.

CHAPTER 3. True Heat Conduction. The Normal Course of
Ground Temperature 26

Survey of the four forms of heat transfer. Laws of heat conduc-
tion in the soil. Heat and thermal conductivity. Diurnal and
annual march of temperature in the ground. Weather and the
soil temperature. The soil as a heat reservoir. Soil temperature
in the mountains.

iv CONTENTS

CHAPTER 4. Eddy Diffusion and Its Significance .... 36
Laminar and turbulent flow. Fundamental transport equation.
The austausch coefficient; its variation with altitude. The laminar
structure of the air layer near the ground. Dynamic and thermal
convection. Examples of the action of eddy diffusion. Seed dis-
persal.

CHAPTER 5. Long Wave Radiation 46

The air as a band radiator. Absorption bands of water vapor and
carbon dioxide. Pseudo-conduction of radiation. Wave-length
transformation at the ground. Formation of the nocturnal inver-
sion. Long-wave radiation by day. Relative importance of mass
exchange and radiation.

SECTION II
TEMPERATURE RELATIONSHIPS NEAR THE GROUND

CHAPTER 6. The Warming Process 51

The boundary layer near the ground. Temperatures in the first
millimeter above the soil. The dark band. Initiation of convec-
tion. The intermediate, and the upper, layer near the ground.
The upward streaming of hot air. Temperature turbulence. Dis-
continuous temperature layers. Cooling at the ground by layer
formation. The theory of very high lapse rates.

CHAPTER 7. The Cooling Process 62

Stability of nocturnal stratification. Importance of the dust con-
tent. Cold convection. Precedence of radiation processes. Minima
above the ground surface. Types of evening temperature distribu-
tion. Formation and destruction of the nocturnal cold air skin.

CHAPTER 8. The Diurnal and Annual Course of Temperature
Near the Ground 68

The need for statistical material. Observational technique. True
air temperature and the temperature of test bodies. Advantages
and disadvantages of both methods. Diurnal and annual march of
temperature in Europe, Egypt and India. Temperature fluctua-
tions by day in proximity to the ground. Frequency of freezing
and thawing. Influence of cloudiness.

CHAPTER 9. The Temperature Gradient Near the Ground . 80

Frequency distribution of the gradients which occur. Diurnal and
annual march. Change of sign of the gradients at morning and
evening. Influence of water-vapor content and air movement.
Gradients with ground fog.

CONTENTS v

SECTION III
OTHER METEOROLOGICAL ELEMENTS NEAR THE GROUND

CHAPTER 10. Humidity Relationships 90

Role of the ground air layer in water economy. The wet type of
moisture distribution as normal type. The dry type. Variation of
vapor pressure and of relative humidity with altitude during the
course of the day. Observations in Germany, Finland and India.
Summary. Humidity fluctuations near the ground. Microclimatic
methods of humidity measurement.

CHAPTER IT. Wind Relationships 102

Variation of wind velocity with altitude. Dependence on tempera-
ture gradient and ground cover. Diurnal march of wind velocity.
Frequency of calms at the ground. Transport of dust and snow.
Temperature effect of wind at night. Destruction of inversions.
Temperature gradient, wind gradient and wind velocity in their
mutual interdependence. A storm destroys the microclimate.

CHAPTER 12. Optical and Acoustical Phenomena. Content of
Dust, Carbon Dioxide and Emanation . . . . . .117

Lack of optical homogeneity in the air layer near the ground.
Stratification. Mirages. Ground rainbows and halos. Sound
transmission. Dust and CO2. Radium emanation.

SECTION IV
INFLUENCE OF THE GROUND ITSELF ON THE CLIMATE NEAR IT

CHAPTER 13. The Temperature of the Ground Surface . . .128
Coefficient of reflection for three spectral bands and different
surfaces. Definition of surface temperature. Direct and indirect
measurement of it. Mercury thermometer, electric thermometer,
flatiron method, wax-cone method. Temperatures on asphalt.
Comparison with black-bulb temperature. Influence of color of
surface. Technical surfaces. Railway tracks. Freezing of the soil.

CHAPTER 14. The Influence of the Type and Condition of
the Soil 138

Temperature march in different kinds of soil. Extreme and mod-
erate microclimate. Influence of soil tillage and soil moisture.
Immediate effect of rain. Observation of soil properties with
snow, frost and glaze.

CHAPTER 15. The Air Layer over Water 153

Water as a surface capable of convection. Depth of penetration
of radiation. Relation of the coefficient of reflection of water sur-
faces to height of the sun. Reflection and vineyards. Tempera-
tures of surface water in ocean, lakes and ponds. Daily tempera-
ture variation over the ocean. Air layer near the water of the
Baltic Sea. Measurements in pools and reeds.

vi CONTENTS

CHAPTER 16. The Air Layer near Snow 164

Albedo of a snow surface. "Light climate" over snow. Perme-
ability of a snow-cover to radiation. Temperatures in and above
the snow-cover. Heat protection. Frozen, thawing and porous
snow covers. Ice-plate formation and melting cavities.

CHAPTER 17. The Air Layer above a Sod Cover . . . .175
Effect of a plant cover. Grass — minimum thermometer. Even a
scanty growth modifies high soil temperature. The "foot-ring
disease." Comparison of runway and sod surface at an airport.
Temperature observations over a sod cover.

SUPPLEMENT. On the Quantitative Determination of the Heat
Economy of the Ground Surface 182

Significance, extent and history of the problem. The four main
factors in heat exchange. Measurements in the snow. Heat ex-
change in the four seasons according to measurements in Pots-
dam and Palkane.

PART TWO

The Microclimate in Its Relation to Topography, to Plants,
Animals and Man 191

Introduction: A fundamentally new kind of microclimatic treat-
ment. Mesoclimatology. Bioclimatology.

SECTION V
THE INFLUENCE OF TOPOGRAPHY 194

CHAPTER 18. Cold Air Floods and Cold Air Dams .... 195
Origin of cold air lakes. Downflow and damming up of cold air.
Frost areas at Munich and Eberswald. The Gstettneralm sink hole.
Effect on plant and animal world. The cold pole of the earth
microclimatically determined. The 1930 catastrophe in the Maas
valley. Gliding speed of cold air. Rhythm of cold-air drops.

CHAPTER 19. Nocturnal Temperature Relationships in Valleys 204

Difference between the movement of cold air and water. Warm
slope zone. Upward evening movement of the inversion. Height
of the warm slope belt and relation to the plant world. Example
from the Arber. Change of temperature with time. Visibility of
nocturnal temperature stratification.

CHAPTER 20. Cold Air Winds (Down-Slope, Down- Valley and
Glacier Winds) 211

Cold air flood, down-slope wind and down valley wind. The
Wisper wind. Air avalanches in Europe and Africa. The glacier
wind as a diurnal cold-air wind.

CONTENTS vii

CHAPTER 21. The Sunniness of Different Slopes . . . .215

Calculation of radiation on a slope. Diffuse radiation and cloudy
weather obscure slope differences. Influence of latitude and alti-
tude. Basic rules of radiation on different slopes. Daily heat totals.
Soil temperatures around a hill. Temperatures on rocky slopes.

CHAPTER 22. Microclimatic Effect of Different Exposures to
Sunshine 231

Ants' nests and termites' dwellings. Temperatures of standing
trees and felled logs. Bark splitting and scaling. Blooming time on
different sides of a tree. Compass- and gnomon-plants.

CHAPTER 23. The Skin of Air on Mountain Slopes . . . .241
Proof of the air-skin on mountain slopes. Influence of plant cover.
Humidity in the air-skin. Precipitation on the favored slope. The
moist east side.

CHAPTER 24. More on the Influence of Topography . . . 248

Daily march of temperature and humidity in a valley, on a slope
and on a peak. Temperature decrease with height in relation to
weather and time of day. Microclimatic scattering of temperature
values obscures the effect of altitude. Mountain atmosphere. Zone
of influence. Up-slope wind and up-valley wind. The plant cover
as indicator of the slope climate. Examples.

CHAPTER 25. Concerning the Range of Validity of Meteorologi-
cal Stations 259

The "representative" observation station. Desire for a closer net-
work of macroclimatic stations. Microclimatology extends the
useful range of the station. Judging the climate of unknown
places. Radiation relationships. Shading by mountains. Average
screening angle, spatial angle of the open sky and amount of com-
pletely diffused radiation. Wind effect. Sampling measurements.
Consideration of the plant cover. Microclimatic special network.

SUPPLEMENT. The Microclimate of Caves 265

Caves open at one place. Ice-caves. Pocket caves. Reservoirs of
cold. Caves open at two places. Wind tunnels.

SECTION VI
THE INFLUENCE OF PLANT COVER 269

CHAPTER 26 .The Heat Economy of Plants, and Plant Tem-
peratures 271

Radiation economy of leaves. Reflectivity in relation to wave-
length. Permeability. Blue shadows and green shadows. Absorp-
tion. Radiation outward. Protective measures of plants. Difference
between plant- and air-temperature. Methods and results of plant-
temperature measurements. Temperatures in bags.

via CONTENTS

CHAPTER 27. Radiation and Temperature Relationships in a
Low Plant Cover 284

Ground cover, lower plant cover and forest. Radiation economy.
Outer effective surfaces. Measurement of temperature fluctuation.
Influence of structure, thickness and height of plant growth. Daily
temperature march. Microclimate in vineyards. Measurements
under tropical conditions in India and Africa.

CHAPTER 28. Humidity and Wind Relationships in a Low Plant
Cover 297

Exchange of water vapor in the plant cover. Influence of density
of stand. Relative humidity between leaves. Series of measure-
ments from South India. Dew. Braking of wind. Height of
friction level. Influence of wind velocity and the kind of plant
cover, on the braking action. Amount of cooling.

CHAPTER 29. Forest Meteorology, Forest Climatology and Stand
Climate 309

Problem of forest meteorology. Concept of habitat climate and
forest climate. Influence of the forest on the macroclimate. Dupli-
cate forestry stations for the study of the trunk-space climate.
Concept of the stand climate. Methods for its study.

CHAPTER 30. Radiation Relationships in an Old Stand . . -317
Weakening and filtering of penetrating radiation by the stand.
Influence of kind of woods and age of stand. Moving spots of
light. Maps of brightness in cloudy weather. Lighting of sloping
surfaces in the stand. Outward radiation from the forest roof.

CHAPTER 31. Temperature and Humidity Relationships in an
Old Stand 326

Temperature march during a summer day in the stand — from the
forest floor to the top of the crown. Temperature stratification by
night and by day. Distribution of moisture in the stand. The
forest floor and the crown-space as distributors of water vapor.

CHAPTER 32. Wind and Precipitation in an Old Stand . . . 336

Braking action of the stand on wind velocity. Quiet air in the
trunk-space. Influence of leaf-growth. Interception of light pre-
cipitation by the crown. Water that runs down the trunks. Pene-
tration of snow to the forest floor. Temperature effect of the
snow cover. Frost-plates under spruces.

CHAPTER 33. The Influence of the Make-up of the Stand on
Its Climate 342

Forest microclimatology as an auxiliary science of forestry. Tem-
perature measuring journeys in the forest. Stands with uniform
canopy, and those of various heights. Groups and screens.

CONTENTS ix

CHAPTER 34. The Microclimate of Circular Slashings, Clear-
ings and Cuttings 350

Critical size of hole-cuttings. Relationships of outgoing radiation,
incoming radiation, and ventilation. Increase of liability to frost
damage, with increased size of hole-cutting. Critical sizes. Out-
ward radiation and sunning of forest cuttings. Wind movement
in cuttings.

CHAPTER 35. The Climate of the Stand Border 357

Outer edge and inner edge. Two causes for the climate of the
stand border. Incoming heat radiation from sun and sky. Width
of shade at edges of stand. Frost protection by minimizing net
radiation. Passive and active influence of the forest on the wind
field. Diurnal forest-wind and nocturnal forest-wind. Seed dis-
tribution at edge of the stand. Fog precipitation and its signifi-
cance. Research problems for the future.

SECTION VII

THE RELATION OF ANIMATE CREATURES AND MAN TO
THE MICROCLIMATE

CHAPTER 36. The Animate World and the Microclimate . . 367

Significance of microclimatoolgy to zoology. Bioclimatic index
forms. Habitat-limited, and fixed-habitat animals. Protection
against radiation, heat, dry ness and winds. Ants, grasshoppers
and chickens as examples. Design of habitations: ants' nests, rabbit
burrows. Hibernation place of bats. Temperature regulation in
wasp and bumblebee nests. Forest entomology and microclimatic
research.

CHAPTER 37. The Unintentional Effect of Man on the Micro-
climate 375

Instinctive reaction to the microclimate and rational microclimatic
search. Man as a disturber of the microclimate. Impoverishment
and monotony of cultivated nature. Creation of new micro-
climate through industrialization and city-building. Dangers from
neglect of microclimatic effects. Soil erosion.

CHAPTER 38. The City Climate 379

Two methods of investigation. Significance of the city climate.
Warming and pollution of city air by combustion. Dust-content
measurements. The "haze-hood." Effect on heat economy and
temperatures. Air humidity in city and country. The peculiar
wind system of the city. City fog. Release of precipitation in
city areas.

x CONTENTS

CHAPTER 39. The Conscious Modification of the Microclimate
by Man 386

The rational climatic search. Health resort climatology and its
dependence on microclimatology. Intentional creation of micro-
climate. Clothing climate and bed climate. Living-room climate.
Expediency of air-conditioning. House and city climate. Layout
of hospitals. Microclimate of animal stables. Greenhouse climate.
Agricultural and forest microclimatic measures. Wind protection.
Action of windbreaks. Frost protection.

CHAPTER 40. Destructive Frost as a Microclimatic Phenomenon 396

Early and late frosts. Summer night frosts. Altitude and late frost
danger. Conditions favorable to the onset of damaging frost.
"Grass frost." Preventive frost protection. Foreplanting of hardy
varieties. Control of frost source regions. Timely frost warnings
by forecasts and alarm systems.

CHAPTER 41. The Battle Against Destructive Frost ... 403

Possibility and prospect of artificial frost fighting. Conditions in
America and Europe. Work of the Imperial Weather Service.
Control of outgoing radiation by covering with screens or caps,
by smudging and by flooding. Production of warmth by artificial
rain, destruction of inversions or heating. Experience and results.

LITERATURE 413

SOURCES OF ILLUSTRATIONS 463

AUTHOR INDEX 465

SUBJECT INDEX 473

PREFACE TO THE FIRST EDITION 1927

I was introduced into the realm of microclimatology by Professor
A. Schmauss. When he put me in charge of the organization and
direction, of the Bavarian special network for investigation of the
air layer near the ground; and later when I had to make two greater
open-air investigations in the realm of forest meteorology, I had
the opportunity of getting in closer touch with people dealing in
forestry, moor cultivation, and agriculture. On this occasion I got
acquainted with the difficulties, everywhere met, in the practical
application of the results of climatological research. This problem
of application is indeed not new, and several valuable contributions
have already been made (I hope that this book will prove this fact) ;
but a systematical study has not been undertaken as yet. The practi-
tioner has neither time nor opportunity to look for the respective
papers from the vast meteorological literature. When, therefore, I
was invited to write a "Climate of the Air Layer Near the Ground,"
I was glad of the opportunity to attempt a first survey of micro-
climatological problems.

With this book I hope to be able to give my best thanks to those
above mentioned men for the manifold suggestions which I have
received from them; especially the scientists and practitioners in for-
estry, highly interested in microclimato logical problems. It is also
with the greatest pleasure that I express here my sincerest thanks to
Professor Schmauss for his constant and unselfish furtherance of
my work.

DR. RUDOLF GEIGER

Meteorologist at the Bavarian Landes wetter warte
Meteorological Observatory of Bavaria

and

Privat Dozent at the University of Munich
Munich, July 1927

FROM THE PREFACE TO THE SECOND EDITION

During the past decade, microclimatology has experienced an ex-
pansion and development to an unexpected degree. Since the ap-
pearance of the first edition, some thousand new works have been
published by meteorologists alone. Perhaps more fruitful still has
been the progress of microclimatic research methods in allied sciences,
in the habitat teaching of botany, in forestry and gardening, in zool-
ogy, biology and medicine, in agriculture, room planning and ar-
rangement— even in the technical aspects of traffic and construction
work.

The book was out of print for several years, so that it required
complete revision. Hardly any sentences are left of the first edition.
Nevertheless it is the same old book, for the purpose is unaltered.
The plan and operation of microclimatology were there stated and
the results so far attained in this new and promising field of research
were presented. But while the attempt of 1927 was justified by the
novelty of the goal, the rounded picture of the new field of endeavor
can only now be completed. The subtitle of "Textbook of Micro-
climatology" has seemed justified.

Certain necessary generalizations have been made as compared
with the former edition. Section IV takes up the influence of the soil,
including the air layers in proximity to water and snow. Chapter 36
deals with the relations between the animal world and microclima-
tology. The relations of man to the microclimate are not merely
touched on but are systematically treated in Chapters 37 through 41.

Everything has been deleted which did not strictly pertain to the
theme. The question of frost damage occupied the whole of the last
section in the first edition. Since then there has appeared, in 1940,
the work of O. W. Kessler and W. Kaempfert in "Protection against
Frost Damage," which gathers up all old and new research and ex-
perimentation in this field. When, on page 7 of this publication it is
said of the "The Climate near the Ground" that it has "brought the
whole frost problem into the front rank," it has done its best for
this question and can therefore rest. Whoever is especially concerned
with the fight against frost should refer to the listed publications
of the Weather Service. In the new edition, consequently, only the
necessary survey of the problem within the compass of general micro-
climatology has been given in the two last chapters.

xiv FROM THE PREFACE TO THE SECOND EDITION

I have taken especial pains with the references. It is particularly
important that the reader should have easy access to original sources.
Those of experience know how much trouble this entails. The litera-
ture cited may still have omissions particularly in the border prov-
inces in spite of every precaution. I hope such will be brought to my
attention. The extent of the references here given could be set at
about 800 items, for there are several separate lists available which
I need only mention — namely, that of H. Lettau on the problem of
mass exchange, of A. Kratzer on City Climate, of B. Huber on the
heat economy of plants, and that of O. W. Kessler and W. Kaemp-
fert on the frost question. In this way 1200 other works have been
included without particular mention. On the whole, a considerably
enlarged content of the second edition remained unavoidable. Both
editor and publisher have, in spite of wartime conditions, made pos-
sible a publication suited to the problem, especially in the tripling
and bringing up to date of the illustrations, for which I wish to
express my thanks here.

RUDOLF GEIGER
Eberswalde, June 20, 1941
Meteorological Institute of the Forestry College

PREFACE TO THE TRANSLATION OF THE
AMPLIFIED SECOND EDITION

The first edition (1927) was translated by Prof. John Leighly and
proved to be very useful. At his suggestion the Air Force Weather
Service obtained a copy of Dr. Geiger's second edition at the end of
the war: only one copy could be found. They were pleased with the
suggestion that a translation be prepared, but they were not in a
position to undertake it. Prof. Leighly could not then translate the
new edition, so the Air Force Weather Service kindly loaned it to
the Blue Hill Observatory.

About this time, Mr. Milroy N. Stewart, of the Rochester, N. Y.,
branch of the American Meteorological Society, indicated a desire
to make the translation; so the book was sent to him, anticipating
arrangements with the Alien Property Custodian for publication by
the Blue Hill Observatory. Dr. F. A. Brooks, of the University of
California, Davis, California, used a portion of his sabbatical leave
in 1947 for checking the heat transfer parts of the translation. We
are further indebted to Prof. Brooks, and also to Prof. James E.
McDonald, Dept. of Physics, Iowa State College of Agriculture and
Mechanic Arts, and Prof. H. C. S. Thorn, University of Maryland
and U. S. Weather Bureau, for critically reading proofs of the entire
book. The expense of publication was borne by the Frank Hagar
Bigelow, Class of '73, Fund for Publication and the Geophysical
Research Fund, both of Harvard University. The main burden of
preparing the edition for publication, including clarification of ob-
scure passages and some other translation, was carried by Dr. Wal-
lace E. Howell, who edited the translations and supervised or him-
self personally performed the numerous minor operations required.

A considerable and widespread interest in the translation of Dr.
Geiger's book developed, and many purchase requests were received.
Then, shortly before the translation of the second edition was fin-
ished, word was received that a third edition had been prepared and
was awaiting paper for publication in Germany. At this time, Dr.
Geiger had been "found." He kindly consented to supply additions
to the second edition that would make it possible for us to make our
translation the virtual equivalent of the third German edition. Mrs.
Victor Conrad translated the additions and prepared the indexes.

xvi PREFACE TO THE TRANSLATION

The photographic department of the Fogg Museum of Art, Har-
vard University, photographed all the illustrations in the second
edition. Miss Ann E. Reiter translated the legends. The Eastern
Engravers, Inc. performed the meticulous task of substituting
English for German words on the numerous diagrams. Mrs.
Barbara Click and Miss C. M. Whalen typed the translation, the
table headings prepared by Dr. Howell, and all but the largest
tables.

Milton, Mass. CHARLES F. BROOKS

August 1950

INTRODUCTORY CHAPTER

THE MICROCLIMATE AND MICROCLIMATIC RESEARCH

When regular meteorological observations began in Europe in the
second half of the 1901 century, it soon became evident that the re-
sults obtained therefrom were influenced by the exposure of the
meteorological instruments. In the larger countries, therefore, com-
prehensive series of experiments were soon carried out to determine
the most suitable exposures. After much trouble, they standardized
on the meteorological shelter which is today familiar to everyone.
Within this shelter, the measuring instruments are il/2 meters —
most of them, 2 meters — above the ground. This great distance was
chosen because at a lower position the variations of the ground, the
physiographic peculiarities, and the nearby surroundings, were too
evident. The air layer adjacent to the ground was a zone of dis-
turbance which should be avoided.

The high location of the instruments made it possible that the data
of the meteorological stations could be regarded as valid for a larger
surrounding district. The results of the points of observation —
separated by 10, 100 and even more, kilometers — made a unified
picture when pieced together. The general features of the climate as
a whole could be recognized for a given neighborhood or a given
country. This climate was therefore called the "large scale climate"
(or, using the Greek term, the "macroclimate").

In the meteorological year-books which the civilized countries
issue regularly, in the works on climatology and in descriptions of
climates in geographical works, it is this macroclimate which is
treated. The macroclimate of Germany is described in a recent ex-
haustive production in several volumes which the Imperial Weather
Service is publishing, entitled "Climatic Information on the German
Empire."

With the progress of science and especially with the increasing use
of scientific data for economic purposes, new needs have arisen. The
plan of the meteorological year-books is no longer sufficient. Indeed
they have even proved misleading when used without further prac-
tical precautions. For instance, the number of frost-days, as pub-
lished in the annuals, give a false picture of the frost danger to
agriculture. The published maximum temperatures are not authori-

xviii INTRODUCTORY CHAPTER

tative in determining the heat available to grapes on the vine. It was
soon found that all plants have their lives conditioned by that very
zone of disturbance which had been so meticulously avoided in
meteorological observations. Within this zone the prevailing climatic
conditions are different from those at 2 meters height. Thus arose
the question concerning the climate near the ground.

The macroclimate is of direct significance to man, who goes up-
right, breathes at a height of 1^2 rneters and is continually changing
his environment. The lower plant, however, bound as it is to one
location, is particularly dependent on the disturbed ground layer at
that period in its growth when it is most sensitive — its youth. Some-
times, therefore, the macroclimate is called the "climate of man";
the ground layer climate, the "climate of plants." These two desig-
nations are illustrative but they do not define.

By the expression "near the ground," we mean in this book, all
that is not more than 2 meters from the earth's surface. By the
"ground air-layer" therefore, we mean the lowest 2 meters of the
atmosphere. This distance serves temporarily to give the reader some
idea of the magnitudes involved. Later we shall have more to say
on the subject. In this use of the words "near the ground" we differ
from the aerologists, who think in terms of such a vast atmosphere
that for them the lower thousand meters are "near the ground."

The difference between the climate near the ground and the macro-
climate consists essentially in the proximity of the earth's surface. As
the lower limit of the atmosphere, this surface plays an important
role in meteorology. The heating and cooling of the atmosphere in
the course of the day and according to seasons, takes place in general
through it as an intermediary. By evaporation from it, water vapor
is given to the air — returning to it again as rain and snow. It acts
as a brake on the winds which pressure differences initiate. It is
therefore no wonder that the ground air layer shows peculiar cli-
matic characteristics. They will be described and explained in the
first part of this book.

But there is something more. While, in the upper air contrasting
conditions which occur are immediately equalized, in the air near
the ground they may continue to exist almost side by side, for every
convective movement which is initiated is tied up by friction on the
surface. Horizontal contrasts are added to vertical. Great climatic
differences can result within the shortest distances by reason of the
kind of soil, its form, the plants growing thereon, variable shading
or sunniness, different wind protection, and many other circum-

INTRODUCTORY CHAPTER xix

stances. G. Kraus coined for it the apt description, "Climate in the
Least Space." This term is somewhat too ceremonious for general
usage. In its place the word "microclimate" is used today. The best
definition of microclimate is, conversely, "climate in the least space."
Microclimatology is the science of the microclimate.

With the rise of microclimatological research, many new expres-
sions have come forward such as: "local climate," "peculiar climate,"
"miniature climate," etc. They all mean the same as "microclimate"
and are best forgotten. If one German word can describe the micro-
climate, that word is "Kleinstklima," but nobody likes to pronounce
these five harsh consonants in succession. The word "Kleinklima,"
which might be suggested, is objectionable on other grounds.

R. Geiger and W. Schmidt (6) (the italic figures refer to the litera-
ture cited at the end of the book) have made an attempt to intro-
duce unified terminology in microclimatology. This attempt has
resulted in confusion rather than clarification. According to their
proposal the word "Kleinklima" should occupy an intermediate po-
sition between the expressions "macroclimate" and "microclimate"
and has been used in this sense in several places. It now appears to
me an unavoidable necessity, sooner or later to settle on such an
intermediate term; the real need is becoming more apparent. The
designation "Kleinklima" is, however, quite unsuitable, for to avoid
the distasteful word "Kleinstklima" or the adjective "Kleinstkli-
matisch," "Kleinklima" has been used in numerous publications as
synonymous with microclimate.

The word "Kleinklima," consequently, having two different mean-
ings, has become useless. H. Scaetta (77) has made an excellent
suggestion: Between the macroclimate and the microclimate should
be the "mesoclimate." I was very much tempted to make the de-
scription clearer by the introduction of this new term. But it is still
too soon for that and the danger is too great that even this expression
would be misunderstood and misused. The terms "mesoclimate"
and "mesoclimatology" appear therefore in neither title nor inscrip-
tion. The attempt has merely been made to inform those same
readers who will actually read the book, in the proper places, in
order to lay the basis for future developments.

In this book everything which does not belong to the macroclimate
and which concerns the climate of a very small space, is brought
together under "microclimate."

It is no accident that microclimatology has been developed in
Germany. The lack of living space, and the consequent necessity of

xx INTRODUCTORY CHAPTER

getting the utmost out of the earth, has favored this development.
If we look back over the history of microclimatology it is no less
noteworthy that it was a scientist who, out of purely scientific inter-
est, first concerned himself deeply with microclimatic problems.
It was the Wiirzburg botanist, Gregor Kraus (1841-1915) .l By the
publication in 1911 of his book (72), "Boden und Klima auf klein-
stem Raum," he became the father of microclimatology, although
he did not use that word himself. Not a practical application but
basic research in the fundamentals of the science guided him in his
first investigations in the Wellenkalk district of the Main at Karl-
stadt. "Having undertaken the task," he writes, "I realized that I
stood here alone, and that, to accomplish something permanent, the
very foundation would have to be laid 'in the egg' and everything
done for the first time. From the first undertaking, working back-
ward toward more solid ground — this is the way the material in
Part I was assembled." These words, applied first of all to his work
on soil conditions in smallest space, apply equally well to climatic
conditions in the same limited space.

If G. Kraus became known as the father of microclimatology, it
was because he was the first to see the problem clearly, to formulate
it and to attack it. Along with him many others have pioneered,
especially Th. Homen, whom V. Rossi (2/1) calls the "Founder of
microclimatology in Finland." Yes, as we look back, we are able to
find references to microclimatological problems in early times as, for
example, B. H. Grimm (7) quotes some such sentences from the
chemical letters of Justus von Liebig. Thus it is with all newly de-
veloping scientific fields. We hope that the factual information in
this book will direct each co-worker in microclimatology to the right
place.

Microclimatology occupies a special position in the realm of the
natural sciences. As part of climatology it pertains to the great
technical province of meteorology and is, systematically, mostly in-
debted thereto. At the same time it is also so closely involved with
numerous kindred sciences that plenty of suggestions and worth-
while projects originate in those fields. Among these, botanists are
some of the first to be mentioned — particularly ecologists. Repre-
sentatives of forestry and agriculture have cooperated. The zool-
ogists, too — among them, the entomologists in particular — find
in microclimatology the habitat condition for the favorable or un-

1 His biography by H. Kniep is to be found in Berichte der Deutschen
Botanischen Gesellschaft, Vol. 33, pp. 69-95, 1915.

INTRODUCTORY CHAPTER xxi

favorable development of animals. In many questions the physi-
cian is interested; in many others, the geographer. Even the
technician and the tradesman come up against climatic peculiarities
in restricted spaces — in street construction, railroad building, house
construction, and the establishment of communication systems. Thus
microclimatology affords an exceptionally fine example of a scientific
community of effort. While formerly the extent of science has led
to the leveling out of research, and its depth to undue specialization,
in microclimatology the two formerly contradictory extremes seem
to join hands. As a special science it can and must deepen; being
rooted in a great number of technical fields it possesses at the same
time an enlivening and enriching breadth.

PART ONE

Concerning the microclimate existing near the ground by
virtue of its proximity to the ground surface.

In order to learn about the climate near the ground, we investigate
in the first part of the book the influence which the ground exerts
on the climate of the boundary layer of air next to it, to a height of
about 2 meters. In order to begin with the simplest conditions, we
shall think first of a completely flat plain, free of plant growth. Sec-
tion I will establish a fundamental point of view regarding the role
played by the ground surface in the heat economy of the atmosphere
and hence of the layer of air next to the ground. In particular, the
manifold ways and means by which heat moves to and from the
ground will be discussed.

Section II shows the consequences of these with respect to the
temperature relationships, which in the layer of air next to the ground
are so completely different from those in the realm of the macro-
climate. The other weather factors, namely vapor pressure and
relative humidity of the air, wind velocity, dust content, visibility
relationships, and so forth, are treated in Section III.

Even omitting the influences of topography, vegetation, and build-
ings, which are to be considered in Part Two of the book, the kind of
ground yet has significance for the climate near the ground. Not
only the material, the water content, and the color of the ground
must be considered, but furthermore above a water surface or a snow
surface the air layer next to the surface has especial characteristics.
These questions are the subject of Section IV.

SECTION I

HEAT EXCHANGE NEAR THE GROUND
CHAPTER 1

MIDDAY HEAT EXCHANGE AT THE GROUND SURFACE
THE INCOMING RADIATIONAL TYPE

At the upper limit of its atmosphere the earth receives a vertical solar
radiation amounting to about 2 calories per square centimeter each
minute. This value is called "solar constant." At European latitudes
normal incidence does not occur. There the horizontal surface
receives at the border of the atmosphere only a portion of the solar
constant. When this radiation penetrates the earth's atmosphere it
suffers a series of losses.

Fig. i shows the heat exchange at noon of a summer day in Ger-
many; the width of the arrows in the figure give an idea of the rela-
tive amounts of the transferred heat totals. First, we consider only
the heat transport caused by short wave radiation (length of waves
below i /A) (in Fig. i widely dotted stripes).

A considerable portion of the enormous incoming sun energy is
reflected by the surface of the clouds and is ineffective concerning the
heat economy of air and ground. As an average for the northern
hemisphere and the year, this amount is 33% of the incoming radia-
tion. In the atmosphere another portion of radiation is scattered in
all directions diffusely by the air molecules themselves and by sub-
stances suspended in the atmosphere (dust, plankton). The radiation
does not suffer a loss in the true sense of the word but only a deflection
from its original direction. But because a portion of the scattered
radiation goes back to universal space (Fig. i) also this portion is
eliminated with regard to the terrestrial heat exchange. Reflections
from clouds and diffuse scattering into universal space make together
42%. The reflecting power (albedo) of the earth, therefore, is 0.42;
for the inhabitant of universal space the earth looks about as bright
as Venus does for our eyes.

The third loss is the absorption of radiation caused by ozone,
water vapor, and carbonic acid; this is a true loss in that the radia-
tion energy is used to increase the temperature of the absorbing

Universal space

Radiation from sky * .

Surface

Supplied

Heat transport by:

absorption

here

Long wave outgoing radiation

Evaporation

Layer close

Convection

to the surface

Radiative pseudo conduction
» Heat conduction

to the ground

*.*.*-*•'] Short wave radiation
*r*rvr;X;*vi Long wave radiation

Molecular heat conduction
Convection

Changes of the physical state of the water

FIG. i. Heat exchange at noon for a summer day. (The width of arrows corresponds
to the transferred heat amounts)

4 THE CLIMATE NEAR THE GROUND

gases and, therefore, is eliminated from the insolation economy.
What happens with this portion is not discussed in this book.

Despite the enormous distance the sun rays have to pass through
from the limit of the atmosphere down to the bottom of the atmos-
phere, a mighty radiation flux penetrates down to the earth's surface
partly as direct sun radiation, partly as scattered radiation from the
sky. The two together represent the main portion of the solar heat
at the disposal of the heat economy of earth and air. Wherever this
immense energy current strikes upon the surface of the solid ground
the radiation cannot penetrate this obstacle. A portion is reflected
from the surface. Most of it is absorbed, changed into heat, and
serves to raise the temperature of the ground.

The earth's surface, then, plays the most important part in midday
heat exchange, but the layer of air next to the ground is that part of
the atmosphere whose temperature relationships are most directly
determined by the relationships of the surface itself. Observations
of this lowest layer of air are therefore indispensable to studies of
heat transfer.

If we glance back over the history of meteorology, we find that
the importance of the air layer adjacent to the ground led to the
first observations in this province. The study of this lowest layer
thereby became a branch of general meteorology, the physics of the
atmosphere. Only later did the climatological side gain attention,
when experience in practical farming made people realize the very
different climate to which young plants are subjected close to the
ground.

It will first be necessary to obtain actual values for the amount of
radiation reaching the earth's surface at noon, here in Germany. For
this purpose we shall use the measurements made at Potsdam from
1907 through 1923 as published by W. Marten (34) and thoroughly
analyzed by J. Schubert (39) . The results are presented in the accom-
panying table, which is divided, on the one hand, as to (A) clear or
(B) partly cloudy weather, and on the other hand as to whether the
receiving surface is perpendicular to the radiation (normal radiation)
or horizontal. In each case the value given is for 12 noon even when,
with a cloudy sky, the highest radiation value occurs during the fore-
noon.

When considering the microclimate at high altitudes, for example,
the living conditions of alpine plants, it must be remembered that
solar radiation increases with height above sea-level. The increase is
most rapid in the lowest, dust-filled air layers. The higher we go,
the less the gain in radiation with increased height. W. Morikofer

MIDDAY HEAT EXCHANGE

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6 THE CLIMATE NEAR THE GROUND

(35) giyes the following values for the noon radiation of a cloudless
day in January, based on measurements at Davos, St. Blasien and
Karlsruhe.

Altitude ... 100 500 1500 4000 m

Radiation (calories per square
centimeter and minute) ... 0.8 1.2 1.4 1.6

Similar data for all the months and for altitudes from 390 to 1577 m
may be found in F. Lauscher ( jj) .

In the mountain ranges still more than in the lowland, the climate
of the layer near the ground is in greater contrast to the macro-
climate on account of increased radiation and a simultaneous de-
crease of air temperature. The saturated flower color of the alpine
plants prove this fact. Unfortunately, systematic observations in
this direction are still lacking.

The radiation-exchange is not fully described by the treatment of
short wave radiation. The sun radiation is accompanied by the out-
going radiation, most effective in the long wave portion between 4 JJL
and 32 fji. The amount of this radiation loss from the earth's surface
is also plotted on Fig. i. In comparison with the enormous incom-
ing radiation, the long-wave outgoing radiation plays only a small
role. The balance of radiation of the earth's surface is strongly posi-
tive at noon in summer. At night, however, when no radiation
from the sun exists, it is just this long-wave outgoing radiation
which controls the exchange of radiation. In the following chapter
we shall deal with this phenomenon when discussing the radiation
balance by night.

The temperature conditions of the layer near the ground are de-
termined by the immense amount of heat which the surface of the
ground absorbs. In summer, this surface is heated in our region up
to 6o°C, sometimes to 70° and 80°; (see Chapter 13). The tempera-
ture of the surface would be increased even much more if a heat
loss — caused and maintained by the temperature contrasts — did
not take place upwards and downwards. Fig. i shows direction and
amount of these various heat currents. One portion of the heat is
conducted from the surface to the deeper layers of the ground as is
further described in Chapter 3. The greater portion serves to heat
the air layer near the ground and thus, indirectly, to heat the atmos-
phere. Partly also here, heat conduction is effective, but as can be
seen from the small arrow of Fig. i it does not play an important
role as far as quantity is concerned. Primarily, convection and

MIDDAY HEAT EXCHANGE 7

radiative pseudo conduction come into consideration. These are
phenomena which are discussed concerning their origin and effect
in Chapters 4 and 5. Furthermore, the ground loses much heat as a
consequence of evaporation since the surface is deprived of 600 gcal
if one gram of water evaporates; this is an amount of heat which
would suffice to heat 6 g water from o°C to the boiling point.

From the significance of the earth's surface for heat exchange it
can be concluded that the highest temperature at about noon is at the

ii i i i im i

FIG. 2. The incoming-radiation (insolation) type. (Tucson, 21 June 1915)

boundary between ground and air; starting from here, the tempera-
ture decreases upward and downward. This kind of temperature
distribution at noon time is called "Incoming Radiation Type." The
real character of this type will be demonstrated by an extreme
example.

Fig. 2 gives the temperature distribution which J. G. Sinclair (40)
observed at the Tucson Desert Laboratory on June 21, 1915, at
i P.M. As we approach the ground from above, the temperature

8 THE CLIMATE NEAR THE GROUND

rises continuously and at an increasingly rapid rate. At the surface
there is a temperature discontinuity between air and earth. The
surface itself possesses the highest temperature, not measured here,
but in any case far above 71.5°, the measurement at a depth of 4 mm
in the ground. In the first 10 cm of earth the temperature decreases
with extraordinary rapidity, so that at a depth of 7 cm it is already
several degrees below the air temperature. The effect of the time of
day, the temporary noon-time heating, extends to a depth of only
about 10 cm, as the break in the temperature curve indicates. In
the lower earth layers the temperature falls again slowly.

Extremely high midday temperatures are therefore, as the illus-
tration indicates, limited to the air and soil layers immediately bor-
dering the earth's surface. Even under our mild climatic conditions
the same holds true, though to a lesser extent. In the consideration
of the daily march of temperature (Chapter 8) further examples are
given of the temperature distribution with an incoming radiation
pattern.

While the laws of heat movement in the ground have long been
known, the rapid decrease of temperature upwards in the lower air
at midday is of particular interest. P. T. Smoliakow (^/) has
recently treated the question theoretically, especially in reference to
microclimatology. The following general facts have been estab-
lished : —

If dry air is moved up or down in the atmosphere adiabatically,
i.e. without the addition or subtraction of heat, its temperature
changes. In moving upwards it comes into a region of lower air
pressure, its volume increases, and work is thereby performed, which
must draw adiabatically on the heat energy of the air itself. It there-
fore becomes cooler. Descending air, on the other hand, becomes
warmer. Thermodynamics teaches that this temperature change
amounts to i°C per 100 m difference in altitude.

If the "adiabatic gradient" of i°C per 100 m prevails in the atmos-
phere, an air parcel moving either up or down will at all points find
the same temperature as it has itself and the same pressure as well.
It is in neutral equilibrium.

If the temperature decrease is less than i°C per 100 m, a rising
air parcel will come into warmer surroundings. It will be heavier
than the surrounding air and will therefore return to its original
position. The air is in stable equilibrium. If the temperature de-
crease, however, is greater than i°C per 100 m, so that the gradient is,
as we say, "super-adiabatic," a rising air parcel must reach colder

MIDDAY HEAT EXCHANGE 9

surroundings and its upward movement is accelerated. Unstable
equilibrium prevails.

These thermodynamic considerations are based, however, on two
assumptions. First, the air parcels put out of balance must have
the necessary freedom of motion so that they can obey the changed
conditions. Second, the above considerations are valid only for iso-
lated air parcels ascending and descending respectively within an
air mass which is essentially not influenced by the processes which
cause the instability of the air parcels. H. Wagemann drew my
attention to this fact. Neither assumption holds true for the layer
near the ground. There, heating from below is so strong and uni-
form above great areas of the surface that superadiabatic gradients
become regular in the presence of intense incoming radiation.

TABLE 2
Month (1925) Feb. April June Aug. Oct. Dec.

Greatest difference of temperature

in °F between 1.2 m and 7.1 m

2.8

2.9

3-6

2.9

2.4 1.4

Calculated lapse rate (°C per

100 m)

23 13

In the free atmosphere superadiabatic gradients not only occur
rarely but they surpass the amount of i°C/ioo m only by a few
tenths of centigrade degrees. Approaching the ground these condi-
tions change. Observations by N. K. Johnson (182) in England in
1925 yielded the following maximum values of the difference of the
true air temperature at the heights of 1.2 m and 7.1 m:

These values occurred throughout at hours between n A.M. and
2 P.M. Even at these heights above the ground each month shows
the formation of a temperature gradient which is from ten to thirty
times as great as the adiabatic.

There remains, however, the fact that the air layer next to the
ground is at this time unstable in the highest degree. There are
two resulting phenomena which serve to demonstrate this particu-
lar condition to every thoughtful observer. One is the formation of
streaks, which we shall treat as an optical phenomenon in Chapter 12.
The other is the formation of dust whirls, also called sand-devils or
according to A. Wegener (43) "small spouts." They are such
a common phenomenon and so well-recognized a sign of superheat-

10 THE CLIMATE NEAR THE GROUND

ing of the air near the ground that they occupy a recognized place
in the "ww group" among the international meteorological symbols.

Instability may be considered as the final preparation for an upset
of stratification, with the warm air ready to eddy upward and the
cold air to sink. At the ground the initial impulse which will put
this overturn into action is still wanting. When, however, such an
upward whirl is initiated through some outside agency, the adjacent
layers are drawn into the movement and the phenomenon proceeds,
borne along by the wind, affecting new layers one after another, and
thus gaining strength. Immediately there begins — for some still
rather obscure reason — a whirling motion which quickly intensi-
fies and so there is formed a whirlwind with an axis which is vertical
or inclined slightly forward with the wind.

This whirlwind first becomes visible when it picks up dust, sand,
leaves, grass or, with further development, even stones and branches.
The formation of a whirl is often easily observed. Haycocks, road-
side slopes, the piles of stone along country roads, are favorite points
of origin, because there the first upward movement of the heated
air is favored by the shape of the surface.

I once made an observation of this sort on a scorching hot sum-
mer noon while walking on the Jura chalk plateau of the Prankish
Alp (at Hetzles near Erlangen). Fig. 3 shows the conditions

General air movement

nigrating dust whirl

._ , layer of superheated air w

Upwind j/ Point of Origin

Plateau of Jurassic Limestone

FIG. 3. The origin of a dust whirl (small spout)

schematically. Over the white, dry chalk plateau, sparsely covered
with vegetation, there formed a strongly superheated layer of air
next to the ground. An upset was initiated at the steep western
edge of the plateau, where the vertical component of the wind
introduced an upward movement.1 A whirl started off from this

1 On account of the tilting of the geological strata toward the east, the west-
ern rim of the Prankish Alp is the highest point of the plateau, thus favoring
vertical motion in that region.

MIDDAY HEAT EXCHANGE 1 1

point, raising dust and leaves with a clearly audible whistling sound
to a height above one's head, and moving off with the general drift
of the air from the edge toward the interior of the plateau. After
leaving the outer zone its behavior became unpredictable. It jumped
to one side or the other, wherever favorable conditions existed for
heated air to break through upward, but died out in a few minutes,
over the uniform plateau. At the edge, however, the formation was
several times repeated.

TABLE 3

NUMBER OF DUST WHIRLS OBSERVED 1927-1932

Jan. Feb. March April May June July

In Egypt and in the Sudan

. . o

In Palestine and Transjordan . .

. . 0

In Iraq

Total: o o 10 27 43 73 86

Aug.

Sept.

Oct.

Nov.

Dec.

Year

In Egypt and the Sudan
In Palestine and Transjordan . .
In Iraq

. . 26
•• 34

27
25

5
5

I
I
o

i
i

164
130
US

Total: 74 70 22 2 2 409

H. Schlichting (57) gives a very interesting account of a whirl
which came under his observation at Liibeck at i P.M. on a certain
day in May, 1934. H. Schober (38) described one which originated
at the boundary between a heated meadow and a shady street.

The most thorough going studies of dust whirls we find in the
works of W. D. Flower (jo) who in the years following 1927 car-
ried out regular observations at 12 meteorological stations in the
dry region between Egypt and the Persian Gulf. The following
results were obtained by months for the years 1927-1932.

The large numbers in those months with the strongest radia-
tion is evident. Just as prominent is the grouping according to time
of day. Fig. 4 shows the daily march of dust-whirl frequency for
the years 1926-1932 in the three above mentioned localities. Also
shown is the mean temperature gradient in Ismailia for those days
in 1932 on which dust whirls were observed. The increasing tend-
ency to dust-whirl formation with increasing temperature gradient
is plainly recognizable.

The dust whirls here observed had mostly a height of between

THE CLIMATE NEAR THE GROUND

25 and 50 m. The limits were <5 and >i,ooo m. Even in a region
so far north as Iceland, A. Wegener (42) observed dust-whirls even
1,000 rn in height, which were, to be sure, over vegetationless plains
of black lava sand where at midday conditions are favorable for
great heating of the air layer next to the ground.

According to W. D. Flower, the duration of a dust-whirl in about
one fourth of all cases is less than 30 seconds. Most of them last

FIG. 4. Frequency of dust whirls (continuous line) and temperature gradient from
4 to 50 feet (broken line). (After W. D. Flower)

several minutes, but not over 20 minutes, at the longest. By watch-
ing pieces of paper and the like which they carried along it was
determined that they whirled at the rate of o.i to 0.6 revolutions
per second. In 175 cases the whirls turned in a clockwise direction;
in 200 cases, counterclockwise. There seems therefore to be a rather
indifferent distribution in this respect. When the rotation was clock-
wise, the course of the whirl as it died out was counterclockwise
away from the wind direction, and vice versa. This corresponds to
the "Magnus effect." This turning away from the course of the
wind continued to the end with increasing curvature, so that in its
final moment the dust whirl sometimes for an instant stood directly
against the wind.

Recently F. Rossmann (56) has been studying the law of motion
of waterspouts.

CHAPTER 2

NOCTURNAL HEAT EXCHANGE AT THE GROUND SURFACE.
THE OUTGOING RADIATIONAL TYPE

From the incoming radiational type, which is most clearly demon-
strated on a hot summer day, we now turn to the opposite condition,
the outgoing type, which is best seen on a cold winter night.

Solar radiation, which governs the heat exchange by day, is lacking
at night. No other natural source of energy is comparable to that of
the sun. In regard to the nocturnal heat exchange, we may say at
once, therefore, that it must necessarily be slight in comparison with
the diurnal, and that even at the most there is no such abrupt tem-
perature contrast in a very short distance, such as occurs by day.

The heat exchange during the night is dependent on heat radia-
tion from the surface of the earth, which is what we must now
consider.

According to the Stefan-Boltzmann law of radiation, every body
radiates heat with an intensity proportional to the fourth power of
its absolute temperature. Since the temperature of the sun is about
6000 °C, while average earth temperature at the surface is only
i4°C, or 287° absolute, it is evident how weak the nocturnal heat
radiation is. But its quality as well differs from that of the sun.
According to the Wien displacement law, the product of the absolute
temperature of a radiating body and the wave length of the most
intense radiation, is a constant. With rising temperature the band
of strongest radiation moves toward the shorter wave lengths. This
maximum intensity of solar radiation lies at 0.5 ju, which is in the
visible part of the spectrum, between green and blue. The maximum
intensity of earth radiation, however, lies in the neighborhood of
10 /A, which is far into the longwave (infrared) part of the spectrum.

As we saw on a preceding page a considerable fraction of the
sun's radiation is able to penetrate the entire atmosphere and reach
the surface of the ground. It is otherwise with the outgoing radia-
tion from the earth's surface. Water vapor and carbon dioxide have
the property of absorbing radiation in certain bands of the spectrum,
which happen to be those of long wave length. Their absorption
capacity is selective. We speak, therefore, of "band" absorption or
of the selective absorption of water vapor and carbon dioxide. The

14 THE CLIMATE NEAR THE GROUND

fact that our atmosphere easily admits solar radiation but lets earth
radiation out only reluctantly, is, as we all know, of fortunate sig-
nificance for the retention of the earth's heat. It is referred to as the
"hot-house" effect of the atmosphere.

According to F. Moeller (67 and also 29) only 12% of the earth's
nocturnal radiation passes out to be lost in space. All the remainder
is absorbed by the various layers of air, in proportion to their water-
vapor and carbon dioxide content. The really difficult question of
radiation exchange within the atmosphere we can pass over and, in
what follows, consider only two amounts: the radiation outward
from the solid ground surfaces and the total radiation of all the air
layers above the place of observation, which is called the counter-
radiation of the atmosphere.

If t in °C represents the temperature of the ground surface, then,
according to the already mentioned Stefan-Boltzmann law (which
in the strict sense applies to black bodies only) the outward radia-
tion, S in calories per sq. cm. per min., is

The constant cr has the value 8.26 X io~n. From this we get the
following temperature-radiation relation : —

TABLE 4

Surface temperature of the

ground (°C) ............ —40 —30 -20 -10 o 10

Outgoing Radiation in cal/

cmfl, min ................ 0.244 0.288 0.339 0.395 °459 °-53°

Surface temperature of the

ground (°C) ............ 20 30 40 50 60 70

Outgoing Radiation in cal/

cm2, min ................ 0.609 0.696 0.792 0.899 1.015 I-I43

When considering the nocturnal heat balance we are quite apt
to attribute the strongest outgoing radiation to the wintertime.
There is danger of confusing the duration of the radiation, which
naturally is considerable during long winter nights, with its intensity
per unit of time. As the above-given figures prove, the latter
is much higher in summer; on a warm summer night it is double
that of a cold winter night.

E. Hasche (5$) has studied the variation in the intensity of the
net outgoing radiation in the shade in the course of the day. Dur-
ing the night it decreased by from j% to 8% on account of the

NOCTURNAL HEAT EXCHANGE 15

temperature drop. After sunrise it increased slowly and reached a
maximum about sunset. From then on it decreased rapidly, reaching
the general nocturnal level early in the night. In the daytime,
naturally, these steps are obscured by solar radiation. They are not,
however, of merely theoretical interest, for at times they play an
important part in microclimatic problems.

The amount of outgoing radiation, which can be determined
theoretically from the Stefan-Boltzmann law, is lessened by the
counter-radiation of the atmosphere. When this is taken into con-
sideration, we get the actual outgoing radiation obtained by measure-
ments, which is called "effective outgoing radiation."

Since the radiation of the atmosphere is very dependent on the
water vapor content of the air as well as on its temperature, the
effective outgoing radiation R is also a function of the water vapor
content. If p is the vapor pressure in millimeters measured near the
ground, and S as before is the outgoing radiation according to the
Stefan-Bolzmann law, then according to A. Angstrom (49) the
effective outgoing radiation R in calories per sq cm and minute
equals : —

Here A, B and y are constants whose values are necessarily more
accurate in proportion to the amount of observational data at hand.
In 1935 P. K. Raman (69) in consideration of the work of Angstrom,
Asklof (57), Eckel (55), Kimball (60), Ramanathan and Desai, as
well as his own measurements, assigned the following values : —

A = 0.23, B = 0.28, y — 0.074

The effective outgoing radiation obtained through the equation is
therefore: —

R = 8.26-io-n(* + 273)4(o.23 + 0.28 -lo-0-074'*)

This holds for a cloudless sky since R = S - G (if G represents the
counter-radiation) , then : —

G = 8.26-io-n(* + 273)4(o.77 - 0.28 -lo-0-074'')

In these equations for -R and G, the temperature t and the vapor
pressure p are measured close to the ground but outside its direct
influence, as is customary at meteorological stations. The tempera-

16 THE CLIMATE NEAR THE GROUND

ture and especially the moisture relationships of the whole atmos-
phere are, however, determinative of radiation. As E. Siissenberger
(7_j) recently stated, the observed values of / and p are only assumed
values for the whole atmosphere, representing a normal distribution
of temperature and vapor pressure with height. The variable im-
purities in the air are, as F. Kriigler (6/) points out, not considered
at all; only with this understanding can the above given equations
be used. Since, however, it is only very seldom possible to get
observational meteorological material from the higher air layers and
since it is usually a question of the more easily obtained ground
values, the equations have the very practical value that with their
aid we can in the easiest way get an idea of the magnitude of the
effective outgoing radiation or counterradiation which prevails.

For general use I have recalculated the outgoing radiation value
JR, using the above-given constants. Fig. 5 shows the result. It
indicates that the effective outgoing radiation is in the first approxi-
mation proportional to the relative humidity, for the correspond-
ing curves are nearly at right angles. This agrees well with the
findings of many authors, e.g. O. Eckel (55) that outward radia-
tion is "independent" of temperature (i.e. when the relative humidity
remains approximately constant). Below a certain limit, which is
about 0.15 calories per sq cm per minute, outward radiation does
not decrease (limited by the 100% line.) With low temperatures
such as those in the polar regions and in central Europe in the win-
ter, the possible range of fluctuation of outgoing radiation is quite
small on account of the steady low humidity. The greatest range
is afforded by high temperature with rather low humidity. This is
the upper, right-hand area in Fig. 5; it corresponds to a desert cli-
mate or, with us, to a spell of dry midsummer weather.

The values of Fig. 5 are probably a little too high. For the layer
near the ground H. Philipps succeeded in computing the very com-
plex processes of outgoing radiation. The basic theoretical paper of
Philipps appeared in 1940. His results have led to an equation for
outgoing radiation and back radiation which corresponds essentially
with Angstrom's empirical formula; thus, its theoretical meaning
is explained. The constants Ay B and y have the values 0.220, 0.148,
and 0.068. Only B shows a difference worth mentioning. This dif-
ference indicates that according to the theory, the water vapor plays
a smaller role than in Angstrom's formula. Using the constants of
Philipps we get also somewhat lower values for outgoing radiation

NOCTURNAL HEAT EXCHANGE 17

than are shown in Fig. 5; this is also verified by the more recent ob-
servations. F. Kriigler (6/) found even 17% lower values than
those resulting from Angstrom's formula. As for the amounts of
outgoing radiation given in Fig. 5, we assumed that the sky was
cloudless. In the presence of cloudiness the back radiation from

-20'

Effective Nocturnal Outgoing Radiation in cal/cm" min

FIG. 5. Dependence of the effective outgoing radiation (R) on temperature (/) and
water vapor content (p, in mm, / in %)

the lower surface of the clouds must be considered, especially in the
not-absorbed portions of the spectrum where, previously, entirely
uncompensated outgoing radiation existed. The back radiation is
increased, the effective outgoing radiation decreased.

If Rw is the outgoing radiation at cloudiness W (W = o, cloud-
less; W = i, overcast) then, according to the observation of Ang-
strom (jjo) and S. Asklof :

1 8 THE CLIMATE NEAR THE GROUND

^ is a constant which depends upon the kind of clouds, height of
the ceiling and temperature in this height. According to A. Defant
(53) k. can De considered as the ratio of the difference of the effective
outgoing radiation with overcast sky on the one hand and the effec-
tive outgoing radiation with cloudless sky on the other hand.

In order to use the equation in practice, one may either consider
the clouds according to their kind and elevation; R. Meinander
(66) employs the following mean values:

with low thick clouds (Ac, Sc, Ns, St) ^ = 0.76
with high thinner clouds (Ac, As, Cs) ^ = 0.52

with thin Ci veils J{ = 0.26

or one may use the relationship between the height of the lower
cloud boundary and the value of ^ which was found by H. Philipps
(68a) by theory and measurement:

ceiling (km) 1.5 2 3 5 8
^ 0.87 .83 .74 .62 .45

In this respect Fig. 6 contains more recent observational results. The
thin broken straight lines are valid for the three kinds of cloudiness
according to Angstrom-Asklof formula. Plotted on the heavier curve
are the results which F. Lauscher (62) derived from measurements
of nocturnal radiation made from Oct. 10 to Dec. 17, 1927, at Steier-
mark on the Stolz Alpe (elevation 1160 m). He calculated for dif-
ferent group averages of cloudiness the average observed outgoing
radiation. His values are given in Fig. 6 as hundredths of the
radiation value with a cloudless sky. High, thin sheets were omitted.

The curve shows very clearly the increasing rapidity with which
radiation diminishes as cloudiness grows. At first the curve coin-
cides closely with the straight line representing high clouds, for
very slight nocturnal cloudiness is usually mostly cirrus in type. A
medium amount of cloud corresponds to a middle-height cloud.
With an entirely clouded sky, the clouds are commonly quite thick.
In the case of fog the radiation was reduced 7 to 8% (o.on calories
per sq cm per minute).2

Fig. 6 can therefore be used in conjunction with Fig. 5 in order
to estimate the magnitude of the effective nocturnal radiation out-

2 The change of effective outward radiation with the height of the lower
cloud boundary in the case of thick clouds was mentioned in 1936 by A. Ang-
strom (50) as a result of observations at Stockholm, 1923-1933.

NOCTURNAL HEAT EXCHANGE 19

ward, under specified conditions of temperature, humidity and
cloudiness.

The considerations thus far adduced assume outgoing radiation
to the whole sky hemisphere. For many problems of microclimatol-
ogy it is necessary to know the radiation toward certain parts of the
sky. Here we can learn from the work of P. Dubois (54), F. Linke
(65), E. Siissenberger (72 and 7^) as well as L. A. Ramdas and
collaborators (7/).

0 1

7 8 9 10

13*56

Cloudiness in Tenths

FIG. 6. Dependence of the effective outgoing radiation on cloudiness (Theory and

observation)

Radiation is strongest toward the zenith, because the atmosphere
is of least thickness in that direction. The more the radiation re-
corder is inclined to the horizon, the more effective is the counter-
radiation. Directly toward the horizon outgoing radiation is zero.
But there is still the dependence on the water vapor content of the
air. If Z is the zenith angle and Ro the radiation toward the zenith,
then the radiation Rz toward the direction Z is, according to F.
Linke: —

Rz = Ro • cos yZ

with the exponent y dependent on the vapor pressure p according
to the simplified equation:

y = o.i i + 0.034/7

The following table gives values which F. Linke assembled from the
data of P. Dubois.

20 THE CLIMATE NEAR THE GROUND

TABLE 5
RELATIONSHIP OF EFFECTIVE NOCTURNAL OUTGOING RADIATION TO ZENITH

ANGLE Z AND VAPOR PRESSURE p (mm).
(Outgoing Radiation toward the Zenith = 100)

Zenith Angle Z

Vapor
pressure p

100

If parts of the sky are screened off, this of course affects the
magnitude of the effective radiation to a marked degree. Under
a tall tree standing by itself, the very "coldest" parts of the sky are
obscured. It is therefore easily understood that a pine or a single
birch can afford frost protection on quiet nights when radiation
is the chief determinant of temperature.

K. Brocks (52) measured night temperatures in narrow furrows
with various angles of side slope. Here, of course, not only radiation
but heat conduction has a part, yet the measurements are of interest
in this connection. The temperature of the nocturnal minimum in-
creased with the steepness of the slope. He found:

TABLE 6

with an angle of slope of

15°

30°

45°

60°

75°

00°

in the mean of 138 nights 1937 . . .
in a particular case (24/^/37) . . .

. 6.23
. 6.6

6.27
6.6

6-34
7.0

6.44
7-3

6.59

7-5

6.67
8.1

The stream of heat (radiant and conducted) from the layers of
soil cut by the furrows produced heat protection. Only when snow-
fall shut off the ground temperatures did the reverse temperature
distribution enter and then not as a consequence of radiation but
of what we shall treat in Chapter 18 as the flow of cold air.

Often the problem arises in microclimatology to determine, with
a certain form of horizontal screening, how much the outgoing
radiation toward the sky will be thereby reduced. F. Lauscher (6^)
has developed general methods for this purpose. From the abun-
dance of calculated data we shall give only a few often noted exam-
ples. The first row of figures in the following table gives the radia-
tion M from a horizontal plane in the deepest part of a basin for
different slope angles (/J). The outgoing radiation M is given in

NOCTURNAL HEAT EXCHANGE 21

percent o£ the simultaneous radiation from an open exposure. The
calculations are based on a vapor pressure of 5.4 mm. In the second
row of figures the radiation from a horizontal plane at the bottom
of a valley is given as T. The valley has mountains on both sides,
up to the angle of elevation /J; its bottom having further a straight-
away course without a grade. The values for T can be used for
radiation in the middle of a straight street, a forest cutting and so
forth.

TABLE 7

(*:

10 15

IOO

98 95

IOO

98 98

For the microclimate in the mountains it is desirable to know the
dependence of radiation on altitude also. In this direction too it is
Angstrom who has been the pioneer, with his observations in Lap-
land, Algiers and California. In general it may be said that with
increasing altitude the mass of air above the place of observation
rapidly decreases. In consequence, the counter-radiation also dimin-
ishes while the effective outgoing radiation increases. This increase,
however, is partially offset by the fact that the temperature falls
with height. There still remains an increase with height of heat
loss through outgoing radiation, just as we have determined a gain
in heat received by day. The microclimate of high levels is conse-
quently not only more extreme in its higher heat reception by day,
but also in its greater heat loss at night.

On page 79 of "Dynamic Meteorology" by H. Ertel (56) there
will be found a sketch showing the change of effective outgoing
radiation with altitude, applicable to all high levels such as are of
concern in mountain microclimatology. It increases slowly at first,
then more rapidly. In the first 3,000 m the change is so slight that
it may be almost neglected. F. Lauscher (64) has calculated this ac-
cording to an Angstrom formula for the eastern Alps in reference
to temperature and water vapor relationships at different altitude
levels. This has been found to be substantiated by numerous meas-
urements in the village of Lunz. We give here an extract from
his results for hot July days (see Table 8) .

We learned that outgoing radiation and back radiation were the
main factors of the nocturnal heat exchange. Fig. 7 gives a survey

THE CLIMATE NEAR THE GROUND

TABLE 8

Average

Effective

Altitude

Temperature

Vapor

Atmospheric

Outgoing

(m)

<°C)

Pressure

Radiation

3000

1.6

4.6

0.275

0.186

2000

8.6

6.3

0.322

0.189

I5OO

I2.O

7-3

0.348

0.189

1000

14.8

8.5

0.372

0.186

500

I7'5

10-4

o«397

0.181

2I.O

14-3

0.429

0.178

of the entire nocturnal heat exchange in the same manner and the
same scale as Fig. i for the heat exchange during day time.
The short wave radiation exchange is entirely lacking. The Stefan-

effective outgoing radiation

back radiation

convection

radiation pseudo
conduction

thermal conduction
formation of dew

evaporation

fera

Surface "m of the Ground

I supplied from the ground

FIG. 7. Heat exchange at night (same scale and same pattern as in Fig. i)

Boltzmann radiation orT4 is mostly compensated by the back radia-
tion. The effective outgoing radiation together with the loss of heat
by evaporation causes the nocturnal decrease of surface temperature.
The width of the arrows representing the effective outgoing radia-
tion in Fig. 7 is a little smaller than that of the long wave outgoing
radiation (which likewise is considered as the difference between

NOCTURNAL HEAT EXCHANGE 23

the Stefan-Boltzmann radiation and the back radiation) in day time
in Fig. i. This fact corresponds with the law, previously mentioned
(p. 14), that the outgoing radiation decreases with decreasing
temperature. E. Hasche (58) who measured separately the long
wave outgoing radiation during day-time established indeed a de-
crease of the nocturnal values of 7 to 8 percent in comparison with
the daytime values.

The temperature decrease of the surface of the ground is dimin-
ished by the heat from the deeper layers of the ground which is
stored up there during daytime and now is available for the benefit
of the surface. As a consequence of the processes (already men-
tioned with Fig. i) of heat conduction, convection, and radiative
pseudo conduction, the air layer adjacent to the ground also par-
ticipates in the process of cooling of the surface insofar as it gives
up heat to the surface. The respective arrows in Fig. 7 are in opposite
direction to those of Fig. i. As a new fact, the surface profits in heat
when dew or frost is condensed upon it; but this gain is negligible
except on nights with copious dew.

Also the air itself radiates some heat but according to H. Philipps
(68a), at the utmost, only a twentieth of the nocturnal fall of
temperature can be explained in this way. The cooling of the
atmosphere starts essentially only from the ground and we can, con-
sequently, conclude that in the case of the heat exchange by night,
the earth's surface plays an important role similar to that of the heat
exchange at noon. Just as the boundary surface between earth and
air was the seat of highest temperature in the daytime, so does the
lowest temperature prevail there at night. The temperature in-
creases thence upward in the adjacent air layer and also downward
in the adjacent earth. The vertical temperature distribution at the
time when the outward radiation type prevails is therefore a mirror
image of that shown in Fig. 2 for the incoming type.

Because a fall of temperature with increase of altitude is the rule,
the nocturnal increase of temperature above the ground is called
"temperature reversal" or "inversion." It is not limited to the air
layer next to the ground, but may extend upward several hundred
meters. (See Fig. 20 in Chapter 5.) The amount of the temperature
fall, however, decreases very rapidly with the distance from the sur-
face of the earth.

Fig. 8 shows the typical inversion curve for the air closest to the
ground, according to the classical investigation of G. Hellmann (59).
The values are from measurements taken every 5 cm upward from

24 THE CLIMATE NEAR THE GROUND

the ground and represent the smoothed mean of 14 clear radiation
nights.

This study of Hellmann's was the first proof that there is no tem-
perature discontinuity at night within the air layer next to the
ground, and that on the contrary the temperature in comparison
with the ground continuously decreases, but at an increasingly slower
rate. A glance at this diagram makes clear how unfavorable the
plant climate is in respect to frost phenomena. We shall deal more
fully with this further on.

40 .£

FIG. 8. Nocturnal temperature inversion over the ground. (After G. Hellmann)

There is a temperature discontinuity at the surface of the ground.
Within the soil the temperature at first rises very rapidly, then more
slowly. Here likewise the outgoing radiation type is the converse
of the incoming type. Hellmann, however, did not extend his meas-
urements into the ground.

How it is the solid ground surface which, through its radiation
outward, occasions the temperature inversion in the air near the
ground, is demonstrated by the following fine observation by S. Pet-
terssen (68). On the night of July 30-31, 1927, at 7 P.M., he was mak-
ing measurements with the Assmann aspiration psychrometer in the
neighborhood of Grotoy (68°N). There was no wind. Scattered
cirrus clouds were of slight hindrance to the outgoing nocturnal
radiation. A thin layer of fog about 3 rn thick, lay on the ground
and was rapidly thickening, a visible indication of the nocturnal
temperature inversion. Above the earth's surface E the temperature

NOCTURNAL HEAT EXCHANGE 25

distribution was as indicated by the fine broken line in Fig. 9. It is
consistent with our outgoing radiation type (Fig. 8). In a ditch
55 cm deep the temperature was only 3.6°C (settling of the coldest
air at the lowest point) .

sew

Temperature f°C)

12 °C

FIG. 9. Double surface produces a double inversion. (Observed by S. Petterssen)

Now in this area there was a barn, to which a wooden bridge,
5 cm thick, led up steeply. While Petterssen was measuring the
temperature at various heights, he found, where the bridge B lay
a meter above the ground, the temperature distribution indicated by
the heavy line in Fig. 9. The bridge was acting as a second surface.
The air at its upperside was 1.8° colder than the air at its underside.
A double inversion had formed, corresponding to the two radiating
surfaces, E and B. The narrow bridge hindered the radiation from
E directly under the bridge only, so that the temperature there was
indeed higher than in the open (broken-line curve) but still the
course of the normal inversion, under the influence of the freely
radiating surroundings, could be recognized.

CHAPTER 3

TRUE HEAT CONDUCTION

THE NORMAL COURSE OF GROUND TEMPERATURE

In the field of microclimatology there are four different forms of
heat transmission: —

1. Conduction (molecular), known also as "physical" heat conduc-
tion or "true" heat conduction.

2. Convection, also called "eddy diffusion" or "pseudo-conduction."

3. Radiation.

4. The heat economy of water in its various states.

It has been shown earlier that every body emits radiant heat in
accordance with its own temperature (Stefan-Boltzmann law).
Radiant heat passes even through airless space, of which the sun
furnishes the best example. Just as every body emits radiant heat,
so also is it exposed to all kinds of radiation directed toward it from
without. The gain or loss of heat through radiation is the resultant
of the incoming and outgoing streams.

Conduction and convection of heat require matter. Conduction
takes place in all bodies, but convection in liquids and gases only;
hence, for meteorology, only in water and air.

According to the kinetic theory of heat, heat energy is conceived
of as energy of molecular motion. Lively molecular motion trans-
mits itself to adjacent more sluggish molecules. The faster moving
molecules lose energy; the slower ones gain it. In other words,
warmer bodies give warmth to the colder ones, with loss of their
own heat. This process is molecular, physical, or "true" heat con-
duction. The bodies, considered as a whole, remain at rest, all their
separate parts maintaining their relative position. Thus it is, for
example, in the case of an iron rod, heated at one end.

When, on the other hand, there is convection in liquids and gases,
the masses themselves are displaced. They retain all their properties;
the air, for instance, its content of heat, water vapor and dust. They
are brought into contact with ever-varying portions of the liquid or
gas. The pseudo-conduction of heat through convection proceeds,
therefore, with many hundred times the rapidity of true conduc-
tion.

If water evaporates at the surface of the earth there results not

TRUE HEAT CONDUCTION 27

only a change in the moisture content of air and ground, but the
energy required to evaporate the water is taken from the surround-
ings in the form of heat energy. Heat passes off with the water
vapor ("cooling by evaporation"). The reverse process takes place
when dew is formed. Finally, all precipitation brings from the
higher air layers where it originates, its lower or higher temperature,
down into the air near the ground, onto the surface itself and finally
into the ground, influencing the temperature which it finds there.
These phenomena are the fourth mode of heat transmission.

On account of this varied nature of heat transmission, the tem-
perature relationships on both sides of the ground surface are not
easily explained. We shall therefore try first to get a clearer
understanding of the first three forms of heat transmission. The
role of water we shall not consider at present.

We shall begin with true conduction.

Conduction accounts almost exclusively for heat transmission
within the earth. Consequently a study of the laws of conduction
is the best way to understand ground-temperature relationships.
Furthermore, heat processes in the ground govern to a large degree
the air temperature near the ground. J. Siegenthaler (9^) calculated
the correlation coefficient between the temperature at 10 cm depth
within the ground and the air temperature of the macroclimate as
0.87. How much closer is the relationship with the climate close
to the soil!

The speed with which the heat is transferred in the ground, up-
ward and downward respectively, depends upon existing tempera-
ture gradients and heat conductivity of the ground. The heat con-
ductivity X is characterized by that amount of heat in cal which flows
through a cross section of i cm2 if perpendicularly to this cross sec-
tion a temperature gradient of i°C/cm exists and no heat is con-
veyed to or removed from any other direction. In the following
table (according to R. Geiger (780)) the values of X are found con-
cerning the most essential substances which are of meteorological
interest.

If we assume that only a vertical temperature gradient dt/dx (t =
temperature, x = depth of soil) is present in the ground — that there
is no horizontal temperature difference, in other words; then the
amount of heat W which passes in one second through the square
centimeter area is given by the equation

W = X- 7-
dx

THE CLIMATE NEAR THE GROUND

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TRUE HEAT CONDUCTION 29

Theoretical physics teaches how the heat cycle which in the course
of a day or year arrives at the upper surface of the soil is delayed
and weakened as it penetrates within the ground.

The time lag of the maximum and minimum value of the heat
cycle is expressed by the following equation. Let Xi and x% be two
depths below the ground surface expressed in cm; T, the oscillation
period of the heat cycle in seconds (T = 86,400 for a diurnal heat
wave) ; z± and z2, the corresponding time of reaching the maximum
values (in seconds); p, the density of the ground and c its specific
heat; X, the heat conductivity as stated above.

Then *2 - *i = (*2 - *i) — V -r x

2TT r T'X

The weakening of the temperature cycle can be found from the fol-
lowing relation : — If the difference between the highest and lowest
value of temperature at depth x^ equals 81, and that at depth x2
equals 82, then

V^5 \
82 = S^*1 *2) v T-\ • The value a =

P'c

is known as the thermal diffusivity. For the greater is the density
p and the specific heat c of a body, so much less is the rise of tem-
perature which will be occasioned by a given amount of heat. Many
an error has resulted from confusing heat conduction and thermal
diffusion. Numerical values for a are also given in Table 9.

In order to represent how this heat movement takes place in the
ground, three different methods are used. Either, as in Figures 10
and 14, we show the progress of the heat cycle by lines of equal
temperature, using time and soil depth as coordinates; or, we use
time as abscissa and temperature as ordinate, as in figures n to 13,
giving the temperature march at definite depths; or, we choose tem-
perature and soil depth as coordinates and show lines of condition
("tautochrones") at a given time. Fig. 15 is an example of this.

Th. Homen (82) the Finnish pioneer in microclimatological ob-
servations, carried on a series of measurements at Wakkarais in
1893 dealing with the temperature march at various depths within
the soil. They are so valuable even today that we have chosen from
them the first example of the variation of soil temperature with
time. Fig. 10 represents soil temperature observations in a sand
heath at two-hour intervals from Aug. 13, at 6 A.M. to Aug. 14 at

3°

THE CLIMATE NEAR THE GROUND

8 A.M. The isotherms penetrating downward toward the right indi-
cate the lag of the diurnal temperature cycle. The lines of small
crosses unite the points of highest or lowest temperatures at the
various depths. Even at 5 cm below the surface the day's extreme
reading is already lagging by two hours; at 20 cm, by five hours.
But the extremes are rapidly weakened by depth; the isotherms of
maximum and minimum temperatures join not far below the sur-

\ t , 13. August

/». August

FIG. 10. The very regular penetration of the daily temperature cycle into the ground
by heat conduction. (After observations by Th. Homen)

face. Fewer and fewer become the penetrating isotherms; greater
and greater the distance between them.

Figs, ii and 12 show the daily march of soil temperature during
certain months, according to observations made at Pawlovsk in 1888
and analyzed by E. Leyst (#5) . They afford a contrast between the
month of May, representing a month of the strongest seasonal
heating under intensive solar radiation, and the month of January
as a winter month with weak radiation.

In May (Fig. n) the temperature fluctuation is still considerable
at a depth of i cm below the surface and follows the march of
radiation quite closely. At a depth of 20 cm the temperature does
not reach its maximum till about sundown. At 40 cm the daily
march is reversed, i.e., noon is there the coldest time of day (as an

TRUE HEAT CONDUCTION

after effect of night). At 80 cm the daily fluctuation is lacking.
That it is spring we know by the cold which is still present in the
deeper earth layers (80 and 160 cm).

Time of day
FIG. ii. Daily temperature course in sandy soil at Pawlovsk in May. (After E. Leyst)

In January (Fig. 12) the daily fluctuation is slight at all depths,
almost disappearing at 20 cm. But the deeper we go, the warmer
the soil — the seasonal antithesis of Fig. n.

^
£

160cm

80cm

/ icm

Ground
snow-free

Time of day

FIG. 12. Daily temperature course in sandy soil at Pawlovsk in January. (After

E. Leyst)

32 THE CLIMATE NEAR THE GROUND

J3> finally, as an example of an annual temperature march,
represents measurements made by A. Schmidt (90) and E. Leyst
(86) at Konigsberg during the years 1873-1877 and 1879-1886. The
extraordinary regularity with which the heat movement in the soil
proceeds is so great that the curves appear to have been plotted

Jan. far. March April May June July August Sept Od Nov. December

FIG. 13. Annual course of ground temperature at Konigsberg. (After A. Schmidt and

E. Leyst)

theoretically. The diminution of the annual fluctuation and the lag
with depth are evident from one measuring level to the next. At
only 7 m below the surface, summer is the coldest season, and
winter the warmest! But the difference between the two has there
dropped to il/2°.

A person glancing at the course of temperature with reference to
time and space as shown in Figs, n through 13 may get the im-
pression that it is almost mathematically regular. In reality the
temperature march proceeds quite otherwise. The results repre-
sented in Figs. 11-13 were obtained on artificially laid-out experi-
mental fields which were kept free from snow during the winter.
Under natural conditions the ground is far less homogeneous than
on such an experimental field. Not only does its property change
with depth but great contrasts can exist side by side. When Wm.

TRUE HEAT CONDUCTION

Schmidt (279) had developed a simple method for quickly obtain-
ing ground temperatures under natural conditions, he could detect
temperature contrasts within the smallest distances, even in the
ground. In addition there is an effect of soil condition, which
changes with the weather, variable water content being the most
important factor. Winter snow acts as an almost entirely heat-insu-
lating blanket on the earth. An entire chapter, (14), is devoted to
the manifold influences of kind and condition of soil.

But even aside from the lack of uniformity in the soil, such mathe-
matically perfect appearing temperature relationships can be ob-
tained only when averaged over a long period of time as in Figs. 11-
13 or when quiet days are selected as in Fig. 10 or Fig. 15. Fig. 14

OF M f. A 12. * 20 Jt. U t 3. & 0. 17. 21. 25. t Ji 9. U 17 21. 25 a i C W

FIG. 14. Ground temperatures under the influence of changeable weather. (Winter

1928/29 at Potsdam)

shows how unsettled the picture of ground temperatures may appear
even in an experimental area, when under the influence of change-
able weather. It contains the ground temperatures at Potsdam for
the winter of 1928-29 according to a sketch by J. Bartels. The scheme
of representation is the same as that in Fig. 10. The heat and cold
cycles penetrate into the ground from the surface. The depth of
penetration depends on the temperature variations at the surface.
At the depth of a half meter, weather variations are mostly cancelled
out; at depths beyond i m the course of the temperature approaches
the theoretically anticipated regular form.

THE CLIMATE NEAR THE GROUND

In the first two chapters the temperature relationships during the
day were designated as the incoming radiation type, while those
prevailing at night were called the outgoing radiation type. We can
now show, in the case of ground temperatures, how these blend
together.

In order to do this we must make use of tautochrones, i.e. lines
which show the relation of temperature to depth at a given time.
In Fig. 15 this is shown, not for a single instant but for each odd
hour of the day, with all the curves assembled for comparison in one
diagram. Fig. 15 is made up of measurements by means of thermo-

^6* 28° 30* 32' 3t° 36° J6*

FIG. 15. Tautochrones of the ground temperature on a radiation day in summer.

(After L. Herr)

couples, at 10 different depths. They were obtained by L. Herr (So)
in natural soil near the Geophysical Institute of the University of
Leipzig at Oschatz, on the loth and nth of July, 1934.

The tautochrone of 3 P.M. corresponds to the incoming radiation
type (See Fig. 2, page 7). After 3 P.M. the temperature first falls
at the ground surface while in the deeper layers it is still rising.
About 7 P.M. the evening cooling is so effective even at 3 cm depth
that there the maximum temperature appears as a critical point.
The point falls in the course of time to deeper layers and becomes
rounded in form. This indicates that the nocturnal radiation out-
ward affects ever deeper ground layers and, down there, loses mem-
ory of the day. About 5 A.M., before sunrise the typical outgoing
radiation type is attained, the converse of the incoming. And now the

TRUE HEAT CONDUCTION 35

cycle begins again as already depicted save with reverse symptoms.

It must be noted that the lower parts of all the curves in Fig. 15
are closely grouped and inclined upwards toward the right. From
this it may be concluded that the day on which the measurements
were made happened to be one on which the weather was increas-
ing in warmth.

For the microclimate near the ground, the ground itself acts as
a regulating reservoir of heat. At times of heat surplus — at midday,
or in the summer — it absorbs great amounts of heat, thus avoiding
unduly high temperature and at the same time laying away calories
for a time of need. At night, or in the winter it gives up its savings
and thus keeps the temperature from falling too far.

The greater the thermal conductivity of the ground, the more effec-
tive is its role as a heat reservoir. Microclimates over soils of good
conductivity consequently show a smooth march of temperature. On
the other hand, microclimates over a poorly conducting soil are ex-
treme — too cold by night, too hot by day. An artificial modification
of the soil's heat diffusivity therefore modifies the microclimate near
the ground as well. We shall come back to this in a later chapter.

W. Meinardus (88) carried out some ground-temperature meas-
urements at Schellal in the extreme desert climate of Egypt during
1914.

In high mountains, with their generally low temperatures, the
plant world can thrive only close to tlie ground. According to
}. Maurer (#7), the amount by which the ground temperature
exceeds the air temperature increases with altitude. With the great
increase in solar radiation and the slight increase in outward radia-
tion, this is to be expected and would prove that mountain vegetation,
even more than that of the plains, is dependent on the climate near
the earth. Measurements during the summer months of 1929 and
1930 have been published by W. Hecht (79). The one series was
made at Korneuburg near Vienna, (167 m msl), and the other
at Davos, on the Schatz Alp (1868 m msl). In his work there are to
be found some new ideas on ground climate and climate near the
ground in mountainous regions. Longer series of systematic meas-
urements are, however, very desirable.

CHAPTER 4
EDDY DIFFUSION AND ITS SIGNIFICANCE

In the foregoing chapter we have dealt with the heat flow resulting
from molecular heat conduction downwards from the warm earth's
surface into the ground, or upward through the ground toward the
cooler surface. A heat exchange of the same sort takes place also
between the earth's surface and the air layer adjacent to it. Heat
conduction in air is, to be sure, decidedly poorer than in the earth,
but air on account of its slight density, does possess good thermal
diffusivity. The stream of heat from the ground surface upward
(and back), resulting from thermal diffusivity, is equal in its order
of magnitude to that flowing downward from that surface.

If we apply this to figures n, 12 and 13 and imagine "upward"
and "downward" there reversed, it follows that the heat of midday
would not be felt till evening in the first story of a house, while
summer temperatures would not be reached till the beginning of
winter. Since this is not true, the heat must be transmitted by some
other method. This method is eddy diffusion of heat or, simply,
eddy diffusion.

There are two kinds of Circulation in water and air: laminar, and
turbulent. That circulation is called "laminar," in which there is no
whirling motion; if whirls are present, it is "turbulent." Such whirls
can be observed in the motion of tobacco smoke in a closed room.
In the open where the wind constantly favors mixing, the air is
almost without exception in a turbulent state. If the wind is light
we do not perceive this turbulence, but if the wind is strong we
recognize its gustiness, both as to direction and speed.

Turbulence causes a continuous mixing of air masses. As the
masses mingle, so do all their properties. The parcel of air that
rises at random from the earth's surface carries with it some heat,
a relatively large amount of water vapor, and perhaps dust, radium
emanations or what have you. All these properties are transferred
into a new location with new suroundings and new conditions.
Heat, water vapor, etc., are carried away in the air by this process
many hundred times faster than heat is carried by molecular con-
duction or water vapor by diffusion. Certain kinds of transportation,
such as the dispersal of dust, pollen and seeds can be explained in
no other way than by this irregular movement.

EDDY DIFFUSION AND ITS SIGNIFICANCE 37

Alfred Wegener (114) has the honor of having pointed out the
importance of turbulent movements for meteorology in general.
Wilhelm Schmidt's book (//j), "Eddy Diffusion in the Free Air,
and Related Phenomena," which appeared in 1925, was of pioneer-
ing significance for microclimatic research problems. H. Lettau
(108) in 1939 treated the problem of atmospheric turbulence in an
entirely new and comprehensive fashion. Anyone with a good
mathematical background will find his book very helpful in gaining
an acquaintance with the whole problem.

It is the particular purpose of this text-book to present to the reader
a clear idea of the eddy diffusion process and what great significance
it has for microclimatic questions. To this end we shall derive the
fundamental eddy diffusion equations according to Schmidt's simple
construction.

Suppose a surface / (Fig. 16) lying horizontally and at rest with
respect to its surroundings. If the air as a whole moves forward,

FIG. 1 6. Diagrammatic representation of the fundamental equation of exchange

the surface moves with it. We shall assume that only eddy diffusion
is active. Let the air have the property s per unit of mass — leaving
open what this property is. The only requirement is that it must
be free from outside restrictions. The property s can therefore be
the content of dust, heat or water vapor, and is generally a function
of altitude.

Suppose that, in consequence of eddy diffusion, the mass m passes
upward through the surface /; it thereby carries with it the quantity
ws of the property. All the particles of air from below which, by
eddy diffusion, pass through / bring 2,m+*s with them, if we con-
sider the direction upwards as positive. In a corresponding manner

38 THE CLIMATE NEAR THE GROUND

all the air particles from above bring 2,m_-s with them. There
passes upward through the surface / therefore, in the very small
time which we are considering, only the difference between the
properties moving upward and those moving downwards. This flux
is 2ra +*s — 2ra_*.f. If we refer this flux to unit surface and unit
time (t = time in seconds) and if the flux is designated by S>, then

@ = — [%m+-s — Sra _-.?]. We assume moreover that the prop-

/'*

erty is arranged above the ground at a height x^ (in cm) precisely
according to a parobola. According to experience the change of all
factors in the climate near the ground which increase with altitude,
can be expressed by a parabola. If s0 be the property at the height
x — o, then

ds i d2s o

s = s0 + x + • xr

dx -2 dx"

If we substitute this in the above-given equation, we get —

@ = — { *0 I 2w+ -2m_ 1 + — I
/•/ L J dx L

2 dx'2

The first expression in brackets [] is equal to zero because, accord-
ing to premise, no mass stratification takes place through eddy
diffusion, and just as great a mass passes downward as upwards.
We shall further assume that eddy diffusion proceeds symmetrically
with respect to surface /. For each mass at the distance x = +q
there will be found an equally large mass which comes to / from
the distance x = — q. The last expression in brackets also becomes
zero and the equation is simplified into

^m x _ ^m _x js

/•* dx

The factor which precedes — -, adds nothing more to the property s

of the moving airmasses but only indicates the liveliness of the
motion. If we call this A, then

r~ A ds

@ = A

EDDY DIFFUSION AND ITS SIGNIFICANCE 39

Comparing this short formula with the one on page 27 we recog-
nize eddy diffusion as a heat conduction having the value A in place
of the constant X. This has led to eddy diffusion of mass being
called also "pseudo conduction."

Now while X is a physical constant, which depends entirely on
the material under consideration, A changes with time and place.
A is called the austausch coefficient: its value varies (if the c-g-s
terminology is used) from o.ooi to 100 — within wide limits, in
short. It is the simplest expression to designate the condition of
irregular motion in the air. The dimension of the coefficient is
cm"1 • g • sec""1.

In the preceding derivation two assumptions were made. In the
first place the property s must be independent of outside conditions.
In general, therefore, temperature cannot be used as such a property,
since it depends on pressure. For the air layer near the ground this
limitation vanishes, since the vertical extent is so little that thermo-
dynamic temperature changes may be neglected.

In the second place it was tacitly assumed that eddy diffusion was
operating alone. In reality, however, the processes of molecular
physics (conduction and diffusion) cannot be eliminated. For the
most part these are quite unimportant in action compared with eddy
diffusion as the considerations at the beginning of this chapter make
clear. The effect of eddy diffusion is 10 to 100,000 times as great as
that of heat conduction. Just at the ground surface, however, this
assumption may not hold, which is something for microclimatology
to consider. Even the supposed symmetry of eddy diffusion does
not always exist close to the ground. Furthermore there is in the
climate near the ground a second kind of pseudo-conduction through
radiation, of which we shall treat in the following chapter. This too
warns us to be cautious in the use of the eddy diffusion equation.

In spite of these limitations the important fact remains : Although
within the ground heat is largely transported by conduction, yet in
the air near the ground, it is predominantly eddy diffusion which
both by day and night moves the heat upward from the earth's sur-
face, and vice versa.

First we shall look at some figures on the magnitude of the
austausch coefficient A. W. Schmidt (//j) has computed its value
according to very different criteria. The following extract from his
compilation shows not only probable values of A, but also the great
number of ways it can be calculated, as well as the general signif-
icance of eddy diffusion. He found : —

40 THE CLIMATE NEAR THE GROUND

(1) from the distribution of smoke streamers above a field where
there was particularly stable stratification about sunrise, A = 0.006;

(2) from the heat transfer over a snow cover on a clear, calm
winter night, according to A. Angstrom's measurements at Abisko,
A = 0.14;

(3) from the daily temperature march at Paris (15 year average)
in the layer between 2 m and 123 m above the ground, A = 9;

(4) from the scattering of the pollen from our forest trees over
the Baltic, A — 43;

(5) from the distribution of wind velocity and direction at dif-
ferent heights at the Eifel tower, A = 90.

More recent measurements substantiate the accuracy of these fig-
ures.

The coefficient A increases with height above the ground. This
increase, as H. Lettau (108) has shown, follows theoretically as well
as according to actual measurements. At the ground, therefore, eddy
diffusion as well as wind velocity is subject to a braking effect. (See
Chapter n.) At the same time, as W. Haude (132) has cogently
remarked, larger units of turbulence are "ground up" at the ground
into smaller and smaller ones.

The linear increase of A with altitude is however only a theoret-
ical law which has decided variations near the ground in individual
instances. It has often been observed that eddy diffusion varies un-
evenly from one layer to the next. Thus H. Berg (98), in his meas-
urements on the Bissendorf moor near Hanover in 1934, proved that
within the first meter above the ground A increased slowly, then
rapidly up to 5 m, but from there up to 16 m was almost constant.
W. Haude (132) made some observations over an area of broken
stone in the Gobi desert, on March 7, 1932, at 2:15 P.M. Next to the
ground was a layer 25 cm thick, showing weak eddy diffusion. Above
this A increased quickly to many times its lower value. But between
70 and 80 cm there was again a much slower increase.

From all this we may conclude that the air layer adjacent to the
ground has a laminar structure. This is suggested by other circum-
stances as well, and explains many phenomena otherwise hard to
understand. One hot summer day I noticed, on the Peiting moor in
upper Bavaria, that if the eye was placed about a meter above the
ground a sharp boundary layer could be seen, below which the air
showed irregular streakiness, and above which there were threadlike
streaks like smoke waving in the wind. When taking temperature
measurements during the day one often observes within the basal
air layer a secondary temperature rise at some height, say il/2 m,

EDDY DIFFUSION AND ITS SIGNIFICANCE 41

above the ground. It is the so-called "secondary temperature maxi-
mum" which was discovered by Hornbergef (106) and described
by P. Vujevic (197) but was attributed to observational errors.
R. Geiger (179, 180) confirmed it repeatedly even in monthly means,
as can be seen in the following 8-hour observations at the Anzinger
Sauschiitte near Munich.

A. Schmauss (///) referring to the "rising current of air," which
is often mentioned in meteorological theory but hard to find in
nature, mentions the research of R. E. Liesegang. He poured into a

TABLE 10

Month, 1924 Mean Temperature (Six's thermometers)

0.05 0.50 i. oo i.5om

May .

I^.OQ

IA.OS;

14.23

n.8<5 °C

June . .

. . . l6.4I

1^.4=?

15.60

I5.IQ °C

July . . .

18.48

17.41

17.45

16.88 °C

beaker 200 g of very fine-grained powder of Caffeine-sodium sali-
cylate with 400 cc of cold water. After agitating a short time, the
beaker was placed in a hot-water bath and allowed to remain there
undisturbed. When, some 10 minutes later, the powder had dissolved,
the liquid showed horizontal stratification into 8 or 10 layers. The
decreasing concentration upward was not continuous, but by steps.
The sharp boundaries between the several strata could be easily
recognized through the varying light refraction. To produce the
phenomenon it was necessary to warm the solution from below. A
similar stratification was evident in other solutions of salts and col-
loids. These processes may be considered analogous to the formation
of a foliated structure in the dust-filled air near the ground when
it is warmed from below at midday. The secondary temperature
maximum would indicate a limiting layer which through local
conditions can have a preferred position and therefore can be
found regularly at a certain height.

Eddy diffusion has two causes which, as early as 1919 were differ-
entiated by A. Angstrom (97). Dynamic eddy diffusion is caused by
the turbulent streaming of the air. According to H. Lettau (fo8) the
austausch coefficient A increases linearly with the wind velocity: at
any rate, this rule is valid for the wind regimes of that portion of the
air layer near the ground. The thermal exchange is added which
originates from the instability of the thermal stratification. This

42 THE CLIMATE NEAR THE GROUND

explains the above mentioned fact that the exchange coefficient is
dependent also upon the stratification of temperature in the air near
the ground.

One remarkable observational fact remains that the computation
of A on the basis of the variation of wind speed with height does
not show such a dependency on the respective temperature stratifica-
tion. Since this is the same process of exchange which causes mo-
mentum and heat transfer (water vapor, etc.), this result is sur-
prising. With different methods different values of A result. As an
example, H. Lettau (ioj) calculated from simultaneous observations
at the geophysical observatory of Leipzig that A = 20.0, if tempera-
ture measurements are taken into consideration; A — 2.8 if the wind
measurements are taken.

To explain this seeming contradiction, one may point to the before
mentioned factors neglected during the derivation of the exchange
equations : there is especially the neglect of the radiation phenomena
which certainly are of importance for the heat process but not
immediately for air motion. F. Albrecht (96^) recently gave us an
explanation. He assumes that within the air current aloft large
turbulence bodies (order of magnitude: 100 m diameter) exist; these
may descend vertically and be broken up to smallest turbulence
bodies near the ground. Such a concept of the turbulence near the
ground can agree with the fact that momentum on the one hand,
heat and water vapor on the other, are exchanged between air and
ground in different ways. It also puts the outstanding importance
of the dynamic exchange, which exists beyond all doubt, in the
right place.

In conclusion let us look at a few examples of how the action of
eddy diffusion of mass becomes visible directly or otherwise. (The
optical side will be treated only in Chapter 12.)

A. Budel (/oo, /o/) followed the life history of individual tur-
bulence units which he made visible by means of smoke and whose
course he observed through motion pictures. He also produced a
vertical smoke band by means of a smoke pot falling from a height
of 40 m and measured its drift and dispersion.

Even earlier W. Schmidt (112) had used movies in the study of
eddy diffusion. He allowed light wire frames connected by half
transparent material, such as a bridal veil, to follow irregular wind
movements, recording their position in moving pictures taken as
rapidly as possible. Fig. 139 in Chapter 28 shows an example of his

EDDY DIFFUSION AND ITS SIGNIFICANCE

measurements. We can see there the limitation which the influence
of the ground imposes on the diffusive process.

Fig. 17 deals with the effect of the weather. In it eddy diffusion
is recognizable in the irregular temperature fluctuations which
R. Geiger (102) observed thermoelectrically on the Main meadows

A. Clear, low wind

# April 1930

tt-20* XT V 40" SO" »' 15V <0*

FIG. 17. Temperature unrest in relation to weather. (After R. Geiger)

at Schweinfurt in April, 1930. The upper strip is the record of a
calm, sunny day. Between 10 and 12 in the forenoon it can be plainl)
seen by the isotherms (i° solid lines, l/2° broken lines) that incom-
ing radiation prevails. At times, however, the temperature fluctuates

FIG. 1 8. Eddy diffusion of mass made visible. (Observed by A. Schmauss on
January 8, 1931 and January 26, 1937)

rapidly. The superheated layer next to the ground at times clings
closely to the surface, at other times it separates from it. Small
masses of warm air ascend (10:52), cold air descends (11:01).

In contrast to this is the lower half of Fig. 17. The cloudy day
brought little heat radiation to the ground, with consequently slight
heating of the adjacent air layer. The wind increased eddy diffu-
sion so that temperature gradients which did form near the ground

44 THE CLIMATE NEAR THE GROUND

were quickly obliterated. Lively eddy diffusion and weather unfavor-
able to radiation account, therefore, for the quiet aspect.

At the Munich Meteorological Institute, I was called to the win-
dow one day by A. Schmauss, who directed my attention to the
following phenomenon on the other side of the courtyard. (See
Fig. 1 8) : Above the gently sloping tin roof B of a wash-house there
was fastened a grating R which formed the floor of a drying place
on the roof. When there was a light snowfall with little wind (as
on Jan. 8, 1931 and Jan. 26, 1937) the grating appeared white-banded
with narrower black intervals, while the roof beneath appeared like-
wise, not black-banded with narrower white strips of snow. The
short distance of a few decimeters which the snow had to fall be-
tween grating and roof sufficed to broaden by eddy diffusion the
snow band falling through the narrow spaces of the grating. Only
when R and B closely approached one another as at the right in
Fig. 1 8, was the fall so little and perhaps eddy diffusion so much
weakened, that black strips with narrower bands of snow between
were seen on the tin roof.

F. Rossmann (no) makes a very original contribution in his essay
on "Circulation in the Matchbox."

The importance of eddy diffusion can be shown, finally, in the
dispersal of pollen and seeds. Assuming a laminar wind movement,
even spores, with their very slow rate of settling, would not get very
far. In no case could they rise higher than their source in the plants
which bore them. Mass convection, however, with its irregular
movements, scatters these particles widely. They find themselves
now in rising, now in falling, airmasses unpredictably. Part there-
fore reach the ground sooner; part considerably later. To observe
light, winged seeds in their flight is one of the most interesting
studies of eddy diffusion which nature gives us an opportunity to
make. The lower the rate of settling and the greater the eddy diffu-
sion, the wider is the distribution. W. Schmidt (//j) who grappled
with this problem mathematically was able to show that with de-
crease of settling speed the distances the particles were carried in-
creased with extraordinary rapidity. If, by average limit of dispersal,
we understand that distance to which at least one percent of the
scattered seeds attain, and if we use an austausch coefficient of
A = 20 and a wind velocity of 6 m per second, the following dis-
persal limits shown in Table n result.

The light spores of the lycoperdon are therefore unquestionably
scattered over the whole earth. Observations on land, on sea and
along the shore have confirmed these theoretical results. H. Rempe

EDDY DIFFUSION AND ITS SIGNIFICANCE 45

TABLE 11

Sinking Rate
Substance cm/sec

Average
Dispersal Limits
in km

Fruit of t

Pollen of
Spores of

he ash (Fraxinus excelsior)

. 200

106

• 57
• 25

5-3
. 1.76
0.23
0.047

0.03
0.09

0-3
1.6
10

40
330

19000
460000

" fir (Abies pectinata)
pine (Picea excelsa)

" birch (Betula verrucosa)
dandelion (Taraxacum officinale) .
spruce fir (Pinus silvestris)

clubmoss (Lykopodium)

(Polytrichum)

(Lycoperdon)

(709) has published a more recent study of this question. The fores-
ter in scattering finely-divided poisonous powders from airplanes
over forest nurseries in the war against insect pests, is making prac-
tical use of the law of mass eddy diffusion. R. Geiger (/oj) has pub-
lished meteorological experiences in this field.

CHAPTER 5
LONG WAVE RADIATION

The heat exchange between ground and air and the heat exchange
within the air layer near the ground is caused not only by heat con-
duction (see Chapter 3) and convection (see Chapter 4) but also
by the exchange of heat in consequence of the long wave heat
radiation of the surface and the air itself.

Since 1931, G. Falckenberg (7/7) hinted at the fact that the depth
of the long wave radiation in the air is so small that the absorption
by air should not be neglected if the thermal economy of the air
layer adjacent to the ground is considered. There can be no doubt
about this kind of radiation exchange; but as far as its importance is
concerned for the entire thermal economy, there exist very different
opinions nowadays.

Generally, the long wave radiation exchange is reckoned into the
effects of convection, and that because the observations, for ex-
ample, of temperature stratification do not permit the separation
of both influences. That means that the radiation is considered as an
unessential additional part of the exchange. G. Falckenberg, how-
ever, and his followers consider the nocturnal cooling of the ground
as caused essentially by such radiation processes. In this chapter we
will get better acquainted with his idea.

From Chapter 2 (page 13) we have learned that the greatest in-
tensity of radiation from ground and air according to the low tem-
perature (in comparison with the sun) lies within the long wave
portion of the spectrum. From Wien's law we calculate:

for a temperature of: —40 —20 o +20 +4o°C

the wave length of maxi-
mum radiation intensity 12.4 11.4 10.5 9.8 9.2 ft

The absorption of the long wage radiation emitted from the ground
during day and night is caused (as has already been briefly men-
tioned in Chapter 2, p. 13) primarily by water vapor and carbon
dioxide of the air. Fig. 19 shows the absorption spectrum of the two
gases according to F. Schnaidt (127). In the upper portion (of the
figure) the absorption coefficient of water vapor, equivalent to o.oi cm
of precipitable water is represented as dependent upon the wave

LONG WAVE RADIATION

length X. The visible portion of the spectrum (0.4 — 0.8 ft) lies on
the left side beyond the figure. In this most effective portion of the
solar radiation the absorption of water vapor is negligible. The first
absorption band is at 3, another, more effective, is between 5 and 9 p

80 100

FIG. 19. Absorption spectrum for (a) water vapor and (b) carbonic acid. (After

F. Schnaidt)

with the maximum at 6.3 /A. Beyond a comparatively diathermic por-
tion the absorption increases, starting from 12 //,, rapidly and con-
tinues remaining high. Carbon dioxide, whose absorption coefficient
is represented in the lower portion of Fig. 19 (but in another scale
regarding wave length), shows two bands with sharp boundaries,
at 4.3 and 14.7 //,. The comparison of the absorption spectrums
with the array of numbers shown above for the wave length of the
strongest outgoing radiation shows that these fall in the region of
rapidly increasing absorption. The absorbed part of the radiation
hence varies with the temperature of the radiating surface.

The relationship between the emissivity ' of a body and its ab-
sorptivity is constant at a given wave length and temperature, ac-
cording to Kirchhoff's law, and the air is thus a "band radiator,"
since it absorbs in bands. It is thus different from solid ground,
for the latter is, in the region of long wave lengths, practically a
"black body"; as will be shown in Chapter 13 (see page 130), it
absorbs all radiation falling upon it. It is thus also a "black body

48 THE CLIMATE NEAR THE GROUND

radiator," that is to say it emits at all wave lengths indifferently.

This difference between the black body radiation of the ground
and the band radiation of the air leads to the phenomenon which
G. Falckenberg (//6, 118) has called the wavelength transformation.
When for instance the earth's surface is cooled by outgoing radiation
at night, heat is returned to it by the warm air next to the ground
in the form of band radiation. The ground surface which receives
this energy transforms it into radiation with a practically continuous
spectrum as it leaves the solid earth which is in effect a "black
body." This radiation emitted by the ground meets a two-fold fate.
As much of it as falls within that part of the continuous spectrum
belonging to the water vapor and carbon dioxide cannot get out. It
is absorbed. Part of this energy is given to the higher air layers and
passes away into space. Another part gets back to the earth.

Those wavelengths, however, which do not belong to the bands
mentioned, pass through the air unhindered. Their energy is "effec-
tively" radiated. The ground consequently is cooled, but only the
lowest air-layer is cooled, for it can now return energy through
band radiation to the once more cooler ground. This, in turn, gives
back only a part as utilizable to the air, while it loses a part for
good as a result of wavelength transformation, and itself cools off
still more.

According to G. Falckenberg (118) and F. Schnaidt (127) the
depth of the long wave radiation is very small. It is only a few
meters and for some wave lengths even less than 85 cm.! The air
layers at a somewhat greater distance from the ground do not cool
immediately by radiation towards the cold ground, but by radiating
towards the lower air layers, which, on their own part, are
already cooled by radiation. Therefore, the cooling process is prop-
agated very slowly upwards. Hence, E. Stoecker speaks of a radia-
tive pseudo conduction] as with the genuine heat conduction, in
consequence of the short path of the molecules, the heat is con-
ducted only slowly, also with radiative pseudo conduction heat is
transferred slowly in consequence of the small range of the long
waves.

The followers of Falckenberg pleaded in favor of the opinion that
the slow rise of the inversion layer in the evening (see p. 49) is
caused chiefly by these radiation processes. Nevertheless, the theory
of H. Philipps (68a) which considers simultaneously mass exchange
and radiation permits calculation of this lifting of the inversion in
the evening in full agreement with the observations. Be that as it
may, G. Falckenberg's observations at the aerological observatory of

LONG WAVE RADIATION

Rostock offer excellent examples of the nocturnal development of
inversions. By means of Fig. 20 we give results of -observations taken
from a paper of O. Steiner (rjo).

The evening of July 20th, 1925, was cloudless with wind from east
and south (from inland). About 6 P.M. a decided fall in tempera-
ture set in at the ground (heavy line) more than two hours before

FIG. 20. Formation of the nocturnal temperature inversion in the lowest 300 m. on
July 20, 1925 at Rostock. (After O. Steiner)

sundown, for by this time the balance was already in favor of out-
going radiation. (How often we observe, while out walking on an
autumn evening that the ground is already stiffening with frost
although to sight and touch the air seems warmer!) The tempera-
ture fall continued, slowing up somewhat as the sun went down,
until about midnight. In comparison with the course of the ground
temperature, Fig. 20 shows the course at heights of 2, 50, 100 and
300 m above the earth. The evening decrease of temperature becomes
less, the farther the air is from the earth's surface. The various de-
grees of cooling result, about the time of sunset, in an equality of
temperature throughout all layers (isothermy) and, a few hours
later, in an inversion. At 300 m above the ground the course of the
temperature has, consequently, more the appearance of accidental

50 THE CLIMATE NEAR THE GROUND

variation than of a regular daily cycle. The effect of radiative
pseudo-conduction scarcely reaches that high.

The effect of wave length transformation extends, it must be con-
cluded, to the daytime, likewise. Solar radiation causes a rise of the
temperature of the earth's surface; this temperature rise leads to an
increase of ground radiation, which occurs as an almost continuous
spectrum. A part of this ground radiation is taken in by the
absorbing bands of the air and this part causes a slowly moving heat
wave to rise from the ground, which is therefore attributable to radia-
tive pseudo-conduction. The portion of the ground radiation not
absorbed in the air is lost to the earth.

The question what share of the heat transport in the air layer near
the ground should be given to the mass exchange and what to the
radiative pseudo-conduction cannot yet be decided. Recently B. H.
Ch. Brunner (1150) estimated mathematically the share of radiation
on a summer day as, at the highest, 5 percent. In any case, the arrows
in Fig. i (page 3) and in Fig. 7 (p. 22) marked as long wave
radiation and which represent the quantitative influence of the radia-
tive pseudo-conduction are rather too large than too small.

SECTION II

TEMPERATURE RELATIONSHIPS NEAR THE GROUND

CHAPTER 6
THE WARMING PROCESS

In the earlier considerations of the incoming and outgoing types of
radiation attention was drawn to the great significance of the earth's
surface to the general heat economy, by day as well as by night. It
was further indicated what temperature conditions are to be found
in both extreme cases. In Chapters 3 through 5 it was then explained
how the movement of heat proceeds within the air layer adjacent to
the ground and in the ground itself. Let us now turn our attention
again to the two radiation types and endeavor to understand the
mechanisms of the heating and the cooling processes.

The movement of heat at midday from the heated ground down-
wards into the deeper earth layers appeared fairly simple. Here true
conduction ruled almost alone. In Chapter 3, therefore, we could
deal with temperature relationships within the ground.

At midday also it is true conduction which causes the flow of heat
from the heated ground to the air molecules adjacent to it. It is
appropriate to designate as the "boundary layer" that thin skin of air
above the ground surface in which heat movement proceeds chiefly
through molecular heat conduction. It will have a thickness of a
few millimeters at most, and on winter nights according to recent
measurements of A. Nyberg (345), even less than i mm. We shall
imagine its upper limit located at the place where the heat transfer
by eddy diffusion equals that resulting from true conduction. The
discussion in Chapter 5 shows that in addition to these two effects
there is also some transfer of heat by radiation in this boundary layer.

The agricultural meteorologists of the Indian school, under the
leadership of L. A. Ramdas have recently been working successfully
from both the theoretical and the practical angle on the subject
of heat transmission from the heated ground surface into this bound-
ary layer. There are great technical difficulties in measuring the tem-
perature distribution close to a surface. L. A. Ramdas and M. K.
Paranjpe (138) have succeeded in determining it optically, through
interference within the first millimeters above a surface without dis-

52 THE CLIMATE NEAR THE GROUND

turbing the natural layering by means of the measuring equipment.
Above an electrically heated plate, in a room temperature of 22.5°C
they obtained the following values: —

TABLE 12

Distance from the heated surface in millimeters:

o.o 0.025 0*05 o.i 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 i.o.
Temperature in °C:
87.58 82.0 79.6 77.4 74.0 71.2 68.8 66.6 64.4 62.0 60.0 58.0 56.5.

There is therefore a temperature jump of io°C in the first tenth of
a millimeter! With such a temperature gradient a strange phe-
nomenon takes place. In dust-filled air the separate dust particles
next to the hot surface receive stronger blows from the more lively
moving molecules on the side with the higher temperature. They
are therefore subject to an excess of pressure from that side and
move away from the heated surface. This results in the formation
of a very thin, dustfree boundary strip which in certain light shows
up dark in contrast with the dust-filled air above it, which reflects
the light. This dark strip affords proof of how heating proceeds
with increasing distance from the ground.

According to the observations of L. A. Ramdas and S. L. Malur-
kar (/J7) the upper surface of the dark strip is in continuous wave-
like motion. We shall return to this subject in connection with
optical phenomena. (See Fig. 60.) In several places the superheated
air now begins to lift the boundary layer in tongue-like forms.

FIG. 21. The beginning of the upward eddies in the boundary layers next to the
ground. (After experiments by L. A. Ramdas and S. L. Malurkar, 1932)

Fig. 21 is a photograph by the author which shows this. The heated
plate, visible below as a bright strip, is overlain by a cloud of
brightly lighted, artificial dust. The dustfree layer, which appears

THE WARMING PROCESS 53

as a narrow dark interval, is, in several places, extended upward in
the form of tongues. It is in these places that the superheated air is
breaking through upward. Here is where eddy diffusion begins.

These experiments have been with entirely uniform surfaces, such
as do not exist in nature. Such surfaces are, however, necessary for
a thorough understanding of the heating process because they show
how, even under these unfavorable conditions, the transition from
pure heat conduction to convection takes place.

Now we take a further step. From the "boundary layer near the
ground" we pass to the "intermediate layer near the ground."

On rough and stony ground in the desert of Gobi, W. Haude
(7^2) undertook measurements of the temperature close to the
ground surface. Fig. 22 shows in the upper part the course of the
temperature at i mm (solid line) and at i cm (dotted line) above
the ground. The Albrecht platinum wire thermometer which he
used was very suitable for the distance from the surface. The regis-
tration was rapid, Fig. 22 covering only 4^/2 minutes at about noon
on Feb. 28, 1931. In the lower part of the figure is an isoplethic
representation of the vertical airlayer between the i mm and i cm
levels.

Even at only i mm above the ground there is considerable tem-
perature disturbance — a symptom of eddy diffusion. We must
remember that the platinum wire used in the measurement was be-
tween 8 and 10 cm long. Consequently it independently integrated
all the inequalities within this horizontal distance. A measuring
point would show a considerably livelier state of unrest. In any case
it can safely be concluded from the observations that the point of
measurement at i mm above the ground is already above the
boundary layer.

But we have still not reached the layer where eddy diffusion is fully
effective. In order to save space, the 3° line in Fig. 22 is superim-
posed on the 6° line. There is actually quite a distance between the
solid and the dotted lines. This indicates that with all the disturb-
ance of the temperature at both places, which are only 9 mm apart,
there is nevertheless no exchange of air between the two layers. At
a height of i mm it is much warmer than at 10 mm. With all the
ups and downs of temperature the lowest point of the fluctuating
temperature at i mm still does not approach the highest tempera-
ture at i cm.

Above the boundary layer, then, there is a region within which
there is already vigorous eddy diffusion taking place under the influ-
ence of the strong temperature gradient. Still, its vertical effectiveness

THE CLIMATE NEAR THE GROUND

fa .fl ;. s.J

. O

2
8,

THE WARMING PROCESS

V V *?

-5
-.9

o «

56 THE CLIMATE NEAR THE GROUND

is restricted by the damping action of the adjacent ground surface. It
is this layer which we call the "intermediate layer" near the ground.
Above this, in turn, is a third layer, which, to distinguish, we
shall call the "overlayer" near the ground. This is where vertical
eddy diffusion comes into full play as compared with the intermediate
layer lying beneath it. The overlayer includes the greater part of

FIG. 24. Upward eddies of hot air, made visible by water vapor. (Photo by L. A.
Ramdas and S. L. Malurkar)

the whole air layer near the ground. In contrast, however, with the
province of the macroclimate which lies next above the overlayer,
the urgency of heating from below is here still so great that, in spite
of lively eddy diffusion, it is possible to maintain a vertical tempera-
ture gradient of considerably more than i° per 100 m during the
period that incoming radiation prevails.

To take an instance from the region of the overlayer, we shall
again use the measurements of W. Haude (132) in the Gobi desert.
Fig. 23 shows a record made with the same apparatus and on the
same day as that in Fig. 22, but 17 minutes later. The measuring
points are now located 8 cm and 100 cm above the ground. The
vertical distance between the two points of measurement is a hun-
dred times that in Fig. 22. Nevertheless the lower temperatures at
8 cm are equivalent to the high temperatures at i m. From this we

THE WARMING PROCESS

may conclude at least that air parcels move back and forth between
the two places of measurement.

Fig. 24 is intended to illustrate the strong vertical mixing in the
overlayer. It is taken from the work of L. A. Ramdas and S. L.
Malurkar (137). Upward streaming of the warm air is rendered
visible by some water which was placed on the hot surface. It takes
place irregularly, according to the nature of eddy diffusion. In cer-
tain places the heated air breaks through upwards. In the darker
places the necessary compensating downward movement of cold air
takes place. From the processes shown in Fig. 24 up to water spouts
and dust whirls, is only a difference in magnitude — not in kind.

We already called attention in connection with Fig. 17 to the ex-
traordinarily unsettled condition of the temperature as a special
characteristic of the whole climate province near the ground. Fig. 25

^ \20.May1934\

. - • .' :

: "•"W740*

/ //•""•

Tl/

3/7°

*\n

jT»*

r-^p^/^^

..- •>;o.-^.; o<y

ivW' ^

^^C'"

^v/

/^70

ft 1 ^^ '

/V/

, i — '/r

FIG. 25. The large temperature unrest after 10 o'clock (incoming-radiation type) is
characteristic of climate near the ground. (Recorded by R. Geigcr in Munich)

will make clear how temperature conditions during the period of
incoming radiation contribute to the development of such a condi-
tion. It covers a record made with engraved stem thermometers by
R. Geiger (/j/) by means of a Hartmann and Braun recorder
located at heights of 0.23 cm, 100 cm, and 200 cm above the ground
at the Munich airport. At o cm the thermometer lay on the ground,
which was covered with a short, dry sod.

At the beginning of the record the curves are so placed that the
highest temperature corresponds to the highest point of the curve
(Inversion, outgoing radiation type). In the overlayer from 23 to

58 THE CLIMATE NEAR THE GROUND

200 cm the transition to incoming radiation type takes place between
5 and 6 o'clock; the curves intersect. From then on, the upper curve
corresponds to the 23 cm height. In the air layer next to the ground,
which is confined by the blades of grass, it remains relatively cold.
Not until after 7 o'clock does this layer share equally in the warm-
ing up process, after which time it proceeds rapidly.

When, after 10 A.M. the incoming type of radiation, with its steep
temperature gradients, comes into full play, it seems as though the
temperature curves are suddenly destroyed. Vigorous eddy diffusion
scatters the readings over a wide range. If a person wanted to find
the average temperature for the time between 12 and i P.M. at a
certain height, it could be only a theoretical figure. The temperature
distribution becomes an essential characteristic and must be deter-
mined independently of the actual reading. R. Geiger therefore
proposed in presenting temperature observations near the ground, to
use not calculated temperature points but bands of temperature,
whose breadth corresponds to the range of distribution of the tem-
perature for a given time at the place in question. This proposal
has meanwhile been accepted and used in numerous publications.

Finally let us reach up in thought still further above the air layer
near the ground and seek to understand the heating process at greater
heights, for the stratification of unstable air masses is not without
reaction upon the ground layer.

H. G. Koch (/£?) using a pair of pilot balloons, sent a radiation-
shielded resistance thermometer up to 100 m and determined the
temperature stratification within this air layer. He was able thus to
demonstrate the great temperature disturbance which results from
the heating process — "temperature gustiness" he calls it. In order to
extend our consideration of the heating process also into these higher
layers we have presented in Fig. 26 a fine example from Koch's work
on the ascent of heated air from the ground. The upper curve of
temperature state I shows the normal type of incoming radiation
(Aug. 3, 1935 at 1050 A.M.). Shortly thereafter there was an up-
heaval in the stratification. It became warmer above, but noticeably
colder in the ground layer. At first the mixing is imperfect; conse-
quently curve II still shows a stratified structure of the air layer.
Only after several minutes (curve III) is adjustment complete, with
the temperature increasing uniformly with altitude.

The process of upheaval shown here coincided with the over-
shadowing of the measuring place by a cumulus cloud. Seven min-
utes after the cloud passed over, the incoming type of radiation I
was re-established.

THE WARMING PROCESS

In conclusion we mention briefly the theoretical work of S. L.
Malurkar and L. A. Ramdas (134) whose purpose it was to compute
temperature stratification above heated ground from the heat bal-
ance and to test their calculations by observations.

30
20
10

60
50
§W
$30
20
10

60
SO
W
30
20
10

0°

?°

9°

FIG. 26. Cooling of the layers of air near the ground by overturning of layers.
(After H. G. Koch)

The assumptions which had to be made before attacking each
theoretical problem are in the present case so few, and correspond
so closely to the natural conditions, that the theoretical work can be
treated here in close connection with practical microclimatic meas-
urements.

It is assumed, namely, that: —

1. The heat balance, which is determined by radiational and eddy
diffusion processes, has come to equilibrium; the temperatures are
constant for a time, as occurs during the midday hours.

2. The water vapor content of the ground layer up to some 20 cm
is uniform. This was tested by measurements with an Assmann
psychrometer and found to be true at the place of measurement, a
bare asphalt floor.

3. The temperature of the air masses lying above the ground
layer can be looked upon as homogeneous, and finally,

4. No horizontal air transport (advection) is disturbing the heat
balance.

THE CLIMATE NEAR THE GROUND

The mathematical treatment takes into consideration the long-
wave temperature radiation of the heated ground surface and all
the air layers concerned — also the convection processes, under the
conventional assumption that heat transfer is proportional to the
vertical temperature gradient.

The computation leads to the following result : — Using the hyper-
bolic sine function,

sinh

+ z)

sinh a • h

z is the height in cm above the ground surface; h is the height of
the ground layer concerned up to about 20 cm; O0 is a constant;
<I> is the variable part of the temperature. Since, for z = A, the value
of <I> = o, <3> is the excess of temperature in the ground layer over
the temperature value which was taken to be constant above the
ground layer and which approaches the temperature curve asymp-

30°

' \

o Calculated

X Observed

1
•f

• \

• o

till

X°^
I i i I

"•"•i •»

35"

Temperature ( C)

FIG. 27. Theoretically calculated and observed temperature stratification over an
asphalt street. (After S. L. Malurkar and L. A. Ramdas)

totically as z increases, a is a coefficient in which are combined

(a) the absorption coefficient of water vapor for long wave radiation,

(b) the Stefan-Boltzmann constant, (c) the absolute temperature
and (d) the austausch coefficient. The value of a varies. In the

THE WARMING PROCESS 61

highest layer, which extends from about i cm above the ground
to some 30 cm, it amounts to 0.25. In the second layer, which ex-
tends from about i mm to 10 mm, it is 4.2. The change is discon-
tinuous. Below i mm there is a layer with a still more suddenly
increased a value.

Fig. 27 shows the temperatures measured by the author at 2 P.M.
on Oct. 22, 1931 above an asphalt street near the Meteorological
Office in Poona, using an Assmann aspiration psychrometer. The
water vapor content of the air amounted to 10 mm. In addition there
is a theoretical curve derived from the above given equation. The
agreement of theory with observation is very good.

CHAPTER 7
THE COOLING PROCESS

The nocturnal cooling near the ground is a result of the outward
radiation from the ground surface, which was described in Chap-
ter 2. Even for some time before sunset the radiation balance of
the ground is negative; in other words, the outgoing radiation
exceeds the incoming. Nocturnal cooling consequently sets in before
nightfall and lasts till after sunrise as E. G. Meyer (/49#) has re-
cently confirmed. Net outward radiation increases decidely as night
comes on and reaches its maximum before midnight.

The ground surface cools through radiation. Along with the tem-
perature decline of the surface goes the cooling of the air near it.
Through radiative pseudo-conduction (Chapter 5) and pseudo heat
conduction (Chapter 4) it gives heat to the colder ground. This loss
is greatest for the layers nearest the surface and decreases with dis-
tance from the ground. Thus there is set up the outgoing radiation
type shown in Fig. 8, in which the cold, and therefore heavy, air
layers form beneath the warmer, lighter ones. In contrast to the
incoming radiation type where the turmoil hinders stratification, a
stable, vertical stratification predominates at night. This stability
increases throughout the night as further cooling proceeds. G. Hell-
mann (59) has described this condition in these apt words: "The air
clings to the ground as though anchored there, and resists all efforts
to move it." Night is, consequently, the time of least wind velocity
at the ground. Horizontal fog banks which last for hours un-
changed, or slowly increasing in thickness, are often the visible mani-
festation of this stable stratification.

Nevertheless, at night a perceptible convection exists although
diminished in comparison with the values during the day time. This
is proved by observation. It is surprising at first glance; because the
thermal exchange does not exist because of the stable air stratifica-
tion and also the dynamic part is essentially diminished on account
of the insignificant motion of the air. By night, however, another
process occurs supporting the exchange to which A. Defant (144)
drew our attention in 1919.

The dust content of the lower air layers increases at day time
when the upward directed exchange currents lift the dust upward.

THE COOLING PROCESS 63

At night, a downward motion must occur because, otherwise, the
dust content of the atmosphere would increase steadily. This sink-
ing down is expected also from thermal causes. The dust which
consists mostly of solid particles of the disintegrated soil absorbs al-
together 5 to 15 percent of the incoming radiation during daytime.
At night, it must play a similar part: the solid dust particles emit as
does the solid surface and cool rapidly, therefore, below the tempera-
ture of the immediately surrounding air. This has a twofold effect :
first, the air directly adjacent is cooled off. Because under normal
conditions the air near the ground contains tens of thousands of dust
particles per liter this kind of cooling process is not unimportant.
Second, each dust particle together with the enveloping air film —-
the "small gas ball" — starts falling because of the lower temperature.
Thus, a current of smallest air threads is fomed, called "returning
convection" or "coldness convection" by A. Schmauss (/5^). There
exists also during night, therefore, a thermally caused exchange.
It is distinguished, however, from that in day time by the small
dimensions of the portions of air which partake of the exchange.
Therefore, it occurs to a certain extent unperceived in the stable air
layers near the ground.

Besides the convection, processes caused by long wave radia-
tion are effective as discussed in Chapter 5. These processes permit
us to explain a striking phenomenon known from the tropics.

In 1932, L. A. Ramdas and S. Atmanathan (752) called attention
to the fact that in India the lowest night temperature is in many
cases not at the ground surface but at some distance above. Some-
times the minimum occurs at a height of only a few centimeters, but
occasionally it may be as much as i m or more. Measurements at
various places in India have proved this so many times that it can
hardly be doubted. I know of no instance, however, of a similar
valid observation in our climate, especially iri Germany. The testi-
mony of measurements which led to similar curves of state is inad-
missible, since the slightest ground cover of plants naturally raises
the minimum into the air, arid since, too, even over bare ground,
surrounding influences, such as a neighboring plant surface may
occasion the same phenomenon. In additibn, similar conditions in
close proximity to the ground may be deceptive on account of diffi-
culties in the technical use of instruments.

L. A. Ramdas, R. J. Kalamkar and K. M. Gadre (/pz and 192)
and later L. A. Ramdas (151) have made rather close estimates of
minima above the surface. In Fig. 28 is given an example from a
new piece of research by K. R. Ramanathan and L. A. Ramdas

64 THE CLIMATE NEAR THE GROUND

(750). The measurements were made in the neighborhood of Poona
on a January night in 1933. The temperature of the ground itself is
indicated on the sketch by a small arrow coming from below. The
small circle at zero height gives the air temperature measured di-
rectly at the surface; it is considerably lower than the surface tem-
perature. From here on the temperature decreases with height still
further up to a height of between 10 and 12 cm above the ground.
Only above this begins the normal type of nocturnal outgoing
radiation, the temperature inversion.

-60

"20

ft W 12 H

Temperature in °C

FIG. 28. Temperature variation with height during the night between January 5 and
6> 1933 near Poona. (After K. R. Ramanathan and L. A. Ramdas)

In the climate of India the heating of the ground by day is so
excessive that at night there is a very strong flow of heat toward its
surface. But even in an extreme case of this kind of a heat transfer,
the minimum can occur above the surface only if the radiation proc-
esses, perhaps in conjunction with the stratification of the water
vapor, displace the maximum zone of outward radiation from the
ground surface into the airlayer above it. The work of K. R. Raman-
athan and L. A. Ramdas already contains the theoretical research
necessary to a solution of this question, and should be consulted for
further details.

It remains our task here to show how the process of nocturnal
cooling leads gradually to that temperature distribution which we
recognize as the normal outgoing radiation type. Going beyond the
province of the ground airlayer, we have already, in Fig. 20, shown
the extension of the nocturnal temperature inversion to greater and
greater heights. In what follows we shall limit ourselves to the region
near the ground.

THE COOLING PROCESS 65

S. Siegel (755) assembled a wealth of observational data at Ham-
burg. On the grounds of the meteorological institute of the uni-
versity, an observation tower was erected, with 22 measuring points
arranged between the ground and a height of 4 m. At these points
the nocturnal temperature was followed by means of radiation-
shielded thermocouples. Fig. 29 shows several types of nocturnal
temperature distribution according to altitude. The wind is assumed
to be weak and the night favorable to radiation.

Curve i, which Siegel calls the "evening wind type," starts at
nightfall before the nocturnal calm has become established. The

300

200

100

1 Z 3 5 5

Types of nocturnal temperature distribution over the ground. (After
S. Siegel)

FIG. 29.

effect of outgoing radiation is yet noticeable only near the ground.
Above this, practical isothermy prevails in consequence of the uni-
form mixing of the air. Only after the wind dies down does the
intermediate type 2 take over. The cold layer deepens from below.
A tolerably uniform fall of temperature is found up to a height of
1.5 m. At greater heights the temperature distribution is the same
as for the evening wind type i.

After the air has been quiet for 20 to 40 minutes we have type 2
passing into type 3, with a nocturnal secondary minimum. This is
an indication that even at night the ground air has a foliated struc-
ture. The secondary minimum corresponds therefore to the second-
ary maximum of the day. When the wind rises suddenly at night
a thorough mixing of the air is soon in process again with a conse-

THE CLIMATE NEAR THE GROUND

quent equalization of temperature ("The wind prevents frost!").
The convection type 4 results.

In Fig. 29 the so-called "meadow-fog type" is given as type 5. It
occurs when a thin sheet of fog i to 2 m thick forms over the
meadows. A uniformly slowed cooling which Siegel attributes to
condensation is evident within this layer. This warming up is
superimposed on the outgoing radiation type and so appears in
curve 5. At the Aspern airport in Vienna W. Kiihnert (/^p), with a
similar experimental setup, studied the temperature gradients above
the ground during the slow growth of a fog bank. His work pre-
sents an opportunity to study the gradient fluctuations in relation
to time and place, which are associated with ground-fog formation.

Fig. 30 shows in isopleths the ideal case of temperature stratifica-
tion in the course of the night. At the upper edge of the chart is

Time

lime

FIG. 30. Ideal case of the nocturnal temperature stratification over the grou

(After S. Siegel)

ind.

the idealized course of the wind velocity. The dying down of the
wind in the evening causes the cooling off of the ground to be in-
creasingly accelerated. As the night progresses the cold air layer at
the ground builds up. It is most fully developed shortly before sun-
rise and usually disappears suddenly as the sun rises. Fig. 31 shows
quite clearly what the actual thermal stratification is in certain cases.
The picture in general is the same as in Fig. 30 but the course of
the isotherms is much more irregular. If we compare them with

THE COOLING PROCESS 67

the curve for wind velocity that same night, as shown at the upper
edge of the chart, we find that each time the wind dies down, the
cold air layer builds up and each time the wind increases in strength

Height

** Time '

FIG. 31. Course of temperature on a night with light wind at Hamburg. (According
to thermoelectric measurements at 23 heights above the ground by S. Siegel)

the stratification is destroyed. It follows that the disturbed state
which we mentioned as a characteristic of temperature relationships
in the microclimate is present by night as well as by day, though to
a lesser degree.

CHAPTER 8

THE DIURNAL AND ANNUAL COURSE OF TEMPERATURE
NEAR THE GROUND

Our remarks thus far have been directed particularly toward an
understanding of the physical laws governing heat stratification in
the microclimatic air above the ground surface. How the heat trans-
fer proceeds by day and night, how the heat is carried upward and
downward from the boundary surface between earth and air, what
contrasting temperature distributions present themselves as the in-
coming and outgoing types of radiation, these have been the sub-
jects of our consideration.

Now, however, we must turn to the climatological side. Between
the two extremes of temperature distribution mentioned above there
are many transitional states. Transition is effected through the
temporary change from one type to the other as sometimes happens
during the morning hours. It is occasioned also by weather condi-
tions which reinforce or reduce radiation in contrast to other factors
such as wind or precipitation.

Whoever interests himself in the temperature relationships of the
microclimate, seeks not only knowledge of the meteorological proc-
esses but also a description of the actual average conditions. It is our
next task to present this.

For this purpose as long a series of observations as possible is de-
sirable. Such a series is unfortunately very rare. Such as exist fall
naturally into two distinct groups, which differ in the method of
approach.

In the first place we seek to observe the "true air temperature."
In the macroclimate we understand by this term that temperature
which is indicated by some measuring instrument, such as a ther-
mometer, which is in good contact with the air to be measured,
while protected from all radiative influences. In practical climatology
the measurement will be carried out in the standard shelter or with
an aspiration psychrometer. The radiation shield, in the case of the
shelter, consists of whitepainted walls and a double roof; in the case
of the aspiration psychrometer, of a double-walled metal protecting
cylinder which is polished nickel outside and blackened inside. Ven-
tilation of the shelter through numerous openings in the walls, floor

THE DIURNAL AND ANNUAL COURSE 69

and roof is fairly good. Ventilation of the aspiration psychrometer
is accomplished artificially by means of clockwork. In the case of
the sling psychrometer the instrument is moved through the air
instead of the air being moved past the instrument. When protected
from radiation it gives good results, at least so long as no precipita-
tion is falling.

Measurements of the "true air-temperature" in this sense are also
carried out as continuous records in proximity to the ground. For
investigations in the larger space relationships it is possible to use
observation shelters located at various heights and equipped with
thermographs. The measurements given by K. Knoch (/#5) will
serve as an example. As soon as we pass to smaller and smaller
spaces we have to use electric thermometers — either resistance ther-
mometers or thermocouples. The thermometers can be artificially
shielded and ventilated. Representative in accuracy and general
features of such measuring equipment are the investigations of N. K.
Johnson (182) and W. D. Flower (778) of which more will be said
later. Or, on the other hand, we can disregard both radiation shields
and ventilation and, instead, use a resistance thermometer of such
a small diameter that radiation errors can be neglected. Such was
the fine resistance thermometer of F. Albrecht (/57). In Figs. 22
and 23 we have already shown what excellent results Haude attained
with this apparatus.

Albrecht's method has the advantage that the thermometer, on
account of its small dimensions and mass, scarcely disturbs the nat-
ural temperature stratification at all, while artificial ventilation is
entirely omitted. The disadvantage of the liability of the 0.015 mm
platinum wire to mechanical damage can be offset by using a pro-
tecting cage. A. Made (174) described such a gadget which had
practically no effect on the temperature readings. Unfortunately we
have not as yet a long record with this apparatus from a ground air
layer free from vegetation.

We are consequently using in the following discussion the diurnal
and annual temperature march according to the measurements of
N. K. Johnson and W. D. Flower. These data do not extend further
than 1.2 m above the ground, so that the artificial ventilation is not
a disadvantage in a comparison with the higher air state. A. C. Best
(776) extended these researches, by means of the same method,
through measurements at 30.0 cm and 2.5 cm above the ground.
Although artificial ventilation at these low altitudes must necessarily
result in mixing unequally warm air layers, and although conse-

70 THE CLIMATE NEAR THE GROUND

quently the height of the place of measurement is not absolutely
definite, we shall refer quite often to these measurements.

To summarize, let us say that in this chapter and the following
one we shall deal with measurements of the true air temperature.
Yet here we must mention a fundamental thought in this connec-
tion.

The "true air temperature" whose definition was a just and neces-
sary precaution for macroclimatology, loses its significance as we
approach the surface of the ground. As is evident from all that has
been said, the neighborhood of the ground is characterized, in its
effect on the microclimate, by the rapid decrease of natural ventila-
tion and the decided increase of the effect of radiation. If by defini-
tion we reject radiation and demand thorough ventilation, we con-
tradict the very nature of the ground climate.

While, therefore, physically minded meteorologists must have
true air temperatures, even close to the ground, biologically minded
students direct their attention to observed values which mean more
in respect to biological processes. Plants and animals, insofar as they
live near the ground, undergo these special conditions of high radia-
tion and deficient ventilation. For biological purposes we are glad to
use test objects. Their temperatures should be subject to the same
kind, even if not to the same amounts, of radiation and wind as are
the plants and animals themselves. The application of ground tem-
peratures to the practical ends of gardening, agriculture and forestry
has led to the use of experimental bodies, just as the application of
macroclimatic temperature measurements to hygiene and biology
have led to the use of the frigorimeter or frigorigraph, which basi-
cally are really test objects.

It is a question of scientific standardization how to judge the two
viewpoints. The precise physicist will always have a horror of using
artificial test objects where an immense number of inextricably in-
volved separate factors are in play simultaneously. The practical
botanist, on the other hand, can not understand why we measure
an air temperature which, by its very definition, gives results far
different from actual natural processes. The microclimatologist,
who stands between the two camps, will perhaps consider it his pur-
pose to measure all the factors involved, both singly and together, —
true air temperature, humidity, wind, incoming and outgoing radia-
tion of all wave lengths, reflected radiation, etc. Until this distant
goal is attained the method of experimental bodies can not be en-
tirely dispensed with.

THE DIURNAL AND ANNUAL COURSE 71

J. Bartels and M. Kohn (162) report that J. Schubert used a ther-
mograph as a test object. The measurements of P. Vujeric (797),
inasmuch as they were made without a radiation shield, may be con-
sidered as measurement of experimental bodies. In the upper Bavar-
ian station network R. Geiger (/7p) used Six's thermometer with
transparent filling liquid whose overheating by day was compara-
tively slight. For continuous microclimatic records R. Geiger (167)
introduced cylindrical electrical thermometers, which were also used
occasionally by J. Bartels (/6/). Technical objections to this manner
of using them in conjunction with recorders have been made by
G. Griindl (/6p) and H. Forster (765), to which, in turn, the late
F. Linke (772) took exception.

In Chapter 17 we shall introduce records from experimental bodies,
but just now we shall turn our attention to measurements of the
diurnal and annual march of the true air temperature at different
heights above the ground.

N. K. Johnson (182) during the years 1923 to 1925, carried on
measurements at heights of 1.2, 7.1 and 17.1 m above close-mown
sod on Salisbury Plain, in southern England. He employed electric
resistance thermometers which were perfectly shielded from radia-
tion through six glazed porcelain casings arranged one over another
in layers, and were artificially ventilated day and night. The ther-
mometers were hung from a steel tower, constructed as light as
possible. The tower was painted white to reduce its effect on the
temperature field. The details of standardization of the apparatus
should be obtained from the original publication which contains
also the recorded data for the three years. A. C Best (776) has con-
tinued this research, making a record after the same manner, at
heights of 30 cm and 2.5 cm from Aug. i, 1931 to July 31, 1933.

The same arrangement was used by W. D. Flower (178) for ob-
servations which he has carried out since 1928 near the airship anchor-
age at Ismailia in Egypt (at the Suez canal). A special steel tower
was erected near the anchor mast in that desertlike, almost flat land.
Records were made at heights of i.i, 16.2, 46.4 and 61.0 m. That
from October 1931 to October 1932 has been worked out in detail.
Flower had most favorable conditions for his experiment in the
uniformly clear weather of Egypt; Johnson and Best's results cor-
respond more closely to our climatic conditions.

In Fig. 32 the diurnal march of temperature in Egypt for the two
contrasting months of January and July is shown as measured at
heights of i.i m (solid line), 16.2 m (broken line) and 61.0 m

72 THE CLIMATE NEAR THE GROUND

dotted line). In summer the incoming type of radiation; in winter,
the outgoing type, occupies the greater portion of the day. During
the summer nights the temperature difference between the several
altitude layers continues to increase till sunrise; the three curves tend
to separate. In the winter, on the contrary, a temperature gradient
is established by midnight, which is maintained, even during the
continued cooling.

30'

-61,0m
-162m

• um

July

0 6 12 18

Time of doy

FIG. 32. Daily course of temperature in Tsmaiiia (Egypt) in three different heights
in January and July 1932. (After W. D. Flower)

In summer, as in winter, the temperature rise at the close of the
night occurs at quite different times at the three different heights.
In July, for instance, the minimum at i.i m is about 5 A.M.; at
16.2 m, shortly before 6; at 61.0 m not until about 7. As the begin-
ning of the warming process is retarded with height, so does the
maximum temperature occur later, the higher the measuring station
is above the ground. In Fig. 32 this is most evident in January.

While the change from night to day always shows marked and
regular retardation with height, the transition from day to night
follows a different pattern. This is true not only for the measure-
ments made in Egypt but as a general rule. The transition to the

THE DIURNAL AND ANNUAL COURSE

nocturnal condition of stratification takes place almost simultaneously
at all heights here considered. The way the temperature lines inter-
sect is accidental rather than the result of strict regularity.

From the temperature scale of Fig. 32 it can be seen that the
measurements are from a subtropical climate. In contrast, Fig. 33
shows the course of the temperature during a summer day in Eng-
land. As is to be expected in a climate where insolation is weak,

20°

mm}

-. ( After N. K. Johnsoi

_ 7.1m i

— — rim }

— 2 5 Cm After A. C. Be

2**

Time of day

FIG. 33. Daily course of temperature in August, 1923-1925. (After Johnson and Best)

and at lesser heights above the ground, the temperature differences
in the various layers are smaller throughout. (Notice the different
temperature scale of the two charts!) The general features of Fig.
33, however, correspond to those of Fig. 32.

An example which brings us to the temperature stratification
nearer the ground is shown in Fig. 34, from a publication by L. A.
Ramdas and M. S. Katti (2/0). From measurements with an Ass-
mann aspiration psychrometer the average values of hourly observa-
tions from the 4th to the 8th of January, 1933 are given in the form

THE CLIMATE NEAR THE GROUND

of isopleths in steps of 2l/20C. The accompanying humidity distribu-
tion we shall study later in connection with Figs. 46 and 48.

In regard to the temperature stratification within the ground, the
sketch reminds us of Fig. 10. The regularities found there reappear
here too — i.e. decreased fluctuation of temperature, and lag of ex-
treme values with depth. A very similar picture is formed by the
isopleths above the ground, yet they, in contrast to conditions within
the ground, are greatly elongated away from the surface. Here is

Time of day

FIG. 34. Temperature layers both sides of the ground surface in the course of the day.
(After measurements of L. A. Ramdas and M. S. Katti)

where the effect of eddy diffusion appears; the air behaves like a
soil of extremely high conductivity. During the morning hours we
find a cold-air dome, at midday, a warm-air dome, above the ground.
During the afternoon temperatures at the ground rise to almost 50°.
This could not be clearly shown in Fig. 34, since the region of time
and place corresponding to these high temperatures is represented as
a black area.

From the course of the daily temperature as depicted it follows
that the daily range of temperature increases rapidly as we approach

THE DIURNAL AND ANNUAL COURSE 75

the surface of the ground. At the Schleissheim observation station,
the average daily temperature range for the months of May-Septem-
ber found by R. Geiger (179) was: —

Height above ground 1.5 i.o 0.5 0.05 m

Daily range 14.3 14.7 15.4 T9-5°C

A. C. Best (776), using his records and those of N. K. Johnson
over a period of two years, calculated the following average values
of the daily temperature range in relation to height, season and
weather : —

Height above ground

17.1

7.T

1.2

0.3

O.025 m

December Average
June Average
8 sunny June days

2.4
7-7

IO.Q

2.7

8-3

II.Q

3-i

9.4
14.0

3-3

10.2
IS. 3

3-7°C
u.8°C
i8.o°C

One of the particular characteristics of the microclimate is that it
becomes more extreme the closer we approach the ground. Proof
of this can be seen everywhere. Fig. 35 shows the railing of a flight
of sandstone steps at the Winterthur city hall, in Switzerland, as
published by F. de Quervain and M. Gschwind (/po). The disin-
tegration suffered by the soft stone increased with nearness to the
ground. The great temperature range is here reinforced in its
action by water. In the first place the lower part of the stone has
been subjected to alternations between dryness and the moisture of
snow and spattering rain more often than the upper part — another
characteristic of the ground microclimate! Furthermore, in the
transitional seasons, the destructive effect of frost through thawing
and freezing is greater, the nearer the ground.

This change of melting and freezing again, the so called "frost
change" is very different from place to place as far as the yearly
frequency and its annual variation are concerned. Even with the
large scale climate basic differences exist so that, now, the fre-
quency of frost changes are considered a significant climatic element.
Besides, the rnicroclimatic differences are effective, for the mechan-
ical formation of the ground, i.e. splitting of the rocks in conse-
quence of the volume increase of water in the fissures when freezing,
plays an important role.

A day with frost change is a day the temperature curve of which
passes the freezing point one or several times independent of the

76 THE CLIMATE NEAR THE GROUND

sense of the temperature change; if the temperature passes from posi-
tive to negative this process is connected with an explosive effect;
changing from negative to positive is the condition for freezing again.
Number of frost changes is the number of passages through the

FIG. 35. Picture of the corrosion of the Bernese Sandstone on the State House in
Winterthur. (After F. de Quervain and M. Gschwind)

freezing point. The number of frost changes is, therefore, equal to
or greater than the number of days with frost change. The ratio of
the two values which is ^ i is called density of frost change. Its
value is (in our latitudes) 1.5-2.0. In the high altitudes of the
tropics the temperatures are above the freezing point during day
time and below the freezing point with the same regularity at night
in consequence of the uniform temperature the year round. The
density of frost change results exactly with 2.0. It can even reach
the value of 2.4 by supplementary irregular temperature variations.
The frost change is most frequent in the top layer of the ground.
E. Heyer (/#/) found for Potsdam how the frost change number
varies with depth:

THE DIURNAL AND ANNUAL COURSE 77

Depth, cm:

IOO

Annual frost change number

... 119

3-5

o-3

Average frost change density:

... 1.8

1.8

i-7

i-5

i.i

I.O

From the synchronous temperature records in the shelter (1.9 m) he
found a frost change number of 131, on the observation tower (34 m)
95; the frost change density was 1.8 at both points. The decrease
of the frequency of frost changes from the surface downwards and
upwards is easily recognizable, but systematic observations in the
air layer near the surface are still lacking. As far as frost change
frequency is concerned climatology at large scale and microclimatol-
ogy approach one another closely.

In 1943 C. Troll (1960) made a thorough study on the importance
and the geographical distribution of this climatic element and ex-
plained (1947) (^96^) its effect upon soil formation. In the dry
highlands of the tropical and semi-tropical mountain ranges rich
in radiation the annual number of days with frost change surpasses
300 although the shelter temperature is used (for instance El Misti in
South Peru (jJ7). The frost change is, there, a whole year phenom-
enon; in higher latitudes it is limited to the transition seasons and
winter. In many places soil structures with polygonal nets are caused
by particular frost effects. The depth of these soil structures is only
10 to 20 cm in the high levels of the tropics, e. g. South Peru on the
western slope of the Andes between 4100 and 5200 m, corresponding
to the small depths to which the daily frost penetrates. But in the
arctic regions, the order of magnitude of the depth of the soil struc-
ture is meters, corresponding to the deep reaching seasonal frost
effects. But with these considerations we change to the realm of
climatology on a large scale and soil science.

The increase of the daily temperature range with approach to the
ground is common to all the macroclimates of the earth. This is to
be expected in the tropics. But it is true of the polar climate also.
The excellent observations of Alfred Wegener in Greenland have
demonstrated the independence of the polar microclimate at the
ground. More recently, H. Slanar (*/95) has carried out temperature
measurement over a basalt ground surface in the polar wilderness
of central Iceland during July 1931. As an average of five clear days
he obtained a temperature range of ii°C at a height of i m and of
at least 26°C at the ground. At a depth of 20 cm in the ground
the range had diminished to 5°.

78 THE CLIMATE NEAR THE GROUND

Now we shall show the influence of cloudiness on the daily tem-
perature range in the air layer near the ground. The effect of the
wind we shall take up later.

The observations of N. K. Johnson (182), which we present in
Fig. 36, show very clearly the influence of changing weather on the
temperature stratification above the ground surface. The summer

Height above the ground

Time of day

1 f f f f

FIG. 36. Dependence of daily course of temperature on cloudiness. (After N. K.

Johnson)

month of June and the winter month of December are placed side
by side, using the same temperature scale. The times of sunrise and
sunset are indicated by a small arrow attached to the recognized
solar symbol. Cloudy weather in summer causes a decided flattening
of the temperature curve. The average temperature, however, is
only slightly reduced. The vertical temperature stratification is less,
to be sure, but by day it is always evident. The displacement of the
times of temperature extremes with height is greater in cloudy
weather than in clear.

In December cloudy weather causes a decided rise of the whole
temperature level. Clear weather brings frost. While on clear days

THE DIURNAL AND ANNUAL COURSE 79

the outgoing type of radiation prevails for most of the day as a con-
sequence of the long night, cloudy weather brings practical uni-
formity of night temperature. Only by day is there still some indi-
cation of a special microclimate above the ground. It should be
noticed, nevertheless, that Fig. 36 shows nothing of the temperature
relationships below 1.2 m. Many observations indicate that even in
stormy weather there are still noteworthy temperature differences
to be found there. Unfortunately we lack sufficient observations.

W. D. Flower has harmonically analyzed the annual temperature
march for observation heights of i.i, 16.2, 46.4 and 61.0 m above the
ground. From the course of the temperature it appeared that at the
four heights mentioned the peak values of the annual temperature
curve fell on July 10, July 29, July 30, and July 31 respectively. By
harmonic analysis of the diurnal march, the corresponding times of
the temperature maxima were: — 2:42, 3:17, 3:34 and 3:40 P.M. Thus
the extension of the diurnal and annual temperature wave can still
be recognized though so far away from the surface and through good
measurements it can be traced as in the ground. It is really an unex-
pected pleasure to be able to demonstrate it so beautifully in the
realm of the ground climate.

CHAPTER 9
THE TEMPERATURE GRADIENT NEAR THE GROUND

In the free atmosphere, decrease of temperature with altitude is the
rule. Temperature relationships near the ground, however, are
characterized by change of the temperature gradient in direction and
magnitude. It has been shown statistically what high values the

between 46.4 and 61.0 meters height

between 1.1 and 16.2 meters height

10° 20° 30° 40° 50°

Temperature lapse rate for each 100 m height
*- Adiabatic gradient

FIG. 37. Frequency distribution of temperature gradients occurring in Ismailia.
(After W. D. Flower)

temperature gradient can attain at midday. W. D. Flower (178) in
analyzing the Egyptian observations, as did N. K. Johnson (182)
before him, paid particular attention to the variation of the tempera-
ture gradient with time. W. D. Flower's conclusions shall be our
guide as to the most important facts in the following discussion.

Fig. 37 indicates, first of all, the frequency distribution of the most
common gradients. As abscissa we have the gradient, computed in

TEMPERATURE GRADIENT NEAR THE GROUND 81

altitude steps of 100 m each. Negative values mean the normal tem-
perature decrease with altitude; positive mean inversions. As ordi-
nate we have the annual percentage frequency computed as the mean
from hourly values. The boundary curve of the shaded area
represents the temperature gradients between i.i and 16.2 m above
the ground. The heavy line represents gradients between heights of
46.4 and 61.0 m.

The shaded area is unsymmetrical with respect to the zero gradi-
ent. Slight inversions are the usual condition at these heights. The
curve declines gradually to the right, for very large inversions are
improbable but, on account of the stable stratification, still possible,
and to the extent of almost 50° per 100 m. The adiabatic gradient
is indicated by the arrow in the lower part of Fig. 37. There are
super-adiabatic gradients far above this rate in the layer between
i and 16 m. But once a value about ten times the adiabatic is reached,
overturning occurs even in this very conservative ground layer. The
frequency curve falls off steeply to the left of the — 10° point.

In the more readily homogenized air between 46 and 61 m, by
far the most frequent gradients lie between isothermy and the
adiabatic value. Toward the left from this point the curve falls off
very steeply since surplus heating from below is quickly equalized,
with a return to the adiabatic gradient. Inversions still occur fre-
quently.

The diurnal and annual cycle of temperature gradients is shown
in Fig. 38. From 1893 to I9°4 simultaneous records were made at
the Meteorological Observatory in Potsdam, of temperature and
humidity at the height of 2 m over a meadow and 34 m on a tower.
K. Knoch (185) has analyzed the results. Fig. 38, accordingly, shows
by isopleths the temperature difference between the two locations.

The time of year has been taken for abscissa, the time of day for
ordinate. The continuous lines connect points of equal temperature
difference between the 2 and 34 m heights. These differences are
small since both records were made in shelters, thus avoiding ex-
treme conditions. Negative numbers signify a normal decrease of
temperature with height; positive numbers, an inversion. The two
heavy zero lines indicate the condition of isothermy or what might
well be called the transition from incoming to outgoing type of
radiation and vice versa. The dotted lines correspond to the times of
sunrise and sunset.

As has already been stated, the outgoing type lasts one or two
hours after sunrise and takes over again about the same length of
time before sunset. According to Fig. 38 difference of time is rela-

THE CLIMATE NEAR THE GROUND

tively independent of the season, since the heavy zero lines and the
dotted lines are practically parallel. In our latitude, the outgoing
type occupies the greater amount of time; in winter it compresses the
incoming type into a few midday hours.

In Fig. 38 the isopleths move far apart at night and stand vertically.
The temperature difference between the upper and lower position

Jon. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Jon. Feb. Mar. Apr. May June July Aug. .Sep. Oct. Nov. Dec.

night

FIG. 38. Difference of the air temperature at 2 and 34 meters height in Potsdam
1893-1904, (After K. Knoch)

shows scarcely any change through the nocturnal hours of stable
stratification, but in the daylight hours when the sun is actively
effective it is quite otherwise; the horizontally lying isopleths lie
close together. It is worth noticing that the course of the curves
follows the dotted lines quite closely. It is sunrise and sunset which
bring about the alteration in the heat balance in spite of the change
from one type of radiation to the other occurring at a different hour.
We shall later see (Fig. 41) that the transition is not always so
regular in the ground climate.

Fig. 39 shows the daily change of the temperature gradients in
three different parallel air layers, according to the observations of

TEMPERATURE GRADIENT NEAR THE GROUND 83

W. D. Flower. In the uppermost layer between 46.4 and 61.0 m
(dotted line), we find that in July there is a temperature inversion
of il/2°C per 100 m before sunrise. Immediately after sunrise the
gradient curve falls off but about 8 A.M. there is a sharp turn toward
the horizontal. In this already freely moving air layer, as Fig. 37
has already demonstrated, the temperature gradient cannot signif-
icantly exceed the adiabatic value. Even in the underlying layer,
down as low as 16.2 m the equalizing influence of convection can be
easily recognized (dot-dash line, for layer from 16.2 to 46.4 m).

2 * 6 8 10 12 K 16 18 20 22 ft

Time of day

FIG. 39. Daily course of temperature gradients in Istnailia

Entirely different is the behavior of the air layer between i.i and
16.2 m (solid line). To be sure, the change between positive and
negative temperature gradients occurs at the same time of day as
in the higher layers. But the mobility of the air, which there exerts
a moderating influence on the gradients, is lacking here. The gradi-
ents in both directions are excessive.

As we come still nearer to the ground, the gradients reach values
of far more than 500° per 100 m. While the rate per 100 m naturally
loses significance for such thin layers, it is nevertheless necessary in
order to be able to compare the temperature gradients, independently
of the apparent thickness of the layers. In the accompanying table
derived from the measurements of Johnson (182) and Best (176) in
England, we give an idea of the gradients at different heights and

THE CLIMATE NEAR THE GROUND

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l>*

TEMPERATURE GRADIENT NEAR THE GROUND 85

for different times of day in January and in July. The table is an
excerpt from a compilation made by F. Steinhauser (196) in which
data can be found for all months and hours of the day.

We shall now turn our attention particularly to the moment when
the transition between incoming and outgoing types of radiation
occurs. Here again we shall depend on the observations which
W. D. Flower (178) made in Egypt.

In Fig. 40 the heavy line indicates the time of sunrise at Ismailia
according to time of day and season. Quite regularly throughout

Sn 6 7

Time of day (morning)

FIG. 40. Time of onset of the isothermal condition in the morning. (After the
observations of W. D. Flower)

the year, at about 1 1/2 hours after sunrise as a result of heating from
below, the same temperature is reached at a height of i m as pre-
vails at 16 m as a consequence of the nocturnal inversion. With as
great regularity, about 20 minutes later, the measurements at i m
and 61 m show equal temperatures.

At sunset the relationships are changed. From Fig. 41 we see that
in the winter the outgoing radiation is so strong toward nightfall
that the temperature gradient for the lower air layer has become zero
by l/2 hour before sunset. In summer, on the other hand, such an
amount of heat is accumulated in the ground and the adjacent air
layer in the course of the long day, that it is quite a while after
sunset before the effective outgoing radiation at last makes its effect
felt in the temperature gradient. The two curves, corresponding to
the two air layers, are most widely separated during the months of
May through August.

THE CLIMATE NEAR THE GROUND

The cause of this difference between morning and evening con-
ditions is this: At sunrise the air lies on the ground in a very stable
state. By means of radiant solar energy it is upheaved from the
ground. Knowing the vain attempts which technicians have made
with heating apparatus to destroy the nocturnal inversion in the
interest of frost protection, it is easier to comprehend the enormous
work done by the sun every morning. The upset of the stratification
proceeds rapidly and steadily upward in correspondence with the
increasing warmth of the sun.

ft* 17» £* ft* ft* ft*

Time of day (evening)
FIG. 41. Time of onset of the isothermal condition in the evening

When the sun goes down, on the other hand, the atmosphere
is at first in relatively unstable stratification. Eddy diffusion becomes
of less and less significance as time goes on. Radiative pseudo conduc-
tion takes its place. The ground gives up the heat stored during the
day. The change in temperature stratification is brought about, not
as by the powerful attack of a single all-compelling energy as is the
case at sunrise; rather, the air layers become the sport of many fac-
tors and it depends on the accidental circumstances of the season
what the total effect of the interplay of various forces amounts to.
This is why the heat supply of the Egyptian ground in summer can
postpone isothermy so long in the evening, while, as is shown in
Fig. 38, the relationships are quite different in the hill and meadow
lands near the Potsdam Observatory.

Referring to Fig. 42, let us consider how the time in the evening
when isothermy occurs is related to humidity and wind velocity.

TEMPERATURE GRADIENT NEAR THE GROUND 87

Once more the basic measurements are those of W. D, Flower (178),
made on clear evenings for air layers between i and 16 m height.

The abscissa of the chart gives the number of minutes by which
isothermy precedes or follows sunset. The ordinate is the magnitude
t/c in which is combined the effect of temperature t as well as
that of the vapor pressure of the air e. High humidity lessens the
value of the expression, since e is in the denominator, so that a
small ordinate value corresponds to a cool temperature and a high
humidity. The three curves, which we shall first consider as a whole,

-100 o 100 200

Minutes before ( — ) or offer (+) sunset

FIG. 42. Time of beginning of evening isothermal conditions in relation to tempera-
ture, humidity, and wind for the layer from i to 16 meter heights

demonstrate the fact that isothermy occurs earlier in the evening, the
drier the air. This is readily understood, since dry air is associated
with strong outgoing radiation. As the humidity increases it be-
comes increasingly longer after sundown before isothermy occurs.

The influence of the wind is clearly shown by the relation of the
three curves to one another. High wind speed (7 m/sec) makes the
curves steeper. This indicates that the influence of temperature and
vapor pressure is less when the wind movement is lively. If, how-
ever, the wind is weak (3 m/sec) temperature and humidity have
a greater effect.

During an evening fog, W. Kiihnert (149) observed at the airport
of Vienna the variation of the gradient in the layer near the ground
up to 4 m height by means of thermocouples. Within the fog layer
near the ground a temperature increase was found in the order of
magnitude of i°C per i m. In the same proportion as the radiation
fog increased in thickness, the inversion layer also was increased;

88 THE CLIMATE NEAR THE GROUND

it was formed, therefore, by air sinking down which was cooled by
outgoing radiation at the upper surface of the fog. On the upper
side of the fog the temperature increase with height was much
smaller (order of magnitude o.3°C per i m) ; isothermy or even an
insignificant decrease of temperature with height was established.

A particularly fine example of the interrelation between tem-
perature gradient and weather is afforded by the night's record at
Ismailia on the i4th and i5th of April, 1932. The change of gradient
during the night is there shown (Fig. 43). Fog began to form
shortly before 3 A.M. The fog increased in density till about 8 A.M.

FIG. 43. Course of temperature gradients in morning fog at Ismailia. (After the
observations of W. D. Flower)

but disappeared quickly as the sun rose. Let us first consider the
course of the temperature gradient between the i and 16 m levels
(heavy line).

In accordance with the normal nocturnal temperature fall, the
gradient increases steadily till 4 A.M. The onset of fog formation
makes no difference at first; but when, at about 4 A.M., the fog has
reached a thickness of 16 m, the upper observation point becomes
involved in it. The temperature gradient suddenly drops (point P)
while at the same time the gradient between the 16 m and 46 m
heights increases (point Q). This indicates that, under the influence
of the fog, it suddenly became cold at the observation point in the
1 6 m level. From then on it belongs within the cold ground layer,
within which the gradient decreases until point /2, is reached.

When, at about 6:30 A.M. the fog reaches the 46 m level, the same
act is repeated: the broken-line curve drops steeply, the dotted line

TEMPERATURE GRADIENT NEAR THE GROUND 89

continues to climb. Under protection of the fog a minimum tem-
perature gradient (/2) is found toward the end of the night in the
layer between 16 and 46 m.

Further development is stopped by the rising sun. The sun shines
first on the upper sea of fog suddenly turning the upper gradient
curve downward at A3. Continued evaporation of the fog appears
as a second shortlived increase of the inversion (Warming above!)
in the broken-line and solid curves. Then they too turn down
(Points A2 and AI) and pass into the incoming radiation type.

Further comments as to the relations between temperature gradi-
ents, wind gradients and wind velocity will appear in Chapter n,
where the wind relationships in the ground air layer are taken up.

SECTION III

OTHER METEOROLOGICAL ELEMENTS NEAR THE GROUND

CHAPTER 10
HUMIDITY RELATIONSHIPS

Looking at the water balance of the atmosphere as a whole, we find
that water vapor is furnished to the air only from the evaporating
surfaces of the land and the water. Therein consists the great sig-
nificance, for the water balance of the atmosphere, of the air layer
next to the ground or the water. It is the producer and first trans-
mitter of the water vapor of the air. When the upper layers thus
become enriched and finally saturated with moisture, the condens-
ing portion returns as precipitation to the earth's surface and is
ready for another cycle.

Through evaporation at the surface, there follows directly an en-
richment of the air with water vapor. Further transport upward in
the air near the ground follows through eddy diffusion (Chap-
ter 4) for, just as with heat transport it is not the true molecular
conduction which is most important but rather mass exchange, so
also for the transport of water vapor, it is not the molecular-physical
process of diffusion which is important, but this same eddy diffusion.

The water vapor of the ground air layer, therefore, always comes
from below. This, to be sure, is true for a definite place of observa-
tion only so long as no foreign influences intrude — according to the
concept introduced by R. Geiger (_?#), only so long as the climate is
"independent." It is precisely in the small spaces with which micro-
climatology has to deal, that it is most evident how moister air and
therefore excess vapor is created from propinquity. In such depend-
ent climates, advection (the importation of water vapor) plays a
part. We shall in the following paragraphs consider this very prac-
tical question.

The water loss of the ground determines the moisture relations
of the air adjacent to it, just as its heat balance was affected by the
ground surface. But while heat from the ground is transmitted to
the air for one half of the day and for the other half it is returned
by the air to the earth's surface, it is otherwise with the air's humid-

HUMIDITY RELATIONSHIPS

ity. Water vapor nearly always goes upward. Its return to the ground
takes place almost entirely as another process, precipitation. It does
happen in the very closest air to the ground that water vapor is led
downwards to the soil, but even here, only under special conditions
of dew formation. This process is confined to short night hours and
its effectiveness as compared with the surrounding mass of water
vapor is quite negligible in contrast to the normal process, by which
the water vapor passes upward from the earth into the air.

So while temperature shows at one time a maximum at the earth's
surface, and again, a minimum, the water vapor content of the air,

Relative
humidity

Vapor \ pressure

wmm

10%

FIG. 44. Daily mean of the relative humidity and vapor pressure in relation to altitude.

(After V. Rossi)

looking at it by and large, decreases steadily with height above the
ground. This fundamental law holds, whether we consider vapor
pressure measured in millimeters of mercury, or relative humidity,
which is the ratio of the prevailing vapor pressure to the maximum
possible at the existing temperature (the so-called saturation pres-
sure) and which is expressed in percentage. Fig. 44 shows the varia-
tion of both magnitudes with altitude, for the lower 2 m of the
atmosphere. The data are the daily average of the observations made
by V. Rossi (211) at Lauttakyla, Finland from the loth to the i6th
of July, 1930, using thermocouple-psychrometers. The two curves
show the rapid increase of both humidity values with approach to
the ground. They remind one of the incoming radiation type when
an overflow of heat is furnished by the earth's surface. Here, where
water vapor is produced from below, we speak of a "wet type" of
vertical moisture distribution.

92 THE CLIMATE NEAR THE GROUND

The two wet type curves are based, as already mentioned, on the
daily average. They are therefore only the result of a cross-section.
Considering the relationships of all the times of day, we find that
there is a dry type as well as a wet type. By "dry type" we mean a
humidity distribution with respect to height, in which the air near
the ground is dry and that above it is moist. The designation "dry
type," just as that of "wet type," applies to both vapor pressure and
relative humidity.

At what times and under what circumstances the dry type enters
the picture depends on whether we are referring to the absolute or
the relative humidity.

I. VARIATION OF VAPOR PRESSURE WITH HEIGHT DURING
THE COURSE OF THE DAY

The amount of evaporation depends principally, according to a
law of Dalton, on the temperature of the evaporating surface. The
daily march of evaporation therefore parallels that of temperature.
The high temperature of the ground by day sends so much water
vapor from the ground air, with its extreme temperature range, into
the overlying air, with its moderate range, that I know of no case
in which the wet type does not predominate during the day.

By night the conditions change. The dew or frost which forms
on the ground is derived, for the most part at least, from the water-
vapor content of the air layers resting upon it. But even when there
is no dew formed, the ground may absorb moisture, or evaporate it
more slowly than capillarity brings it up, for, instead of the dry
evening soil, we ordinarily find a moist surface in the morning.

At some time early in the night, therefore, there is a transition
from wet to dry type of vapor pressure distribution in the ground
air. Observations of H. E. Hamberg (203) made in the summer of
1875 in connection with his study of dew, show this drying out of
the air layer near the ground. As an average of four July nights he
obtained the following values of vapor pressure in millimeters
in relation to height above ground and to time of day: —

TABLE 14

Hour of the night
Height 2o!/2 21 22 23 24 i 4 5 6

7.7

7 Q

7.6

7.7

7.5

7.3

6.8

8.3

7.8

8.0

7.8

7.7

7.3

7.1

6.6

8.0

Q.I

0-3

rhr

m
eround

7.8
8.3

8.2

8.4

7.8

7.6

7.«%

7.2
7.1

7.0
7.0

6.5

6.4

7.8

7.5

9-3

HUMIDITY RELATIONSHIPS

The maximum value for each hour of observation is shown in
bold-face type. We see that in the evening the maximum is still at
the ground. As dew begins to form the maximum moves quickly
upward. After midnight it probably is higher than 31 m, for at that
height the humidity is still decreasing till 4 A.M. Observations at
greater heights are lacking. As soon as evaporation begins in the
morning in place of condensation, the return of the maximum to the
ground gives evidence of the wet type which characterizes daytime
conditions.

We can thank M. Franssila (577) for some recent measurements
in Finland. In the lower half of Fig. 45 which we shall next con-

Time of day

FIG. 45. Daily course of the relative humidity and vapor pressure in Finland.
(After M. Franssila)

sider, there is represented the daily course of vapor pressure at three
different distances above the ground. The measurements were made
in the Palkane parish in Finland; the values shown are averaged
from three August days in 1934. We notice first of all that the wet
type shows a decided increase during the day, particularly around
midday. The dry type prevails rather weakly from 10 P.M. to 7 A.M.
The lower part of Fig. 45 shows a further regularity. The broken
line, which corresponds to a height of i m above the ground, shows
the well-known double wave of vapor pressure. The principal mini-
mum occurs in the morning at the same time as the temperature

94 THE CLIMATE NEAR THE GROUND

minimum. A second, weaker minimum occurs in the early after-
noon. This indicates that vigorous midday convection moves moist
air upward from the ground and drier air downward. In the so-
called "continental" or "desert" type of climate this midday mini-
mum becomes the principal one.

Within the realm of the microclimate existing within 5 cm of
the ground, the daily cycle of vapor pressure is a simple one. Instead
of the secondary minimum there is a midday maximum. The reason
for this is doubtless the fact that the weak convection does not reach
quite to the ground and hence does not carry upwards the consider-
able amount of water vapor which is there available, consequently
the maximum, which corresponds to the temperature maximum, and
also the evaporation maximum, is maintained.

L. A. Ramdas (209) has furnished a record of vapor pressure
relationships in the lower air at Poona, in southern India for the
winters of 1933-37. From the measurements I have shown the aver-
age for the months of November-February. At this season the clear,
undisturbed "winter" weather of India prevails — rich in radiation
and unaffected by the ocean wind which comes with the spring. For
nine different heights we get the following values: —

TABLE 15

Height above the ground
in cm:

122 QI

7 *;

Vapor pressure in mm:
At sunrise

8.8

8.2 8.0

7.6 7.=?

7.4

7 c

At noon ...

8.3

8.5 8.6

8.Q Q.O

Q.4

10.0

At 3 m height the difference between the vapor pressure at sun-
rise and that at noon is only 0.5 mm. The slight difference is an
indication that there is only one homogeneous air mass present at
that season. Directly on the ground the difference between day and
night amounts to five times as much (2.5 mm) . By day it is moister
on the ground than in the higher air; by night it is somewhat drier.

The measurements of H. Berg (98) also show the dry type at
night. An instance is mentioned by S. Petterssen (68) of a case in
which within the lowest 3% m air layer there was a vapor pressure
increase of 5.9 below and 10.4 mm above!

Finally, Fig. 46 shows by isopleths the daily course of the vapor
pressure according to the observations of L. A. Ramdas and M. S.
Katti (2/0). The concurrent temperature distribution has already
been shown in Fig. 34. The 9 mm vapor pressure line arches up

HUMIDITY RELATIONSHIPS

above the ground at midday. The maximum height coincides not
with the temperature maximum (Fig. 34) but with the radiation
maximum. At night the air layer at 2 m above the ground shows
the dry type, whose development becomes progressively stronger
toward sunrise. As the sun rises, the air undergoes as sudden a

06 U 18 &

Time of day

FIG. 46. Daily course of the vapor pressure in India. (After L. A. Ramdas and

M. S. Katti)

change in water vapor content as we found, according to the meas-
urements of S. Siegel, taking place at the distintegration of the
nocturnal inversion.

We find measurements at greater heights in the old observations
of S. A. Hill (204). There the daily march of vapor pressure at
Allahabad is given in hourly values for the heights of 1.2, 14, 32
and 50 m.

In Fig. 49, where the daily humidity cycle in the ground air layer
is represented according to types, only one type, the normal, is shown
for vapor pressure. It will be quite different now, as we turn our
attention to the relative humidity relationships.

2. VARIATION OF RELATIVE HUMIDITY WITH HEIGHT DURING
THE COURSE OF THE DAY

The relative humidity is influenced by the absolute humidity as
well as by temperature. If we imagine the water vapor content of the
air unchanged, the daily course of the relative humidity is the con-
verse of that of the temperature. In the ground air layer, the closer to
the ground, the more extreme the temperature variation. Conse-
quently the wet type prevails by night, the dry type by day. The in-

96 THE CLIMATE NEAR THE GROUND

fluence of temperature on relative humidity is so compelling that
this course of relative humidity represents the normal type.

For an example we first refer to the 12 year series of Potsdam
observations at 2 and 34 m heights, studied by K. Knoch (/#5).
In Fig. 47 we find the isopleths of relative humidity difference
for all months and hours — just as Fig. 38 showed the temperature
difference between the same heights. Most of the surface is occupied

3an. Ftbr. March April M«y Dune 3ul> Auq Sept. Oct. Nov.

Mid

night 3«n fet>r. M«rth April M»y 3un> July Aug. Stpt. Oct. Nov Otc nighl

FIG. 47. Difference in relative humidity between the heights of 2 and 34 m. in
Potsdam. (After K. Knoch)

by negative values; the wet type predominates. About the middle
of the day, however, the dry type appears — being strongest in the
dry spring months. While a difference of —9% is attained in the
wet type, it is only in March and April that the dry type exceeds
+2%. This is quite different from the temperature differences de-
picted in Fig. 38, which extended equally in each direction both by
day and by night. Otherwise the curves approach closely, as did
those of temperature, at sunrise and sunset, while the lines of equal
humidity difference make right-angle bends at the times of transi-
tion between day and night.

HUMIDITY RELATIONSHIPS 97

There are two exceptions to this daily course of relative humidity
in the microclimate, which in Fig. 49 is accepted as the normal type.
They are occasioned by the effect of vapor pressure on the relative
humidity.

In climatic provinces with low temperatures or high humidities,
the wet type predominates throughout the day, even at midday. We
can turn to Fig. 45 and see an example of this in the daily march
of relative humidity at Palkane, Finland. At 5 cm above the ground
the humidity even at noon is still 20% higher than at i m altitude.
A contributing factor may be that these measurements were taken
over clipped sod. D. Szymkiewicz (213) found something similar
in his 1929 observations over a meadow in the Czerme peat bog. He
found the mean relative humidity at 2:30 P.M. for the three summer
months to be : —

TABLE 16

Height above
ground in cm

July

Relative Humidity
August

September

200

The second exception is found in climates with high temperature
and low humidity. The Indian observations of L. A. Ramdas and
M. S. Katti (2/0) will again serve as an example here. L. A. Ramdas
(209) has called attention to the fact that the soil of India, particu-
larly the black, cotton growing soil, has an extraordinary capacity
for absorbing water vapor. With such enormously high noon tem-
peratures the soil dries out greatly but at night it is able to withdraw
large amounts of water vapor from the air layer resting upon it.
By daily measurements of the moisture content of the ground sur-
face from January to March, 1935, Ramdas found that the afternoon
average water content in the soil was 3.8% as compared with 7.8%
in the forenoon.

Fig. 48 shows the daily course of relative humidity in the air at
2 m above the ground just as the temperature is shown in Fig. 34
and the vapor pressure in Fig. 46. We find the dry type most prom-
inent by day, as it is in our climate also. But even at night the dry-
ing effect of the ground on the lower air is so noticeable that even
at the ground the relative humidity is somewhat lower than at a
height of 2 m. Average values for the months November-February

98 THE CLIMATE NEAR THE GROUND

gave a difference of 6% in relative humidity between the heights of
8 mm and 3 m. We can probably assume that this is seldom true.

Summarizing the data, we find a distribution of moisture in the
air of the ground climate such as is shown schematically according

'o 6 12 18 21

Time of day

FIG. 48. Daily course of relative humidity in India (dry climate type)

to types in Fig. 49. For vapor pressure we have only the normal
type. This is a combination of the wet type by day and the dry
type by night. In the case of relative humidity, the normal is a com-
bination of dry type by day and wet type by night. Here again
we find two exceptions. The daily march of moisture distribution
in which the wet type prevails is most deserving of the designation
"wet climate type" of daily range. It is, as we saw, limited to moist
and (or) cold regions. Correspondingly, we designate the excep-
tional type, in which the dry type of vertical moisture distribution
is to be found during the whole day, as "dry climate type." It has
been observed only in southern India.

In discussing temperature relationships, we mentioned great fluc-
tuations and unsettled conditions of the temperature as one of the
chief characteristics of the microclimate near the ground. This state
of unrest which, in spite of high gradients, resulted from lack of
convection, is likewise to be found in connection with humidity.
Measurements which A. Biidel and R. Geiger (199) carried on in
the neighborhood of Munich, showed sudden, violent fluctuations
in relative humidity. Although the hygrometer which was used,
on account of the length of hair, gave an average reading for a
relatively large air layer, the quick succession of moist air masses

HUMIDITY RELATIONSHIPS

from below, and dry air masses from above could be noticed in the
oscillations of the hygrometer pointer.

6* Kh It" It*

FIG. 49. Types of humidity distribution in the layer of air next to the ground

The following figures, taken, again, from the measurements of
L. A. Ramdas (209) in Poona, will give some information on the
daily fluctuation of humidity values in relation to height.

TABLE 17

Average value for day
(6 January 1933)

Fluctuation during day
in % of mean value

Height above
ground in cm

Vapor
pressure
in mm

Relative
Humidity
in%

Vapor
pressure

Relative
Humidity

305

7.0

48
48
43

12
20
21

7.1

7.5

6.8

0.8

7.1

100 THE CLIMATE NEAR THE GROUND

The daily ranges of vapor pressure and relative humidity increase
rapidly with approach to the ground, just as is the case with the
temperature.

Reference should be made, in passing, to the manifold difficulties
in adapting the technique of humidity measurement to the needs
of microclimatology. The usual hair hygrometers fail to work in the
ground air layer because they are too large; psychrometers because
they require circulating air. In moisture measurement, then, tech-
nical difficulties arise from the same grounds as in the case of tem-
perature measurement. The biologists in particular, often wish to
measure humidity in very confined spaces, such as glass vessels in
which imaginary conditions have been simulated. The publication
by P. A. Buxton and K. Mellanby (207) gives an enlightening re-
view of the biologists' needs in this respect. Today there are a num-
ber of solutions for this problem, no one of which can be considered
entirely satisfactory.

A. Biklel (799) adapted the hair hygrometer, through a horizontal
arrangement of the hair, to use in measurements near the ground.
R. Geiger (</) published observations made by the use of this instru-
ment, which show an extraordinarily pronounced stratification of
humidity. D. E. Howell and R. Craig (205) (according to a refer-
ence in the 1940 bioclimatic supplement) describe a hair hygrometer
whose most important part is the balance of a wrist watch. The
dimensions of this instrument (6 x 8 x 0.5 cm) permit measurements
in small spaces. V. Rossi (2/7) used thermocouples as a psychrom-
eter. In 1932 H. Wald (2130) in Munich, developed the theory and
technique of the electric psychrometer without artificial ventilation.
W. Koch (206) also describes a similar arrangement. In a complete
calm the psychrometric difference increases according to the decreas-
ing diameter of the thermocouple used. By introducing the "wet"
thermo-element into a porous clay tube of i mm diameter (better
than a cloth covering) the psychrometric difference soon reached its
maximum, which was not exceeded by later ventilation. This
method, which was tested by Koch in the laboratory, has not yet,
to my knowledge, been used in microclimatology.

Very recently E. T. Nielsen and H. M. Thamdrup (20$) proposed
a new method. If dilute sulphuric acid is in contact with air whose
vapor pressure is greater than the saturation pressure of the acid,
the air will give up water to the acid until equilibrium is attained
and vice versa. The authors used small capillary tubes, 3 to 5 mm
long, which were filled with sulphuric acid solutions of various con-

HUMIDITY RELATIONSHIPS 101

centrations, varying by steps corresponding to 5% on the humidity
scale. These tubes could be introduced into very small research ap-
paratus, such as glass jars, insect nests, etc. After 10 minutes it can
be observed with a magnifier whether the liquid surface, which was
just even with the end of the tube, has risen or fallen. From this
the relative humidity is determined. The temperature error is neg-
ligibly small. It would be a great help in microclimatology if this
new method should prove satisfactory.

CHAPTER 11
WIND RELATIONSHIPS

The most violent wind of the free atmosphere is to some extent
slowed down by the ground. Directly at the surface, the air is en-
tirely, or almost entirely, at rest. Through eddy diffusion the braking
effect of the ground is transmitted upward, for each parcel of air
which moves upward, carries with it the lesser horizontal motion
which it possesses and, coming in contact with faster moving layers,
exerts a braking action on them through its inertia. Conversely, each
descending parcel of air carries down the higher velocity of the upper
air currents. Just as through eddy diffusion the heat content, water
content, dust content, etc. of the air is equalized upward and down-
ward, so is it with the energy of motion.

The nearer to the ground, the more is all movement hindered.
We have already recognized an instance of this in the "grinding up"
of eddies at the ground. M. Franssila (^77) has determined the
air temperature at heights of 5, 20 and 100 cm above the ground
by using an Assmann aspiration thermometer as well as electric
resistance thermometers. The comparison proves that the air
drawn in by the Assmann during the day comes, on the average,
from an air layer 4 cm higher than that corresponding to the heights
of the suction tube. The higher, more mobile air, therefore, flows
more readily into the inlet tube than the lower lying, less mobile
air; the suction is unsymmetric. When, at night, the air, as a result
of temperature stratification, is at rest and cleaves tenaciously to the
ground, the air drawn in originates in a layer even 10 cm higher.1
This neat measurement of Franssila demonstrates the braking effect
of the ground surface on air movement.

The air near the ground is the part of the atmosphere in particular
where wind velocity shows a great increase with height. A glance at
the rime formation shown in Fig. 50 shows this strikingly. As is
well known, rime results from the deposition of supercooled water
drops floating in a driving fog which are carried by the wind
against some solid object. The size of the rime flags which grow

lOn this account J. Bartcls (160) proposed moving slowly forward with
the Assmann so that the orifice of the suction tube could be kept constantly
at the desired height.

WIND RELATIONSHIPS 103

against the wind is greater, the more drops freeze on in a given
time. This, in turn, depends on the wind velocity. It is not unusual
for the flags to grow according to their height above ground as
Fig. 50 shows them on telegraph poles. The deposition of rime can
be regarded as a natural record of wind velocity.

FIG. 50. Rime banner, which demonstrates the increase of wind speed with height
above the ground. (Photographed on Mt. Washington)

For all practical purposes the variation of wind velocity with
height can be expressed in this simple equation : —

z>2 signifies the wind velocity in m per sec at the height of z meters;
vl9 the velocity at a height of i m. a is an exponent whose value
must be determined from observations on the actual variation of
wind with height.

G. Hellmann (2/6) was probably the first to carry on systematic
measurements of wind velocity in the ground air. He located regis-
tering anemometers on the Nuthe meadows at Potsdam, at eleva-
tions of 5, 25, 50, 100 and 200 cm. The anemometers were at least
4 m apart horizontally, in order to avoid mutual interference. The

104

THE CLIMATE NEAR THE GROUND

experiment lasted from July to October, 1918; there was a total of
1488 hours' record.

Fig. 51 gives the results. If we express the above-stated equation
logarithmically, then

log z>2 — log v j = a log z

In the logarithmic system of coordinates, such as that chosen for
Fig, 51, the curve of state therefore appears as a straight line. Con-

456 8 10

200
150

200

0.2 0.3 0.4 05 1

Wind speed in m/sec

IB 2 3 4 5 6 8 10

FIG. 51. Variation of wind speed with altitude

versely, if the law is actually fulfilled, all observed values must lie
on straight lines. We see that this requirement is substantially ful-
filled by the observations. The line b represents the mean value of
the 1488 hours; a and c, the same values separated into the calm
hours of the night and the windy hours of the day. Observations on

WIND RELATIONSHIPS 105

the windiest day are located along line d\ those during the windiest
hour (Sept. 30, 1918, from 10 to 11 P.M.), along line e.

The value of the exponent a is equal to the tangent of the angle
at which the straight line is inclined to the ordinate. In Fig. 51,
a = 0.3. The inclination of the line K corresponds to the value,
a — 1/7; this is the lowest value thus far observed.

The value of a, as has already been mentioned, is not constant.
It depends principally on height, for with increasing height the effect
of ground friction diminishes, and consequently a becomes smaller.
G. Hellmann (2/7) showed, however, in reference to the air near
the ground that, for at least the lowest il/2 m, a may be considered
constant.

O. G. Sutton (2^7) has emphasized the dependence of the ex-
ponent a on the temperature gradient. He calculated the daily
range of the exponent, from the observations of G. S. P. Heywood
(218). The wind measurements used were at heights of 12.7 and
94.5 m above the ground at Leafield. During the summer (April
through September) the change of exponent from midday to mid-
night was from 0.07 to 0.17. During the winter (October through
March) the corresponding values were 0.08 and 0.13. This range
seems slight. B. Ali (2/5) found a large range in observations made
at Agra, in India. A. C. Best (776) in a recent thorough investiga-
tion of wind variation with height and wind structure near the
ground, has determined the increase of velocity in relation to simul-
taneously occurring temperature gradients. Since this research
touches on the peculiar province of the microclimate, it has particu-
lar interest for us. The following figures give the average wind
speeds for the lowest 2 m, expressed in percentage of the speed at
i m. When the temperature decreases decidedly with height (as in

TABLE 18

Height above ground in cm
Temperature gradient 2.5 5 10 25 50 100 200

Wind speed (in % of wind speed at i m height)

— 3°F/m (Temperature decrease) 43 52 67 81 90 100 107

o (isothermy) 36 49 63 ' 79 90 TOO 112

-fi°F/m (Inversion) 34 48 60 77 89 100 114

line i), the variation of wind with height is less than in the case
of an inversion (line 3). A. C. Best rightly remarks that it is quite
impossible to separate the effects of temperature gradient and wind

106 THE CLIMATE NEAR THE GROUND

gradient, for they mutually affect and determine one another. We
shall return to this question when considering Fig. 59.

Besides the above mentioned research of G. Hellmann, we have
older measurements by Th. Stevenson (250) which have been made
accessible through the work of W. Schmidt (228). A. Peppier (225)
has also determined wind variation with height through observa-
tions on the Eilveser radio tower. The most recent and careful in-
vestigations in the air near the ground we owe to W. Paeschke (224) .
He has, in particular, made a comparison of all available measure-
ments as to the dependence of the exponent a on the ground cover.
It appears that, in general, a lies between 1/5 and 1/3. The value
1/5 occurs above a snow cover, which offers least resistance to the
wind. The upper limit of 1/3 was obtained over a turnip field. We
shall return to this in Chapter 28. The largest value for a, of which
I know in the literature, is that of 0.46, given by B. Ali (2/5) ; the
smallest (0.07) has been mentioned above.

The numerical values for a make it possible, if we know the wind
speed at one height, to calculate it for any other height within the
ground air layer. A graphic method such as that shown in Fig. 51,
is useful. It should be noted, however, that for microclimatic meas-
urements, a height of i m above the ground has been taken as
normal. It is desirable in all kinds of investigations, first to locate
the anemometer at this height in order to avoid corrections so far
as possible.

Finally it should be emphasized that the law of wind variation
with height, as stated, is only a statistical law. It holds good in a
long series of observations, but not necessarily in individual in-
stances. G. Hellmann (2/7) remarked that "small currents of faster
moving air often underlie others with lower velocity." Great varia-
tion in the values of the exponent a were found by P. Michaelis
(3430) in wind velocity measurements over a snow cover in the
little Walser valley (Alhgau Alps). Fig. 52 shows examples pub-
lished by W. Schmidt (817). The upper half of Fig. 52 (a) repre-
sents wind variation with height on the night of May 10-11, 1928,
in the neighborhood of Vienna. In the air layer from the ground
up to 2 rn, there is low wind speed with no regular dependence on
height apparent. Above this level a strong wind is blowing. The
lower half of Fig. 52 shows four examples from the following night,
on which frost occurred. At about 8 P.M. (b) there was an almost
linear increase in wind speed from 0.6 m per sec at the ground to
1.8 m per sec at a height of 7 m. A half hour later (c) there was
a calm at 7 m while the layer within 2 m of the ground was the

WIND RELATIONSHIPS

107

one in most lively motion. Thus quickly does the picture of wind
velocity distribution change. Here also the "stratified structure" of
the ground air is strikingly in evidence.

Wind speed, m/sec

May 11, 1928
b

Mast »

19:50

Masfy

22:00

Wind speed, m/sec

FIG. 52. Irregular wind stratification above the ground. (Measurements by Wilh.

Schmidt)

It is of great significance for microclimatology that F. Albrecht
(214) has built a hot wire anemometer which is very suitably de-
signed for field work in meteorology. By means of this instrument
it is possible to make direct measurements of the lowest wind speeds
with great accuracy. W. Viereck (233) has described a recording
wind apparatus based on the hot wire principle. Details of its appli-
cations are lacking.

Now we shall turn to the daily range of wind velocity in the
ground air.

100

THE CLIMATE NEAR THE GROUND

It is known that near the ground a maximum of wind velocity
is found at about midday, while at night the strength of the wind
usually diminishes. A. Wagner (2^4) has shown that it is not eddy
diffusion which accounts for this range, which is directly the opposite
of that in the higher air layers. Although eddy diffusion is stronger
at midday, yet stronger eddy diffusion means only stronger braking
effect. It is rather caused by the greater increase of eddy diffusion
with height in the middle of the day as contrasted with less increase
by night.

Fig. 53 is a graphic representation of the already mentioned meas-
urements by G. Hellmann (2/7) of the daily range of wind velocity
at various heights above the ground. It clearly shows the midday

4" 6* 8h 10h 12* 2* 4n 6" d"

FIG. 53. Daily course of the wind speed at various altitudes. (After G. Hellmann)

maximum and the nocturnal calm in all layers. For practical ques-
tions, especially in plant physiology, it is noteworthy that the time
when quiet hours predominate gets longer with approach to the
earth's surface. In Fig. 54 the number of calm hours according to
Hellmann's observations is shown as percentage of all the recorded
hours in their relation to height and time of day. The calms are
more numerous the darker the shading. It is plain how the midday
increase of wind velocity is much less pronounced near the ground.
In the transitional hours of morning and evening, the nocturnal
calm close to the earth extends into the daylight hours.

Here, a particular property of the air layer near the ground should
be mentioned which is in close correlation with the wind conditions.
Strong wind is able to lift up and carry away loose particles of
the ground such as dust, loess, sand or snow. Dust at first decreases
the visibility; then one speaks of sand-sweep and snow-sweep as

WIND RELATIONSHIPS

109

long as the particles drift along so close to the ground as not to
hinder the horizontal visibility greatly; with further increase of the
wind, drifting sand or drifting snow decrease the visibility. If snow-
fall joins the drift we speak of a snowstorm.

W. Haude (4260) gives the following description of sand drifting
high up around the winter camp of Edsengol (Gobi Desert) :
"Turbidity of the air by dust and sand starts when the wind freshens
up above a certain threshold which naturally is lower with dust
whirling than with sand and even with coarse gravels. During day
time, the threshold is also lower than at night. Everywhere where
terminal lakes exist into which the streams empty, at least tem-
porarily, the water of which is enriched by finest silt, great quanti-

12 2 4 6 8 10 12 2 4 6 8 10 12

FIG. 54. Frequency of calm hours in the air layer near the ground

ties of finest dust particles are available on areas occasionally or
previously inundated. Naturally, it is here most likely that great
quantities of silt are whirled up. Freshening wind meets here
smallest particles which can be carried along. As a consequence
the decrease in visibility is the most intense in the immediate vicinity
of the terminal lakes. This is true near the Gaschuun-nor and Koko-
nor and is the most strongly marked at the Lob-nor (chara buran,
black storm).

"Thus, at the Edsen-gol, a hazy banner cloud could be observed
north of the terminal lake region which developed mostly in the
morning of many days. The manner in which it developed and
changed was one of the obvious indications of the beginning of sand
drift. When the dust cloud grew rapidly, drifting sand could be
counted upon to start soon, since in the region of Edsen-gol sand
was abundant. While on the crest of the dunes the well-known
whirls and eddies occurred, drifting of sand near the ground devel-
oped on the area with gravel. At eye level, the visibility remained
still good, while near the ground, large masses of sand were moved

110 THE CLIMATE NEAR THE GROUND

along the fields over the gravel ground according to the intensity of
the individual wind squalls. With each more intense squall long
chains of dust approached, which, however, did not reach a greater
height. Concavely curved on the front, they moved over the free
gravel surface carrying with them a long trail of dust but taking off
only few dust particles from the gravel area. The motion was mostly
in straight lines; only now and then, small variations of the direction
for only a few degrees could be observed. Sometimes, a subsequent
dust wave had a somewhat other direction than the preceding one."

It is now of a special microclimatic interest how he describes the
transition from this winter-time drifting sand to the first small
spout (see page 9) in earliest spring when the daily temperature
variation was intensified : "During the last days of January, a change
of this echelonlike straight forward movement set in. With the in-
dividual squalls a trace of rotation could be seen. Seemingly, it was
developed incompletely, so that a circular shape was seen clearly
only in the highest-lifted dust. In the beginning of February, how-
ever, some squalls represented already genuine small spouts and
were seen within some of the described straight lined dust squalls;
or they occurred independently when the general wind speed had
decreased. Their appearance occurred between 10 and noon. The
radiation had become already so effective that the heating of the
ground caused superheating of the lowest air layers. Most fre-
quently, however, the motion of the air must reach a certain speed
to cause their development."

Fig. 55 is a reproduction of a photograph by P. Michaelis (344).
The pine shown is growing at timber line in the Allgau Alps. The
dotted line indicates the position of the snow surface in winter. The
growth of the tree shows the influence of the two-fold surface. The
entire absence of branches on the right, just above the winter sur-
face, shows the abrasive effect of the wind, loaded with drifting
snow. The trunk lacks the growth of lichens which elsewhere are
abundant; often the bark is deeply cracked. This is the north side.
On the south side, however, which, in the photo, is at the left, the
branches, though withered, are still present in part as dead wood
covered with lichens. The great fluctuation of temperature above
the highly reflecting snow cover is to blame for this damage.

In conclusion it is our task to point out the influence of the wind
on the temperature of the ground^air.

Higher wind velocity means, as we have seen, increased dynamic
convection. Increased convection results in decreased temperature

WIND RELATIONSHIPS

III

gradients. This means lower temperature at the ground by day, and
higher at night. It is the night effect which is of great practical
importance. The farmer is not afraid of frost when there is wind,
but he is, if the wind goes down with the sun.

F. Katheder (2/9) tells of the following observation: On Septem-
ber 23, 1936, just after 6 P.M. a shallow fog, i to il/2 m thick formed

FIG. 55. The microclimatic damage on this alpine fir tree above the winter (dotted
line) snow cover is evident. (Photograph by P. Michaelis in Allgau)

in the quiet air covering the ground at the Nuremberg airport. Above
this layer there was excellent visibility. In the instrument shelter 2 m
above the ground the relative humidity was 86%, the air tempera-
ture, 15.2°. The ground temperature was about 12°. At 6:40 a three-
motored Junkers started on its scheduled trip to Munich. "During
the take-off there was formed along the runway behind the plane a
channel entirely free of fog. In the course of four or five minutes
the sharp boundary between fog and fog-free space disappeared and

112

THE CLIMATE NEAR THE GROUND

after a short time the original condition again prevailed. The width
of the clear channel was just about the span of the Junkers." The
stirring up of the ground air by the three propellers of the plane
in this case brought warmer and drier air down from above. Per-
haps the hot exhaust gases had something to do with the temporary
fog dispersal. It was a visible demonstration of the law we have
stated.

The effect of the wind in raising the temperature is not limited
to the air near the ground. We shall first take an example from
the more abundant data at normal heights. A. G. McAdie's record
(222) reproduced in Fig. 56 covers three nights with uniform

20
IS/
10
5
0

11.

12 6

12.

12 6 12 1

2 (

13. |

12 6 1

14.
> 12

Noon

Noon ^e

\ 15

r\r

FIG. 56. Night temperatures at Kentfield in California from nth to i4th of December
1911. (After A. G. McAdie)

weather conditions. Between the calm nights of December 11-12
and 13-14, 1911 there was one with a brisk wind. The wind meas-
urements recorded at the nearest station are reproduced at the lower
edge of the chart, and show the increase in strength of the wind
from 2 P.M. on the i2th until about noon the next day. Now while
the temperature reached o°C the preceding night and almost as low
on the night following, the wind, whose fluctuations are apparent
in the temperature curve, kept the temperature above 10° on the
1 2th and i3th.

Fig. 57 refers to the fruit growing district of Los Angeles, Califor-
nia. The district is bounded on the north by the San Gabriel and
San Bernardino mountains, between which lies the Cajon Pass. The
night of January 19-20, 1922, brought a heavy, killing frost to the
whole region. F. D. Young (2^5) has furnished the temperature
minima observed in numerous orange groves; the figures naturally
vary with the locations. If we treat them in small groups so as to

WIND RELATIONSHIPS

screen out local influences we get a unified picture. On the sketch
the nocturnal minima (in C°) are given in oblique figures; the
small adjacent figures indicate the number of observations from
which the values are averaged.

By drawing the broken-line isotherms it becomes very evident
that in the areas where the wind blowing through the Cajon Pass

Gabriel

iiiiimiiiiiiiiiiiiimii

Range

FIG. 57. Temperature distribution during the frosty night of the igth to 20th of
January 1922 in Los Angeles

was effective (see arrow), the temperatures, as indicated by darker
shading, were generally higher than in the neighboring, unaffected
districts. The action of the wind, here probably reinforced by foehn
warming, in destroying the inversion is easily recognized.

The heating effect of the night wind depends on its velocity. The
temperature change is great as we go from a calm to a steady breeze;
it then decreases if the velocity increases further. Finally, there is a
limit beyond which increased velocity has no more effect on tem-
perature. This occurs when a thorough mixing of the different
warm air layers has been attained.

This law can best be studied in relation to the change of tempera-
ture gradients with increasing wind velocity. At first we shall con-
fine ourselves to observations within the province of the macrocli-
mate. A. Angstrom (46) has studied the temperature difference
between the Swedish station of Wassijaure at 519 and Mt. Wassit-
jakko at 1372 m msl. Fig. 58 shows the result in relation to wind
velocity which is chosen as abscissa. The ordinate is the temperature
difference between the two stations — positive when the lower sta-

114 THE CLIMATE NEAR THE GROUND

tion was the warmer. In a calm there is a strong inversion amount-
ing to o.6°C. At a velocity of il/2 m per sec the lower, colder air
layer is so stirred up that the same night temperature is found above
and below. When about 6 m per sec is reached, further increase
of wind has no added effect on the temperature difference. The

1.0

0.5

6789

FIG. 58. Warming effect of the winds, detectable by macroclimatic temperature
differences. (After A. Angstrom) x

adiabatic value of temperature decrease with height is then prac-
tically attained.1

S. Siegel (755) from 77 separate measurements taken on four
windy nights has deduced that the following relation exists in the
ground air between the amount of temperature inversion within
the layer from 6 to 220 cm above the ground, and the wind velocity
measured at a height of 225 cm : —

Wind velocity .
Amount of invc

irsion

0.3
3.1

2.2

2
1.6

1.2

4 m per sec
0.9 °C

A series of observations over a snow surface made by A. Nyberg
(^45) is given in a later table.

We get a better insight into the relation between wind movement
and temperature stratification if we measure the wind, not at one
place only but consider its variation with height. For this wind
gradient is in close mutual relation to the temperature gradient. A
strong inversion must plainly be accompanied by a decided wind
change; yet this too depends on the absolute wind velocity above.

W. D. Flower's measurements (178) in Egypt which have been
fully described on a preceding page, give us a good idea of the inter-
relation of the various factors. The results from the observations

*In Fig. 58 the temperature gradient is shown with signs the opposite of
those used elsewhere in this book.

WIND RELATIONSHIPS

made in the winter of 1931-1932 are shown in Fig. 59. They con-
sist not merely of night observations, but of those made at all hours
of the day.

The abscissa is the wind velocity at the upper observation point
62.6 m above the ground. The ordinate is the wind increase from

S E

tfc <N

o s w

Wind speed at 62.6 m height in m/sec

FIG. 59. Relation between the wind speed, the wind variation with altitude and the
temperature gradients. (After W. D. Flower, 1937)

15.2 rn to 62.6 m. The temperature gradients (negative = temperature
decrease with height) are those existing between the altitudes of
16.2 and 61.0 m. They are computed for C° per 100 m. The observa-
tions plotted in Fig. 59 are grouped according to four values of this
gradient. The four combined curves are marked with the corre-
sponding value of the temperature curve.

The four curves appear to approach one another at the left of
the zero point. This must be so, for if there is a calm at 63 m, it is
normally quiet in the underlying air also; there is therefore no in-
crease of velocity. If the temperature gradient is negative, then the
midday decrease of temperature with height is slight (— 2°C), and
rate of variation of wind speed with height is small; an increased
velocity does not greatly alter it, for the vertical mixing is good.

In the case of the nocturnal inversion, however, (+ 4°) the in-
crease of velocity with height is very marked, for the cold air remains
at the bottom, quiet and viscous. If the wind freshens, it only be-
comes noticeable at some distance above the ground. The gradient

Ii6 THE CLIMATE NEAR THE GROUND

consequently increases rapidly with increasing velocity and attains
its maximum with a high wind aloft.

The normal condition of nocturnal temperature rise occasioned
by the wind, we might here remark parenthetically, must not be
confused with the rare, abnormal case of cold advection on a rising
wind. This usually plays by no means as important a part in the
weather picture as does, for instance, the outbreak over Germany of
easterly wind from a Russian winter "high." This occurs on a
small scale, with short-lived gusts, when for instance, air moves out
of a cold hollow, or in the case of air avalanches as A. Schmauss
(414) pointed out with reference to Alpine valleys, and as H. Scaetta
(4/2, 413) later found in the mountains of central Africa. C. Hallen-
beck (395) gives a good example, telling how in the Roswell fruit
district (U. S. A.) the temperature suddenly fell several degrees
shortly after sunrise, as some gusts of northeast wind brought in air
from some of the colder surrounding country. (See the temperature
curve of April 22, 1917, as there published.)

In all the discussion up to this point, the change of wind velocity
within the ground air has been emphasized. We must here state a
fundamental microclimatological law which has to do with the abso-
lute value of the wind velocity.

The role played by the ground surface in the balance of radiation,
of heat and of water, accounts for the temperature and humidity
contrasts found within the air layer near the ground. These con-
trasts must, however, be caught on the spot. For this, quiet air is
needed. In a storm all differences vanish; the microclimate of the
ground air is suspended (with no prejudice to the fact that wind
change with height is still its characteristic) . Windy or stormy days
are therefore unsuited to observations of the microclimate, designed
to discover still unknown contrasts. To be sure, it is all the more
enticing to the experienced observer to see with what tenacity the
ground air layer attempts — and is able — to maintain its identity
in the face of the oncoming wind.

CHAPTER 12

OPTICAL AND ACOUSTICAL PHENOMENA
CONTENT OF DUST, CARBON DIOXIDE AND EMANATION

W. Koppen (247) has fittingly remarked concerning the air layer
which, aerologically considered, may be called the lowest : — "It may
be analyzed into characteristic subdivisions: ist, the layer from the
ground up to a height of i or il/2 m, in which most of our culti-
vated plants grow, and in which contact with warm water or heated
ground produces a mirage directed downward." Here the nature
of the ground air layer is characterized by an optical phenomenon.
It is therefore well — and, indeed, essential — that we do not en-
tirely omit optics, as was done in the first edition of this book. We
shall take the opportunity, at the same time, to mention other proc-
esses — acoustic, electric or radioactive — insofar as their importance
in microclimatalogy is today recognized.

The great variation of temperature, water vapor and wind veloc-
ity, with height is the occasion of a great lack of homogeneity in
the lowest air layer. H. Goldschmidt (243) beamed the light from
a searchlight parallel with the ground and determined the turbidity
of the atmosphere from the decrease of light intensity with distance
from the searchlight. He found that the turbidity factor in this
ground air was at least ten times greater than the turbidity factor of
the air layer above the place of observation, which was calculated
from the weakening of the solar radiation. F. H. Bielich (239)
called the attention of flight meteorologists to the fact that visibility
as determined at the ground is not of much use to a pilot in deter-
mining reliable visibility in an oblique direction. He proves it with
the words, "because the most noted inequalities of the air are found
in the neighborhood of the earth's surface, where forest, meadow,
marsh and open water make their own peculiar little climates."

Air masses of different temperature have different densities.
Where there is discontinuity of density, the light rays will be re-
fracted toward the denser medium. If, therefore, a ray of light passes
through a nonhomogeneous air mass consisting partly of warm air
parcels, and partly of cold, it will deviate far from a straight path.
This lack of homogeneity in the ground air is especially prevalent
about midday. The heat which the ground gives off so generously

Il8 THE CLIMATE NEAR THE GROUND

is not transported upward rapidly and smoothly enough. If we
glance along a heated country road, over a sandy surface, or across
a sunny field of grain, the objects in the background seem to be in
constant unrest. The degree of inhomogeneity varies rapidly as
small parcels of warm air detach themselves from the ground, so
that a restless shimmer results. Stationary lines, such as the corners
of houses, seem to be in irregular, wavy motion. This phenomenon
is called "streaking" or terrestrial scintillation. The more the gaze
of the observer wanders over a large area the more readily it is ob-
served. One should not hesitate to lie down on the ground to get a
better view when the phenomenon is well developed.

Fig. 60 is a photograph of this apperance obtained by L. A.
Ramdas and S. L. Malurkar (137) in the following manner: A hori-
zontal iron plate, 135 by 45 cm, could be heated from below. A long
glass rod lay at a distance of 4 or 5 m. It was placed horizontally

FIG. 60. Luminous line (above) with reflected image (below), that appears in form
of wave motion because of "streaking"

before an open window and showed as a bright line of light. The
picture of this line was taken with it just grazing the top of the
iron plate. While the plate was heated, the picture shown in
Fig, 60 was made, using i/io sec exposure. The bright upper line
is the direct image of the line of light. Under this appears its reflec-
tion on the plate which has a wavy outline, the wavelength in this
case amounting to 2 cm.

Another optical phenomenon, peculiar to the air near the ground,
is that of air reflection downward.

In a gas, the density of which decreases with height as in the
atmosphere, the visual ray between two points A and B is not a
straight line but slightly curved, as shown in Fig. 61, upper left.
As a consequence of this process which is called refraction of light

OPTICAL AND ACOUSTICAL PHENOMENA

119

or simply refraction, the visual objects seem to be lifted; the observer
at B sees the object A in the direction of A'. This is superior mirage.
In the layer near the ground also the reverse process appears, such
that superheated, thinner air lies under cooler, denser air. The
visual ray has then the reverse curvature (Fig. 61, upper right).

If in this case the angle of incidence of the visual ray is very small
so that the visual ray enters the heated layer near the ground nearly

Cool (dense)

-0,1

Hot (thin)

Overheated

'surface layer

FIG. 61. Path of the rays with mirage ( schema tical). Mirages are to the right.

grazingly, then it may happen that the ray is curved upwards from
the ground. Total reflection occurs. It is as if the visual ray (Fig, 61
bottom) were reflected from a mirror SS. Let the small angle with
which the total reflection sets in be i//. It has the order of magnitude
of some minutes of arc. An observer at A sees the object, there-
fore, twice; the first time directly A to R, the second time indirectly
reflected along ATR, and appearing below the object seen directly.
It is, therefore, called inverted or inferior mirage. It is observed
over heated roads, but mostly on shores, where, we have always a
more or less free horizon and, therefore, the necessary small angle */i.
The necessary stratification of temperature is also often present on
or near shore when the sand is strongly heated by the sunshine or a
cool wind blows off the land and over the warmer water. How this
mirage develops is amplified in Fig. 62. Let the observer's eye
be at A, AF is then the eye level above the sea. AH is the direction
towards the astronomical horizon, as the angle (HAF) is a right
angle. The range of sight over the sea is determined by the weak
curvature of the ray AKW, caused by normal refraction, which

THE CLIMATE NEAR THE GROUND
H

Sky

True horizon

3 m Reflecting strip

Apparent horizon

Water

FIG. 62. Development and appearance of the inverted mirage

touches the surface of the water at K. AK is the horizontal visibility.
The angle v is the dip of the horizon. In the case of normal re-
fraction the dip of the horizon and the range of sight are dependent
upon the height of the eye level in the following way :

eye level (above sea) (m)

dip of the horizon (arc minutes) .
range of sight (nautical miles) . . .
range of sight (kilometers)

. 1.8

. 2.1
. 3.8

2-5

3-o
S.6?

4.0

4-7
8,6

5.6
6.6

12.2

9-7
11.4

21.2

12.6
14.8
27.3

Now let us suppose that air mirage occurs. Let angle $, as in
Fig. 61, be the greatest angle with which total reflection still occurs;
then all visible rays incoming within the range of the angles \fj-v are
reflected. Let TK be the width of the reflecting strip. Since the
mirror is convex in consequence of the earth's curvature the mirages
appear distorted, i.e. shortened in the vertical direction. Everything
in the space WKZ is invisible, everything within the space RTKW
is directly visible and miraged. The resulting pictures are repre-
sented in the sketch. The lighthouse is miraged only below L
if L is the intersecting point of the visible ray RT with the lighthouse.
The steamship, nearer than the horizon, is visible directly, and the
sailing ship, beyond the horizon, for its upper portion, and both
are miraged. The line where direct image and mirage touch
each other is lifted upwards within the reflecting strip. At the dis-
tance T it coincides with the borderline between sea and (reflected)

OPTICAL AND ACOUSTICAL PHENOMENA 121

sky, i.e. with the apparent visual horizon, at the distance K and
beyond with the true sea horizon.

The theory of mirage is discussed by A. Wegener (2590). W. E.
Schiele (257) gave a bibliography, worthy of thanks, of the most
recent literature. The curves published by him, the result of all
measurements hitherto made on the appearance of mirages, corre-
spond perfectly to incoming radiation conditions. He points out in
addition that the superheated air layer really responsible for the
mirage is only a few centimeters thick. This explains why the
phenomenon is not destroyed by the wind or by street traffic. It is
very frequent over asphalt pavements around noontime and is then
called a "street mirage." The mirrored image of the sky in this case
gives the impression of a great puddle of water. L. A. Ramdas and
S. L. Malurkar (256) have published an excellent photo of such a
street mirage. W. Findeisen (241) using an airplane camera, took
pictures of the coast at Cuxhaven from a distance of 6 to 12 km.
Fig. 63 gives an example. In the upper part is a stretch of the coast
at Cuxhaven shown under optically normal conditions as taken
from a distance of 12.2 km. At the left appears the 30 m beacon.
Below is a photograph from the same point with an inferior
mirage. The mirage as outlined in Fig. 62, is easily recognized at
the lighthouse as well as in the outline of the coast at the right. The
dark stripes below the mirage correspond in their upper boundary
to the visible horizon (the point T in Fig. 62).

In order to give an approximate idea of the value of the magni-
tudes involved, we quote a concrete example from A. Wegener. For
a height of 10 m above sea-level (boat deck), a temperature jump
of 5° at the surface and a horizon depth of v — 5.6/, the maximum
angle ty — 12.2' and the breadth of the reflecting band = 12 km.

That this mirage is a phenomenon of the heated air near the
ground is most apparent from the fact that it also occurs at a sunny
wall. J. M. Pernter and F. M. Exner (254) published a photograph
in which a boy leaning against a heated wall is visible both directly
and doubly reflected. The objective of the camera in this case was
only 16 cm from the wall. The line of sight therefore grazed the
wall and could consequently give rise to a mirage just as though
directed along the ground.

The rainbow occurs also as a phenomenon near the ground,
namely in fountains or wherever water is sprayed. Because the arti-
ficial water drops are much larger than the largest natural drops in
the case of showers the artificial rainbow is extraordinarily rich in
colors. In the air layer near the water it is often seen in the spray

122

THE CLIMATE NEAR THE GROUND

from the waves. What is more beautiful than to sail through the
sea still heavy after the storm of the rear side of the depression
when the sky is clearing and the sun, behind, is breaking through
the clouds while, ahead, a rainbow appears magically again and
again in spray tossed up with the dark sea as background.

The halo too, caused by reflection from and refraction in ice
crystals, may sometimes be observed as a phenomenon near the

FIG. 63. Above: Coast at Cuxhaven at a distance of 12.2 km. Below: The same
coast with an inverted mirage. (Photographer: W. Findeisen)

ground. H. Seilkopf (March 6, 1931) observed a halo according to
W. Portig (2540), within the soft frost crystals shaken down from
the trees by a gusty wind. Portig himself observed both parhelia,
parts of the 22° halo and the upper tangential arc in an ice fog
originating from evaporation of water when gas coke was extin-
guished in the humid, cold ( — i3°C) atmosphere in the region of
the port of Hamburg. Seemingly, halo as well as rainbow near the
ground is marked by an unusual brilliance.

From the optical phenomena we pass on to the acoustic phe-
nomena within the layer near the ground.

It is generally known that the propagation of sound is dependent
upon weather. The mighty thunder is very rarely heard beyond
10 miles (15 km) because of the peculiar stratification of temperature.
On the other hand, heavy artillery cannonade can be heard over a

OPTICAL AND ACOUSTICAL PHENOMENA 123

distance of some hundreds of kilometers. However, this is not
always true. The propagation of sound is determined by the varia-
tion of temperature and wind with height, and probably also by the
intensity of mass exchange. Therefore, the air layer near the ground
influences the audibility by means of its often unusual stratification
of temperature and wind. This for example was of great impor-
tance for the overwater signals formerly much used for safeguard-
ing navigation.

When the temperature decreases quickly with height the audi-
bility is low; the ray of sound is deflected from the surface. If, how-
ever, temperature increases quickly with height, generally at night,
the sound ray, directed upwards, returns to the ground. Wind in-
tensely increasing with height has a similar effect downwind; this
also occurs mostly with inversions of temperature during the
night (compare Chapter 7 and n). In this case, the range of sound
is unusually great. Some time ago, A. Schmauss drew my attention
to the extraordinary audibility which is observed in streets of great
cities during the night. The step of a wanderer or whispering
human voices are heard at great distances. The "putt-putt" of the
motor of a small fishing boat is heard even if it is far off the shore.
According to a personal communication of H. Wagemann, this is
the case especially in spring when warm air lies above the still cold
sea and the normal temperature inversion is intensified by the
weather situation.

In the polar climate, where extreme inversions occur, unusual
audibility is a generally striking phenomenon. In the diary of Cap-
tain Scott (2580) we read of such a weather condition in the
Antarctic (August i, 1911) :

The light was especially good today; the sun was directly reflected by
a single twisted iridescent cloud in the north, a brilliant, and most beau-
tiful object. The air was still, and it was very pleasant to hear the crisp
sounds of our workers abroad. The tones of voices, the swish of ski, or
the clipping of an ice pick carry two or three miles on such days — more
than once today we could hear the notes of some blithe singer — happily
signalling the coming of the spring and the sun.

L. Aujeszky (257) points to two practical cases when the observer
compulsorily realized the local differences of the propagation of
sound, one time during the First World War in the evaluation of
listening posts for the sound-measuring troops, and again in the
evaluation of noise-free plots for building construction. In the first
case it was a question of selecting the place where most could be

124 THE CLIMATE NEAR THE GROUND

heard. The "often deceptively large differences in acoustics between
places quite near together" were sought out and studied. In the
second case, it was just the opposite, an attempt to find the quietest
places in the neighborhood of large cities, for instance.

Local acoustics in general are not dependent entirely on the con-
dition of the atmosphere, such as the uniform occurrence of a
temperature inversion in a valley, or favorable local winds. More
important are topography, vegetation and buildings. Sound waves
bend around obstructions such as houses, hills and woodlots. The
deep tones of artillery fire which govern the suitability of a listening
post, get around such obstacles with comparative ease on account of
their long wave lengths. The high-pitched, short wave, racket which
annoys people, cannot do this. Sound shadows result. They must
be sought out in selecting building sites which will be free from
noise. L. Aujeszky has given various directions to this end. Refer-
ences to other literature on this subject are found in his work.
B. Hrudicka (777) has something on the acoustic peculiarities of
city climate.

The dust content of the layer near the ground is determined under
normal conditons by vertical temperature stratification and wind.
Therefore, it has a daily variation. At the time of the nocturnal in-
version and calm air the dust drops down to the lower layers.
According to M. Rotschke (2560) the content of dust increases in
the layer near the ground with beginning of the nocturnal out-
going radiation and reaches its maximum at sunrise. As soon as
incoming radiation sets in and temperature increases, the dust, as a
consequence of the intensified exchange, is lifted up from the air
layers near the ground and, therefore, the dust content is smallest
in the late afternoon. Unfortunately no observations at different
heights within the layers near the ground exist so far. According
to E. F. Effenberger (240^) the daily course of the content of con-
densation nuclei is reversed.

Strong wind lifts dust, sand, snow and water over the ocean (as
already mentioned, page 108) and carries them into the lowest
air layer. In all these cases the boundary between air and ground,
snow cover or water surface respectively disappears. With the ma-
terial of the surface also its properties are brought into the lower air
layer. No doubt the scorching heat with sand storms of the deserts
is intensified by the fact that the sand of the surface, the tempera-
ture of which is higher than the air ever reaches, transfers its heat
to the air layer near the ground which carries it along.

OPTICAL AND ACOUSTICAL PHENOMENA 125

Under such abnormal conditions, unfortunately, no measurements
of the content of sand, snow or water in different heights have been
made, as interesting as they might be. Only from Central Iceland
I know of measurements by H. Slanar (/95). On the occasion of
strong NE winds which carry fine basaltic dust in greater quanti-
ties he fixed on a pole paper boxes the openings of which with a
cross section of 25 cm2 were directed towards the wind. During
the time of July 21 to 27, the following quantities of basalt dust were
accumulated there:

at the bottom 10 cm 30 cm 50 cm (height)

13 cc 2.5 cc 0.5 cc only traces

The content of carbon dioxide in relation to height has been in-
vestigated by W. Kreutz (2470), at Giessen in the years 1939-41.
Within the layer near the ground the amount of CO2 decreased
with height and increased again with further increasing height.
The average values were:

height (m)

o.o

0.=;

2.0

14.0

CO2 — content (volume percent) .

. 0.0461

0.0431

0.0417

0.0444

If all CO2 values at 0.5, 2.0 and 14.0 m height (c^c^c^) are corre-
lated with the value at the ground (c-0), (according to W. Kreutz),
the following relations were found by means of the method of least
squares :

Ci — 0.92 c0 -f- 0.2

Ca - 0.84 C0 + 2.8
Ca = 0.69 CQ + 12.9

Therefore, the content of CO2 is composed of two components: the
CO2 emanating from the ground decreased with height as is proved
by the decrease of the factor of c0. Additionally carbon dioxide is
advected originating from gases escaping industrial plants and home
heating contrivances; this CQ2 comes into the air layer near the
ground from above and its amount increases, therefore, with height
as it is shown by the second term of the equations mentioned.

Above the ocean the CO2 supply from below is often lacking.
In the air layer near the water only a little increase of CO2 with

126 THE CLIMATE NEAR THE GROUND

height is observed. A series of measurements by K. Buch
July 7, 1935, in the waters of New York yielded :

Height above sea (m) 0.3 1.5 4 8 30

CO2 — content (volume per-
cent) 0.0307 0.0312 0.0313 0.0314 0.0329

Several authors have been interested in the distribution of radio-
active material directly above the ground. J. Priebsch (255) has
made a brief summary, and I shall follow his conclusions.

Gaseous, radioactive materials in the atmosphere are derived solely
from the ground. Through convection, the ground air-layer plays
the same part in transmitting these emanations as it does for water
vapor. It has recently been discovered that the radioactive substances
are subject to decomposition. The shorter the time of disintegration,
the less the height to which radioactive material can be carried by
convection. Long-lived radium emanation is therefore more richly
distributed at a given height above ground than is thorium B, while
this again is more abundant than the very short-lived thorium
emanation. Under the assumption that the exponent a has the
value 1/3, we can expect the following distribution of radioactive
material in the lowest air, considering the amount present at a height
of i cm as 100: —

TABLE

Height above ground in cm:

100

IOOO

IO,OOO

Radium Emanation

100

Thorium Emanation

100

Thorium B

100

Experiments have proved that the actual distribution is in close
accord with this law.

Since radium emanation originates in the ground, the condition of
the ground is of great influence on the emanation content of the
ground air. We must expect considerable variation between local-
ities. If the soil is very wet or frozen, the emanation content is
small; it becomes zero when there is a snowcover of only a few
centimeters thickness. When the soil is dry, it depends on the
weather and the kind of soil how much emanation escapes from
the pores. H. Israel-Kohler (245) has given a summarizing report

OPTICAL AND ACOUSTICAL PHENOMENA

127

of measurements made near the soil surface to find out fluctuations
in the subsoil.

F. Becker (238) followed the daily range of radium emanation
content. At the Meteorological Institute at Frankfurt on the Main
he made observations i m and 13 m above the ground. The results
of April 4-5, 1934, are given in Fig. 64. Curve I gives the emanation
content at i m height. It is greater in this layer near the surface
than it is at 13 m (Curve II). Actually, mass exchange governs the

0 Z

6 8 10 12 W 16 18 &

FIG. 64. Daily course of radium emanation content of the air near the ground.
(After F. Becker)

content. During the calm night hours with stable temperature strati-
fication, the difference between upper and lower stations is great.
The nocturnal enrichment with emanation which takes place at this
time in the neighborhood of the ground moves upward in the morn-
ing hours, somewhat weakened and with a three-hour lag. The
strong midday convection irons out the difference. The minimum
emanation content, however, still lags three hours behind that at the
ground.

In a high Thuringian pine forest, C. Schmid-Curtius (25$) has
measured radioactive precipitation at different heights on a 20 m
scaffold reaching above the tree-tops. His original work, which was
done from a health-resort viewpoint, deserves study.

SECTION IV

THE INFLUENCE OF THE GROUND ITSELF ON THE CLIMATE NEAR IT

CHAPTER 13
THE TEMPERATURE OF THE GROUND SURFACE

All discussions up to this point in regard to the physical condition
of the air layer near the ground have been under the assumption that
there was no plant cover and that the ground was quite flat. Both
these assumptions still hold in what follows.

We wish now to focus our attention on the influence of the earth's
surface on conditions in the ground air. Hitherto we have assumed
that all observations have been made over a uniform, solid ground —
fine sand, for instance. Although we could not avoid mentioning now
and then the influence which the kind of soil exerted on the lower
air, it is only at this point that such effects are to be thoroughly
examined.

In nature we find three different kinds of surface on the earth —
land, water and snow. Among these, land shows the greatest varia-
bility, even without considering the varied vegetation which may
cover it. There is no end to the varieties of soil; its variation with
depth is different in different places. The condition of the soil is
affected by varied cultivation. Moreover, changes of humidity result
in different ground conditions from time to time.

While, in the case of land, it is only the uppermost layer which
receives and gives off radiation — in fact merely the boundary surface
adjacent to the atmosphere, a different condition exists in regard to
water and snow. Solar radiation can penetrate both water and snow
and the heat exchange between earth and air is not merely a surface
matter, but has to do with a vertical distribution to a considerable
depth. Both water and snow vary with depth. The water in a shal-
low puddle has an influence on the adjacent air which is quite dif-
ferent from that exerted by deep water. Standing water acts differ-
ently from running water that carries its heat relationships with it.
As for a snow cover, it is its age, especially, which markedly affects
the physical condition of the surface and of the air adjacent to it.

THE TEMPERATURE OF THE GROUND SURFACE I2<)

Conditions over water and over snow are considered in Chapter
15 and 16. Nevertheless, for the sake of a proper perspective, we
must interject a few pertinent remarks here and now. We shall
devote the present chapter to the processes at the surface of the land,
and shall begin by investigating how the surface affects radiation.

By "reflection number/' "reflectivity" or "albedo" is understood
the ratio of reflected radiation to the insolation; it is usually expressed
as a percentage. A reflection number of 0.4 (or 40%) indicates that
the ground reflects 40%, and absorbs 60%, of the radiation which
strikes it. According to Kirchhoff's law, the ratio of emissivity to
absorptivity is constant for a given wave length and temperature.
If, therefore, a body has low absorptivity and high reflectivity for a
certain wave length band, it has low emissivity in the same range of
wave lengths.

Three spectral bands should be differentiated, i. The ultraviolet,
with wave lengths below 0.36 //,; 2. the visible spectrum, with wave
lengths from 0.36 to 0.76 /*, and finally; 3. the long wave (infrared)
from 0.76 to about 100 //,. We shall begin with the ultraviolet.

According to the measurements of P. Gotz (334) and F. Lauscher
and O. Eckel (341), the reflection number of a snow cover in the
ultraviolet is from 80 to 85%. All other surfaces have only a small
reflectivity in the ultraviolet. W. Hausmann and F. M. Kuen (27^)
22 to 25% for stone (gravel, granite, chalk), and 6% for garden
soil. K. Biittner and E. Sutter (^07) observed 17% on dry dune
sand, and 2% in dune heath. H. Voigts (283) estimated, from com-
parative measurements along the Bay of Liibeck, that on clear July
days, the reflection of the sandy beach caused an 8 to 9% increase
in the ultraviolet.

Most of our observations are for the visible range of the spectrum,
those of A. Angstrom (260) and K. Biittner (264) for example.
J. Bartels (267) made a compilation in 1930. The following figures,
selected from all the measurements, will serve at least as a rough
table of comparative values.

REFLECTION NUMBER (ALBEDO) OF VARIOUS SURFACES FOR THE
VISIBLE PORTION OF THE SPECTRUM

Fresh snow cover 80-85%

Cloud surface 60-90

Older snow cover 42-70

Fields, meadows, tilled soil 15—30

Heath and Sand 10-25

Forests 5-18

Surface of the sea 8— 10

130 THE CLIMATE NEAR THE GROUND

On later pages we shall give further data on the albedo of snow
and also that of water, particularly as to mirages when the sun is
low.

If dry sand is moistened, it appears darker. This is an indication
that the albedo of moist surfaces is less than that of dry ones.

A. Angstrom (260) observed that a certain gray sand had an
albedo of 18% when dry, but 9% when moist. For a high, brightly
colored grass carpet, the corresponding figures were 32% and 20%.
K. Buttner and E. Sutter (307) determined the albedo of dune sand
at Amrum:

TABLE 20

For the total
radiation
(0.5-3-0 /A)

For ultraviolet
radiation
(0.3 /*)

In dry condition

. 37%

17%

In moist condition

. 24%

We shall mention this again in connection with Fig. 73.

Angstrom has also given an explanation of this fact: When the
particles of the soil or plant surface are covered with a film of water,
light rays can enter the water film in all directions, but the only rays
which can emerge are those which can reach the surface of the water
film within the limiting angle of total reflection. The water film
therefore retains part of the radiation.

For the infrared portion of the spectrum, we have the observa-
tions of G. Falckenberg (269). Most surfaces are practically "black
bodies" for this spectral range, i.e. they absorb almost all radia-
tion which strikes them. For instance, Falckenberg's observations
show: —

for light colored sand, an albedo of 11%

for light gray limestone, an albedo of 8-9%

for coarse gravel, an albedo of 8-9%

for clods of earth with sod, an albedo of 2%

for snow, an albedo of 0.5%

Snow, in particular, devours practically all radiation. Hence this
paradox of Falckenberg: "Fresh-fallen snow is the 'blackest body'
we know." An exception seems to be a living vegetation cover, which
will be treated in Chapter 26.

THE TEMPERATURE OF THE GROUND SURFACE 131

In regard to the body surfaces of animals we may say that F.
Riicker (277) found a minimum of the preponderantly diffuse re-
flection between 1.9 and 2.2 JJL for beetles, between 2.6 and 3.0 ju, for
butterflies and between 1.7 and 2.2 \L for snailshells. For example,
for a butterfly's wing (f orewing of Pieris brassicae) he found : —

for wave length (//,) .... i.i 1.5 1.9 2.2 2.6 3.0 3.5
an albedo (%) 69 70 61 55 31 27 35

The different reactions of various soil types to radiation is notice-
able in the heat economy of the air near the ground. A soil surface
with a high index of reflection heats up by day much less than one
with high power of absorption. For example, we find very high
temperatures over dark moor soils by day, and this is responsible
for the extraordinary demands upon plants in the frost-endangered
mucklands.

It has already been mentioned how important it is in regard to
the whole heat economy on the earth's surface, to know the tem-
perature of the surface itself. It is best defined as the "temperature
of the boundary surface between earth and air." To measure it
accurately is a matter of considerable difficulty.

All earlier measurements made with mercury thermometers are
useless. On the one hand the temperature "on the earth's surface"
was measured — which meant placing the thermometer flat on the
ground. In this case the measurements obtained were those of the
lowest airlayer, influenced by radiation and dependent on the con-
struction of the particular thermometer. Measurements "/'« the
surface" were carried out by placing the thermometer within the soil
but covered by only a very thin layer. Such a thin cover is easily
carried off or heaped up by wind or rain. But even when there is a
careful observer to watch the exposure of the thermometer, it is
only the temperature close beneath the surface of the soil which is
obtained.

By means of thermocouples, made so tiny that their radiation
errors are vanishingly small, the surface temperature can be ob-
tained electrically with quite satisfactory accuracy. Great care must
be exercised to make sure that the thermo elements are in closest
contact with the surface. Wilh. Schmidt (279) used inserted glass
tubes to determine the temperatures of the surface, the air layer
above, and the earth layer beneath, by touching the tube wall with
the thermocouple.

It is an intriguing idea to measure the surface temperature by
day or night, not directly at the surface but indirectly as a tempera-

132 THE CLIMATE NEAR THE GROUND

ture boundary. It is possible to do this by observing the gradient of
temperature in the ground or in the air in very close contact with the
ground and then extrapolating for the temperature of the surface
itself. A. Nyberg (^5), for example, did this. Or we can determine
the temperature of the surface from its temperature radiation. G.
Falckenberg (270) has made and used apparatus of this nature.
K. Wegener (75) and H. Trojer (74, 76) used a parabolic mirror
at whose focus the radiant heat was concentrated.

In India K. R. Ramanthan (274) followed the suggestion of G.
Chatterji by inserting a mercury thermometer in a well-conducting
copper plate of 1.5 sq cm area. The thermometer was as close as
possible to the under surface of the plate. By means of a sheet of
felt which rested upon the plate, insulating it from heat and also
serving as a handle, the copper plate could be moved about here
and there over the heated ground. This "flatiron" method gives a
mean value over a rather large area and is at any rate the best way
to use a mercury thermometer in measuring ground surface tem-
peratures. Mention should here be made of the original method by
means of which an English biologist in the Syrian desert was able,
without dismounting from his horse, to determine approximately
the temperature of the ground. He carried with him a great quan-
tity of wax balls whose melting points varied by regular steps. Thus
he could measure the surface temperature to within the difference
between two successive melting points.

It is on the surface of the ground that the highest midday tempera-
tures are found — unexceeded in the neighboring air. Fig. 65 repre-
sents a temperature measurement made by G. S. Eaton (268) in
Riverside, Illinois on Aug. 7, 1918. It is an interesting example since
asphalt pavements play an important part in the life of the modern
city-dweller. The dot and dash curve gives the air temperature as
measured in the shade, 10 m to one side of the street. It shows the
normal march of temperature with a maximum at about 3 P.M.
Considerably higher are the air temperatures at 120 and 30 cm
above the street, while the surface of the street at noon is about 20°
warmer than the air layer a few decimeters higher.

Notice the time of the temperature maxima as indicated on the
chart! The maximum on the surface follows the daily period of solar
radiation more closely than does that of the air temperature, and is
therefore earlier than that of the air. Most striking, however, is the
broad maximum in the air near the ground, which tends to persist
till evening. The reason for this is probably that the asphalt pave-
ment stores up so much heat around midday that it continues to

THE TEMPERATURE OF THE GROUND SURFACE

133

give off heat to the air lying above it, during the afternoon. In
Fig. 65 the street is 8° warmer than the air 30 cm above the ground
at 4 P.M., and is still 5° warmer at 6 P.M.

The high temperatures existing in the solid pavement result in
phenomena which A. Schmauss (278) has described. The ground
under the concrete is almost entirely sealed off from atmospheric
breathing. "The result can be seen in bulges and bubbles of the
asphalt which is evidently subjected to a gas pressure from below.

8a 9 10 11 12 1 2 3 4 5 6 7 8 p.

FIG. 65. Temperatures above an asphalt street. (After G. S. Eaton)

This condition occurs particularly where there are little holes in the
material with rounded edges which must have been caused by escap-
ing gas and which look like the "eyes" that occasionally crack out
on a viscous liquid left standing over a burner. But in that case the
flaws close up again, while in asphalt they are permanent."

The midday temperatures of more than 50°, which are indicated
in Fig. 65, are by no means the highest experienced in our climate.
According to a recent compilation by Br. Huber (5/4), surface
temperatures of 70° C and even more have been repeatedly observed.
On southern exposures in our climate temperatures up to 8o°C can
be expected under favorable conditions. A reference to the harmful
effect of this on young plants is made in Chapter 17. The following

134 THE CLIMATE NEAR THE GROUND

example shows how surface temperatures may work out in polar
climates.

In his report on the German Antarctic Expedition of 1938-39,
A. Ritscher (276) states that in New Swabia Land, 100 nautical
miles inland from the edge of shelf ice a number of pools were dis-
covered between dark red rounded peaks in the midst of the glacial
ice. "Our first impression, that the evident melting process was
attributable to heat from within — vulcanism, in other words —
seems to have changed to the hypothesis that it is the consequence of
heat storage due to intensive insolation, with which the dark reddish
brown color of the surrounding rock would best agree."

"Black bulb" thermometers are ordinarily used in measuring
radiation. These are mercury thermometers whose bulbs have been
blackened in order to absorb as much as possible of the incident
radiation. The bulb is surrounded by a second glass bulb; the in-
tervening space is evacuated so that the thermometer can transmit no
heat to the air.

There is a common impression that the surface of the ground,
which of course gives off heat to the air, corresponds in the highest
degree to the temperature of a black bulb thermometer exposed to
the same conditions. A. F. Dufton and H. E. Beckett (267) have
shown that this is a false opinion. In the case of the black bulb
thermometer there is an equilibrium set up between the heat intake
through insolation and the heat output through radiation to the
surrounding glass bulb. In contrast to the black bulb thermometer,
a natural surface is subject to heat loss by conduction and convection.
A plane surface, however, can radiate heat only toward half a hemi-
sphere, i.e. upward, while the blackened bulb can radiate to all
directions. If the natural ground surface is concave, the storage of
heat is still greater. A hindering of convection and a poorly con-
ducting soil have a similar effect. Dufton and Beckett present the
following data: — Air temperature, 20.6°C; Black bulb thermometer,
56.1 °C; Surface of a tar-paper roof over a heat-insulating base,
65.5 °C. One more extreme instance: If you construct a well in-
sulated box with blackened walls, and cover the box with a pane
of clear glass, you can cook a blackened egg in it — reaching a tem-
perature of i20°C. So the black bulb thermometer does not indi-
cate the extremes of surface temperature which are possible under
peculiar local conditions in the microclimate.

Many attempts have been made to determine how the nature of
the surface affects ground temperatures. Thus, for example, E.
Wollny (285) colored three different kinds of soil partly black and

THE TEMPERATURE OF THE GROUND SURFACE 135

partly white and studied the temperature range beneath the surface.
Fig. 66 shows a recent attempt of the same sort, made by L. A.
Ramdas and R. K. Dravid (joo) under the strong sun of India.
The left half represents the temperature range during the 40 days'
experiment, on the same test surface; the right half, an untreated
control surface. Both surfaces had "black cotton bases." Five days
after the beginning of the measurements (Point A), a very thin
layer of white powdered lime was dusted over the test surface. This
caused the isotherms to turn suddenly upward, continuing to rise

Test surface

Control surface

§ 5

20t

J il

5 10 15 20 25 30 35
Number of experiment day

0 5 10 15 20 25 30 35
Number of experiment day

FIG. 66. Change of the ground temperature owing to scattering (A) of white lime
powder. (After L. A. Ramdas and R. K. Dravid)

for nearly 10 days until the change is complete. At the ground it is
then about 15° cooler than at the surface of the black soil. The sur-
face effect is felt to a depth of at least 10 cm. At Point B the powdered
lime was removed. It had already weakened in effect by reason of
wind and humidity, but after its complete removal it was still i to
2 weeks before conditions were the same on the test surface as on
the control surface.

C. Dorno (266) has investigated the effect of painting on the
temperature of wood. For this purpose he placed four small, cylin-
drical wooden blocks, 3 cm high by 2^ cm in diameter, in the sun
on a south-facing balcony at Davos. Thermometers were inserted
in mercury-filled holes in the wood. He found that the effective
radiation amounting to one gram-calorie caused the temperature
of the wood to rise above that of its surroundings by the following
amounts for the various colors: —

136 THE CLIMATE NEAR THE GROUND

White lead paint io.8°C

Rosepaint (zinc white with dammar lacquer) ii.o°C

Yellow ochre paint I4.8°C

Red oil paint i5-7°c

Lamp black i6.9°C

K. Schropp (287) carried out a series of measurements for tech-
nical finishes. The surfaces in question were placed on an insulat-
ing cork plate, 5 cm thick, while the temperatures were measured
in sunshine and quiet air by means of a thermocouple. He found
that, under similar daytime conditions, black paper or black enamel
attained a temperature of from 45° to 55°C; white surfaces, 15° to
2O°C; while polished aluminum foil showed only i5°C. By night
all the surfaces had temperatures 2 to 4° lower than the air.

Railroad tracks heat up strongly in sunshine. The only known
measurements are those of K. R. Ramanathan (274) in India. In
Agra he placed a rail 1.5 m long on broken stone 10 cm above the
ground. A hole, drilled vertically 25 mm into the rail was half -filled
with mercury. In this the thermometer was inserted. The following
table gives an abstract of several average and absolute monthly ex-

TABLE 21

Maxima Minima

Mean Absolute Mean Absolute

Month

Air

Rail-
road
track

Air

Rail-
road
track

Air

Rail-
road
track

Air

Rail-
road
track

May, 1927

• 414

56.1

45.1

59.8

25.6

23.2

23.1

20.2

June

. 41.0

54.6

44-3

61.1

27.8

26.2

20.9

17-3

August

31.5

47.7

36.6

54.4

25.2

24.7

22.5

2I.Q

September

34.0

4Q.6

37.7

56.5

22.5

21.7

TQ.4

18.1

November

. 26.6

39-6

31-7

47-4

II.4

9-5

5.8

3-2

January, 1928 . .

. 21.6

34-2

25.0

40.9

7-i

5.2

2-7

i-5

tremes in degrees centigrade for both the railroad rail and an air
temperature control measured within a Stevenson shelter.

In Geisenheim on the Rhine, H. Schanderl and N. Weger (2770)
experimented in 1938-39 with a 3 m trellis wall of light brown
quartzite facing toward the southwest. It was partly painted black
and white. In front of it tomatoes were planted, whose growth and
yield were measured. The true air temperature was observed by
means of a platinum wire thermometer, while the counter-radiation

THE TEMPERATURE OF THE GROUND SURFACE 137

of the wall was obtained with a black bulb resistance thermometer.
At a distance of 10 cm from the wall the difference between the air
and plant temperatures in front of the three different parts of the
wall was not very great but that between the amounts of counter
radiation was. If we consider the total radiation of the black wall
as 100, that of the natural colored wall on the sunny igth of June
1940 was no, while that of the white wall was 156. In the short
wave part of the spectrum the differences were still greater.

At first the tomatoes in front of the black wall grew considerably
faster; their yield, however, was less. The amount of radiation, in
conjunction with the overpowering long wave counter radiation
stimulated the plants here to purely vegetative growth. The greater
amount of radiation (short wave, especially) in front of the white
section of the wall retarded growth in height but stimulated pro-
ductivity. The greater yield of tomatoes in front of the white wall
justified the cost of painting.

Mention should be made here of the movement of the ground
surface by frost. In the spring it plays an important part in agricul-
ture at times ("heaving"). R. Fleischmann (277, 272) has described
a simple arrangement by means of which the vertical movement of
the ground can be easily measured, and has himself carried on
numerous observations. Under "Heaving" he writes as follows:
"The action of frost on water particles in the pores of the soil results
in an increase of their volume; thawing, on the other hand, occasions
a sinking of the surface. The greater the difference between the de-
grees of frost at -2 P.M. and at 7 A.M. on the following day, the greater
the amount of ground frozen, and the deeper the scene of this action
lies beneath the surface, the greater the heaving effect." It appears
that tearing of roots in the soil, with consequent damage to agricul-
ture, begins when the heaving of the soil amounts to about 15 mm.
To give an idea of the amount of heaving which ordinarily takes
place, the following figures are taken from R. Fleischmann's find-
ings for the years 1931 through 1935: —

Heaving Movement 0-5 5-10 10-15 15-20 20-250101

Number of cases

I93i~i935 57 38 4 3 2

Concerning the process of soil respiration, which we shall not
discuss here, the reader is referred to the recent work of M. Diem
(265) and W. Schmidt and P. Lehmann (280).

CHAPTER 14
THE INFLUENCE OF THE TYPE AND CONDITION OF THE SOIL

In the preceding chapter we treated only the surface of the ground,
its characteristics and temperature relationships. The temperatures
in the ground below the surface were considered in general in Chap-
ter 3. There we mentioned the effect of the heat conductivity of
different kinds of ground.

The influence of the kind of soil and its condition upon the
microclimate is, however, so great that we feel we should deal with
it in the present chapter in more detail. The following computation
by H. Philipps (68a) (from his "Theory of heat radiation near the
ground") will well show this influence. Under the assumption that
at sunset the temperature is n.5°C and the water vapor pressure
5.8 mm he finds the following decreases of temperature of the ground
in the course of 10 night hours for different kinds of soil in depend-
ence on the intensity of mass exchange:

TABLE 22

Cooling of ground (°C) within 10 hours with

a coefficient of exchange
Kind of Soil A — o.oi A = 0.70

Granite 7.6 7.0

Loamy Sand 10.9 9.6

Peaty Soil 12.5 10.9

Dry Sand 13.6 11.7

Wet Sand 16.2 13.5

When the exchange is greater more heat is supplied from the air
layers near the ground; the decrease of temperature of the surface is,
consequently, smaller. What is so significant with these numbers
is the dominating influence of the kind of soil. A first glance at the
nocturnal thermal economy in Fig. 7 (page 22) explains this fact,
The supplementary heat supply from below is dependent upon the
kind of soil, the amount of evaporation (water content of the ground)
and (indirectly through the surface temperature) also the effective
outgoing radiation. When taking into consideration the width of
the arrows of Fig. 7, we see that the nocturnal thermal economy is

TYPE AND CONDITION OF THE SOIL 139

— in principle — already determined by the three elements already
mentioned. The kind of soil and the conditions of the ground are,
therefore, more important for the danger of night-frost than the
more or less intensive exchange within the air layers near the ground.

The temperatures of the ground consequently govern the climate
near the ground to the greatest extent; this is valid not only for the
night, as in the above mentioned example, but for any time. W.
Kreutz and M. Rohweder (297) have proved this close relation be-
tween the temperature of the ground and that of the air, calculating
correlation factors. In the following, we deal more in detail with the
soil conditions and investigate, first, the influence of the soil, i.e.
sand, clay, humus, etc.; then, we discuss the influence of the ground
conditions, its state of cultivation, its water content, etc.

From the loth through the i2th of August, 1893, Th. Homen (#2)
carried on a series of clear-weather temperature observations in Fin-
land, both on and above three different kinds of soil. These old
experiments, which were far in advance of our knowledge in those
days and which have not been surpassed since, will serve as our first
example of the influence of the nature of the soil on the temperature
cycle. Fig. 67 gives graphically the highest and the lowest daily tem-
perature for the three days mentioned.

For granite rock (dotted line) the maximum of 35° occurs (as is
to be expected) on the rock surface; the temperature falls rapidly in
the air above. The coincident maximum air temperature at a height
of 2 m, which is only somewhat over 23°, is indicated by a small
double circle. Going into the rock from the surface, the tempera-
ture at first decreases rapidly, then more slowly. By night its course
is reversed. Within the rock the temperature increases with depth.
The minimum occurs, not on the rock surface but at the level of
the surrounding grass, which cools off more than the rock does.

The minimum air temperature of not quite io°C at 2 m height,
which is indicated by a small double circle shows, abnormally, a still
lower temperature than the rock surface. The air temperature is not
measured over each kind of soil separately, but is observed once for
the three places. The rock, with its good heat storage, however, has
relatively high temperatures. The two dotted curves consequently
lie well to the right in Fig. 67; the maximum and minimum
curves are widely separated and do not meet, even at a depth of 60
cm. This is an indication that daily heating penetrates deeply into
the rock. The result is that even by night a good deal of heat is
passed out from within, thus accounting for the high level of tem-
perature in the rock at night — higher than that of the air. If one

140

THE CLIMATE NEAR THE GROUND

passes close to a stone-wall in the evening or near a house which
stands by itself, he can feel directly the return of the stored up day's
heat as it is being given back to the adjacent air.

Swamp land
Sand heath
Granite rock

FIG. 67. Temperature maxima and minima in three different kinds of ground.
(After Th. Homen)

Sandy soil (solid lines) heats up to an extraordinary degree in its
uppermost layer, more so, even, than does granite. But the tempera-
ture decreases very rapidly both upward and downward. Like
granite, it is a dry soil, but has much lower heat conductivity on
account of the air spaces between the sand grains. The day's heat
does not penetrate so deeply as in granite; at 60 cm depth there is
practically no evidence of daily fluctuation.

How peculiar is the behavior of the damp moor! The broken-

TYPE AND CONDITION OF THE SOIL 141

line curves lie at the left in Fig. 67 — in the cold region. To be sure
the maximum at the surface (which is here the surface of the grass)
is quite far to the right. The temperature drop within the soil,
however, is very abrupt; even at a depth of only 5 cm the 'daily
range, on account of the low heat conductivity, is as insignificant
as at a depth of 45 cm in granite. At 25 cm it has disappeared en-
tirely. During the night the moorland shows the lowest tempera-
ture of any; again the minimum occurs at the top of the short grass.
The fact that the curves for the interior of the earth slope in general
from upper right to lower left is occasioned by the observations
being made during a time of warming up — fair weather after
cloudy days; the interior temperature lags behind this warming
process.

Because of the dark color (see p. 134) the daytime temperatures
on the peaty soil are comparatively much higher in the climate of
Germany which is richer in radiation, than could be expected
according to these observations in Finland. The extreme daily varia-
tion of temperature and the low thermal conductivity are properties
of the moor despite its high water content. Therefore, drainage
of the moors intensified generally the microclimatic disadvantages,
especially the frost danger. It is diminished most effectively by
sanding the moors because in this way the nature of the surface
layer of soil, which chiefly influences the adjacent air layer, is
changed. At Emslandmoor, W. Kreutz (296^) studied the daily va-
riation of temperature over experimental areas that were covered
with sand, 5 cm, 10 cm, and 15 cm deep and could prove that the
thermal conductivity and the storage of heat in the ground was in-
creased with the depth of the sand layer. Also, the open water sur-
faces of the trenches in the moor moderate the frost, at least for a
distance of a few dekameters.

Another example of soil effect is given by N. K. Johnson and
E. L. Davies (292). In 1925 they made temperature observations in
six different kinds of soil on Salisbury Plain in England. The vari-
ous soils were in boxes i meter square, filled to a depth of 15 cm.
A maximum and minimum thermometer were each inserted in a
brass tube, i cm in diameter and 10 cm long, and placed i cm be-
neath the surface of each type of soil. The following table gives the
monthly temperature range in C° for June and January.

June typifies summer conditions. By far the greatest temperature
range is shown by the tar-macadam which we have already identified
as the extreme, artificial ground cover. Next comes sand, then earth,
and later the grass-covered soil, in agreement with Homen's results.

142 THE CLIMATE NEAR THE GROUND

TABLE 23
MONTHLY RANGE OF TEMPERATURE (°C)

June 1925 January 1925

Tar — Macadam 32.6 6.8

Sand 25.9 5.4

Earth 25.0 5.4

Gravel 21.1 5.7

Grassy ground 16.0 3.3

Clay soil 1 1.5 5.0

For comparison: air temperature at 1.2 m

(Stevenson shelter) 14.2 6.6

Gravel, on account of the many poorly-conducting air-filled spaces,
does not have the characteristics of rock, which we learned from
Fig. 67. Particularly low is the range for the moist clay soil. The
simultaneous range of the air-temperature is appended in the last
line of the table. It is slight in comparison with that of the soils,
as we might expect from our knowledge of the temperature range
in the lower air as compared with that in the ground.

In January, however, (that is, in winter) the temperature range of
the air is almost as great as that of the macadam. While in summer
the air is in next to the last place, in winter it is next to the first.
In summer, radiation and the earth temperature governed by it, de-
termine the air temperature near the ground. In winter, however,
when radiation is weak (particularly in the cloudy maritime climate
of England) the influence of the ground diminishes. The air tem-
perature is governed by the change of air masses and consequently
shows a relatively large monthly range.

Recently W. Kreutz (296^) has investigated the annual course of
temperature in loam, sand, and humus at the agrometeorological
research station at Giessen. He published the annual temperature
variation on the average of the years 1939-1941, based on 10-days
means for the depths 5, 10, 20, 50 and 100 cm. The ideal temperature
variation derived from these observations resulted in the tempera-
tures shown in Table 24. The values for the surface (not observed)
are extrapolated.

Fig. 68 gives, according to Wilhelm Schmidt (//j), a picture of
the different fractions of the total heat which can be utilized by the
ground and the air, respectively. The latter are shaded in the cut,
Ground types which show a large amount of white, tend toward
a mild microclimate; those types showing considerable shading,
toward a microclimate of extremes.

TYPE AND CONDITION OF THE SOIL
TABLE 24

143

Average

Annual Temperature

Annual Range of Temperature

Depth

(°C)

Loam

Sand

Humus

Loam

Sand

Humus

9.1

9-3

IO.I

20.0

21.6

23.0

9-3

9-5

10.3

19.6

20.8

22.0

9-5

10.4

18.8

19.8

21.0

9.9

10.3

10.8

17.0

17.4

18.2

IOO

10.7

H-3

n-3

14.2

144

In top place is the ocean. For reasons which will be explained in
the following chapter, water retains almost all the heat radiated to it.
Consequently, there is practically no daily range of temperature in

Sect

Granite

Sand

Still water

Sphagnum

Snow

Leaf litte

Portion of heat given op to the air
FIG. 68. How the different kinds of ground utilize incident heat radiation

the air near water. Over the ocean the daily fluctuation of air tem-
perature is not over l/2°C. In second place is granite, which is known
to us, from Homen's investigations, for its favorable heat balance.
Close behind sand, which stands in third place, comes quiet water.
By this is understood water which is not being in any way stirred

144 THE CLIMATE NEAR THE GROUND

up. This condition is met by shallow pools on land. Again it is
Wilhelm Schmidt (^27) who has been able to prove these theoreti-
cally stated facts by practical measurements. When a person is sailing
over a land-locked bay on a hot summer midday, he can experience
this very impressively in the oppressive, sultry heat which prevails
in the air just above the water.

In next place, according to Fig. 68, comes the sphagnum bog,
which has an important, and indeed unfavorable, influence on tem-
perature conditions of the moorland. In next to the last place is
snow. The extreme temperature range in the air above snow is
known from the very low winter temperatures which occur as soon
as the ground is insulated by a snow-cover. (More of this in Chap-
ter 16.)

For many practical questions it is of great significance that the
decayed vegetation which covers the soil, leaf litter, for example, has
an even lower heat conductivity than snow. Over such a cover there-
fore the temperature range within the province of the microclimate
is greater than over snow.

F. Firbas (2$$) has shown, through a long series of microclimatic
temperature measurements in oak and beech forests, that the rapid
heating of the sun-irradiated leaf litter enables the spring vegetation
in these forests — such as anemones, hepaticas, etc. — to produce their
flowers before the leaf buds of the beeches and oaks. In the early
days of May he found temperatures up to 43° within the leaf-litter.
The disadvantage of strong outgoing radiation is in this case lessened
by the fact that even the leafless trees form an effective screen against
outgoing radiation.

The foregoing is however a particularly favorable instance. Nor-
mally, poor heat conductivity is a great disadvantage, especially in
forestry. When a newly laid out culture of grass and weeds has
grown up and dies down in the autumn, increasing the ground cover
by all its organic material, such a culture becomes a center of frost,
for the exceptionally high noon temperatures, which entice the
plants to push upwards, are offset by exceptionally low night tem-
peratures. In a laboratory district of the Forestry College at Ebers-
walde, R. Geiger and G. Fritzsche (290) have recently furnished
numerical proof of this.

In a weed-covered, frost-damaged pine plantation, one part of
the ground had been plowed deeply as for a tilled crop. This deep
cultivation had torn apart the dead, poorly conducting surface layer
of the soil and thoroughly mingled it with the more highly mineral-
ized subsoil. In 1937 and in the late winter of 1939 parts of the

TYPE AND CONDITION OF THE SOIL 145

planting were treated in this manner. In the spring and summer of
1939 the nocturnal temperature minima were measured at a height
of 10 cm above these two areas and also above the unplowed surface:
Four observation stations situated at exactly the same altitude
afforded the data for the next table.

In all the figures the influence of the kind of soil is clearly and
constantly in evidence. The alteration of the soil through deep
tillage affected the heat economy of the adjacent air favorably and
lessened the frost damage to the vegetation growing there.

TABLE 25

Late season frost
nights, 1939

Mean of
30 cold
nights

Number
of frost
nights

Last
late
frost

Type of Ground

I3/M
May

29/30

May

10/11

June

Weedy frost area in
a shallow bowl .
Weedy cultivated
plain
Soil deeply plowed
in IQ37

-3-5
-1.6

~o-3

+0.2

-8.1
-6.5
-3-9
"3-4

-4.1

~3-7
— i.o

+0.5

-0.6

-f 0.2

+3-3
+4.0

J7
15
9
6

12 July
28 June
15 June
15 June

Soil deeply plowed
in 1939

The second deep-tillage proved more effective than the former. The
land plowed up in 1937 settled in time and began, as could be seen,
to cover the consolidated surface with weeds again. Here there is,
on the one hand, a further proof that it is actually the disturbance
of the soil which is responsible for the change in the night tem-
peratures, and, on the other hand, a practical hint that deep tillage
loses its frost-protecting properties to the extent that the vegetation
growing thereon becomes grosser and more frost-hardened.

The different thermal conductivity of the kinds of soil finds its
expression by the manner in which the winterly frost penetrates the
ground. At the experiment station of the agrorneteorological re-
search station at Giessen, W. Kreutz (296^) made the observations
in the hard winter 1939/40 shown in Table 26.

In Fig. 14 (p. 33) the trend of the o°C — isotherm indicates the
range of winter frost in the ground as to space and time. Similarly,
W. Kreutz gave many representations of the frost phenomenon for
many kinds of soil and for a number of stations for the winter
1939/40 in his paper.

146

THE CLIMATE NEAR THE GROUND

TABLE 26

Depth

.owest temperature in

of
Frost

Speed of

the depth (cm)

Kind of Soil

cm/day

in spring

100

Humus ....

0.6

Mar. 22

— 2.0

-0.5

0.4

3-o

Loamy sand .

i.i

Mar. 7

~5-2

~3-3

i.i

1.8

Loam

T.I

Mar. 16

-9-6

-4.6

— O.I

I.O

Sand

i-7

Feb. 28

-9-6

-7.0

— O.I

2.1

Basalt dust .

2.0

Feb. 25

— T2.0

-8.1

— 1.2

2.1

Along with the kind of soil we must consider its condition.

In the preceding example, one kind of soil was changed to an-
other by plowing; i.e. a two-layer soil, (a layer of humus-bearing top
soil over a mineral-bearing subsoil) being changed into a homo-
geneous prevailingly mineral soil. If, however, we confine our atten-
tion to one kind of soil, the effect of plowing is to make it looser,
so that it contains more air. Since the air conducts heat much more
poorly than any kind of soil, tillage results in poorer heat economy.

This is the explanation, for instance, of K. Bender's observation
(808) when he writes: "I shall always remember how, after a frosty
night, the plants in a certain potato field which had been weeded the
day before were all frozen, while those in a piece which, fortunately
in this case, had not been worked on, escaped with no damage."
Similar observations are often made. Wilhelm Schmidt (^02) carried
out comparative temperature measurements in an implanted, and
unplowed, field. The temperatures on the 23-241!! of August, 1924
were as follows :

TABLE 27

Time of day
Condition of ground

Early, about 5 A.M.

Afternoon 3 P.M.

Firm

Loose

Firm

Loose

Ground Surface
5 cm depth
10 cm depth

11.6

13.8
ic 4

9.6

12.4
16.4

31.0
27.6
24.2

36.4
29.0

23.8

In both places the early temperature increases with depth, while the
afternoon temperature decreases. (Outgoing and incoming types of
radiation.) But on the loosened ground, the diminished heat con-
ductivity causes it to be colder by nights (hence more danger of
frost!) in the upper 8 cm or so of soil, and hotter by day, than on the

TYPE AND CONDITION OF THE SOIL 147

untilled, denser soil. Below a depth of 8 cm within the ground, the
conditions are reversed, for in the loosened soil the heat exchange is
mostly confined to the layers near the surface, while in the denser
soil the deeper penetration of the daily range of temperature leads
to higher day temperatures and lower night temperatures in the
deeper layers, than is true for the looser soil.

Next to tillage, it is the water content of the soil which particu-
larly influences its temperature conditions and those of the adjacent
air.

Rainwater and snowmelt carry along their own temperatures as
they penetrate the ground and affect its temperature. F. Becker
(2^7) published the temperature record which is reproduced as Fig.
69, and which was obtained from electric thermometers at Potsdam.

\f\

•'V,

7*10 10 Jfl ¥0 50 IS 10 *0 30 ¥0 50 1$ Iff 20 30 M 50 20"
Middle East Time

Jff

18
IS

70cm

FIG. 69. The penetration of cold thunderstorm rain is marked in the recording of
ground temperature. (After F. Becker)

The lower half shows the temperature march, the upper half that
of the precipitation, on July 3, 1936, at which time a thunderstorm
caused a 20.8 mm rainfall. Nineteen minutes after precipitation
began, the thermometer at a depth of i cm shows the first effect
of the cold rain water. This relatively long time includes the wetting
resistance of the ground surface, for the water requires only 3 minutes

148 THE CLIMATE NEAR THE GROUND

to penetrate from i cm to a depth of 2 cm, 6 minutes from 2 to 10 cm
and 10 minutes from 10 to 20 cm. Such splendid automatic records
of rain water penetration are very rare, and can be obtained only
after such a heavy downfall as that in the preceding example. T.
Balanica (286) could find no similar instance among the ground
temperature records made near Munich.

F. Albrecht (574) gives the month of July 1937 as an example of
one in which there was a close correlation of precipitation and soil
heat conductivity. Fig. 70 in its upper portion gives the rainfall

J
I

0
2500
2000
7500

7000
4
500

ecipita

Jl , 1

Jul;

^ J

•''.

!*%

S
-...—I-''

''•

70cm

\ •* *

'-K

'•..•" N

'•• 1^

ii A 4 A * 4

HAA/

U*/J\

VWV

/^ «• ^ & JO

FIG. 70. Relationship between precipitation (at top) and heat conductivity of
the ground (lower group of curves). (After F. Albrecht)

for each of the 31 days. Underneath is the record of the heat con-
ductivity of the soil at depths of i, 10 and 50 cm. They were ob-
tained by means of the Albrecht heat-conductivity meter.

It is at once evident that high conductivity corresponds to a
moist soil. In other words the poorly conducting air in the pores of
the soil is replaced by better conducting water. In the layers near
the surface the ground is generally — and especially in dry weather
— drier and consequently of poorer conductivity than at a greater
depth. The i cm curve follows the precipitation closely and without

TYPE AND CONDITION OF THE SOIL

149

lag. With depth there comes a phase displacement; at a depth of
50 cm (dotted curve) it amounts to several days. In dry weather the
i cm curve has a decided daily cycle as Fig. 70 clearly indicates.
Albrecht explains it as "the stronger pressure of the then highly
heated sand," but perhaps a simpler explanation is the daily range
of soil humidity, for which L. A. Ramdas and M. S. Katti (jo/)
have furnished many recent data.

Since the water content of the soil undergoes constant fluctuation,
the heat conductivity of the soil varies with time and weather.
J. Schubert (22) proposed separating the heat capacity of the soil
into two components to begin with — namely, the heat capacity of
the dry soil, and the supplementary component resulting from the
water content. The former is a soil constant which does not change
for a given place. The latter takes into account temporary varia-
tions. It is equal to the water content of the soil if we relate heat
capacity to unit volume rather than as is customary to unit mass.

Many previous attempts have been made to determine the in-
fluence of soil moisture on soil temperature. The earliest measure-
ments known to me are those of E. Wollny (505). Fig. 71 shows

61. 2 S
May 1934

FIG. 71. The influence of artificial watering («/) on the ground temperature. (After
tests by L. A. Ramdas and R. K. Dravid)

an experiment made by L. A. Ramdas and R. K. Dravid (300) in
1934. The left half of the chart shows the experimental surface; the
right half, an untreated surface for comparison. The temperature
curves are drawn from ground temperature measurements at about
2 P.M. from the first through the sixth of May, 1934. At 6 A.M. on
the second day of the test (Point W), the ground was artificially
watered. The resultant cooling causes the isotherms in the ground
to rise suddenly. As can be seen from the control surface, the weather

150 THE CLIMATE NEAR THE GROUND

caused a cooling on the following day. The effect o£ the watering,
however, was outstanding from the beginning and was still evident
on the sixth day.

O. Fuchs (2^9) studied the influence of soil moisture on the
adjacent temperature field for a limited area. His purpose was to
determine the connection between the rising wind fields which
favor gliding and the condition of the soil in the neighborhood of
Darmstadt. In this connection thermal convection appeared closely
dependent on ground water. A fall of temperature appeared from
dry to moist areas; in places of abrupt temperature contrasts in par-
ticular the removal of heated air from the ground set in.

The work of G. Krauss and collaborators (296) shows how soil
humidity can vary within a limited area and so determine forest
habitats.

The variations of the soil and their influence on the heat economy
of the ground are made evident to an attentive observer through
three meteorological processes. These are: the melting of freshly
fallen snow, the formation of frost, and the formation of glaze.

Snow forms in the higher air layers, so that snowfall is independent
of the microclimate: If we travel through mountainous country
when the temperature is somewhat above o°C, and wet snow is
falling, we can observe over wide areas that the lower limit of
snowfall coincides with an isohypse.

As soon, however, as the snow covers the ground and the action
of radiation, wind, ground warmth, etc. set in, the microclimatic
variations quickly become noticeable — and the more quickly, the
thinner the snow cover. The lower limit of snowfall becomes ragged.
On soils of good heat conductivity the boundary quickly rises to
higher levels and under the influence of heat streaming up from
below, just as on slopes where the influx of radiation brings heat to
bear on the snow cover from above and removes it.

H. Mayer (299) has recorded in a photograph a very fine observa-
tion on this subject, which we have reproduced in Fig. 72. In April
1933 the members of the Frankfurt Meterological Institute had come
to visit the Research Institute on the Jungfrau pass in the Lauter-
brunner valley. H. Mayer writes: "A final snow cover of about 10
cm thickness and o°C lay on the warm earth whose temperature
was above zero. The air temperature was also o°C. On account of
the low clouds there was only a weak, diffuse sky radiation, so that
the snow was being melted principally by heat coming from the
ground. The first snow to be melted was that on the living rock,
then that on the meadows and the overgrown talus slope. It was

TYPE AND CONDITION OF THE SOIL 151

already completely melted away everywhere when we noticed one
final snow-covered area in the midst of the green slopes before the
Staubach valley. The falls of the Staubach dashing over 300 m down
the vertical walls of a former glacial trough had formed a small
erosion valley at its foot, in the solidified, overgrown talus slope.
The talus newly formed in this cut bore a final snow cover. The

FIG. 72. The different thermal conductivities of the ground are evident by the melting
snowcover. (Photograph by H. Mayer)

stone forming this cone, which has been brought down by the
water in comparatively recent times, appears in its various features
to have quite the same high heat conductivity as the ground. Never-
theless, the loose structure of this still unconsolidated talus deter-
mines its total heat conductivity through the air present in its in-
terstices, and this conductivity is much less than that of the solid
earth. The snow did not begin to melt until the weather cleared
and the sun got to work."

Similarly conclusive observations can be made on the subject of
hoarfrost. While snow falls everywhere and only its manner of
melting discloses the microclimatic variations, the formation of frost
is of limited occurrence. The time when frost melts should be ob-
served. In the morning, board piles are still all white long after the
well conducting earth has become dark.

152 THE CLIMATE NEAR THE GROUND

A pipe connecting parts of a heating system betrays its location
by a stain through the white covering of frost.

Several trees, with their root-balls of earth, were removed in
autumn from a lane in the Munich court garden. Uniform care of
the ground had soon covered up all traces of the filled holes. During
the following spring, however, A. Schmauss observed that after cold
nights the whole area of the former pits was white with hoarfrost.
The still loose earth in the pits had lower heat conductivity than the
surrounding older soil. Consequently, at night there was less heat
transfer from the lower layers of the ground and the surface cooled
off more.

Glaze is probably the most sensitive symptom of changing ground
conditions. It is recognized that glaze forms in two ways, either
through the solidification of super-cooled precipitation on the warm
ground, or through the freezing of rain drops (above o°) on the
very cold ground. Whoever walks the streets with his eyes open
when there is much glaze present cannot avoid astonishment, ques-
tions and research. Every street, every curb-side, every kind of
ground, every kind of stone has its own glaze formation. Houses
with central heating show an effect clear out to the sidewalk. Long-
filled excavations at the side of the street are plainly visible. Surface
roughness, the thickness and type of stone facings, the inclination
of the ground — everything shows up. Truly, if anyone wants to
take a hard test in microclimatology, let him take a walk when glaze
has formed, and answer all the questions that Nature propounds!

We may add, incidentally, that such things as accidents resulting
from glaze formation demand the attention of even the meteoro-
logical experts to these phenomena.

CHAPTER 15
THE AIR LAYER OVER WATER

While on land insolation and sky radiation are caught by the ground
surface, on water the radiation penetrates. To be sure, long-wave
heat radiation is almost entirely absorbed by the first centimeter of
water and even short waves are absorbed to a great extent. But
visible light, as every swimmer knows, can penetrate to quite a
depth. E. Sauberer (525) has made a classification of light relation-
ships in inland waters. G. Dietrich (j/o) has recently published a
complete description of such relationships in the ocean.

Ten to 40% of the total radiation from above penetrates to a
depth of i m, depending on the purity of the water. Thus the ab-
sorbed insolation ib distributed through a considerable vertical
extent, in contrast to what takes place in the ground. But of still
greater importance is the movement of the water. Under normal
conditions there is always convection taking place in water, just as
in air. It rapidly transmits to the lower layers the heat which is
absorbed at the surface. Consequently the diurnal and annual tem-
perature cycles make themselves felt more deeply in the water than
in the earth. We shall first recall these facts from macroclimatology,
which account for the difference between continental and maritime
climates.

From the standpoint of microclimatology, however, it must be
emphasized that besides the difference between land and water,
there must be considered the difference between water and water.
Conditions over the open sea are quite different from those over
a lake, a narrow river, or a pool. While the interest of macro-
climatology in water-surfaces is in proportion to their size, because
then all opposing factors can be so much more easily weighed,
microclimatology, here also, shows its love for the small. It is the
very small water surfaces which give us the key to many questions
of heat exchange in water and soil — questions which cannot be asked
of the sea. The plant world, too, has closer relations with small
waters than with great ones.

Just as it was necessary to touch the borders of geology in study-
ing microclimatology on land, so here too, we verge on oceanography
and limnology in orienting ourselves on the waters. But we shall

154 THE CLIMATE NEAR THE GROUND

mention only what is indispensable to an understanding of the air
adjacent to the water.

We have already spoken of the low albedo which characterizes a
water surface. It is about 9% for the visible spectrum and 5% for
the ultraviolet. Reflection is partially diffuse and partially direct.
It depends on the altitude of the sun. The lower the sun, so much
more effective is direct reflection from quiet water. Consequently the
albedo, in general, is a function of solar altitude. Though, as K.
Bihtner and E. Sutter (^07) have shown, for the ultraviolet alone
the albedo is practically independent of solar altitude. Their explana-
tion for this is that the greater part of the ultra-violet radiation falling
on a horizontal surface comes not from the sun, but from the sky
and hence is not from any particular direction. Perhaps the greater
ease with which the short waves are scattered has some bearing
on the question.

On the other hand a great increase of albedo with decreasing sun
height is to be observed for the total radiation from l/2 to 3 p. Fig.
73, taken from the work of Biittner and Sutter, shows this depend-
ence according to numerous measurements of the authors, made in
the North Sea. Besides the data for naturally moved water, figures
are also given on the same chart for wet and for dry sand, both of
which have significance for coastal climate. As we already know,
sand has a higher albedo than water, and dry sand higher than wet.
From the zenith position of the sun down to an altitude of about
40°, the reflectivity does not change greatly, but from there on it
climbs more and more steeply as the sun goes down. One hundred
per cent for o° altitude of the sun is the theoretical limit to which
the measurements approach.

This fact has a practical meaning for the beaches, for inland lakes
and the banks of the rivers. At steep vineyards the "underlighting"
reflected from the river can yield considerable additional radiation.
At the vineyards of the Steinberg near Wiirzburg, O. H. Volk (328^)
observed by means of a Lange photo cell on a sunny day in February,
i.e. at low altitude of the sun, an illumination by the sun of 16800,
by the sky (light from above) of 8800 and by reflection from the
river Main (light from below) of 16400 units (lux.). Five measure-
ments brought about the averages of 6520 for light from above and
4280 from below. "The best situations for a vineyard (according to
Volk) profit from this additional light. East and west of the Main
valley, we find slopes in a few kilometers distance of the river
which have equal exposure, inclination, geological substratum and

THE AIR LAYER OVER WATER 155

soil conditions which, therefore, have in no way characteristics dif-
ferent from the south — or west slopes of the Main-valley, and, de-
spite this, have no vine culture at all or yield only very mediocre
kinds of wine. Macroclimatic differences between Main- and Wern-
valley do not exist. I was unable to explain the difference in vine

7Wr

SO
70
\W

30
20
Iff

"S, *

Dry

sand

"~v —

- W

et sane

*"«-

-MOV

ing w

ater

10 W 30 10 SO SO W 60

• Solar altitude

FIG. 73. Dependence of reflection number for total radiation on the altitude of the
sun. (After K. Biittncr and E. Sutter)

cultures until I became aware of the differences concerning the
light from below and from above." Also with the wild growing
plants this difference was significant.

As for the lakes, there the west shores receive a noteworthy addi-
tional reflected radiation by the morning sun and the east shores by
the evening sun. In this connection, the preference for a westward
sloping shore is, as H. Frey (j/j) mentions, mostly self-delusion;
the western slopes are more often observed, because modern man is
not an early riser by choice and seldom sees the eastern slopes in
morning sunshine.

We shall now return to the temperatures in the surface layer of
water, which forms the boundary of the adjacent air.

156 THE CLIMATE NEAR THE GROUND

The recent data from the 1925-27 voyage of the German "Meteor,"
as worked out by E. Kuhlbrodt and J. Reger (316) showed a daily
temperature fluctuation of the ocean surface amounting to only
0.26 °C. Near the equator, where radiation is powerful, a maximum
of o.34°C was reached. There is therefore practically no difference
between day and night in the upper layer of sea water.

It is somewhat otherwise in the larger lakes. On the part of
meteorology, we have the studies of Lake Constance by E. Klein-
schmidt (j/5), and W. Peppier (320) and the work of V. Conrad
(308} and Wilh. Schmidt (328} on the Austrian Alpine lakes. The
temperature range in these lakes does not depend entirely, as in
the sea, on insolation and heat loss through outgoing radiation and
evaporation. There is a heat exchange between the water and air,
and between the water and the lake bottom. The air is partially
controlled by the temperature conditions of the surrounding land
with its more extreme diurnal and annual fluctuations. The effect
of the lake bottom is greatest along the shore and in shallow parts
of the lake and is extended up to the surface by means of convection
within the water.

The diurnal temperature range of the surface water is chiefly
governed by radiation. It is greatest at, or just before, the season
when the sun stands highest; it is least in winter. According to
measurements of F. M. Exner (311) in the Wolfgangsee, of W.
Peppier (321) in Lake Constance and of V. Conrad (308) in four
lakes of the Austrian Alps, the diurnal range in the surface water
during the summer amounts to from i to 2°C; in winter it is only
a tenth of that, or even less.

On a lake near Leipzig called the "Kirchenteich," which is i.i km
long, about 200 m wide and has an average depth of 2 m, J. Herzog
(314) carried out temperature measurements at seven depths from
i to 250 cm. The place of measurement was 90 m from the shore.
Fig. 74 shows the course of the temperature on a clear, almost calm
summer day (July 17, 1934). The abscissa is the time of day; the
ordinate, the depth of water. Along the upper edge are weather
notes. The generally horizontal course of the isotherms in the lower
part of the chart indicate the colder, deeper water, which is not
affected by the daily range. In the surface layer the fluctuation is
about 2°C, which is noticeably more than in the larger lakes.

W. Pichler (322) made a series of measurements near Leoben,
Obersteiermark, in a shallow pool of about 12 sq m area and 40 cm
maximum depth, lying at 650 m msl. At a depth of i cm (Aug.
1936) he found a daily temperature range of more than io°C. It

THE AIR LAYER OVER WATER 157

decreased with depth to about 4°C at a depth of 40 cm. The pool
was thickly filled with horse-tail rushes (Equisetum paludum)
which greatly diminished convection in the water. For this reason,
and not merely on account of the smaller lake basin, these pool
measurements differ from those in the Kirchenteich and more re-
semble those on dry land. A. Merz (318) observed, in the algae-
filled Pontelsee at Walkenried, even in cloudy weather, a daily
fluctuation of 11.8° in the surface water.

SW wind (Beauf. 2) Catm ,ncreasing wfnd

Cloudless Cu appears o

, 1934

FIG. 74. Course of diurnal temperature in a small lake. (After observations by

J. Herzog)

A glance at Fig. 75 will show how great is the similarity to tem-
perature conditions in the ground. The temperature condition curves
are for a fine summer day and a pool 40 cm deep. From the water
surface down to a depth of about 30 cm, the lines run very similarly
to those for the solid earth as shown in Fig. 15. The solid lines in
Fig. 75 indicate warming from above in the course of the forenoon;
toward midday they are bowed increasingly to the right. About 4
P.M. the first cooling at the water surface is indicated by the cur-
vature of the tautochrone toward the left. The broken line curves
which correspond to the afternoon and night, indicate the cooling
off upward of the water layer near the surface.

There are two differences between Fig. 75 and Fig. 15. One is
that the decrease of the daily range in water with increasing depth
is much less than in the ground. We have only to compare the
close crowding of lines at 30 cm depth in Fig. 15 with the spread at
the same depth in Fig. 75. The cooler surface layer is evidently very
shallow. Both are results of convection, which, in spite of the

i58

THE CLIMATE NEAR THE GROUND

braking action of water plants, is still active. It apparently increases
the heat conductivity noticeably and at night permits immediate
sinking of the colder, heavier water from the surface. Consequently
from midnight to 7 A.M. there appears in the water a relatively thick
isothermal layer below the surface, its thickness increasing until sun-
rise. The bend of the tautochrones to the left is confined to the first
centimeter below the surface.

FIG. 75. Curves of the temperature condition on a clear summer day in a 40 cm. deep,
pool of water in which horse tail rushes were growing. (After W. Pichler)

Fig. 75, on the other hand, shows an irregularity of the curve
below a depth of 30 cm, which is quite absent in that for the ground.
The lines are inclined above the bottom of the pool — mostly toward
the right, but in the evening toward the left also. This indicates
that the water is receiving heat from the bottom except in the eve-
ning, when the reverse is true. Since the maximum bending toward
the right occurs about midday (2 P.M.) it is W. Pichler 's opinion
that the phenomenon results from a warming up of the bottom by
direct radiation which penetrates the water. In the case of such
shallow water, so filled with plant growth, we must also consider
heat conduction by way of the more highly heated ground along the
edges of the pool. This seems the more probable, since the maxi-
mum temperature at the bottom is not reached till 4 P.M.

With different depths of water, different subsoils and different
plant growth, these measurements might have shown different re-
sults. They are well suited, however, to demonstrate the fact that

THE AIR LAYER OVER WATER 159

as water surfaces diminish in size the water temperatures are likely
to approach those of dry land while still retaining their peculiar
characteristics.

From a consideration of temperatures in the upper layers of the
water let us now proceed to those of the adjacent air.

On the open sea the temperature difference between water and air
depends on the origin of the air lying above the water. If it has
come from a region lying poleward from the place of observation,
the air is, in general, colder than the water. If it comes from nearer
the equator, it is, in general, warmer. That is why, in the weather
service, the temperature difference between water and air is used
as an indication of air-mass origins.

The air adjacent to the water has to span the transition. In this
equalization of advectively occasioned contrasts the significance of
the air next to the water degenerates, for the influence of radiation
processes upon it is hardly worth mentioning. It is not so long
since we thought that, judging by the appreciable daily range of
air temperature over the ocean, direct absorption of insolation by
the air was important. But E. Kuhlbrodt and J. Reger (316) showed
in the "Meteor" observations, that this apparent daily fluctuation of
air temperature was due to the method of observing temperatures on
board ship. Actually, it amounts to only 0.3 to o.5°C which is
scarcely more than that of the surface water.

On the open sea the air next to the water is of special character
on account of the motion of the waves. The boundary surface be-
tween air and water is in motion; the lowest air is increasingly
mingled with spray as the waves rise. When, in a heavy sea, the
foam is beaten from their crests and thrown like mist over the sur-
face of the water, when the horizon disappears and the air is filled
with spume, then there is no more "air adjacent to the water" in a
microclimatological sense. Water and air are at battle together.
Consequently it is only under special weather conditions that it is
possible to take measurements in the air layer next to the water.

In the Baltic, particularly the Bay of Mecklenburg, G. Wiist
(^29) has made temperature and humidity measurements with
an Assmann aspiration psychrometer from the little, flat, ship's boat
of a schooner. The result of the 26 series of observations for the first
2 m of air above the water is given in Fig. 76. The upper portion
shows the temperatures; the lower portion, the vapor pressures —
separately for the 17 series in which the water was warmer than the
air, and for the 9 series in which the water was colder, the latter

i6o

THE CLIMATE NEAR THE GROUND

chiefly in the midday hours. There are no evidences of definitely in-
and out-going diurnal types of radiation in either diagram, depend-
ent on the time of day.

Up to 20 cm the two temperature curves are practically opposites.
From 20 to 50 cm the air temperature increases, slowly at first,
then suddenly faster. At zero height in Fig. 76 is the tempera-

""j 1

v^-_.

J^I'M^ series)

v-isSr^s^

i?5^^^^^^^^!?5^5^5^^5^^5^

?^^55^^f?f^fe5^^^^5^^^^5!^

1 1 1 l 1 1 1 1 1

^5^^^^5^5^^/y5^^^

1 1 i 1 11

f/> 145 /.

?^ «J /

S^ /*J f •

series)

o o Water colder than air
(9 series)

50 ~

no 11.0 uo 149

FIG. 76. Course of temperature and vapor pressure in the layer of air directly over
the water — in the Baltic. (After G. Wiist)

ture of the ocean surface, which in most cases is warmer than the
air at 20 cm, but in other cases is cooler. G. Wiist draws this con-
clusion: "Normally the temperature stratification close above the
water of the open ocean represents a condition of unstable equi-
librium." Perhaps later observations will show to what extent the
summer season (the measurements were made between the i4th
and i9th of Sept. 1919) and the nearness of land are responsible for
this.

In the lower half of Fig. 76 the saturation vapor pressure corre-
sponding to the temperature and salinity of the ocean surface is taken

THE AIR LAYER OVER WATER l6l

for zero height. The course of both curves corresponds to the normal
wet type, which we have already found for a solid surface which is
giving off water vapor. (See Fig. 44.)

W. Findeisen (312) has demonstrated the probability, on the
basis of observations of strip like wave formation on the thin water
layer of the Neuwerk tidal flats, that the boundary layer of air
lying on the water is partially laminar and partially turbulent.
Water strips with a smooth surface correspond to laminar bound-
ary layers; those with a wavy surface, to turbulent layers.

The investigations of W. Peppier (321) on the temperature of
the air and the water on Lake Constance belong in the realm of
macroclimatology, so we shall only mention them here. R. Mar-
quardt (3/7) has studied heat and water convection over a water
surface from ship and shore observations on Lake Constance.

In conclusion we shall give some temperature measurements of
Wilh. Schmidt (327) which he made on the shores of the Lunz
lakes, according to the observational procedure which he has given
for ground temperatures (279). Those listed in Fig. 77 were all
made on Nov. 13, 1926 — a perfectly calm, warm and sunny autumn
day. The temperature is taken as abscissa, with the 10° line solid
and single degrees indicated by short strokes. The depth below the
surface is taken as ordinate. The position of the measuring points
is given at the right-hand margin of the chart.

Letters a through d correspond to the successive times of day —
a being at 11:12 A.M., a time of strong insolation, while d was at
4:27 P.M. The upper series (al through d±) refers to measurements
in a shallow bay in which the water on this calm day was abso-
lutely motionless. Under the influence of insolation (a^) the tem-
perature maximum does not occur at the water surface, but several
millimeters beneath. Wilh. Schmidt explains this as the combined
effect of heat radiation penetrating the water, and evaporation cool-
ing the surface. Part of the radiation penetrates as far as the lake
bottom, 20 cm down, and there induces a secondary temperature
maximum. This has already appeared in Fig. 75, and has come up
under different, but similar circumstances in the air, in Fig. 9 for
instance.

With decreasing radiation (b± — d±) the temperature contrasts in
the water diminish. The surface cools more and more, while the
air above (d^) cools still more under the influence of the neighbor-
ing land. (Compare d± with d^)

In the lower row there are given the simultaneous observations at
three locations not far apart. Position II (heavy line) was in a pool

162

THE CLIMATE NEAR THE GROUND

which, a few weeks previously, had been dry. The mud was still
soft and impassable. Line a<2 on both sides of the ground surface,
shows a finely developed incoming type of radiation such as char-

acterizes solid ground. At a depth of 5 cm there is still a recog-
nizable minimum, due perhaps to the effect of the wet ground
(evaporation!) and of the preceding weather. A comparison of a2
with #! shows how slight are temperature contrasts in water as com-

THE AIR LAYER OVER WATER 163

pared with dry land, and how completely different is the simul-
taneous temperature distribution in the first few centimeters of air
above water from that at a similar height above land.

As the evening cooling process sets in, there occurs at Position II
a temperature minimum at the radiating surface, which is at first
a secondary (<r2) but soon becomes a chief, minimum (d2).

The dotted lines in Fig. 77 belong to Position III which was lo-
cated on a dry shelf , for the most part devoid of vegetation. Trie
air near the ground at this point is somewhat cooler around noon
(£2) on account of the influence of plants near the ground surface
(shading), yet at a higher level somewhat warmer (absorption of
insolation) than above soil without vegetation (Position II). In the
evening, however, (d2) there is a decided cooling of the shelf and
the cold air sinks between the grass blades till it is close to the
surface.

The fine solid lines refer to Position IV which was near the lake
shore on a shelf about 10 cm below the surface of the water. The
temperature lines do not correspond to those in the open water
(upper series in Fig. 77), but to those for dry land (II and III). The
water held by the shelf loses its mobility; the shelf protects the water
surface from radiation and becomes itself a medium of radiation
exchange. It is all the same whether the foundation of the shelf is
solid ground, mud or quiet water. Thus the temperatures in the
uppermost water layer and in the lowest air layers merge, without
noticeable discontinuity, into temperature conditions as we already
know them to exist in and above the land.

CHAPTER 16
THE AIR LAYER NEAR SNOW

We have numerous measurements on the albedo of a snow surface,
such as those of A. Angstrom, C. Dorno, P. Gotz, N. N. Kalitin, H.
Lunelund, H. Olsson, F. Sauberer, and Ch. Thams. F. Sauberer
(^50) has published a recent compilation of results. The value for
new snow ranges between 75 and 88%. P. Gotz (334) obtained
100% several times during winter measurements in Arosa (1800 m
msl). For old and wet snow the value goes down to about 43%.
Plainly the figures for new snow are quite uniform within the spec-
tral range from 0.35 through 2.5 ft. 80 to 85 was found in the
ultraviolet, which is of the same order. But, as G. Falckenberg
(269) showed, in the infrared around 10 /*, the snow is "black" —
that is, it absorbs all the heat radiation which strikes it. According
to Kirchhoff's law, this results in snow being an exceptionally good
radiator for long-wave, nocturnal heat radiation.

The properties of the snow surface consequently have a like effect
by both day and night on the heat balance of the snow cover. By
day the insolation is to a large extent reflected, so that the snow can
absorb little heat. By night, on the other hand, it radiates outward
strongly, which lowers the temperature. This is where the high
insulating power of the snow cover comes into play, as witnessed
already by the table of heat conductivity of various types of ground
cover. The nightly transfer of heat upward from the ground is
thereby regulated and any storage of the day's heat worth mention-
ing is rendered impossible.

It is otherwise with the air above the snow.

Solar radiation which is reflected from the surface of snow returns
into the atmosphere. Part comes back again to the snow surface,
especially when a high degree of cloudiness favors reflection. The
process is repeated, with the resulting high radiation readings which
are obtained from measuring apparatus in the presence of a snow
cover. Since they occur in the visible portion of the spectrum, we are
accustomed to speak of a favorable "light climate" above the snow.
A. Angstrom (jj/) calculated that insolation with an original value
of i increases to: —

THE AIR LAYER NEAR SNOW 165

1.02 through reflection from a snow-free ground under a clear

sky,

i. 08 under similar conditions, except a cloudy sky,
i. 2 1 with a snow cover and clear sky,
2.10 with a snow cover and cloudy sky.

He took only 70% as the albedo for a snow cover. Actually ob-
served radiation measurements substantiate these calculations. See
further literature cited by F. Lindholm (342) .

Air just above snow is therefore, subject to great contrasts. The
heat balance of snow may be considered unfavorable. We found in
Fig. 68, as we surveyed the different kinds of ground cover, that
snow was in next to last place, it could absorb so little heat. The
adjacent air is consequently influenced from underneath by very
low temperatures. Nevertheless, incoming radiation is exceptionally
great. Before we get into a discussion of this paradox we need to
know even more about the snow cover — first of all as to its per-
meability by radiation.

Solar radiation and diffuse sky radiation can penetrate snow just
as they do water. According to H. Olsson (364) : — If /0 represents
the radiation penetrating the snow surface, and dl the depth of
snow in centimeters, then the radiation /, which reaches the depth J,
has the value, / = J0.e~kd.

This simple absorption law, mentioned by F. Sauberer (350) is
strictly applicable only for an optically homogeneous substance, and
hence is only approximately accurate in respect to snow. Further-
more, in practical measurements of nature, penetrating radiation,
exclusive of the direct solar rays, there is always included the sky
radiation, which has a complicated and continuously changing dis-
tribution over the face of the heavens. Nevertheless, the absorption
coefficients ^ which have been derived, according to this law, from
the observations are on the whole quite consistent.

Measurements with a photo-electric cell gave H. Olsson a value of
^ = 0.074, while a pyranometer gave ^ = 0.114. The cell is sensi-
tive to a spectral range of from 0.3 to 0.7 ja, the pyranometer, from
0.3 to 4 ft. The difference is this: since the long waves are absorbed
by the snow, an instrument which is sensitive to longer wave lengths
will give a higher ^ value. Ch. Thams (352) found 0.083. F. Sau-
berer showed, for the range between 0.38 and 0.76 /A, that pene-
trability on the average was not closely related to the wave length
of the penetrating radiation, and that individual values varied widely.
For example he once found, under 7.5 cm of wet, fresh snow, more

i66

THE CLIMATE NEAR THE GROUND

blue and violet radiation than at another time beneath 3.5 cm of
drier snow. His mean ^ value is higher (0.150). Pyranometer
observations of N. N. Kalitin (338) gave doubtfully high /(-values.
Summarizing all available measurements, we may assume that
the value of the absorption coefficient \ is somewhere between 0.07
and 0.12. Fig. 78 shows, for both of these extreme values, how much
radiation gets through to a given snow depth d. These percentages
do not refer to the amount of radiation striking the surface of the
snow, for the greater part of this is lost by reflection from the sur-

Percent of transmitted radiation
nO 20 W 60 SO 100

'k- a

FIG. 78. Transmissivity of snow cover for radiation

face. The value 100 means, rather, the radiation which gets through
the surface. Of this amount, as Fig. 78 shows, up to 50% reaches
a depth of 10 cm, and (with ^ = 0.07) 10% reaches a depth of 30
cm. These are considerable amounts of radiation.

Solid objects such as twigs or stones lying in the snow may, under
the influence of this penetrating radiation, attain a temperature
above o° and thus cause the overlying snow to melt from below. As
the snow disappears on a sunny spring day it is common for stones
and plants first to become visible in depressions and even cavities
in the snow cover. We shall return to this later.

We still know far too little about snow's permeability to air. I
can mention only the laboratory experiments in its determination,
by O. Gabran (333), according to which it is equal to that of an
equal thickness of splinter-free sawdust. Air permeability is of great

THE AIR LAYER NEAR SNOW 167

importance for the wintering of plants under the snow. The rotting
of winter grain, for instance, is not a question of insufficient light
but mostly a lack of air. If the snow glazes over or if several layers
of ice form within it, its permeability to air is much impaired. No
figures on this are at hand, however.

Alterations with and in the existing snow cover stand in close
relationship to microclimatic processes. Whoever is interested in
this subject should read the comprehensive and very interestingly
written book of W. Paulcke (^7).

Temperature measurements within the snow and on its surface
are nowadays usually attempted only with electrical thermometers
in order to avoid radiation errors and melting due to heating up of
the instruments. In this way it is possible, by distant readings, to
keep the field of measurement untrodden and untouched.

Recent measurements of snow temperatures we can credit to J.
Keranen (8j, JJ9), E. Niederdorfer (380), and L. Herr (80) as well
as O. Eckel and Ch. Thams (332). Fig. 79 gives an excerpt from the
last mentioned work — the course of the isotherms in the snow at
Davos during the winter of 1937-38. The measurements were made
at 8 A.M. The upper boundary curve gives the depth of the snow
as a function of time. The ordinate scale is in meters; at the middle
of January, therefore, a snow depth of 65 cm was reached. In gen-
eral the course of the isotherms is similar to that in the earth, as a
comparison with Fig. 14 will show. Yet snow has some distinctive
characteristics when considered as "ground."

In the first place its poor heat conductivity results in a crowding
of the isotherms near the surface. (Fig. 79 gives them for unequal
intervals — every 4° near the surface!) Deep within the snow, its
temperature is only slightly below freezing, even when the air tem-
perature may be as low as —33°. This illustrates how great pro-
tection is afforded seeds by a winter snow cover. Heat waves pene-
trate the snow considerably faster than do cold waves, for while
the latter are transmitted only by true heat conduction, the former
have the benefit of a pseudo-conduction through infiltrating water
from melting.

Fig. 79 gives no indication of the daily march of temperatures
within the snow. For this we must turn to Fig. 80 which is taken
from the observations of E. Niederdorfer (380) at Eisenkappel
(Karnten) on Jan. 16, 1932. For a snow thickness of 20 cm he
derived the arrangement of tautochrones there shown, based on
measurements of the heat balance and temperature.

Radiant heat penetrates the snow deeply, for at a depth of 20 cm

i68

THE CLIMATE NEAR THE GROUND

/S 10. 15. J/. S tO. tS

December January

FIG. 79. Course of the isotherms in the winter snow cover in Davos. (After Eckel

and Thams)

Temperature

FIG. 80. Tautochrones of the snow cover temperature. (After E. Niederdorfer)

THE AIR LAYER NEAR SNOW 169

there occurs a temperature rise of 2.5° during the forenoon. The
nocturnal type of radiation outward which is well represented by
the 9*45 curve, is replaced by the incoming type. The temperature
maximum does not occur at the surface of the snow, however, but
at a depth of i cm. The cooling effect of evaporation is present on
the surface. We have the same set of circumstances which we have
previously described as applying to air above the water. The type
of temperature distribution designated in Fig. 77 as a^ corresponds
near the surface exactly to the midday tautochrones shown here in
Fig. 80. In the case of the snow we have here the added effect, that
the long-wave heat radiation of the snow takes place from only a very
thin surface layer, while the short-wave incoming radiation pene-
trates into the snow. E. Niederdorfer suggests, on this point, that
a snow distribution corresponding to this temperature stratification
is often met with — namely 2 or 3 cm of powdery snow lying over
wet snow.

From the temperatures of the snow itself we now return to tem-
peratures in the air lying just above it.

In the first 25 mm above the upper surface of the snow, A. Nyberg
(345) made careful and enlightening measurements at Upsala, em-
ploying electric resistance thermometers like those of F. Albrecht
(757). Even in this thin layer the stratified structure of the air
overlying the snow stands out clearly as can be seen in the charts
which Nyberg has published. At night the outgoing type of radia-
tion was well developed. Averaging numerous observations, he
arrived at the following relation of temperature stratification to wind
velocity:

TABLE 28

Height in

mm above

At a height

Wind Number of

the snow surface

of 1 40 cm

(m/sec)

tions

polated)

Full calm

-17-6

-17.0

-16.4

-16.1

-15.9

-15.7

— 12.1

0.3—0.6

-II.5

-10.7

— 10.1

- 9-8

- 9-4

- 9.2

~ 6.7

0.9-1.2

~ 9-3

~ 8.7

- 8.4

- 8.2

- 8.1

- 8.0

- 6.4

1.8

— 4.1

- 3-7

- 3-5

- 3.4

- -3-3

- 3-3

- 2.7

These figures demonstrate beautifully both the decrease of the tem-
perature gradient and the increase of temperature as the wind velocity
picks up. The temperature variation with height could be repre-
sented very satisfactorily by an exponential function.
P. Michaelis (j^j and j^) has, from a botanical standpoint, made

170

THE CLIMATE NEAR THE GROUND

a thorough study of the air layer adjacent to snow in the mountains.
R. Geiger has made measurements at Munich with thermometers as
test bodies, of which we have already spoken and of which more
will be said later. Fig. 81 shows three examples which illustrate the
most important processes.

The upper record of Jan. 9, 1935 was made as snow was beginning
to fall. At about 2 A.M. all temperature differences in the air just
above the snow have disappeared under the influence of cloudy
weather. The stem thermometer lying on the ground is covered
with snow in the succeeding hours. In the air adjacent to the snow
the temperature is slightly retrogressive for the snowfall produces
cooling, the thermometers become moist and lose heat by evapora-
tion. The protection of the snow cover immediately makes itself
felt in the thermometer. The snow insulates it against the action
taking place in the lower air layer; outgoing radiation ceases and
ground heat from below becomes effective. Five hours after the
beginning of snowfall, the thermometer on the ground is already
5°C warmer than the one in the air just above the snow. The old
rule: "Snow saves the seeds" can be read directly from the record.

The second record in Fig. 81 is an example of conditions in freezing

10 //

Time of day

FIG. 81. Temperature recordings in the air layer over the snow at beginning of snow-
fall (above), during frost-weather (middle) and during thawing weather (below)

THE AIR LAYER NEAR SNOW 171

winter weather. On Jan. 20, 1935 the depth of snow cover amounted
to 9 cm. In snow the temperature fluctuations are very slight; the
diurnal march appears in the rise of the heavy line till 2 P.M., with
subsequent decline. In the air near the snow, however, there is great
temperature unrest, which is familiar to us from our acquaintance
with the air layers just above the ground. The greatest fluctuations
occur, not at the surface of the snow (for there air friction is too
great) but directly above. A. Nyberg (345) found i cm; M. Frans-

sila (377) > 5 cm-
In the course of the forenoon, incoming radiation prevails, though

irregularly and not very clearly. After the weather clears up, at
about 4 P.M., outgoing radiation is quite evident. The thermometer
at a height of 10 cm cools to —20°; that at 2 m, only to — 10°.

The third record in Fig. 81 represents a thawing snow cover 6 cm
deep, on Feb. 12, 1935. The temperature throughout the snow is o°.
The thermometer lying on the ground records a few tenths of a de-
gree above zero at times, because it is absorbing the penetrating
radiation. The daily course of the temperatures above the snow pro-
ceeds very irregularly. At noon the temperature of the stem ther-
mometer a few centimeters above the snow rises to +io°C. In the
meteorological shelter the air temperature reached only +o.9°C.
If the ground beneath the snow is still frozen, vegetation is in
great lack of water, for the movement of water from below is hin-
dered by the grip of winter. In the air near the snow, however, the
evaporation requirements of spring are ushered in by high plant
temperatures.

Toward evening the temperatures again drop below freezing, and
cut the line of the snow temperature at a sharp angle. Incoming
radiation, which is particularly evident between 2 and 3 P.M., gives
way to the outgoing type.

From observations at the observatory of the Air Weather Service
in Munich during the winter of 1934-35, tne average distribution
ranges of air temperatures above the snow have been worked out by
R. Geiger (535) as shown in Fig. 82. There are three different
groups of observations: in the upper portion of the chart, days with
freezing weather and an old snow cover (22 days); in the middle
portion, days with air temperature prevailingly above o° and an old
snow cover (9 days) ; and below, days on which remnants of a snow
cover still lay on the ground (7 days).

The shaded columns show, in relation to height above ground
(ordinates) the temperature province within which the tempera-
tures of the thermometer varied. The left-hand column refers to

172

THE CLIMATE NEAR THE GROUND

the hours from 5 P.M. to 7 A.M.; the right-hand column to those
between 9 A.M. and 2 P.M. The temperature scale is at the bottom;
the frost line is especially marked by the vertical line in each
of the three sections of the chart. The thickness of the snow cover
is indicated by the dotted areas. The positions at which measure-
ments were taken are shown by small circle?.

The two small stars above each portion of the chart give the aver-
age true air temperature in the meteorological shelter for the same

Snow cover

-8 -6 -4 -2 0 2 4

Temperature degrees in Celsius

FIG. 82. Range of temperature distribution during the day (right) and night (left) in
the air space near snow

periods of time in order to facilitate comparison with the macro-
climate. The two points below the ground surface correspond to
simultaneous ground temperatures at a depth of i cm.

For all three parts of the sketch, the common characteristics of the
climate close to the snow are these: —

1. In contrast to temperatures within the shelter the tempera-
ture picture of the air near the snow is one of extremes, particularly
so at the snow surface.

2. The temperature scattering (indicated by breadth of the
shaded areas) is greater by day than by night, and as we leave the
surface of the snow, it decreases upward slowly, but downward very
rapidly.

3. In the air close to the snow, incoming radiation predominates
by day and outgoing by night, just as over earth.

THE AIR LAYER NEAR SNOW 173

The lower part of Fig. 82 especially is practically the same as for
bare ground. The diurnal surface maximum is very pronounced.
Here, for the first time, positive temperatures appear at a depth of
i cm within the ground. It is a situation favorable to snow~smotyng
according to F. Rossmann (348) . If the amount of bare surface con-
siderably exceeds the amount of snow surface remaining in isolated
banks, if temperature and humidity are high, and if the wind is very
light, a very fine fog may be seen at times over the snow banks. The
veil of fog forms on the windward side of the snow surface and
dissolves not far beyond the lee edge. On May 26, 1931, F. Ross-
mann succeeded in apprehending the conditions of this micro-
climatological process on the summit of the Feldberg in the Black
Forest, using the Assmann aspiration psychrometer. To windward
of the snow bank he observed, as the average of several series of
measurements, i8.i°C and 82% humidity; in the lee, 15.2° and
89%. The phenomenon arises therefore from a cooling against the
snow bank of the warm air current close to the ground.

Now let us return to Fig. 82 and consider conditions during freez-
ing weather. Imagine the conditions facing a young plant that looks
out over the snow. Its foot is in the province of winter rest and
protected heat. The part which extends up to the top of the snow
is exposed to the sharpest radiation frost. A few millimeters higher
is full insolation and the strong reflected radiation of the snow sur-
face on stem and branches. Then there is the wind to consider, to
which is added drifting snow at times. All this is against plant
parts which have not been accustomed throughout their growth to
the demands of a microclimate close to the surface but, on the con-
trary, have been unexpectedly subjected to them by the accidental
height of the snow. In Fig. 55 we saw evidence of the consequences.

Through the heating action of those parts of the plant which
appear through the snow, the snow which lies against them is
melted away. This induces the formation of a cavity melted out
around every stem, twig and blade of grass, in which the plant
stands as in a funnel. This funnel usually extends further on the
sunny side than on the shady side.

This process is diagrammatically shown at the left in Fig. 83.
At the right we have another frequently observed phenomenon of
melting. Suppose there has been fresh snowfall during the night.
(Sketch i). The snow heaps up somewhat about a blade of grass.
The next day let us suppose the temperature of the snow is such that,
under the influence of insolation, it just begins to melt on the slope
facing the sun. The following night the water which has formed

'74

THE CLIMATE NEAR THE GROUND

at the surface freezes into a thin sheet of ice (Sketch 2). On the
next day this is penetrated by the insolation almost perfectly. As
proof of this, F. Sauberer (550) found, behind a 25 mm sheet of ice,
84 to 87% of the radiation falling on its face. The little ice plates

FIG. 83. Melt-craters and ice-sheet formation at the thawing surface of snow.

to which we refer are, however, only some i to 3 mm thick. They
melt not at all or very slowly and remain a long time on the chang-
ing and settling snow cover. When they reach the points of grass
or other plants the ice plate is left there while the transmitted in-
solation melts away the snow from the grass beneath. There often
result great "glass-covered" cavities in which high temperatures
must prevail (Sketch 3). Sometimes the ice plates remain for a
while after the snow has all melted away (Sketch 4). On stubble
fields whose uniform rows give rise to a series of ice plates on the
south side, one can see rank after rank of such ice plates. On account
of their permeability to radiation they last a long time in freezing
weather, melt where they lie and crumble with time.

CHAPTER 17
THE AIR LAYER ABOVE A SOD COVER

How relationships in the air layer near the ground vary in the
presence of a plant cover and how the interaction proceeds between
the microclimate and the living plants will be the subject of the
sixth section. Here we shall speak of plants only insofar as they
alter the nature of the ground surface. In this case the plant cover
causes no changes within the lower air, but this air layer as a whole
is affected by the living ground-cover just as the air above sand
possesses different properties from that over rock or over an asphalt
street. Consequently we shall have much to say about this ground-
cover in this fourth section, which treats of the influence of the sub-
stratum on the microclimate.

Observations in the air adjacent to the ground are frequently
carried out over a clipped turf, for this kind of "ground" can not
only be kept uniform and level without much care, but, best of all,
it is not altered by rain and storm. Most civilized countries make
use of observations with a "grass-minimum" thermometer at 5 cm
above the ground, in order to verify microclimatological night tem-
peratures in the macroclimatic network. The international commis-
sion for agricultural meteorology has recommended for observations
in the lower air the following standard procedure (23) : "There
should be a uniform ground cover within a distance of at least 15 m
about the installation. In climates where it is possible, the ground
cover should be a uniform sod." The previously mentioned measure-
ments of N. K. Johnson and A. C. Best were made under such con-
ditions. Under these circumstances a living ground cover is of par-
ticular, practical significance in microclimatology.

Now, what changes does the ground surface undergo when it is
overgrown with plants? Back in Chapter 13 we mentioned the
change in albedo of the surface of the ground. Of much greater
importance for the heat economy is the fact that even most plant
growth greatly alters the form of the surface. Short blades of grass
or the leaves of very small plants, even if only a few millimeters
high, capture a portion of the insolation and shade a corresponding
part of the soil surface. An absorption layer of several millimeters
in thickness is thus created, in place of the infinitely thin absorption

176 THE CLIMATE NEAR THE GROUND

surface which bare ground presents. This prevents the occurrence
of such harmfully high maximum temperatures around midday.

E. Leick and G. Propp (^62) investigated the reciprocal relations
between ground temperatures and plant growth at the biological
research station on the Hiddensee Island. They say: "From measure-
ments in very different localities it appears that a ground cover of
vegetation exerts a strong influence on the heat characteristics of the
substratum. Even the most scanty plant growth can considerably
modify the extremes of bare ground." On May 28, 1928 at 4 P.M.,
for example, the authors found at different points a few decimeters
apart (but all lying on a steep coastal cliff exposed to the afternoon
sun) the following ground temperatures at a depth of 2 cm : —

Under bare, loamy sand , 25.o°C

At a place partially overgrown with moss and grass . . 23.6 °C
Under a thick turf i2.3°C

The true air temperature was 13.1 °C. It may be assumed that the
temperature difference which at 2 cm depth amounted to i2.j°C
within a very small area, was much greater still at the surface.

Young plants, such as pine seedlings or beans, growing in par-
ticularly hot places sometimes burn off where they come through
the soil, and die. E. Munch (363, j6^) called this phenomenon
the "foot-ring disease." Careful notice discloses that the deadly
burn does not occur at the surface of the ground but several milli-
meters higher up. The plant conducts heat relatively well and the
ascending sap is as cool as the deep ground from which it rises.

E. Rouschal (356^) measured the temperature of the transpiration
current thermo-electrically in old trees of the "Forstgarten" (fores-
try experimental station) at Tharandt near Dresden. The cooling
effect of this current could be authenticated up to 3 meter height
above the ground. With the foliaceous trees the pores of which
are ringlike and easily passed through by the sap current, the effect
was three to five times greater than with the coniferous trees and
those foliage trees with scattered pores. He found e.g. at the root-
neck of a chestnut tree a difference of i5°C between the conducting
sappy wood and the non-conducting sun heated wood; even at one
meter height still more than 3°C.

The relationships here are the same as in the measurements which
K. R. Ramanathan (274) determined at Agra, on railroad rails set
vertically in the ground. The following table shows the observed
results on two selected days : —

THE AIR LAYER ABOVE A SOD COVER 177

TABLE 29
TEMPERATURES ON A VERTICALLY PLACED RAILROAD RAIL AT AGRA

Oct. 25, 1926 Feb. 5, 1928

Height above 2:45 P.M. 2:10 P.M.

the ground (clear weather) (after a rainy night)

305 cm 26.5

183 cm 38.8 27.0

122 cm 39.4 26.7

61 cm 40.0 27.9

Rail on the ground 39.3 26.2

Ground surface 47.1 25.7

In the shelter (Macroclimate) 31.1 20.5

The highest temperatures were measured, not at the surface of the
ground but at the first measuring point above it. The great height
of 6 1 cm is occasioned by the excellent heat conductivity of the iron
— probably by the accidental choice of a measuring place. The dis-
placement of the maximum is practically the same as in the case of
living plants.

When the plants have once covered the ground in mutual con-
tact— when, as the forester is accustomed to say, they have "closed
in," the foot-ring disease is no longer possible. For in place of the
heat-absorbing ground surface we now have the heat-absorbing
layer of vegetation covering the ground.

Besides the heat economy, the water economy of both the soil and
the air next to it are altered by the living ground cover. It is almost
superfluous to say that air humidity over living plants is higher than
above sterile ground, for every plant must breathe in order to live.
The observations of D. Szymkiewicz (2/5), given in Chapter 10,
demonstrated this by the steep humidity gradient found in the air
near the ground. Further data appear in Chapter 28.

Here, where we are concerned with a sod cover as a surface prop-
erty of the ground, we shall first point out that the water economy
of a bare soil is radically different from that of one covered with
short turf. We have proof of this through measurements of water
economy under natural conditions, which were made at a lysimeter
site in Eberswalde. J. Bartels and W. Friedrich (557) laid out this
installation at the Meteorological Institute of the Forestry College.
Boxes il/2 cubic meters in volume were sunk flush with the ground,
resting on scales by means of which they could be counterbalanced
up to a weight of 100 g — corresponding to a precipitation depth
of o.i mm. Since the precipitation on the surface and the penetra-

178 THE CLIMATE NEAR THE GROUND

tion of the water to the deeper soil layers were to be observed directly,
conclusions could be drawn from the change of weight, as to actual
evaporation or dew-fall.

J. Bartels (355) worked out the results of comparative measure-
ments, over a three-year period, of bare sand as against a close-cut
sod surface. Evaporation from a Wild cup — i.e. an open water
surface — placed in the meteorological shelter, was used as a stand-
ard for comparison. For the months of May-August, 1930-32, he
found the average evaporation in mm to be: —

TABLE 30

From sand
surface

From sod
surface

From water
surface

On days after rain

2.38

2.80

2.24

On clear days

0.47

2. IS

3.61

On drought days

0.26

1. 14

3.80

The drier the weather, the more the sandy soil reduced its output
of water, while the open water surface gave up all the more water.
The drier it was, the more difficult it was for the sod to get water
from the subsoil. It resembled the sandy ground in that its evapora-
tion decreased with increased dryness. But its evaporation was al-
ways more than that of the sandy ground — as much as four to
five fold, in times of drought, "The oft-repeated observation," says
J. Bartels, "that the bare ground in every respect suffers by com-
parison with that which is covered with vegetation, is completely
refuted by our data, in respect to water content."

The average yearly evaporation from the sod surface was 189 mm
more than from the sandy surface. This excess equaled 28% of
the annual precipitation. These 28% were consequently extracted
from the soil by the growth of the ground cover. This amount of
189 mm was actually greater than the total annual evaporation from
the sand surface. During the growing season — April through Sep-
tember— the excess amounted to 39%.

The relationships of temperature to atmospheric humidity in the
air layer near the ground — over sterile, as compared with living,
ground — may be readily understood from the sampling tests of
W. Knochenhauer (j6/) at the Hannover airport. One evening
when the wind was still and a light dew was forming, he made some
measurements with an aspiration psychrometer over both the run-

THE AIR LAYER ABOVE A SOD COVER

179

way at the airport and the adjacent sod, at four distances above the
ground. His results are given in Fig. 84.

w^M^)MM^mmm®**. -

Distance from building

FIG. 84. Influence of runways and grass cover on the air layers near the ground on
an airfield. (After W. Knochenhauer)

The upper half of the sketch shows the distribution of relative
humidity; the lower half, that of the air temperature. The under-
most air layer, 1.5 m in height, is in both cases shown in cross-section,
from a building at the extreme left, across the field (which is about
90 m long) and on to a distance of some 300 meters. The observa-
tions were made between 10 and n P.M., when the microclimatic
differences appear most clearly.

Looking at the sketch as a whole, the first thing to attract atten-
tion is that the lines of equal temperature and equal humidity run
vertically rather than horizontally. The fact seems to be that it is
warm and dry in the neighborhood of the building, but cool and
moist out over the sod.

In the air closest to the ground, the iso-lines bend over to a hori-
zontal position so that near the ground the contrast with conditions
at a greater height is increased. Over the sod there is a cool, moist
air layer; over the concrete, a warm, dry one. The influence of the
latter is greater; for we have to go some 130 m from the building
before the two conditions balance, with resultant vertical lines of
equal temperature and moisture. About i m above the sod there is
a region of maximum temperature; it is also noticeable on the
humidity chart as a dry zone, though rather a weak one. It looks
as though the warm, dry air which has formed over the concrete,
moves slowly out at this height above the sod, overrunning the cold
ground air.

i8o THE CLIMATE NEAR THE GROUND

H. Runge (^67) has published a fine example from which we can
recognize the effect of such microclimatic differences on local weather
conditions. In an article written for the press in 1936, speaking of
the danger of fog for motor cars and its alleviation, I found the re-
mark:— "Careful observations have shown that even when there
is heavy fog close to the ground, a layer of clear or only slightly
cloudy air often forms up to a height of 35 cm or so. A high candle-
power light reflector mounted very low at the front of a car with
its light-beam directed obliquely downward and permitting no up-
ward scattering may be of great help to visibility in driving on foggy
nights." If this observation is true, it is no doubt based on the
microclimatic distinction between the dry concrete road and the
surrounding moist, cultivated land as shown in Fig. 84. Up to at
least 35 cm the pavement controls the adjacent air layer.

As soon as the living ground cover is a few millimeters or, at
most, centimeters high, an air skin close to the ground is formed.
R. Geiger (358-360) has made some temperature records at Munich
with cylindrical test-bodies which established quite well the tem-
perature relationships in this thin air layer and the air directly above
it. The test-bodies were cylindrical resistance thermometers in a
nickeled sleeve 5 mm in diameter and 65 mm long (167) .

Fig. 85 represents the distribution pattern of the temperatures for
several hours on summer days in 1935 when there was no precipi-
tation but plenty of sunshine. Only the lowest 40 cm of the ground
air are considered. The ground temperatures (5) at points about
i cm below the surface are on the chart displaced to a depth of 5 cm
for greater legibility. The right and left hand boundaries of the
shaded areas are the average absolute extremes within the given
hours; in the case of ground temperatures the instantaneous values
for the moments of beginning and end of the observation period
are chosen as left and right end, respectively, of the heavy line. The
slight spread of the ground temperatures makes them comparable
on the chart.

Between 2 and 3 A.M. the nocturnal curve of outgoing radiation
is recognizable only at a considerable height. Near the surface of
the ground the temperature again decreases vertically upward and
the already narrow spread is still further restricted. The effective
zone of outgoing radiation at night is at the top of the grass. The
air beneath this level is "anchored fast" as G. Hellrnann says. In
the morning (6 to 7 A.M.) it is first of all the surface of the grass
which is struck and warmed by the slanting rays of the rising sun.
There is incoming radiation above, but at the ground the outgoing

THE AIR LAYER ABOVE A SOD COVER

181

type still can be found. The temperature spread is very wide—-
corresponding to its rapid rise. As soon as the angle of incident radia-

FIG. 85. Temperature stratification over sod in the course of the day

tion becomes steeper, the nocturnal situation in the living plant cover
disappears, the conservative action of dew evaporation on ground

182 THE CLIMATE NEAR THE GROUND

and grass is gone, and the air next to the ground begins to heat up
as is shown by the curves of condition from 8 A.M. to 5 P.M. The
small vertical arrow in Fig. 85 indicates the time of occurrence of
the maximum temperature at the thermometer just above the ground.
The ground and the air next to it follow this sudden heating only
slowly.

The cooling of the grass-covered ground proceeds quite differently
from its heating. For, while in the morning, the sluggish night air
masses at the ground can be only gradually warmed up by means
of heat radiated to them from above, the reverse process which in-
volves their evening stabilization, takes place in all layers alike. The
symmetrically proportioned figure for the hours between 6 P.M.
and 7 P.M. shows this very nicely. The mean temperature is almost
the same at all points. The right-hand boundary of the distribution
area still reflects daytime conditions. With increasing cooling off
the outgoing type of radiation sets in, appearing in its pure form
between 8 and 9 P.M.

This brings us to a consideration of the influence of plant cover
on the climate near the ground. Since this is reserved for Section VI,
we shall break off at this point for the present.

SUPPLEMENT

ON THE QUANTITATIVE DETERMINATION OF THE HEAT ECONOMY
OF THE GROUND SURFACE

All the processes in the air adjacent to the ground are to our belief
intelligible only if the heat economy of the ground surface is not
only understood but also quantitatively apprehended. How the sur-
face heat is introduced or dissipated has been described in the course
of the 17 preceding chapters. The relative significance of the various
factors has been pointed out. But the picture is not complete until
the share of each factor in the total heat exchange is numerically
known at every moment.

This desire of microclimatological research carries, however, far
beyond its immediate tasks. It is a fundamental problem of general
meteorology to investigate this exchange of heat at the surface
since the condition of the whole atmosphere is determined by it.
The problems of the quantitative description of the problem are here
only outlined as a supplement. Further, one must point to the
publications related to this subject.

In the nineties of the past century, Th. Homen (378) of Finland,

THE AIR LAYER ABOVE A SOD COVER 183

tried to determine the heat exchange with three different kinds of
soil by observations. His results are now obsolete; with respect to
the present state of science he underestimated the significance of the
radiation process. F. Albrecht, Potsdam, (369-374) did pioneer
work in this field; he inaugurated the first comprehensive observa-
tions (some results of which will be mentioned later on); his
technical talents brought about a great number of instruments
which serve exclusively to measure directly certain elements of
radiation and heat economy; most recently, he succeeded in de-
termining the heat economy, as far as its main elements are con-
cerned, for all geographical latitudes for the solid ground as well
as for the ocean. Besides, a Finnish scientist, M. Franssila (377)
has carried on the heritage of Homen in a greater series of experi-
ments at Palkane.

The heat economy of the ground surface is made up of four parts.
The exchange of radiation should be mentioned first. Chapter i
dealt with short-wave insolation from sun and sky; Chapter 2 and 5,
long-wave outgoing radiation, radiative pseudo-conduction and wave
length transformation. Accordingly as in- or out-going radiation
predominates, the radiation exchange is positive or negative. F. Al-
brecht (372) constructed a radiation-exchange meter which per-
mitted direct observation of the balance. S. Sauberer (382-385) in
particular, in a series of works has studied radiation exchange as one
factor in heat transfer.

Second is the heat gain (or loss, as the case may be) of the ground
surface, which results from the influx of heat from deeper layers of
the earth or its return in that direction. The third chapter was de-
voted to this.

As a third factor we may mention heat exchange with the adjacent
air. It occurs chiefly through convection but also by advective
processes — the moving in of warmer or colder air. Chapters 4, 6
and 7 dealt with these questions.

Finally there is the heat loss resulting from evaporation of water
from the surface of the ground. In order to change i gram of water
from the liquid to the gaseous state it requires an amount of heat
which depends on the temperature of the water.

for a water temperature of o 25 40 °C

the vaporizing heat in calories is .... 595 582 575

This heat is withdrawn from the ground surface. In dew and frost
formation this same amount is returned to the ground as heat of
condensation. Consequently the heat exchange through the con-

184 THE CLIMATE NEAR THE GROUND

densation or evaporation of water may be positive or negative. It
cannot be overlooked.

Normally the heat economy of the ground surface is not in equi-
librium. The interplay of the various factors greatly increases or
diminishes the heat supply of the surface at a given moment. Its
temperature is always rising or falling. Only on long winter nights
in quiet weather can equilibrium finally be attained. It will be
readily understood that it is much easier to compute the exchange
with actual figures in such a case, than add it when in flux.

If there is a snow cover the task becomes still easier for then the
surface is of uniform nature and form, while the transport of heat
from the ground is small. Consequently a new series of heat ex-
change measurements has been carried out at night, directly over
the snow. A. Angstrom (jjo) in 1919 published measurements at
Abisco during the polar night when exceptionally stable conditions
rule. In 1932-33 there followed the observations of G. Falckenberg
(376) and F. Kriigler (148, 579) at Rostock and of E. Niederdorfer
(j&>) at Karnten. According to these the results of the nocturnal
heat exchange of the snow cover in calories per sq cm per min. are
as follows : —

TABLE 31

Heat Loss

Heat Gain

of the Ground

Surface

Time Mean
of ' V
Separate
measurements

Effective
Outgoing
Radiation

Through
conduction
from snow
cover

Through
convection

Through
heat of
condensation

E. Niederdorfer

January, 1932
Scattering
Mean (n = 7) ...

0.013-0.060
0.052

0.004-0.037
0.024

0.009-0.035
0.028

disregarded

K. Kriigler
Winter, 1932-33
Scattering
Mean (n - 8)

0.115-0.163
0.135

0.050-0.094
0.075

0.039-0.076
0.059

0.000-0,003

O.OOI

The four numbers correspond to the four factors of the heat exchange
mentioned above. Among these, the influence of hoar-frost formation
on the snow cover is very slight and consequently was at first
neglected by E. Niederdorfer. Radiation is the main factor and a
negative one. In a condition of equilibrium such as prevails on a
winter night the heat lost by radiation from the snow surface comes

THE AIR LAYER ABOVE A SOD COVER 185

in about equal parts from heat conduction through the snow from
the ground and from convection on the part of the adjacent air
layer.

Further recent data on the heat economy of the snow cover may
be found in the work of O. Eckel and Ch. Thams (332) and A.
Nyberg (#5).

The best and most recent information on the heat exchange over
the course of a full day is to be found in the measurements of F.
Albrecht (570) and M. Franssila (#7). Fig. 86 gives their results.
The four circles at the left are based on F. Albrecht's measurements
at Potsdam on a clear day in each season, i.e. Apr. 5, 1925, July 19,
1925, Sept. 30, 1924 and Dec. 16, 1924. The upper right-hand circle,
which represents Palkane in the middle of Finland (Lat. 61° N),
is an average of seven daily series made by M. Franssila in June and
August, 1934 and is placed for comparison with the summer
measurements at Potsdam.

The upper semicircle in each case shows by the area of its several
sectors the amounts of heat brought to each square centimeter of
ground surface in the course of the day — namely through heat trans-
mitted upward from lower ground strata (line-shaded), through
radiation from sun and sky (white), through contact with the ad-
jacent air (cross-hatched), and finally through dew and frost forma-
tion (black). The lower semicircle shows in a similar manner the
heat loss of the ground surface through the soil (conduction),
through radiation to the overlying air (convection and conduction)
and through evaporation. The unit of area is given at the lower
right of the illustration.

In the heat economy of a normal day the intake and output bal-
ance. Therefore the upper and lower semicircles are equal in size.
But according to season the amount of total exchange varies as is
natural in view of the determining influence of the sun — in winter,
small; in summer, great. The areas of the semicircles in Fig. 86
correspond to ±307 calories per sq cm per day in spring, in summer
±374 at Potsdam and ±394 at Palkane, in fall ±235 and winter
only ±155.

In winter strong outward radiation rules the heat economy; incom-
ing radiation is vanishingly small. The heat loss through radiation
must therefore be made up by a return from the ground and by
accession from the adjacent air layer. The illustration represents a
winter day without snow. In the presence of a snow cover the part
played by the adjacent air would be relatively more important. At
no other time of year is it so closely concerned in the heat exchange of

i86

THE CLIMATE NEAR THE GROUND

the ground surface. This explains the especially strong winter reces-
sion of temperature near the ground with a snow cover.

If winter is the season when heat gain through frost formation can
be counted on, it is in summer that the effect of evaporation comes
to the fore — in Palkane more evidently than in Potsdam. The reason
for this may be that in Palkane the June preceding the experiments

Spring

Summer

Autumn

Heat to and
from ground
In and outgoing

radiation
To and from nearby

air layer

Yield of heat by dew
and frost formation

Heat loss through
evaporation

= 20 cal/cm 2 day

FIG. 86. Daily heat exchange of the ground surface in the different seasons

was a very wet month so that the ground was quite saturated. In
Potsdam, consequently more heat could be carried to lower earth
strata and into the air, while in Palkane it was used for evaporation.

In summer the most important factor is insolation. Indeed this
determines the amount of the heat exchange. A considerable portion
of it is again lost through nocturnal radiation outward on account
of the high ground temperature, in spite of the short nights. The
heat exchange in the ground alone remains essentially in equilibrium
as in the transitional seasons. The heat surplus is chiefly used up in
evaporation.

Finally we cast still a glance at the annual heat exchange in the
different geographical latitudes. The following table contains the
results of the investigation of F. Albrecht (3740). The third column
of the table shows the entire annual heat exchange, i.e. the total of
heat in cal/sq cm which passes through the surface of the ground
in both directions.

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l88 THE CLIMATE NEAR THE GROUND

Since, above all, the sun determines the heat economy, the numbers
are the highest at the equator and decrease polewards. The same is
valid for the radiation economy (Column 4 and 7) if considered
separately. While the balance is positive from the equator up to
and beyond Finland, in "Eismitte" (station of the Wegener Expedi-
tion in the middle of the Greenland ice-cap) the outgoing radiation
prevails by far in the total of the year. There conditions are valid
for the year round as for our region in winter time, according to
Fig. 86.

Immense heat amounts are used up for evaporation (see last col-
umn). Because of the increase of precipitation with decreasing
geographical latitudes also these values increase generally towards
the equator. But the climate at large scale is also influencing these
conditions. In East Siberia, poor in precipitation, the heat amount
used for evaporation per year is despite equal latitude and equal
annual heat exchange essentially smaller than that at Potsdam which
belongs to the humid climate of Central Europe. In the Gobi Desert,
it is still much smaller than in Potsdam, despite the differences in
latitude of 10°. It is very noteworthy that in the total of the year
in the climate of Potsdam all heat spent by the sun is used up for
vaporization of water. The amounts which the ground and the air
layer near the ground absorb during the summer and which both
give back during winter compensate each other finally.

But this is not true for all climates of the earth. Moreover, there
are two regions where the air layer near the ground is obviously
effective also for the total of the year, i.e. the frost climate and the
dry climate. In the frost climate (Eismitte) the air layer near the
ground must compensate the heat loss by outgoing radiation of
the ground and the snow cover respectively. This amount of heat
which must be compensated can be supplied only by advection of
warm air from lower latitudes.

Conversely, in steppes and deserts, where the incoming radiation
heat is only partly used up for vaporization, the air layer near the
ground receives enormous amounts of heat. According to W. Haude,
these heat amounts at his experimental station in the eastern
Gobi exceed by 50% the entire heat exchange at Potsdam. There
is a source region for the heating of the atmosphere and, therefore,
it is worth while to study here the conditions of the air layers near
the ground.

If we take into consideration column 9 of the table, we find, even
in Batavia, a rather great transfer of heat from the ground to the air.
It is to say that there the consumption of heat for evaporation is

THE AIR LAYER ABOVE A SOD COVER 189

enormous. Also about 4000 cal/sq cm which are mentioned in
Column 8 as being supplied to the ground are used for heating the
cold rain water originating from greater heights so that the surface
layer does not essentially profit in heat. But as the tropics are abund-
ant in heat something remains for the air layers near the ground.

In the paper mentioned before, F. Albrecht has determined the
heat economy also for the individual months. There are also data
given for some sea-stations. In this respect one must be referred to
the original paper.

PART TWO

The Microclimate in its Relations to Topography, to Plants,
Animals and Man.

Whoever reads the title of the second part of this book may at first
have the impression that very dissimilar things have been included
in this part. Land, vegetation, animals and man are united in their
relation to the microclimate almost as in the case of a filing cabinet
labeled "Miscellaneous."

However, as we enter the second part, we turn to a fundamentally
different kind of microclimatic phenomenon, which will occupy our
attention from now to the end of the book. In the first part we dis-
cussed the microclimates which exist near the ground as a conse-
quence of their locations. We limited our considerations to the thin
air layer, not over 2 m deep, which in the introductory chapter had
been designated from the standpoint of the macroclimate as a "zone
of disturbance."

Now we come to a second group of microclimates which are to be
differentiated from macroclimatology not simply as a disturbing
feature to be disregarded, but which were earlier considered by it
and observed. The portion of the atmosphere in which these new
microclimatic phenomena occur may exceed the limitation of a 2 m
layer. This we shall show by some examples.

In Section V the influence of topography on the nature of the
microclimate will be described. From this we choose our first ex-
ample. In an alluvial valley, the climates on the flood plain, along
the edge of the stream, on the slopes, and on the heights above, are
quite different. It is indeed a climate of a very small space, since it
varies with every meter that we ascend the slopes, and with every
meander of the stream's course. But macroclimatological shelters
can be put everywhere and variations determined according to
accepted climatological methods. This has not been possible for
any of the microclimates described in Part I.

Section VI will treat of the influence of the plant world. Imagine
a little pine forest surrounded by meadows. For the macroclimate
it is all one whether this little forest is there or not. But within the

192 THE CLIMATE NEAR THE GROUND

forest there prevails a microclimate completely different from that
of the surrounding meadows. The space from forest floor to roof is
occupied by this climate within the timber-stand and we can again
determine its properties according to the usual climatological
methods.

Section VII is devoted to the relations of animals and man to the
microclimate. When an architect builds a convention hall he creates
within it, by the nature of the building, a special microclimate. The
volume of air having the characteristics of such a microclimate may
be enormous.

We come therefore to microclimates of a new order of magnitude.
One might well ask whether the designation "microclimate" is
justified on the whole, for this new kind of phenomena. It might
seem desirable to insert between macroclimate and microclimate an
intermediate classification which, according to the proposal of
H. Scaetta (77) would be best called a "mesoclimate." We should
than be tempted to entitle the first part of this book "Microclimat-
ology," the second, "Mesoclimatology." This would immediately
indicate the unifying characteristic of the second part and wherein
it differs from the first.

Yet it is only at first glance that this difference is justified. The
nature of the country, the plant cover, etc., produce not merely
mesoclimates only, but microclimates in the old sense. A furrow in
the field has a special slope climate on either side; an anthill has one
on all sides — both of which are, as far as they go, decided micro-
climates, which cannot be apprehended through the macroclimatic
observation method. A single currant bush modifies the climate of
its immediate vicinity even to the smallest volume relation. Yes,
every leaf is surrounded by a film of air with its own special pecu-
liarities. In this second part of the book, therefore, we deal with
mesoclimates as well as with microclimates. Since, beside this, as was
stated in the introductory chapter, the introduction of a new designa-
tion meets with difficulties of a general nature, it is best to retain the
expression "microclimate" and employ it in the broadest sense. This
is what we shall do in the second part.

In these days we hear and read a great deal about a "bioclimate."
According to F. Linke, as he expresses it in 1934 in the foreword to
his newly established "Bioclimatische Beiblatter of the Meteorolog-
ische Zeitschrift" it is "the science of the influence of natural forces
on organic life." Bioclimatology is intended as a link between the
so-called "exact" and the biological natural sciences, as is medicine.
Since the microclimate is of decisive importance in the life of plant

THE AIR LAYER ABOVE A SOD COVER 193

and animal, bioclimatology and microclimatology, as Wilh. Schmidt
(20) among others has carefully explained, find themselves in closest
fellowship. In Sections VI and VII of this part, these bioclimatic
questions will come more into the field of view of our consideration.

SECTION V

THE INFLUENCE OF TOPOGRAPHY

In investigating to realize the influence exerted by changing topog-
raphy in the nature of the microclimate it is necessary to make a
distinction as to the time of day. During the day, slopes facing in
different directions and at different angles receive very different
amounts of heat radiation. This is the most important factor in
differentiating climates according to location. At night, on the other
hand, it is the cold air which moves downhill and, independently
of slope orientation, produces a variation of climate according to
zones of elevation.

The following description takes into account this distinction as to
time of day. Nocturnal relationships, being easier to understand,
are treated first — in Chapters 18 through 20. Then in Chapters 21
and 22 comes a discussion of the microclimate resulting from action
of various exposures to the sun. Only in the last chapters of Section
V are the general questions of topographic influence taken up.

CHAPTER 18
COLD AIR FLOODS AND COLD AIR DAMS

Air of lower temperature is heavier than air of higher temperature.
Cold air consequently endeavors to push itself under warm air. The
result, if opportunity permits, is a circulation of different air bodies
until equilibrium is attained. This is what happens at night in hilly
country. Outgoing radiation first causes the formation of a cold
layer of air next to the ground. Since this, equal ground conditions
being assumed, at first is of equal vertical extent at all points, the
cold air over the higher portions of ground is at the level of the
higher warm air over the lower ground. This difference of density

FIG. 87. Air drainage at night on both sides of a railway embankment crossing a

sloping surface

in a horizontal plane results in a balancing movement. The cold air
from the high ground flows to the lower places and is replaced by
warmer air from above these lower places. The potential energy
thus expended is so small however, in consequence of the small
dimensions and temperature differences involved, that it takes a
long while for the exchange to take place and it cannot continue if
there are other meteorological factors to disturb it. The process
works best on calm nights when the air pressure is high and the sky
clear. Under such circumstances there are formed the widespread,
often observed phenomena, known as "cold islands," "frost holes/1
"cold lakes," "cold air puddles" or whatever other name may be
given the local formation of areas of low temperature at night.

The expression "cold lake," implies that cold air behaves like
water, which always flows to the lowest point. We speak of a "flood
of cold air." This comparison, as we shall see in Chapter 19, is only
partially correct. It teaches us two things, however: i. That concave
land forms are always cold islands at night, 2. That objects which

196 THE CLIMATE NEAR THE GROUND

impede the flow of air may be of great importance to the distribu-
tion of nocturnal temperatures.

An example may show just how far the analogy holds good
between the circulation of cold air and flowing water.

Where a railroad embankment crosses a gently inclined plain at
right angles to the slope the adjacent area above it where the air is
dammed up is usually colder and more liable to frost than that on
the down-slope side where the air cooled by radiation is free to flow
on down and make room for warmer air from above (Fig. 87). Gard-
eners on opposite sides of the embankment must raise different kinds
of flowers, for what luxuriates in the favored location, freezes in a
nearby area.

This "cold air flood" is noticeable in the distribution of nocturnal
minima within very limited bounds. As our first example we shall
mention observations which R. Geiger (796) made in 1925 on a
"frost area" in the neighborhood of Munich. These frost areas are
young pine plantations of large extent. They originated at the time
when the "Nun" ruled the forests around Munich between 1889
and 1891 and reforestation was slow in recovery. In many pine
nurseries the young shoots of the plants froze year after year, even
in June, so that many died, although part of them survived with
great difficulty. Such plantings, almost destroyed by frost, are known
as frost areas or frost fields.

The frost flats which were the subject of investigation were located
in the Anzing-Ebersberg forest, some 22 km eastward from Munich.
Fig. 88 is a sketch of the experimental field. On the right side
is shown the contour map according to the data from a special
survey. To the eye the surface appears flat but, as the contours
show, there is a slight slope toward the northwest (notice the north
arrow in Fig. 88). The air which at night flows approximately at
right angles to the contours is dammed by the high growth of pines
which surround the frost flats on the north and west, as shown in the
left-hand portion of Fig. 88. This cold air dam results in the forma-
tion of a cold lake every night in the acute angle between the older
plantings and changes the cultivated area into a frost flat.

In order to find out the temperatures to which the plants were
there subjected at night, thermometers were placed 5 cm above the
ground at the points indicated by the large numerals. This was in
the spring of 1925. They showed unexpectedly low temperatures as
the summary in the following table indicates.

We can see from these figures what extraordinarily large tempera-
ture differences can occur at night between places within the same

COLD AIR FLOODS AND COLD AIR DAMS

197

G
I

«J

THE CLIMATE NEAR THE GROUND

TABLE 33

May

June

£j

.be

,t (rt

oil

!§

°£

aj .^P

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OJ S

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3 O

s ^

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Munich

city 8.4 m 8.8

4-2.1

10.6

4-8.2

Munich

outer station ... 1.4 m 6.5

-1.8

9.0

+4-2

B. In the Anzing Forest near

Munich

Anzing

pig-sty 5 cm 1.6

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4-5

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At the frost flat

Point No. 30 "

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climatic province. In the presence of such low temperatures and
such great frost frequency, it is easy to understand why the plants
suffered such damage. The effects of freezing, estimated (before
temperature observations began) by forest gradations and mapped
in the left-hand portion of Fig. 88, agree well with the observed
temperatures.

If we consider the relationship between altitude and temperature,
we find the greatest cold at the lowest points insofar as such
points are not protected by nearby old plantings, such as points 7
and 4. A difference of elevation of the land amounting to only a few
centimeters exerts a marked influence on the nocturnal temperature.
This differentiation of the low spots is permanent as the monthly
mean values show. We must conclude from this that the nocturnal
cold air movement occurs with great regularity even when it
escapes observation and in spite of the fact that the general weather
may be under the domination of such other factors as wind or rain.
Numerous observations on the part of forestry and agriculture as to
the permanency of cold islands substantiate these facts.

In i939> R. Geiger and G. Fritzsche (290) made some measure-

COLD AIR FLOODS AND COLD AIR DAMS 199

ments on a frost damaged pine plantation in a teaching district of
the Eberwald Forestry College, which led to very similar results.
How great here too was the effect of the smallest differences in
height is proved by the following results from measurements at
five places which lay within a distance of not over 100 m from one
another.

TABLE 34

Measuring site no. 8 9 10 n 12

Elevation above sea

level (m) 36.1 36.1 36.3 36.6 37.1

Temperature minima (°C)

Individual frost nights,

May 23/24, 1939 -7.6 -6.9 -5.4 -5.1 -3.7

June 2/3 ~94 ~7-9 —8.2 —6.7 —5.0

July 2/3 — 2.1 —1-3 —LI o.o +0.1

July 11/12 ~2-5 ~T-4 o.o +1.6 +1.9

Mean of the 30

coldest nights —0.6 -0.4 +0.1 +0.7 +1.7

At point 8 there were 17 nights of damaging frost in the spring; at
point 12, only 14.

Fig. 89 is a cross-section of a "sink hole," a rock kettle shut in on
all sides, resulting from subsidence. It is near Lunz in lower
Austria and is called the Gstettneralm (1270 m above sea level).
Wilhelm Schmidt (415) initiated there a great bioclimatic coopera-
tive project of temperature measurements on the slopes of the sink
hole and was able to demonstrate relatively very low night tempera-
tures in the kettle. The cross-section shown in Fig. 89 exaggerates
the altitude somewhat. The temperatures which were taken with
an Assmann aspiration psychrometer before sunrise on Jan. 21,
1930 are entered at the points of observation. Simultaneous data on
wind relationships are given as well. The left side of the illustration
gives the section from north-northeast to the middle of the sink hole.
On the upper part of the slope for some 70 m down the temperatures
are from i° to 2° below zero. As we descend still further the tem-
perature drops with extraordinary rapidity and on the floor of the
kettle reaches — 28.8°C. The cold air from the slopes accumulates
there and cannot escape. The heavy frost which formed in the
lowest 40 m was a visible evidence of this stratification.

In the right hand half of Fig. 89 is a cross-section from the middle
toward the west-southwest. Here the sink hole is intersected by a

200

THE CLIMATE NEAR THE GROUND

saddle. Temperatures below freezing prevail up to the height of
this saddle. Inasmuch, however, as the cold air can flow over the
saddle at this point, the temperatures above the saddle increase
rapidly. If we look across at the left half of the illustration we can
recognize the effect of this overflow on thai side of the sink hole.

Section from NNE to middle

•c

Calm

Light NNE
Calm

- 150

Light NNW

Quickly
fluctuating S W N

W.O

Section from the middle
toward WSW

Height of the
v saddle

toward
Lechnergraben

Altitude 1 270 m

-J9.9

FIG. 89. Temperature distribution in the Gstcttncralm sink hole near Lunz on
January 21, 1930. (After Wilhelm Schmidt)

The Gstettneralm and Schmidt's measurements have attained fame
in that during the well-known severe winter of 1928-29 the lowest
minimum temperatures of all middle Europe were observed there,
— 48°C. A microclimatic phenomenon has here, as so often else-
where, taken the record away from the rnacroclimate. It is signifi-
cant, also, that during the following winters as low as —-51° was
observed at the same place — an indication that it is not so much the
winter weather conditions as a whole, as it is the local, temporary
conditions which lead to such extreme temperatures. In the work of
W. Schmidt mentioned above, we see in particular the peculiarly
conclusive thermogram from the bottom of this sink hole.

Even in midsummer temperatures below freezing are reached in
the sink hole, and it is self evident that the plant world and the
animal world must adapt themselves to these local conditions. At
the bottom of the sink hole the plant growth consists of only a few
hardy grasses and a few herbaceous plants which can maintain
themselves under protection of the snow cover in winter, while in
midsummer they hurry through their growing season in a few
weeks. As one ascends the side of the sink hole, knee pines appear
first, then stunted pines and snow roses. Farther up the pines be-

COLD AIR FLOODS AND COLD AIR DAMS 201

come larger and are mingled with alpine roses. At the upper rim
of the sink hole is a normal forest. The reversal of normal tempera-
ture stratification resulting from the flood of cold air is thus reflected
in a reversed plant stratification. Whereas the forest usually ceases as
we go upward, it comes to an end here as we descend into the sink
hole. Even in the animal world there appears a similar dependence
of kind and number of kinds on the relative height in the sink hole.
(See Chapter 36.)

F. Innerebner (457) has shown, for the meteorological station
of Igls, at Innsbruck, that a cold air lake can form even on a
slightly inclined plateau "especially at those places where the air is
hindered in its flow by apparently insignificant obstacles." The
results which were obtained at the macroclimatic stations of the
country-wide network may therefore very well be influenced by
such cold air accumulations. Even he who is interested only in
macroclimatology, will do well to study these phenomena. Yes, a
generally accepted fact of macroclimatology can be traced back to
such cold air processes: The cold pole of the earth is, according to
the recent determination of S. Obrutschew (408), no longer Verk-
hoyansk but Oimekon. This place, like Verkhoyansk, is situated in
northeastern Siberia and is surrounded completely by mountain
chains. Obrutschew remarks that it forms a "sink most favorable
for the formation of a stagnating lake of cold air." There at the
macroclimatic station, which is here entirely subject to the action of
microclimatic conditions, an air temperature of as low as — 7o°C
has been observed.

We already have learned that nocturnal cold air on account of its
thermal stratification, is in a stable condition. If it lies in a sink or
kettle, this stability is intensified. At the bottom of cold air lakes a
perfect calm prevails. Motionless fog banks often attest to this.

Many will remember when this microclimatic phenomenon cost
the lives of not a few people. The general weather conditions dur-
ing the early days of December, 1930, favored stagnant air and fog
formation in the narrow valley of the Maas near Liege to such a
degree that the fluorine-bearing waste gases from the zinc and
superphosphate factories located there were unusually enriched.
Hundreds of people became ill of respiratory complaints, and over
60 died. That this was only an unusual intensification of a normal
microclimatic condition may be seen from the fact that in 1911 there
was also much harm done in the same area. (See references jpj,

394* 399> 401*)

Up to this point we have only indirectly deduced the facts of

202

THE CLIMATE NEAR THE GROUND

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COLD AIR FLOODS AND COLD AIR DAMS 203

cold air movement from their effect on nocturnal temperature dis-
tribution. Just how does this movement take place?

Wilh. Schmidt ($77) has investigated the method of flow of cold
air by means of wind pressure surfaces (cf. p. 42) in the region of the
lower Lunz lake and at Gumpoldskirchen near Vienna. He con-
cludes that the downflow of cold air is to be classed as a quiet, uni-
form movement which may be considered almost laminar. The lack
of turbulence results in wind speed being subjectively underestimated.
Such a movement assumes of course that the weather permits a quiet
air and that the land is uniform and gently sloping.

On steeper slopes — at inclinations over i% according to A.
Defant ( 390) — the downflow of cold air often occurs by bursts or
drops. On this point we have some exceptionally fine studies made
at the Geophysical Institute of Gottingen by M. Reiher (^//). On a
steep slope with a 1/3 pitch he placed platinum resistance thermom-
eters at heights of 10, 30 and 50 cm above the bottom. Fig. 90 shows
a result of his measurements, which permits us to draw a conclusion
from the temperature field as to the nature of the air flow. Time is
taken for the abscissa, it amounts to only 5% minutes. In the lower
half, the course of the temperature at the three measuring points,
during this period, is represented. Outgoing radiation prevails. It is
plainly seen that shortly after 7:04 P.M. a "drop" of cold air passes
the station. The temperature stratification above the ground, as a
function of time, is shown in the upper half of the figure. We can
readily imagine that this picture also shows the form of the air mass
passing the measuring point. It flows from right to left. In the
middle we recognize the highly arched drop of cold air. It pushes
forward a tongue which raises the passive warm air previously on
the ground. This we learn from the compression of the isotherms
above the cold air (at 7:02 P.M.). After passage of the cold air drop,
at about 7:07, warm air again occupies the lowest half meter above
the ground.

This dropping of cold air was repeated rhythmically every 4 or 5
minutes, in the case under observation. The speed of flow of the
cold air was 1.4 m per sec; the length of the cold air mass, about
300 to 400 m.

CHAPTER 19
NOCTURNAL TEMPERATURE RELATIONSHIPS IN VALLEYS

Cold air floods and cold air dams, as described in the preceding
chapter, were of small dimensions. Cold air movement gains signifi-
cance when it occurs in great volume. This is the case in valleys.
The present chapter will be devoted to the description of such con-
ditions, while the one following will take up the "down-valley"
wind.

First let us return for a moment to the explanation of cold air
flow. It has been already pointed out that the comparison of cold air
movement with that of flowing water is only partially correct. C. F.
Marvin (405) was probably the first to show clearly the difference
between the two processes.

In contrast to water, air is a compressible medium. In up and
down movements, consequently, there is always a question whether
the change of state of the air, due to this displacement, is of signifi-
cance. For air which sinks on account of its weight is dynamically
heated (foehn) while rising air experiences cooling. To be sure, this,
if it is to be practically effective, assumes quite large vertical dis-
placements and adiabatically controlled changes. The first assump-
tion is seldom fulfilled by the slow, gentle movements of cold air;
the latter, never.

In the second place, the energy of air movement — on account of
air being a thousand times less dense than water — is very small.
If we assume with M. Reiher (411) that cold air flow occurs only
under the influence of gravity, then the velocity of flow V in m per
sec is obtained from the expression

V = V2/F

where g is the downward acceleration acting on the air mass and
h is the distance of fall. If T is the absolute temperature of the cold
air, T" that of the surrounding air, and g the normal acceleration due
to gravity (981 cm/sec2) then

, T-T

& 'TV * O

TEMPERATURE RELATIONSHIPS 205

In his experiments M. Reiher found that the equation was con-
firmed in general by the results of his investigations. As mentioned
above, he had measured a velocity of 1.4 m per sec. G. S. P. Hey wood
(397) found, from measurements in the English Cotswold Hills,
speeds of from 1.2 m to 1.6 m per sec. Using probable values for T and
T" we always get similar experimentally justified values for cold air
movement in flat or hilly country. (Chapter 20 will cover the more
extensive "cold air winds" which attain considerably higher
velocities.)

Finally, water movement and cold air movement differ in this,
that a space can be empty of water but cannot be empty of air. Cold
air movement consequently is the beginning of a circulation between
cold and warmer air and only the initiation of such a circulation can
keep the cold air in movement very long.

The last difference is clearly visible when we study the nocturnal
temperature stratification in a valley.

Fig. 91 shows diagrammatically the cross-section of a valley. On the
plateau, sections of \vhich are shown to the right and left, along the
valley walls and on its floor, the lower air cools off at night at the
same time as the ground surface. If the air behaved like water, there
would have to be a circulation like that at the upper left of Fig. 91
and the temperature distribution would be arranged in horizontal
layers according to density as shown at the upper right. Such a sim-
ple circulation does not develop however. On the contrary, a series
of smaller circulations form on the slopes. In these, the cold air on
the slopes is mixed with the neighboring warm air, of which there
is a great reservoir between the valley walls, as shown at the lower
left. On the floor of the valley cold air accumulates. The cold lake
which forms there is deepened by the adjacent circulation on the
slope. The intermediate condition depicted on the slopes reaches
even to the edges of the plateau. The resultant temperature distribu-
tion is shown at the lower right of Fig. 91.

The plateau is cold and the valley floor, very cold, but the higher
part of the side slopes are warm. We speak therefore of a warm
slope (thermal belt). It is the safest place in areas and at times
where there is danger of frost. It is often indicated by the vegetation.

F. W. Nitze (407) was able to make a direct observation of the
nocturnal circulation shown at the lower left of Fig. 91. Small rubber
balloons, which were carried with the drifting air without upward
lift, carried little lights at their lower ends. The light from these
lamps traced the course of the balloons on the sensitive plates of a
stereophotogrammetric measuring apparatus. In this way it was

206

THE CLIMATE NEAR THE GROUND

possible to determine accurately the course of nocturnal air circula-
tion. Such pilot balloons were released at various places on a rather
steep slope and their course showed the equalizing movement which
was taking place between the cold air on the slope and the heat
reservoir.

Occurrence of nocturnal cooling
in a valley

Corresponding distribution of
nocturnal minima

1. Under the hypothesis that cold air behaves in the same manner as cold wafer

is not express

the
actual conditions

2. Corresponding to the best observations

Cold

Plateau
Warm slope
region

Cold lake

Radiating surface
Air movement

Cold 111 I l-m Warm
Nocturnal minima

FIG. 91. Schematic representation of the origin of the warm slope zone [thermal belt]

The temperature distribution over the valley cross-section is con-
firmed by observations. In a valley in Oregon, U.S.A., there was a
radio tower about 100 m high near the middle of the valley. F. D.
Young (42^) in 1918 made temperature measurements along the
sides of the valley, and on the radio tower. As an average of 32
nights in April and May he found the temperature distribution
represented in Fig. 92. It corresponds in general to the condition
shown at the lower right of Fig. 91.

The height at which the warm thermal belt is found, depends on
the time and the locality.

As the cold air gathers at the bottom of the valley the warm
thermal belt in the course of the evening moves upward. Fig. 93
shows the result of the measurements of Wilh. Schmidt (466) on a

TEMPERATURE RELATIONSHIPS

207

microclimatic experimental field situated on the eastern slope of the
Vienna forest at Gumpoldskirchen. Several observation stations
were distributed along the slope. Fig. 93 gives the temperatures at
three different times during the night of a late frost on the n-i2th

450

400m

FIG. 92. Nocturnal temperatures in a valley near Medford, Ore. (After F. D. Young)

of May, 1928. About 8:12 P.M. the temperature of the air on the
bottom of the valley has already retreated to nearly 2°, while at
240 m msl on the slope it is still almost 7°. In the course of the night
the whole temperature curve corresponding to the continued cooling
moves toward the left on the chart. Through the influx of cold air

-4.° -2° 0° 2°

Air temperature in Degrees Celsius

FIG. 93. Nocturnal upward migration of the warmest zone on a slope at
Gumpoldskirchen near Vienna. (After Wilh. Schmidt)

on the valley floor the most favorable zone recedes at about 10:30
P.M. to a height of some 300 m, and at about 4:08 A.M. to around
350 m. In the lowest part there is a heavy frost during these early
morning hours, although the warm thermal zone enjoys the advan-
tage of a +3° temperature.

At any given place this upward migration of the temperature
maximum goes on to a certain extent every clear night. Although
there are certain differences in individual cases, depending on

208

THE CLIMATE NEAR THE GROUND

weather conditions, yet over a long period the thermal zone has an
average height at the end of the night, which is the time of the
temperature minimum. The vegetation is adjusted to this average
position.

R. Geiger, M. Woelfle, and L. Ph. Seip (455) in the springs of
1931 and 1932 studied these relationships on the slopes of the Gross
Arber in the Bavarian forest. Twenty-three measuring points for
the determination of minimum temperature were set up at heights
between 639 and 895 m msl on the side slopes of the great Regen-
fluss near the "Seebach slide." The left half of Fig. 94 shows a
cross-section of the slope; the points of observation are indicated by
small vertical strokes.

Diagram of slope

Frequency

•Points of observation

FIG. 94. Position of the warm slope zone. (After R. Geiger, M. Woelfle, and

L. Ph. Seip)

The right half of the figure contains a frequency curve showing
what heights the thermal zone attained. There is a weak frequency
maximum at the bottom of the valley. When the warmest tempera-
ture occurs below, it means that the nocturnal temperature decreases
steadily upward. This is the case when in very stormy, and particu-
larly in rainy weather there is no true thermal stratification in the
valley. This situation does not interest us here. On the other hand
it is readily seen that the thermal zone is normally over 800 m and
that it fluctuates only slightly up and down.

In the case we are considering, beeches are found at just above
this height while both above and below they are always frozen back
by late spring frosts. In order to establish still better the influence
of microclimatic temperatures in the plant world, simultaneous
phenologic observations were carried on by the authors. Fig. 95
illustrates the results obtained. At the left we have the change of
nocturnal minima with height. At each measuring point the average
of 68 May and June nights in 1931-32 is entered. Not only clear
radiation nights were used, but all available data. The thermal zone
between 800 and 850 m is very evident.

TEMPERATURE RELATIONSHIPS

209

On the right hand side of Fig. 95, the phenological observations
are reproduced. For better comparison with the temperature curve
the time is drawn consecutively from right to left. Early budding and
high night temperatures therefore lie further to the right than late
plant development and lower temperatures. The similarity between
the phenologic and the temperature curves is striking; the thermal
zone is preferred in each.

•750

650

Temperatures

Wortleberry shoots

attaining
4 em in length

5° 6° 7° 8°C 30.

Average minimum nocturnal *~ June
temperatures in May and June

eo.

May

FIG. 95. Relation between nocturnal temperatures (left) and plant growth (right)
on a slope in the Bavarian Forest.

Fig. 96 shows how the nocturnal fall of temperature proceeds at
the various points along the valley slopes. It represents the course
of temperature during the night of Dec. 27-28, 1918 on the slopes of
San Jose mountain in the Pomona valley (California, U. S. A.). The
therrnogram was published by F. D. Young (423). The tempera-
ture increases up to a height of 68 m. The record at a height of 84 m,
however, already shows lower temperatures, an indication that the
thermal zone has been passed. It should be particularly noted, that
at the two lowest stations the temperature curve is almost horizontal
just before sunrise. The cold air is firmly , anchored to the valley
floor, while higher up the slope the small circulation currents make
the course of the temperature uneven.

Under favorable circumstances the nocturnal temperature dis-
tribution over the country, which we have been able to demonstrate
as a result of special investigations, can be observed directly. Hoar-
frost, rime or snow render the microclimatic zones of elevation
visible. They are most easily recognized when fog fills the cold

210

THE CLIMATE NEAR THE GROUND

hollows and valleys. We may read the lively description which
C. F. Brooks (475) gave of an early morning auto ride from Gape
Cod, on the eastern coast of the United States, toward the interior
of Massachusetts. "Light fog," he writes, among other remarks,
"was to be seen here and there in shallow basins. While it did not
hinder the driving, yet every time the auto passed from a colder

Midnight

-5-

FIG. 96. Nocturnal thermograph records during [a frosty] night in the Pomona valley
(California). (After F. D. Young)

lowland to warmer and higher ground, the quick condensation of
water on the windshield was annoying. A rise of only 3 to 6 m
sufficed to cause a temperature increase of from 5 to 6°C, thus
causing a thick deposit in the form of drops on both sides of the
windshield." W. Malsch (403) has recently described a similar in-
stance. While passing through an inversion in a valley of the
Bavarian forest, the windshield of his open auto suddenly iced over
to such a degree that it was impossible to see through it and a stop
had to be made to clean it off. Thus does the microclimate at times
enter directly into our everyday life.

CHAPTER 20

COLD AIR WINDS

DOWN-SLOPE, DOWN-VALLEY, AND GLACIER WINDS

In Chapter 18 we recognized a cold air stream as slow, nocturnal air
movement at a speed of from i to il/2 m per sec. In a large valley
this movement results not only from outward radiation from the
valley floor but the radiating side slopes also have cold air layers
close to the ground, which flow downward and hence are called
"(nocturnal) down-slope" winds. From these down-slope winds
there develops the down-valley wind which, under the formerly
used designation of "mountain wind," is one of the best-known
diurnal periodic winds described in meteorological text books. It is
a wind of local occurrence and to a great extent determines the
microclimate of the region affected by it.

A. Wagner (420), in cooperation with his school of meteorology
at Innsbruck, during the 1930*8 published a great number of
valuable papers giving a new and complete picture of periodic
mountain winds. Fig. 97 is taken from his summarizing work of

FIG. 97. A. Wagner's explanation of the nocturnal down-valley winds

1938. It represents diagrammatically the normal circulation in
valleys at night. The finely dotted area indicates the region of
down-slope winds which have potential energy with respect to the
valley floor and which are fed from the central reservoir of heat.
The coarsely dotted area represents the region of down-valley wind
which we are to imagine as flowing at right angles to the plane of
the illustration. It is made up of two parts, differing in origin. The

212 THE CLIMATE NEAR THE GROUND

one is a down-slope wind all along the valley floor, which is fed
from the side slopes; the other is a wind resulting from the pressure
difference between mountain and plain just as the ocean breeze in
its macroclimatic scope is dependent on the contrast of sea and land.

Thus the large-scale down-valley wind results from small-scale
cold air streams. Its velocity may be more than 1.5 m per sec which
we have set as the upper limit for cold air streams. In vertical extent
it sometimes builds up to several hundred meters.

As an example we cite the "Wisper wind" which has been care-
fully studied by H. Schultz (416).

In the Wisper valley, which opens into the Rhine from the east
at Lorch, a down-valley wind sets in with great regularity in the
evening shortly after darkness comes. This wind attains a velocity
of 3, or sometimes even 4, m per sec. It represents the downflow of
nocturnal cold air out of the cool Wisper valley into the relatively
warmer main valley of the Rhine. It is stronger, the clearer the
night and the weaker the gradient wind (i.e. wind resulting from
pressure gradient). The Wisper wind decreases in strength with
elevation and has a depth of 100 to 150 m in all.

H. Schultz was able to show in addition that the velocity of the
Wisper wind increases in direct relation to the magnitude of the
nocturnal temperature inversion in the Wisper valley — a proof
that the local wind is governed by local temperature contrasts. Since
the night temperatures in turn depend on cloudiness, there followed
an increase of wind speed with decrease of cloudiness.

In a similar manner, R. Luft (402), analyzing 18 years of observa-
tions at Bonn on the left bank of the Rhine and Beuel on the right
bank, proved the significance of the "seven-mountain wind" in the
local climate. L. Schulz (417) studied the down-valley wind at the
Braunlage sanitarium in the upper Harz.

In mountainous country the downflow of cold air can, under
certain conditions, be at first dammed and then suddenly loosed so
that it rushes violently down in what A. Schmauss (414) has
aptly termed an "air avalanche." He discovered the phenomenon in
the German Alps and has described it more fully. Such downrushes
of cold-air bodies have also been observed in the high mountains of
central Africa — by H. Scaetta (412, 413), for instance, at Karisimbi
(4,000 m) northeast of Lake Kiwu. He reports a case when his tent
was almost carried away by such an evening air avalanche. The
same kind of a storm was repeated on succeeding evenings at the
same hour, although with less violence — a further indication that it
was a daily periodic phenomenon.

COLD AIR WINDS

213

We must here mention a particular wind which is also a cold air
stream, though not a result of nocturnal radiation — the glacier
wind, or "firn-wind." The air close to a glacier is in summer cooled
by the glacier ice far below the temperature of its surroundings and
begins to move downward in the same direction as the glacier. The
hotter the summer and the finer the weather, the more this wind is
developed, as H. Tollner (419) has shown in the first thorough

Up-valley wind

x. x

Glacier wind
Glacier

x Wind direction away from observer • Wind direction toward observer

FIG. 98. Arrangements of glacier winds in the mountain wind system during the
day. (After E. Ekhart)

description of this process. While the usual cold-air stream is a night
wind, however, the glacier wind is a day wind. Both are fair
weather winds.

E. Ekhart (^92), using pilot balloons on the Hintereis and
Gepatschferner glaciers in the Otz valley has made a thorough in-
vestigation of the nature of the glacier wind. In summer it begins
at about 8 or 9 A.M. and is at first a gentle current only a few meters
deep. It grows rapidly in depth and strength to a depth of 200 to
300 m and a speed, near the ground, of 4 to 5 m per sec. The steep

214 THE CLIMATE NEAR THE GROUND

slope of the glacier favors high speed and a considerable gustiness as
well. The greatest velocity is found at a height of about 2 m above
the ice. Below this, the friction causes it to diminish rapidly. It also
diminishes upward, being at 50 m only half its maximum value and
at 120 m only a quarter thereof. Even when most fully developed,
the glacier wind does not extend far down valley beyond the front
of the ice, but it causes a marked drop in temperature within the
area it reaches. Toward evening this wind declines in strength and
depth, dying out at about 8 P.M.

Fig. 98 is a diagram of the glacier wind as E. Ekhart has given it
to us from his experience. It shows in the upper part a longitudinal
section of wind relationships on a summer midday, and, in the
lower part, a cross-section of the valley. The length of the arrows
and the size of the crosses and points indicate the relative magnitude
of the wind speeds involved.

Strongest of all is the upper, gradient wind resulting from the
pressure gradient. This has nothing to do with the local fair-weather
winds and so does not interest us here. The mountain valley is
filled at the assumed midday hour with an up-valley wind with
which we shall not become acquainted till later. It pushes over the
downward flowing glacier wind. The observationally confirmed
fact should be noted, that the glacier wind directly over the middle
of the glacier is shallower than along the mountain slopes. (See the
cross-section.) E. Ekhart explains this through divergence which
must develop above the heaped up center of the glacier and forces
the air sideways toward the slopes. A further cause is the greater
friction along the slopes, which allows the air at the center to flow
away more easily.

Since the glacier wind is a true cold air wind it has been treated
here in connection with down-slope and down-valley winds. As a
daytime wind, however, it belongs in the following chapter, in
which we begin to investigate the influence of topography by day on
the nature of local climates.

CHAPTER 21
THE SUNNINESS OF DIFFERENT SLOPES

During the day topography has a great effect on climate in that
the sun delivers different quantities of heat to sloping and flat
ground. To what degree the sloping ground or hillside is favored
or the contrary, depends on the direction and inclination of slope.
Together, these constitute the exposure. When these two factors
are known, together with radiation intensity on a surface at right
angles to the insolation (the so called "full radiation 1), or that on a
flat surface (so called "horizontal radiation") the radiation on the
slope can be calculated.

Differential sunning has such a great effect on climate that it was
this meaning that the ancient Greeks attributed to the "climate."
For climate is of course derived from K\IVCIV, to slope. Side-hill
climate, or exposure climate was to them merely the climate. Today,
as then, it has the greatest practical significance for many questions
of agriculture, forestry, gardening and other technical occupations.
Since it is now relatively easy to determine the insolation on different
slopes, there are a number of works on the subject. The amount of
radiation received depends on five factors — i.e. the time of day, the
season, the degree of cloudiness, the direction of slope, and the angle
of slope. In addition, we now ask for momentary values of radia-
tion, again for totals for days, months or for longer periods of time.
There is no one representation which can be used exclusively for all
practical purposes. In order to help the reader to find what he re-
quires, we must first glance over the computations which have been
made.

There is just one work — that of J. von Kienle (429) — which is
devoted to a calculation of the duration of sunshine on different
slopes; all the others are concerned with radiation intensity. Of the
latter, two are based chiefly on theoretical considerations, proceeding
from astronomical calculations; these are the works of R. Gessler
(426) and M. R. Pers (432). Four other publications are based on
actual measurements of radiation intensity, and consequently are of

1 The concept of full radiation is not to be confused with that of the total radiation
(radiation from sun and sky together) nor with that of the sum of radiations (solar
radiation summed over all wave lengths), which will be made use of in the following
pages.

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THE CLIMATE NEAR THE GROUND

particularly practical importance. Of these, G. Perl (</_?/) consid-
ered all latitudes in working over the radiation data from 80 different
parts of the earth. For individual locations within the range of our
climate, computations have been made by H. H. Kimball and I. F.
Hand (430) for Washington in 1922, by W. Schmidt (433) for
Vienna in 1926, and by J. Schubert (39) for Potsdam in 1928.
C. Schoy (434) has studied radiation received by mountains of dif-
ferent shapes. In the accompanying table we submit to the reader a
survey of what is to be found in the works mentioned.

In calculations of this sort there is usually something left out
which is nevertheless of great importance in their practical applica-
tion.

The amount of heat which a slope receives is made up of two
parts — direct insolation and diffuse sky radiation. The former
varies with the direction and angle of slope; the latter with angle
only. A 20° north slope receives just as much diffuse radiation as a
20° south slope and the amounts of heat received by each do not
differ greatly from that falling on a horizontal surface. Sky radia-
tion, therefore, moderates differences of exposure. The greater the
ratio of diffuse sky radiation to total radiation, the more are the
differences between various slopes effaced.

It follows from this that large differences in exposure will be
encountered in clear weather, small differences in overcast weather.
Fig. 99 furnishes a proof of this. In 1926 R. Geiger (454) made a

90 80 70 60 50 40 30

FIG. 99. Influence of cloudiness on the irradiation of a slope. (After measurements
on the Hohenkarpfcn, 1926)

series of observations on the Hohenkarpfen, a symmetrically round
mountain cone of the Swabian Alps. He mounted Eder-Hecht
optical wedge photometers toward the eight main points of the
compass — all inclined 35°, which corresponded to the average

THE SUNNINESS OF DIFFERENT SLOPES 219

slope of the mountain. From the average of 116 days of observations,
which were divided into four groups, according to the degree of
cloudiness, he obtained the percentage distribution of light falling
on the slopes, which is represented in Fig. 99. The amount which
fell on the south slope in clear weather was taken as 100%.

As the figure shows, the amount of light decreases with increase
of cloudiness. With an entirely clouded sky, it is, for all directions
of slope, between 1/4 and 1/5 of that falling on a south slope on
clear days. As cloudiness increases, the difference of slope direction,
which is in general symmetrical with respect to the north and south
axis, decreases. In clear weather the difference between north and
south exposures amounts to 46 units; with a clouded sky, to only 2.

In the investigation referred to, this law was at first established
only for the short wave radiation to which the optical wedge photo-
meters are sensitive. We can assume, however, that it holds for total
radiation as well.

In addition to the law of the influence of cloudiness on the differ-
ence-of-exposure, there is the influence of latitude.

In those tropical regions where the sun stands in the zenith, the
differences due to direction of slope are small; at midday they dis-
appear entirely; there are no sunny and shady sides. Consequently,
in those very regions which have the strongest insolation, the differ-
ence of exposure is least important to climate. In the far north, on
the other hand, where the position of the sun occasions the greatest
differences, the ratio of direct to diffuse radiation is relatively small.
Equalizing diffuse radiation predominates, and the total radiation is
not great. The consequence is that in polar regions, also, the matter
of exposure is not as significant for plants, man or beast as right in
our own middle latitudes.

As we ascend a mountain the radiation increases, while the air
temperature decreases. The importance of climatic differences re-
sulting from different amounts of radiation received on slopes of
various exposure, therefore, increases with altitude. In the Alps,
north slopes and south slopes are two fundamentally different
habitats for all life dependent on the sun. At a certain time in the
spring when everything is dormant on the snowcovered north
slopes, the first flowers are in bloom on the south slopes between
banks of melting snow or even under them. It is no wonder, then,
that it was in the mountains that climatic variation according to
slope was first noticed and first studied.

Let us now consider the amounts of radiation, corresponding to

220 THE CLIMATE NEAR THE GROUND

our latitude, which the slopes of various inclination and direction
receive.

We use for this purpose the computations of W. Kaempfert,
(426^) (published 1942) for Trier (49°45/N) which are based on
radiation observations of the years 1930-33 and were made with
regard to practical agrarian-meteorological requirements. We can
deduce the fundamental laws from Fig. 100; it gives also immedi-
ate answers to all practical problems, at least as far as the order of
magnitude of the amounts of radiation in question is concerned.

In Fig. 100 the time is plotted on the ordinate in true solar time,
the angle of inclination of the slope in degrees on the abscissa. In
nine separate figures are shown the maxima of sun radiation which
can be expected with cloudless sky and normal turbidity of the
atmosphere, in gcal/sq cm hr related to the surface of the slope.
These values are based on the measurements on the Petersberg
near Trier (267 m altitude) ; there, the normal turbidity corresponded
approximately to the turbidity factor 3, i.e. pure country air. (The
turbidity factor equals the number of ideally pure dry atmospheres
which would cause the equal depletion of sun radiation as the ob-
served real atmosphere.)

The three figures on the left side concern June 21; those of the
middle series are valid for the 21 st of March (approximately also
for the 23rd of September) ; those at the right side for December 21.
The three figures in the upper series are valid for the northern slope;
those in the middle are for the east slope; (if forenoon and afternoon
are exchanged one for the other also for the west slope); those of
the lower part of the picture are valid for the southern slope. In
each individual figure slopes are considered starting from o° (plain)
up to 90° (vertical wall). The left side of each individual figure
represents therefore the radiation upon the plain; consequently, it is
the same for each of the three pictures one below the other, regard-
ing the daily duration of sunshine as well as for the lines of equal
intensity of radiation starting from the left side. The right boundary
of each individual figure corresponds with the radiation upon the
vertical wall. The upper border corresponds with the time of sunrise
the lower with that of sunset. Symmetry to the noon line naturally
exists for the northern and the southern slope, not for the east slope.

Let us start with the lower series, the southern slope. On the 2ist
of March (in the middle) when the sun rises exactly in the east and
sets exactly in the west, as well as during the entire winter half year
(right-side figure) the sun appears on the southern slopes of all
inclinations in the same moment, namely when it rises up above the

THE SUNNINESS OF DIFFERENT SLOPES

221

horizon. In the morning of the summer half year, however, the
sun needs the more time the steeper the southern slope is, to move
from its northeastern azimuth towards the east point and to rise so
high that it strikes the southern slope. The upper and lower border
lines are, therefore, curved in the left figure, and are almost straight

Summer solstice March 21

Winter solstice

0 30 60 "90 JT* JO

Angle of slope

30 60 90

FIG. 100. Maximum amounts of insolation in Trier for N-, E(W)- and S-slopes of
all inclinations for three selected days. (After calculations of W. Kacmpfert)

in the middle and right figures. The intensity, always greatest at
noon, is the greatest on that slope which is perpendicular to the sun.
The maximum is, therefore, shifted from the flatter slope in the sum-
mer when the sun is high (left) to the steeper slope, in the winter
when the sun is low (right). December 21, at noon, the southern
wall receives a radiation intensity which is received by the plain
only after 9 A.M. at summer solstice. We shall come back to this
practically important fact.

222 THE CLIMATE NEAR THE GROUND

First, we consider the northern slope in the upper row of Fig. 100.
In the mid-summer (left) time of sunrise and sunset are identical for
all slope inclinations. If the slope is very steep, however, the sun
at noon, standing in the south, does not reach it; therefore, the
right border shows the remarkable "collar-yoke." On the steep
northern slope, sunshine exists only in the early morning and late
evening hours. For most northern slopes, the maximum intensity
occurs at noon, the same as with the southern slope; but in contra-
diction to the southern slope the maximum intensity of the northern
slope occurs at the inclination o° (the plain).

The eastern slope (middle row) is distinguished from the northern
and southern slope in that the time of peak intensity of the incident
radiation varies according to inclination and season. Also here, simi-
lar to the southern slope, the shifting from flatter slopes in summer
to steeper ones in winter can be recognized. Sunrise is invariable for
all inclinations, sunset occurs the earlier the steeper the slope is.

It is up to the reader to plunge into the figure more and more
and to interpolate between the selected extreme seasons and the
slope directions, for example the southeast slope.

The inclination of the slope is represented, in Fig. 100, in degrees
of angle and is often wrongly estimated. A lawn of 10° inclination
is often estimated as very steep. In the high mountains an alpine
pasture reaches rarely more than 25° inclination. Greater inclina-
tions occur practically only with rocky slopes and buildings. Gradient
and angle of inclination are correlated this way:

Gradient: 1:500 1:100 1:50 1:20 1:10 1:5

Inclination (°) o.i 0.6 i.i 2.9 5.7 11.3

The angle of 90° becomes an important factor when the indoor
climate and plants trained on trellis-work are taken into consideration.
W. Kaempfert (4260) published a special study about the sun radia-
tion on walls covered with trellis work facing the south.

As a supplement to Fig. 100 the following table, computed by
J. Schubert (_?p), may serve. In contradistinction to Fig. 100 allow-
ance is here made for the average cloudiness conditions and, more-
over, the daily amounts are given for all months. The radiation
measurements of 27 years at Potsdam (52° 23'N) are used in this
paper.

From the amounts of radiation for vertical walls we find that in
midsummer the east side of a house is most favored. Compared
with a horizontal surface, an east or west wall receives less heat
throughout the whole year, while a south wall, from Sept. through

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224 THE CLIMATE NEAR THE GROUND

March, receives more. The highest totals of radiation falling on
vertical walls occur in early spring and late autumn on a south
wall. This explains early flowering on south walls. (Compare
Chapter 35.)

J. Schubert has computed the amounts of heat received by slopes
and walls on a clear day in the middle of May, the month which is of
prime importance in plant development. The following daily
amounts in cal per sq cm are arranged in preferred order.

(For comparison, full radiation, that is, the radiation on a
surface which is continuously perpendicular to the sun's rays

South slope, inclined 23%° ... ...................... 594

South slope, inclined 30° ........................... 591

Horizontal surface ................................ 547

East or west slope, inclined 30° ..................... 500

South slope inclined 60° ............................ 486

North slope inclined 30° ............................ 361

An east or west wall ................................ 278

A south wall ....................................... 264

A north wall ...................................... 39

The varied sunning of different slopes affects ground tempera-
tures, primarily. Unfortunately we have few measurements of this
effect.

As far back as 1878, E. Wollny (435) prepared, in a garden, eight
areas of sifted soil which sloped at a 15° inclination toward the eight
main directions. He took temperatures three times a day (7:30 A.M.,
noon, 5:30 P.M.) at a point 15 cm beneath the surface. The average
temperatures for the most important months were:

TABLE 37
DIRECTION OF SLOPE

Month N NE E SE S SW W NW

May ............. 10.7 10.9 1 1.2 11.4 11.3 11.3 i i.o 10.9

June .............. 20.4 20.6 20.8 21.4 21.4 21.4 20.9 20.6

July . ............ 18.7 18.9 19.1 19.4 19.3 19.2 18.9 18.7

August ........... 19.2 19.4 19.8 20.4 20.5 20.5 19.9 19.4

September ........ 12.3 12.5 13.2 13.7 13.9 13.7 13.2 12.7

South or southeast slopes appear warmest. We shall soon discuss
the reason. A. Biihler (424) in 1895 made similar studies at Adlis-
berg in Switzerland but considered only four directions of slope.

THE SUNNINESS OF DIFFERENT SLOPES 225

We have the open country measurements of A. Kerner (427, 428),
carried out at Judenbiichel near Innsbruck from 1887 through 1890.
They were made, however, not on the ground but at the considerable
depth of 70 to 80 cm. Nevertheless on account of the close relation
between surface and ground temperatures they are of some assist-
ance to us.

The results of Kerner 's measurements are represented in Fig. 101
in a modified way (461). The circular form of the figure may sup-
port the idea of the direction of the slope. The concentric circles
correspond with the months. For each month the temperature of
the ground is calculated as the average of all directions. The differ-
ence between the individual temperatures of the slopes and this
average is plotted on Fig. 101. The hatched negative portions are
relatively cold, the dotted positive are relatively warm.

The maximum temperature differences between the separate direc-
tions of slope occur in the summer (at the center of the circular
surface), in opposition to the theoretical figures of Gessler, which
did not consider the weakening of radiation in the atmosphere.

The coldest direction of slope, as might be expected, is the north-
ern. The warmest direction varies, however, in the course of the
year. From January till spring the temperature maximum lies in
the southwest: then it moves quickly toward the southeast where
it is found in June. During summer and autumn it completes the
cycle back to the southwest. This phenomenon which we also found
in the previously quoted measurements of E. Wollny, may be ex-
plained as follows: —

The ground temperature depends not only on the intensity of
insolation, but also on the condition of the ground — particularly on
its highly variable moisture content. The morning sun finds a moist
ground. A great part of the solar energy radiated during the fore-
noon is therefore used up in evaporation with a drying out effect on
the soil. But when in the afternoon the sun does its greatest work on
the southwestern slopes of the mountain the ground is already
comparatively dry, the heat used in evaporation is scanty and most
of the absorbed heat energy is applied toward raising the tempera-
ture.

For this reason the temperature maximum is usually not in the
south, but is displaced toward the southwest. This is also the reason
why a directly western exposure is more than average warm. (In
Fig. 101, the o° line lies in the west-northwest the whole year.) The
eastern exposure has more than average cold; only in the mid-
summer months does the value there exceed zero; in winter the nega-

226

THE CLIMATE NEAR THE GROUND

tive areas on the chart extend into the southern exposures. Here is a
fundamental difference between radiation and ground temperatures.
According to Schubert's data, a 30° east slope receives more radiation
than a west slope because the morning atmosphere is clearer. The
ground temperatures, nevertheless, are lower in the east because of
the shielding effect of moisture.

North

FIG. 1 01. Ground temperatures at a depth of 70 cm. in relation to direction of slope
and time of year. (After measurements by A. Kerner near Innsbruck)

Proceeding from the normal location of the temperature maxi-
mum on a southwest slope, it would seem that the maximum in the
southeast during the summer is abnormal. The cause, as E. Fritsch
(425) and J. von Hann have shown, is the afternoon maximum of
cloudiness during summer. It is precisely in the mountains, where
these observations originated, that it is most usual for the afternoons

THE SUNNINESS OF DIFFERENT SLOPES

227

of even the fine days to be cloudy ("Fair weather cumuli"), fre-
quently accompanied by thunderstorms and precipitation. This
regular diminution of irradiation in the afternoon results in a dis-
placement of the temperature maximum toward the southeast. It is a
function of the macroclimate of Innsbruck and should not be ex-
pected throughout all Germany.

-10°

heavy lines 2 mm depth
thin lines 40 mm depth

10" 12" 14h 16"

FIG. 1 01 (a). Daily variation of ground temperatures in a sand dune of The Gobi
Desert. (After W. Haude)

W. Haude (4260) has regularly made measurements of the temp-
eratures of the ground in the dunes of the Gobi desert at the
Edsengol stream (42°O4/N, ioi°i7'E) about 1400 m) on the south
slope, east slope, west slope and on the top of a big sand dune during
winter 1931/32. Based on the publication of the original values by
F. Albrecht (4230), I calculated (50) the daily variation of tempera-
ture at the four observation places (Fig. loia) for 12 undisturbed

228 THE CLIMATE NEAR THE GROUND

days between December 18, 1931 and February 18, 1932. The aver-
age cloudiness of these days did not even reach one tenth. The
course of temperature simultaneously recorded by a thermograph
in a shelter on the sand dune is also given in Fig. loia and indicates
that in wintertime the air-temperature maximum is reached after
i5h and is still 3°C below freezing point.

The temperatures of the ground are entirely different! The meas-
urements at 2 mm depth, which correspond approximately to
those of the surface, are drawn with heavy lines, those for 40 mm
depth with fine lines; the same observation places are marked much
alike. While the air is always below freezing temperature, the
southern slope of the dune is heated up to nearly 22°C at noon;
February 18, at noon, even 32.8° was reached. Here as well as on
the top the temperature approximately paralleled the radiation. The
maximum occurs at 13!!. Eastern and western exposures show the
expected shifting of the maximum toward forenoon and afternoon
respectively. The greater heat of the surface on the eastern slope is
noteworthy and is perhaps connected with the fact that towards east
beyond the Edsengol stream an open gravel plain is located while
towards west a region of dunes is spread.

The temperatures at 40 mm depth are parallel to those at the
surface. Only they are shifted downwards and to the right corre-
sponding to the decay and the lag of the descending heat wave.

Unfortunately, we do not have systematic series of ground temper-
atures for our climatic region, made with modern devices (see p.
125). In default of such series we want to mention an investigation
made by a botanist.

From 1910 through 1917, A. Schade (446) took maximum and
minimum thermometer readings in moss clumps on different slopes
of the Elbsandsteingebirge. The instruments were inserted in the
moss. The observed temperatures lie between those of the rock
substratum and those of the air but far closer to the former than to
the latter. The figures shed a clarifying light on the fundamentally
different living conditions to which the plants growing close to the
ground are subjected on various exposures.

A liverwort sod of Leptoscyphus Taylori covered a shady north-
east-facing rock wall at Teufelsgrunde near Wehlen. At 50 m dis-
tance was a foliaceous moss clump of Webera nutans on a narrow
south-facing shelf which formed part of a bell-shaped, rounded
rocky summit exposed to the full strength of the sun. The maximum
and minimum thermometers in the moss were read from time to

THE SUNNINESS OF DIFFERENT SLOPES

229

time. The table at hand gives, as an example of the observed data,
the results for 1913.

TABLE 38

Temperature Maximum

Temperature Minimum

Period
1913

south-
facing
rock

NE slope

south-
facing
rock

NE slope

Nov. 15, I9i2-Mar. 5 ....
Mar. 5~Apr. i

. . . 16.2
. . . 28.0

5.2

9-3
13.0
1 6.0
16.4

14-7
13-6
9.1
8.9

-7-7
-1.6

— 2.0
2.9
7.0
6.9
2.6
I.O
-2.6

-6.0

— 2.0
— 2.2
1.8

6.9

7-3
4.8

2.5
o.o

Apr. 2— May 2

47.1

May 3-June 2

55.1

June 3~July 6

55.7

July 7— Aug. 31

47.0

Sept. i-Oct. ii

44.3

Oct. 12-Nov. 2

24.0

Nov. 3— Nov. 30

15.3

The difference between the minima for the two exposures seldom
exceeds 2°. But by day, when incoming radiation is effective the
differences between the maxima are much greater. The average
yearly maximum for the period from May 1912 through May 1917
in the case of the foliaceous moss on the rock with the southern ex-
posure was 52.6°, while in the case of the liverwort on the northern
slope it was only

CHAPTER 22

MlCROCLIMATIC EFFECT OF DIFFERENT EXPOSURES TO SUNSHINE

Before we discuss the temperature of ground air on slopes (Chapt.
23) we must mention certain natural phenomena in which the
varied insolation on different slopes has a directly visible effect within
a very limited space.

Ant hills in our climate and termite dwellings in the tropics ex-
emplify tiny mountains on whose sides the most varied micro-
climates can be observed. We shall refer at present only to the ex-
posures used by these animals in the care of their young. In Chapter
36, which is devoted to the relations of animals to the microclimate,
this reference will be more fully developed.

The trunk of a tree standing in the open is circled by the sun in
the course of a day. The bark receives a continuously changing
radiation, which can be conceived as that falling on a vertical
surface. Half the trunk at a time is under the influence of radiation;
it is greatest on that portion of the bark turned toward the sun.

K. Krenn (444) has used the measurements of the total intensity
of solar radiation at Vienna (202 m msl) and on the Kanzel summit
in Karnten (1474 m msl) in computing for several seasonally im-
portant days (cloudless weather being assumed) how much heat in
calories is received by a standing tree trunk in the course of a day.
The imaginary tree was considered as a circular cylinder with a
diameter of i cm, and divided into 16 sectors, corresponding to the
16 main directions. For each sector the total heat was calculated
from hour to hour and also summarized for the whole day. Fig. 102
shows, according to Krenn's beautiful method of representation, the
relationships on the Kanzel top on Apr. ist.

In the center of the sketch appears the tree in cross-section. The
heat totals which build up hour after hour in the several sectors
are plotted continuously outward from the bark along the radii
and the corresponding hour points are connected. The spaces be-
tween the various hourly curves are black and white alternately for
the sake of better visibility. The figures refer to hours of the day.
The gradual working around of the sun from the eastern side of the
tree to the western side is easily recognized. The outermost border
line represents the amount of heat radiated to the part of the trunk
in question, during the course of the day; what proportion belongs

MICROCLIMATIC EFFECT 231

to the various hours, can be read directly from the diagram. The
boundary line is symmetrical with the north-south axis, since, in
order to simplify calculations, the measured forenoon and afternoon
values of radiation have been equalized.

This boundary line is repeated in Fig. 103 as a broken-line curve
designated "Apr. i," but on account of bilateral symmetry with

5V/ x ^^^^ / ^^^^k. i "^^^^^^^^^ .^ SE

FIG. 102. Hourly progress of the warming of a standing tree trunk on a cloudless
April first on the Kanzel summit. (Determination by K. Krenn)

respect to a N-S line, only the right half is drawn in Fig. 103. This
illustration also contains for the Kanzel top the curve of July ist as
a characteristic of midsummer conditions and of Jan. ist as charac-
teristic of the winter. In the left half, the corresponding curves for
the low-lying Vienna basin, where sunshine is much less frequent.

In the portion above the east-west line, the arrangement of the
three curves is not surprising. In other words on the level as on the
heights the northern part receives most radiation in midsummer
because then the sun rises in the northeast and sets in the northwest.
As winter comes on, the northern portions of the trunk are less
favored. In general the mountain location (at the right) is more
favored than that in the lowland (at the left). The January irradia-
tion of the east and west sides of a tree on the Kanzel top are more
than twice that similarly received at Vienna.

It is on the south side of the trunk that the most remarkable condi-

232

THE CLIMATE NEAR THE GROUND

tions are found. Even on the plain the trunk receives more radiation
in winter than in summer. The difference becomes very striking in
the mountains. In spring (April) the part of the trunk facing
directly south receives more than twice as much radiation as in mid-
summer and even this is exceeded on Jan. i. No other portion of the
trunk receives more radiation at any time. This is due to the low

North

or west

South
50

Cal/day
Low position
(Vienna, 202 m)

JOO..V-'

High position
(Kanzel summit, 1474 m)

FIG. 103. Diurnal total of heat which a standing tree trunk receives on different por-
tions of its surface on the plains and in the mountains. (After K. Krenn)

height of the sun at midday in the winter as has already been dis-
cussed in connection with fig. 100 (lower portion). (See page 220.)

The result of this is a great danger to the trees, particularly in
early spring, when the nights are still very cold while the midday
sun shines powerfully through the clear wintry atmosphere. The
bark splits and loosens; it peels. M. Seeholzer (448), after the un-
usually cold winter of 1928-29, observed this phenomenon among
red beeches at Spessart and has described it in detail.

Only some of the stronger trunks of a breast-high diameter exceed-
ing 25 cm were affected, since only such have enough surface ex-
posed to the sun. The body of the trunk and the bark were still
frozen on account of the excessively low temperatures which held

MICROCLIMATIC EFFECT 233

over from winter into spring. In certain cases, where the beeches
were standing at the southern edge of the wood, so much heat was
received through radiation that the temperature in the bark rose
above the dew point. "This condition," writes Seeholzer, "must
have resulted in the bark's cracking and separating from the wood
in blisters. Contributing to this effect was the fact that from January
on, the beech has the highest water content of the year, and that the
cell structure between bark and wood contains so much water when
the sap starts that the bark easily separates from the wood. After a
few hours, however, this part of the bark has again dropped below
freezing and after sunset quickly followed the air temperature to a
low point. The consequence was a very rapid and decided freezing
with renewed water displacement and considerable shrinkage. The
loosened bark returned to its original volume. But since the bark
which was torn from its organic union with the wood could not be as
closely knit again as before its separation, when the two were an
organic unit, a weak bond resulted which could be broken by only a
slight perpendicular pull of the bark." The same phenomenon of
successive days strengthened the effect.

Fig. 104 shows such a bark wound on a 135 year old beech in the
municipal forest of Lohr, which has already attained a length of
2.6 m. The bark is evidently bulged over a length of 1.4 m. The
rent occurred in Feb. 1929; the picture was taken by Seeholzer in
the following July.

E. Gerlach (440) observed the daily temperature range in the
cambium layer on different sides of a tree. In n series of measure-
ments on an old fir tree in the summer of 1926 he determined the
following relation between time and place of occurrence of the daily
temperature maximum: —

Hour of the day 2:30 3 3:30 4 4:30 P.M.

Place of the daily max. .... SE S air SW N

Amount of the max 31° 31° 24° 32° 24°

According to this, the various sides of the tree-trunk are related in
amount and time of their maximum temperature values in the same
way as are the different slopes on the sides of a circular hill. The
shady north side receives its heat in the main only from the sur-
rounding air — merely by conduction, not radiation; the tempera-
ture maximum there occurs even later than that of a southwesterly
exposed tree trunk. E. Gerlach has also followed the penetration
of heat from the bark into the interior of the tree by means of
temperature measurements to a depth of 10 cm below the bark. It

234

THE CLIMATE NEAR THE GROUND

does not differ essentially from the penetration of daily temperature
fluctuations from the earth's surface into its interior.

In the case of logs lying on the ground, the insolation conditions
are still different, but of no less significance in problems of forestry.
K. Krenn (444) has calculated the figures for a log lying in a north

FIG. 104. Bark scale and cracks on the south side of a red beech as the result of
the strong spring radiation with low air temperatures. (Photograph by M. Seeholzer)

and south line and also for one in a east and west direction. Fig.
105 shows an example of how the varied sunshine works out. It
represents a cross-section of a tree trunk which lay in a NW-SE
direction. On the southwesterly side there is a hot microclimate in
and above the bark; on the opposite NE side it is shady and cool.

MICROCLIMATIC EFFECT 235

E. Schimitschek (706) has investigated the felling of such "captured
trees" by the bark beetle (Ips typographus) and has shown that the
development of the beetle around the trunk varies according to
microclimatic conditions.

A 3

FIG. 105. Microclimatic zones on a fallen tree trunk. (After E. Schimitschek)

On segment i, which is most exposed to the sun, the beetle has
laid no eggs at all. The temperatures here reached 50° while the air
temperature at a distance of 5 cm from the bark was not over 35°,
and that at a distance of i m was only 30°. On the adjacent seg-
ments marked "2" eggs had probably been laid but had died. On
segments "3" larvae had developed but later had dried up. It was
only on that part of the log marked "4" (which is narrow on the
sunny side but wide on the shady side) that the beetle developed
normally. Yet on the underside of the log the death rate in the
brood amounted to from 75 to 92% because in wet weather the log
at and near the place of egg-laying was too damp for the beetle.

Now let us proceed a step further — from the trunk to the whole
leaf-covered or needle-covered tree. The tree, as well as its trunk, is
circled by the sun in the course of a day. The leaves on different
sides of the tree consequently are subjected to quite varied radiation.
Surveying the form of the tree, we can look at its upper surface as a
slope which is not only exposed in every direction, but also possesses
a varying slope, whose angle is a function of the distance from the
ground. Observations of its microclimate are best made by observing
its blossoms, for their development is the most sensitive indication
of radiation and temperature relationships.

In May 1937, A. Scamoni (445) followed the blooming process of
a 15 year old pine standing in the open at Eberswald. The tree,
which had grown up standing free had developed 181 male blossoms
on the four whorls which were studied. The following table shows
the number which had bloomed in the several quadrants.: —

236 THE CLIMATE NEAR THE GROUND

Up to the evening of in the Quadrants

N E S W

May 15 .- o 4 14 2

May 16 6 27 36 37

May 17 2* 44 53 56

The greatest number of blooms was on the south and west sides, in
agreement with temperature conditions. The first flowers to open
were, however, on the south side, in accord with the radiation. Fig.
106 shows the blooming sequence for the fourth whorl of the tree,

Sequence of blooming

May 17

FIG, 1 06. Sequence of blooming of a 15 year old pine standing in the open at
Eberswald in May 1937. (After A. Scamoni)

which was at a height of i.i m above the ground. The twigs, with
the blossoms on them are diagrammatically represented as projected
on the ground. Four stages of blooming are identified by suitable
symbols, so that it is possible to get a picture of the entire process.
In addition to the influence of exposure we have here the influence
of shading by parts of the tree which extend farther out, the influ-

MICROCLIMATIC EFFECT 237

ence of the density of blossoms, of the flow of sap and of the indi-
viduality of each blossom. In this diagram, and even more so in
nature there is a fertile field for microclimatic studies.

P. Filzer (439) has recently investigated the daily temperature
march in the air surrounding a polygonum bush and dwarf pine
growing in the botanical garden at Tubingen. O. Hartel (441) at
Munich has described the blooming sequence in a circular tulip-bed
with a sloping border, in connection with temperature and humidity
measurements.

Our last example of the microclimatic result of varied exposure
to the sun is the "compass plant." Very recently H. Schanderl
(447) has given a comprehensive discussion of the whole problem
from the microclimatic and botanical side. We shall follow his
conclusions.

On southerly slopes it can be observed of the wild lettuce (Lactuca
scariola) and several other plants in Germany, that they orient their
vertically growing leaf-sprays in a north and south direction. The
name "compass plant" was chosen under the assumption that direc-
tions could be deduced from the position of its leaves. The phe-
nomenon has naturally nothing to do with the earth's magnetism
but is a combined effect of direct shortwave solar radiation and long-
wave heat counter-radiation from the earth. The ability to turn
their leaves into one plane is a peculiarity inherent in certain plants
and therefore only certain kinds are known as compass plants. But
single plants are affected by their environment. Those growing in a
moist habitat can easily regulate their heat by evaporation. In dry,
stony habitats, however — especially on a sunny, southerly slope —
they may lack the necessary supply of water from the soil and in
such cases it is of advantage to orient their leaves so as to reduce
their irradiation.

If such a compass plant is growing on a stony, steep, westerly
slope or against a west wall, the counter-radiation of the wall (from
the east) may be more unendurable than the direct radiation which
at midday is greatest from the south. In this case the leaves take a
position at right angles to the wall — east and west; botanists call
it the "transverse compass position." Fig. 107 is a photograph of
such a condition, taken by H. Schanderl. The wild lettuce which
has grown beside a west wall has placed its leaves perpendicular to
the wall. The name "compass plant" we can see is not quite suitable.
For this reason H. Schanderl proposed the more accurate designa-
tion of "orienting plant," but since the older name has been estab-
lished since 1850 he let the matter drop.

238 THE CLIMATE NEAR THE GROUND

That both a north-and-south and an east-and-west arrangement
can be present in the leaves of one and the same plant has been
beautifully demonstrated by H. Schanderl through some specimens
of this same Lactuca scariola. They were growing on a 30° westerly

FIG. 107. Compass orientation of the leaves of a wild lettuce in front of a wall.
(Photograph by H. Schanderl)

slope in the Wurzburg province of Wellenkalk. Four plants close
together had in all 627 leaves. On the sunny loth of July 1931,
Schanderl determined the compass position of each leaf. The result
of his enumeration (the frequency distribution) he has presented as
a percentage for each of the 16 main compass points. Fig. 108 shows
this in graphic form with appended figures. A distinction was made
between the leaves growing at a height of less than 50 cm. and
those growing at a greater height from the ground. The black part
of Fig. 108 applies to the former; the part enclosed by the broken
line, and the figures in parentheses, belong to the latter.

MICROCLIMATIC EFFECT

239

A glance at Fig. 108 immediately shows that the lettuce leaves in
the lower half meter stand predominantly east and west, while the
higher growing leaves are prevailingly north and south. The latter
are protecting themselves mainly from the direct radiation of the

1,9(6,0)

7.7(1,6)

10,1(6,6)

FIG. 1 08. Leaf orientation of the leaves near the ground (black) and the upper leaves
(broken line) of the lettuce on a westerly slope. (After H. Schanderl)

sun; the former, more against the counter radiation of the west
slope. This indicates, as Schanderl has proved by further experi-
ments, that the leaf position depends on the radiation climate of the
particular location where the plant is growing.

A special variety of compass plants is that which Br. Huber (442)
has called the "gnomon plant." Aster linosyris, the "golden mane"
aster, has narrow lanceolate leaves which normally lie horizontally.
In dry habitats, exposed to strong radiation, they assume a vertical
position. On steep southwestern slopes, however, the leaves on the
side next to the slope are combed forward uniformly toward the
south and at the same time stand quite exactly in the direction of the
maximum midday height of the sun.1 At the time of strongest

xThe ancient gnomon had a vertical pin. It was the predecessor of the sun-dial,
whose pin should be parallel to the earth's axis. The word "gnomon" is used here in
the general sense of "sun-pointer" since a plant in "gnomon setting" points to the
sun.

240 THE CLIMATE NEAR THE GROUND

radiation the plant presents the least possible surface to the sun. In
contrast to the compass plant, the gnomon plant, even on a western
slope, takes the same position with reference to the south, deviating,
at the most, not more than 10° from the direction of the midday
sun. There is no doubt that the phenomenon exists, as H. Schanderl
has demonstrated repeatedly; its explanation is still uncertain.

In these last remarks we have wandered far into the field of
botany. Now we shall return to the ground temperatures on the
various slopes, and in the following chapter shall investigate the rela-
tionship of slope to the air near the ground.

CHAPTER 23
THE SKIN OF AIR ON MOUNTAIN SLOPES

Although sloping ground favors the sliding down of cold air and the
rise of warm air, we find, even on the slopes, a layer of air near the
ground which has the peculiar properties described in the first part
of this book. Like a skin of air it clothes even steep cliffs and deter-
mines the climatic habitat of the plants growing thereon.

We have a systematic investigation of the properties of this air
film which R. Geiger (454) carried out in 1926 at the Forestry
Meteorological Institute of Munich. It was directed by A. Schmauss
under the sponsorship of Th. Kiinkele (461). The experimental
area was the Hohenkarpfen, an isolated mountain cone on the
border of the Wurttemberg Alps.

There were 34 observation stations located on slopes of different
directions and at different heights. The sketch map (Fig. 109)
shows by the configuration of the contours how regular the cone is
on all sides. Beside station A on the summit, there was a circle of
8 stations (marked W) on the uppermost slope, which lies at an
average inclination of 30°. On the shoulder of the cone, where the
steep cone of the white Jura passes into the gentler 11° slope of the
brown Jura there were 16 stations (marked H), while still further
down and outside the limits of Fig. 109, there was another circle of
8 stations. At all points maximum and minimum temperature read-
ings were taken at a height of 25 cm, while at 8 H stations and on
the summit, measurements were made at i m height also. The ther-
mometers were mounted at places where the slope was quite per-
pendicular to the desired compass direction and where there was no
disturbing bush or tree in close proximity.

Most of the mountain was in sheep pasture; only on the east and
west slope was there shrubbery with occasional trees, as shown in
Fig. 109. The south slope was the only one entirely free from
vegetation.

Fig. no shows a lateral section of the mountainside; the vertical
scale of the profile is doubled. In order to be able to show tempera-
ture relationships in the air layer near the ground its height had to
be magnified 50 times. In the upper part of the diagram the daily
maxima are represented; in the lower part, the minima, based on
the summer observations of 1926 — all directions of slope being aver-

242 THE CLIMATE NEAR THE GROUND

aged together. The course of the isotherms, which follow the con-
tour of the ground, shows that a ground air layer with pronounced
temperature stratification, is to be found on even the steepest slope.
Within this layer a temperature decrease with increased height pre-
vails by day even as it does over flat ground, while by night the

, Extreme thermometer at 25 cm height

• Extreme thermometer at 25 and 100 cm height

• Optical wedge photometer High bushes*""
a Second order meteorological wjth trees &

station without barograph
j Self recording anemometer
i Rain gauge

.clf-recording rain guage Footpath •

Trail open path ZZr:
High tree <

-H16-- X\Scattered bushes

\5 meter contour

FIG. 109. Arrangement of observing stations for a climate-measuring demonstration
at Hohenkarpfcn in the Swabian Alps. (After R. Geiger)

opposite is true. The temperature distribution at night on a valley
wall, which is represented at the lower right in Fig. 91 is what we
find to a lesser degree in the air skin on the Hohenkarpfen. By day
the distribution corresponds perfectly, with cold and warm air inter-
changed; the valley and the upper plateau are now warm while a
"cold storage" lies in front of the slope.

AIR ON MOUNTAIN SLOPES

243

The data from all directions of slope, which were all summarized
in Fig. no, prove the existence of the air skin. We are next inter-
ested in the influence of slope direction on the temperature relation-
ships within this skin layer.

Profile of hill, 2x vertical exaggeration,-
the layer of air near the ground, 50x

Diurnal maxima
(average of 61 days)

Point of measurement
1 ° Isotherms f (25 and 100 cm over the ground)

Scale 100 m
0 100m

wiiriii .mt*Timiixv\\^"" i

.TTT*M MM ^XX^ 900 M

Diurnal minima
(mean of 70 days)

warm,*.

Cold

FIG. no. Formation of air film by day (above) and night (below) on the slopes of

the Hohenkarpfen

It is best to start from our earlier remarks on direction of slope
and ground temperatures. The air temperatures near the ground
will vary approximately as the ground temperatures. Hence the air
will be coolest in the northern slope and the hottest daytime temper-
atures are to be expected on a slope between the south and south-
west. This is confirmed by Fig. in. But beyond that we must re-
member that winds and convection are easily able to remove the air
which is heated or cooled at the surface, because slopes are particu-
larly exposed to wind and the inclination of the ground favors con-
vection movements.

For this reason vegetation plays an important role in the micro-
climate of sloping ground. Where plant growth restricts convection,
the locally controlled slope climate is better developed, but where a
slope is entirely free from vegetation, the differences disappear.

244

THE CLIMATE NEAR THE GROUND

Fig. in will serve to prove this. It shows the distribution of
temperature maxima on the Hohenkarpfen on the average for 70
summer days. The upper circle represents diagrammatically the
measurements at a height of 25 cm above the ground. Small circles
indicate the positions of the various observation points, easily located

10.2 19.6 200 204 206 212 216

Temperature scale

Maxima

o Point of observation

FIG. in. Distribution of the highest temperatures of the day in the air film at

Hohenkarpfen

on the map in Fig. 109. The circle at the lower left in Fig. in con-
tains the temperature distribution at a height of i m according to
the observations on the summit (A) and at the H stations. In com-
paring the observations at the two heights we are to imagine the
smaller, lower circle as expanded to the dimensions of the larger
one.

That the air skin is universal, appears from the generally lower
temperature at the i m height (dotted surfaces only!), as compared

AIR ON MOUNTAIN SLOPES 245

with that at 25 cm. Here at 25 cm the ground temperature makes
itself felt — the more so, the steeper the slope. As the inclination
increases, so does the influence of stronger insolation as compared
with the greater possibility of equalizing movement outward from
the slope. In Fig. in we may deduce this from the fact that near
the W stations the isotherms are crowded more closely than near
the H stations. On the steepest part of the slope the different direc-
tions of slope vary the most. The observations on the lower, flatter
part of the slope, which are not reproduced here, do not show any
directional effect to speak of. There the temperature of the air near
the ground is entirely dependent on such surface conditions as
grass, tillage, etc.

It was to be expected that the lowest temperatures would be
found on the north slope. It is surprising, however, that the maxi-
mum on the southerly slope is divided in two — one in the southeast
and a stronger one in the southwest. Here it is the influence of the
lack of vegetation on the south slope which modifies the extremes
of a purely southern climate. A proof that this is really the case is
afforded by the measurements at the intermediate station Zw (Fig.
109). This Zw station was inserted between H^ and H6 where a
bare channel ran down the mountain from W4. If the unhindered
air movement on the south slope were really the reason for the miti-
gating of the midday temperature, a similar phenomenon must
necessarily present itself at the Zw station, where the up-slope wind
must be guided into the gap between the thickets and so over Zw,
while the two neighboring stations H 5 and H6 lay in front of the
bushes which hindered the air flow. Fig. in shows that the two
stations H$ and H6 had a higher average midday temperature than
the intermediate Zw station, thus confirming the theory. The
uniform distribution of night temperatures, which are not indi-
cated here, gives assurance that the daytime measurements referred
to are not fortuitous nor the result of errors in the method of
measurement.

Systematic measurements of atmospheric humidity as found in
the air skin on slopes in different directions are unfortunately
lacking. They would be very helpful in many practical questions,
such as the furtherance of forestry in dry climates. O. Hartel (441),
in his measurements to which we have referred, made in a circular,
slightly mounded tulip bed of i m diameter, found that on the
southern slope the noontime humidity 2 cm above the ground was
10% lower than on the northern slope. It is evident that consider-
able differences are to be expected.

246 THE CLIMATE NEAR THE GROUND

A word should be said here about the distribution of precipitation
around a hill.

From macroclimatology we are familiar with the fact that in
middle Europe the prevailing west winds result in the west side of
the mountains receiving the most precipitation. The air has to rise
up the side of the mountain; its pressure falls; it cools and ap-
proaches the dewpoint. If this point is passed, clouds and precipita-
tion follow. The cooling with ascent amounts to i ° per 100 m vertical
rise. On the small scale with which we have to do in considering
hills and rolling country — particularly in microclimatology — such
thermodynamic considerations are obviously out of place. The dis-
tribution of precipitation is determined rather by two other factors
— winds and ground slope.

Measurements which R. Geiger (454) initiated on the Hohen-
karpf en led to the following results : — If the precipitation be meas-
ured with rain gauges whose mouths are mounted horizontally, as
is customary, the slope toward the wind receives less precipitation
than the slope away from the wind. On the windward side the pre-
cipitation is carried away by the wind which strikes the slope and
attempts to flow around and over it. On the lee side, however, a
quiet area with irregular, weak air-movement forms in the wind
"shadow." Here is where the precipitation falls, which was whipped
over the hill. On a hill, therefore, the distribution of precipitation
is exactly the reverse of that on a high mountain; the east side re-
ceives more, the west side less, if the observation is made with a
normally placed rain-gauge. This microclimatic rule applies to
easily drifted snow to an even greater extent than to rain.

Everyone has noticed that the snow lies especially deep behind
fences, boulders and ridges of ground. As a general rule, which
works out practically in forestry and agriculture also, the micro-
climatic phenomenon just described is given too little consideration.
The often made statement that the west side of a forest is favored
with precipitation in comparison with the east side is based on a
confusion between macroclimatic and microclimatic experience and
at least in this general form is unjustified.

In the distribution of precipitation around a hill, the influence of
ground slope is to be added to that of the wind. What is always of
most interest to the practical man is the precipitation falling on the
actual inclined slope, not that which falls on the artificially located
horizontal mouth of the rain-gauge which, moreover, is a meter
above the ground.

Comparative measurements of a horizontal rain-gauge and one

AIR ON MOUNTAIN SLOPES 247

whose mouth was parallel to the ground surface on the Hohenkarp-
fen showed that the 20° slope on the side of the hill turned toward
the wind received more precipitation than the level ground. The
excess resulted from the wind velocity prevailing during the rain-
fall. This excess was as follows : —

At wind-speeds below 4 m per sec 3%

At wind-speeds of 4 to 5 m per sec 1 1 °/o

At wind-speeds of over 5 m per sec 27%

In a single case (thundershower) 34%

The brisker the wind, the more obliquely the rain beats down and
the more this favors the sloping ground. On the side of the hill
which is sheltered from the wind, however, where the rain falls
straight down, 5% less was measured on the slope than in the hori-
zontal gauge.

It is recognized that the just-mentioned effect of ground slope
opposes the wind effect and partially annuls it. The two factors
must be weighed one against the other.

CHAPTER 24
MORE ON THE INFLUENCE OF TOPOGRAPHY

In Chapter 19 we studied temperature relationships by night in
valleys and on hillsides. The effect of land form on the microclimate
by day was presented in the three preceding chapters, so that we can
now describe the temperature relationships throughout the whole
day. To this we now add a consideration of the other meteorological
elements insofar as they are of microclimatic interest and observa-
tions are available.

To represent the daily temperature march on slopes, in valleys and
on mountains we can again make use of a series of experiments
which, at the instigation of Th. Kiinkele (460), were carried out in
1931 and 1932 by R. Geiger, M. Woelfle and L. Ph. Seip (455) of
the Forestry Meteorology Institute at Munich. The site of the experi-
ment was the Gross Arber in the Bavarian Forest. A meteorologi-

Principal station

Line of observation
* points

I Highest points

(summits)

Ridge line
Streams

0 1 2 3 4fr

FIG. 112. Survey of the experimental arrangement at Arber

MORE ON THE INFLUENCE OF TOPOGRAPHY 257

the full force of the cold northeast and east winds, from which the south
flank of the moraine wall is protected; on the other hand the location in
the shadow of the foehn, to which the neighboring south slope is fully
exposed, and finally — of less significance — the cold mountain wind
sweeping over, which comes out of a gully that continues into the notch
between the moraine wall and the northerly valley slope, accompanied
by a brook which follows the same course as this stream of air. When the
brook is frozen, this proximity may have a cooling effect. Both flanks
are stone walls some 20 m high, and on the north flank is where the
Alpine rose grows, in a situation of local contrasts. The mild south
slope, is mostly covered with fir and larch needles — of brownish tone;
the steeper north slope, thickly overgrown with moss — is clad in shim-
mering green.

When a snowfall occurs in summer or early autumn, the white cover-
ing lingers longer on the habitat of the Alpine rose than elsewhere on the
northern side of the valley. In spring the locality is said to hold the
snow three weeks later than its surroundings. Sleds are still used on the
nearby roadway — according to reports — after all the other roads in
the neighborhood are open.

Summer temperature measurements of the upper ground layer at
midday showed a lag of 3 to 4° of the mossy moraine wall slope as com-
pared with the dry slope. Measurements of the relative humidity in the
foehn gave values of 32 to 36% on the flank wall openly exposed to its
impact, as contrasted with 54 to 62% on the side protected from the
wind. Numerous Piche evaporation measurements at one place where
the foehn blows and another place protected from its force, showed that
at the latter place the amount of water evaporated was, on the average,
39% of that evaporated at the former. The least and greatest ratios
were 28% and 46%. The meadow separating the cool, moist, mossy
slope from the above mentioned brook, is called "Vernail," an old
flower name with a certain significance. "Vernail" comes from "ver-
nalis." This may be thought of as a reference to the locality being
still spring-like after all the surroundings have passed into summer, or
in the sense that spring flowers bloom in the meadow while snow and
ice are still in the neighborhood.

It seems harder to explain in terms of local climate, the occurrence
of the Alpine rose at the foot of the inner side of the southern moraine
wall, which is turned toward the Gschnitz valley floor — i.e. at the foot
of the south side of the valley, more than 306 m below its normally
lowest limit of occurrence on this side. It may be that the ground
moisture, here near the valley water table, is greater than higher up on
the slope. The ground formation may have a decided thermal effect.
The valley floor, surrounded by U-shaped moraine walls, is the site of
the development of strong winter inversions. They were discovered
many years ago at the place where the Gschnitz brook now breaks
through the stone wall of the moraine, that is, at the outlet of the

258 THE CLIMATE NEAR THE GROUND

winter cold lake. In 15 cases out of 55 (from mid-January to the end of
March) the nocturnal minimum temperature at this point was more
than 3° lower than at a place 50 m higher on the left-hand valley slope;
in seven cases this difference was more than 5°, and in one case it was
more than 7°.

CHAPTER 25
CONCERNING THE RANGE OF VALIDITY OF METEOROLOGICAL STATIONS

In the network of meteorological observation stations which now-
adays covers every civilized country, the disturbing influence of the
air layer near the ground is avoided with comparative ease by locat-
ing the instruments at least 2 m above the ground. It is more diffi-
cult in considering the microclimatic influences of topography, plant
cover and population to find a station which corresponds to the
average relationships between its nearby and more distant surround-
ings — which is, as we say, "representative."

The chief requirement of a representative station is that it shall
have a wide range of validity. This is the ideal of macroclimatolo-
gists. The variations of topography make the proper choice of such
a station difficult. It is therefore necessary to take up here the ques-
tion of the range of validity of a station where macro- and micro-
climatology are most closely related.

The better the influence of topography on climate became recog-
nized and the greater the demand for accuracy and utility in
meteorological observations, the louder also became the cry for a
denser network of stations. This desire originated in mountainous
country. But the farther climatology progressed, the more limited
became the range of validity of stations on the plains. As early as
1911, K. Knoch (484) showed how important, even on the plains
of northern Germany, were slight variations in topography. And in
view of the increasing number of legal decisions and the services
required by agriculture, commerce, business and industry, attempts
were made for every place in question to have a meteorological sta-
tion close at hand. M. Topolansky (500) said, "There can never be
enough stations."

There are, however, decided difficulties in the way of such a wish.
A. A. Hettner (481) once demanded the expansion of the station
network in the name of geographers, K. Knoch (485) pointed out in
the name of meteorologists that it would require a lot of money.
And even if the money were available the observers must first be
found. With the severe requirements made of each observer as to
faithfulness, carefulness, and tenacity, whereof the user of the ob-
servations has for the most part no conception, this is a difficult task.
But even if personnel and money are available, still new installations

260 THE CLIMATE NEAR THE GROUND

are justifiable only if the data obtained can receive the essential
amount of attention and consideration. This too requires much
means and strength.

We must therefore face the fact that there are practical limits to
the density of a station network. There is, however, another remedy.
Instead of setting up new stations, we can try, as our knowledge in-
creases, to extend the range of validity of those we have. To this end
the words "range of validity" must first of all be given proper
interpretation.

Originally this meant that the numerical data of the observation
stations could be considered applicable to a wider territory. The
range of validity of a station ends where the numerical departures
from the station become too great to be neglected on the basis of the
accuracy required.

But the words "range of validity" may be used in another sense.
When two stations, A and B, are situated so far apart or — what
amounts to the same thing — are in such different microclimatologi-
cal provinces that the observations of A and B differ substantially,
these differences are not of a random nature. Station B may perhaps
many times be warmer, much colder or more moist than A, but the
deviations group themselves according to definite laws, which are
based on the physical nature of the atmosphere and the soil. These
are microclimatic laws. If they are known, then the relations of the
various meteorological elements at stations A and B to each other
can be discovered. Microclimatology in turn now makes it possible
to draw conclusions as to meteorological conditions at one place by
a study of known conditions at a neighboring place.

Instead of expanding the station network by the installation of
new stations we have an expansion of the useful range of each sta-
tion by greater knowledge. Microclimatology even today is fre-
quently called upon to furnish information as to the climate of an
unfamiliar locality. Let us mention here a few precepts which in
such a case will aid in arriving at a practical and reasonable judg-
ment concerning the unknown microclimate.

The first thing to do, naturally, is to consider the macroclimatic
relationships of the nearest observation points. They always furnish
the essential basis for all microclimatic studies.

We next determine radiation relationships at the unknown place.
In mountainous country they play a deciding part. To illustrate the
procedure we shall refer to an investigation of this sort which F.
Lauscher (487) made at the climatic station of Lunz and which rep-
resents the finest and most creative sort of work of its kind.

RANGE OF VALIDITY 261

Radiation depends first of all on the macroclimatic radiation fac-
tors which are in turn dependent on altitude, temperature, atmos-
pheric humidity, cloudiness, amount of turbulence, etc. and which
are uniform over a wide area. Local effects are: i. Albedo of the
ground, for which the table on p. 129 gives an approximation; 2. Di-
rection and inclination of slope, concerning whose sunniness the
necessary comments have been made in Chapter 21; 3. Shading by
surrounding mountains, forests, buildings, etc., of which we shall
now speak.

Using a theodolite, we determine the natural horizon, in doing
which we measure azimuth and angle of elevation of all heights and
depressions. The result is plotted as shown in Fig. 118, which is a
reproduction of four examples according to F. Lauscher.

The method is most clearly understood by reference to the upper
left-hand view. The outer circle represents the horizon; the middle
of the circle represents the zenith. Between horizontal and zenith
the latitudes of 30° and 60° are drawn at equal distances. The sun's
path is shown as of June 21 st, the equinoxes, and Dec. 2ist. The
hours are marked on the sun's paths and at certain points connected
by dotted lines.

In Meisterau, which lies high upon the Dachsteinkalk Plateau,
the natural horizon is restricted only in the northeast where the edge
of the plateau is somewhat elevated. Toward the west and north-
west the view across the plain goes even below the horizontal.

The Gstettneralm (upper right) lies at the bottom of that great
sink hole whose unusually low night temperatures have already
been mentioned. (Compare Fig. 89). Consequently the horizon is
quite uniformly restricted in all directions. Mitterseeboden at the
lower left lies in a narrow north-and-south valley and therefore has
a free horizon only toward the valley ends. Toward the east and
west the outlook is hindered by the mountain sides up to a 30° eleva-
tion. The station of Hohersteinschlag at the lower right is situated
in a clearing in the midst of the Hochwald and furnishes an example
of an exceptionally well shaded station. In this case it is the sur-
rounding mixed forest which furnishes the shade.

The degree of screening of the natural horizon can be made into
a formula by means of which different stations can be easily com-
pared. There are three methods used.

i. Let h be the angle of elevation by which the natural horizon is
higher than the plane horizon. If we find the mean of the h values
obtained from measuring in all points of the compass, we get the
average screening angle (ftm). In practice it is enough to determine

262

THE CLIMATE NEAR THE GROUND

h for the eight main directions, and then average these eight. For
the four stations named in Fig. 118, the mean screening angles are:
4.0, 16.6, 30.5 and 49.0.

2. The solid angle of free sky is obtained as a percentage of the
hemisphere (<*>). On a perfect plain to = 100. When the screening

Meisterau

Mu, 1530m

Gstettneralm N

Om, 1270m

Mi/ferseeboden N Mn,77Qm

Hdherstein- SchlaaN Hg, 970m

FIG. 1 1 8. Different constrictions of the horizon chosen from four of the Lunz

observation stations

angle is equal on all sides h (h = hm), the solid angle o> = 100
(i — sin h). If the natural horizon goes up and down, to cannot be
calculated from hmj but the calculation for each of the eight direc-
tions must be carried out separately and these partial results aver-
aged. For the four stations mentioned above, o> = 93.2, 71.5, 49.8
and 26.6.

3. Recently, F. Lauscher has proposed the "amount of perfectly
diffuse radiation" (D) as the best measure of horizon screening. In
obtaining this it is assumed that the radiation from all unscreened

RANGE OF VALIDITY 263

parts of the sky is equally intense. The quantity D then gives the
amount of diffuse radiation from the open sky which reaches a hori-
zontal surface, as expressed in percentage of diffuse radiation on a
horizontal plane when the horizon is entirely open. The quantity D
is better than o>, because D takes into consideration that the parts of
the sky near the horizon are less concerned in radiation exchange
than are those near the zenith. The quantity D is calculated as
100(1 — sin2A). For the same four stations, it equals 99.5, 91.8, 74.2,
and 43.0. Naturally the difference between stations is less with D
than with o>.

From diagrams similar to Fig. 118, if they are laid out on equal
scale, it is possible to determine at once how long the sun shines at
each station on the three days mentioned., Assuming a cloudless
sky, we can then find the number of hours of sunshine resulting
therefrom.

TABLE 39

For the station On June aist At the equinoxes On Dec. 2ist

Meisterau

147

11.8

8.2

Gstettneralm

I2.O

9.1

1.2

Mitterseeboden

7-4

5-1

0.8

Hohersteinschlag

6.9

0.7

o.o

The local differences in duration of sunshine are therefore extra-
ordinarily varied. They can now also be measured by the "day-
protractor" of Wilh. Schmidt (493)) or the simple altitude finder of
W. Kaempfert (4813).

For heat supply, however, the intensity of irradiation is much
more important than duration of sunshine. We must consider the
fact that restriction of the horizon always screens the sun in the
morning and evening hours when its intensity is least. Conse-
quently the difference between the four stations with respect to the
amount of radiant energy received is less than with respect to its
duration. If the sun is hidden half the time behind the mountain
horizon the amount of irradiation is reduced, not to 50%, but in
winter only to 70% and in summer only to 75%, according to
Lauscher.

After radiation relationships, the next most important element to
investigate is the wind. The less the local wind movement and con-
vection, the more closely the microclimate follows the given radia-

264 THE CLIMATE NEAR THE GROUND

tion pattern. The essential points to consider in judging it are the
macroclimatic wind relationships (prevailing direction, frequency
of wind forces), the topographic position (peak, saddle, windward
slope, etc.) where special attention must be given to local winds
(Chapters 20 and 24), restriction of the horizon (wind protection by
surroundings) and the roughness of the surface.

Another good method of judging the microclimate of an unfa-
miliar place is by test measurements. For this we need an Assmann
aspiration psychrometer and an Horn hand anemometer, which
gives direct measurement of wind velocity. For the measurements
of a clear day should be chosen and an hour when the meteorological
elements are not changing rapidly — either early afternoon, very
early morning, or late evening. Then the observations made at
various points can be compared without too great errors. As a pre-
caution, it is well to take the measurements in figure-of-eight loops
so as to get measurements from one or several places at different
times and so to be able to relate all measurements to the same
moment. (See Chapter 38 as to the use of the research auto as a
microclimatological aid.)

At each place observations are made of air temperature, air humid-
ity and wind force preferably at breast height and also at about 10
cm above the ground in the manner advised by J. Bartels (/6o) : —
One goes forward slowly with the Assmann psychrometer, its
clockwork running, holding the aperture constantly at the desired
height above the ground. Incidentally, in such measurements, all
hints are to be observed which are offered by the nature, composi-
tion, and condition of the plant cover. It has also been mentioned
that valuable conclusions may be drawn from the presence of snow,
frost or ice formations.

Such studies as those here proposed are of particular value in
forming independent judgments. In recent literature there are fine
examples of how one can evaluate such "temperature hikes." The
pioneer work of Gregor Kraus (72) on the climate of restricted areas
resulted from walks and observations in the country. Chas. F.
Brooks (474) in the United States has reported his experiences with
geography students who regularly made such experimental measure-
ments as part of their school work. W. Hartmann (480) made tem-
perature measurements on a journey over the Arlberg road. Local
variations of radiation in the mountains have been observed by F.
Lauscher, F. Steinhauser and M. Toperczer (488). F. Lauscher
(486) described other journeys of similar nature.

The best information as to the range of usefulness of meterologi-

RANGE OF VALIDITY 265

cal stations is certainly afforded by auxiliary networks which have
been established to investigate microclimatic differences.

Wilh. Schmidt, in cooperation with H. Gams, W. Kiihnelt, J.
Furlani and H. Miiller (497-494) established a network of, at first,
13 and later, 23 microclimatic stations in Austria, for the study of
bioclimate. The stations were located along the northern border of
the high Kalk Alps, at altitudes between 610 and 1780 m, in the
neighborhood of the Lunzer Untersee. This network can serve as a
model. We can only hope that the results are given sufficient pub-
licity. In Upper Bavaria R. Geiger (779, 180) operated a series of
stations from 1923 through 1927 for the study of air layers near the
ground. The 99 stations on the Gross Arber (455) have already
been mentioned. On the Karst plateau of the Biikkgebirge in north-
eastern Hungary, F. von Basco and B. Zolyomi (47^) erected seven
stations and carried on observations there during the summer of
1934. The stations were distributed over 4000 sq meters of the
plateau at altitudes between 761 and 783 m msl. Tinn (499) has
compared five stations in the Nottingham district of England on the
basis of several years' observations. Several other examples have
been mentioned in previous chapters. Altogether they furnish
plenty of material for study on the question of the useful range of
meteorological stations.

SUPPLEMENT
THE MICROCLIMATE OF CAVES

There is one topographic feature whose influence on microclimate
has not thus far been mentioned. This is a cave, whose climatic re-
lationships are of interest not only as giving further information
about the cave itself, but also on account of its being the natural
habitat of many animals.

The microclimate of caves is, first of all, a ground climate. It is
characterized by high atmospheric humidity and slight fluctuations
of temperature. Caves may best be classified as having one opening
or several. In the former the air is quite at rest, and the microclimate
is of great uniformity except near the entrance where it is transi-
tional between open country climate and ground climate. If the
cave leads downwards from the single opening, the cold air at a
certain season falls into the cave and remains there. Such caves are
called simply "cold storage" or "static caves.'* In caves which are
open on more than one side there are often uniformly high wind

266

THE CLIMATE NEAR THE GROUND

velocities, since narrow passages allow equalizing currents between
warm and cold parts. Such caves as these are called "wind pipes"
or "dynamic caves."

The cave at Jenin in Palestine of which a longitudinal diagram is
reproduced in Fig. 119, may serve as an example of a cave open on
one side. P. A. Buxton (507) measured temperature and humidity

26'
26°
24

zo°

60%

Dry bulb thermometer

Temperature

s-
Wet bulb thermometer

•/*

Relative humidity

Distance from cave entrance

I 1 1 i i i . t i
0 W 20 30 m

FIG. 119. Temperature and relative humidity measurements in a single opening
cave. (After P. A. Buxton)

in it about midday on June 7, 1931. The data from four measuring
places are given in the lower half of Fig. 119. At A where the day-
light penetrates and where a man could stand upright the air
showed the characteristics of the hot and dry outer atmosphere.
7 m from there, from point B the cave became smaller so that one
must go on hands and knees and daylight diminishes. While the
wet-bulb temperature remained practically constant, that of the dry
bulb approached it. At 20 m from the entrance the air was satu-
rated and from there on the microclimate remained constant. At B
there was a pool in which frogs and the larvae of water insects
were found. Measurements by the same author in many similar
caves showed that within them the daily fluctuation of the meteor-

RANGE OF VALIDITY 267

ological elements was below the normally required accuracy of
measurement.

W. Paulcke (506) in 1932 investigated a cave 21 m deep, 1.2 m
wide and 1.9 m high placed like a gallery in the glacier ice on the
Jungfraujoch. At the entrance to the cave the temperature had a
winter average of —12°. Consequently the temperature of the
whole cave was below the freezing point but rose to — 4° in passing
from the entrance to the inside. The vapor pressure (over water)
amounted to 1.8 at the entrance and 3.4 mm at the innermost part.
The super-saturation with reference to ice was 11% at the entrance
and 3% at the inner end (according to information furnished by
letter). There was cave frost on the walls, consisting mostly of
hollow prisms near the entrance with cup-shaped crystals and leaf-
shaped ice forms farther in. The various forms are described in
Paulcke's wonderful book, with illustrations and explanations.

H. Mrose (504) has studied the temperature relationships of the
"Eisbinge" at Flatten in the Sudeten district. This is a cleavage cave
which contains ice the year round. It is open at the top, i m wide
and 20 m deep and is situated in the Erzgebirge at an altitude of
1000 m msl. Mrose calls it a "sock" cave since the cold air falls into
it from above but cannot escape. What cave experts call "cold
storage" caves are sock caves, also. In damp summer weather a thin
layer of fog, 5 to 10 m deep, appears over the glacier snow within,
as far as exchange of air with the exterior extends. The average
annual temperature of the rock at this altitude amounts to +4°. At
the end of winter therefore, in spite of the sock cave acting like a
frost hole, the snow soon begins to melt clear to the bottom of the
cave. However there is so much hindrance to the movement of heat
from above and the temperature difference with respect to the sur-
rounding rock is so slight that it takes three fourths of the year be-
fore the melting of the il/2 m winter snow is completed. By this
time the first of next winter's snow has arrived so that the glacial
snow never leaves the bottom of the cave.

R. Oedl (505) has described as follows (somewhat condensed) the
caves or "wind tunnels" which have several openings: "'Wind
tunnels' are all those caves which have more than one exit, so that
an air circulation results in them on account of temperature differ-
ences between the cave air and the air outside. In most cases these
wind tunnels have one lower entrance in the side of the mountain
and another entrance into a more or less horizontal system of pas-
sages, domes and labyrinths. From these there are flues branching
off — almost vertical, circular pipes which lead upward to the sur-

268 THE CLIMATE NEAR THE GROUND

face of the mountain and at their exit end in earth funnels — on
high plateaus, in snow funnels or little sinks. The so-called "world
of ice giants" in the Tennengebirge may serve as a model wind
tunnel. Passageway caves with only two openings such as we find
in the huge Frauenmauer caves of Steiermark and the Mammuth
cave of Dachstein are true wind tunnels with a strong air current
although their entrances differ only slightly in elevation.

The alternation of air currents is a peculiarity of wind tunnels. In
warm weather, when the air outside is noticeably warmer than that
within, the cold and therefore heavy inner air falls out the lowest
opening sucking outside air in at the upper opening; this is cooled in
turn by the cave walls. In the "world of ice giants" this process is
intensified by the fact that at the time of snow melting, and during
periods of heavy rain, a great amount of water passes through the
plateau gorges, carrying outside air with it. This is strongly cooled
in the snow funnels which at this altitude easily persist throughout
the whole summer, so that I have never encountered a temperature
higher than +2.0° in the inner cave system of the "world of the ice
giants." Here therefore the geothermal stages are completely done
away with to a depth of almost 800 m.

In winter, when the outside temperature is very low, the relatively
warmer cave air within the mountain will rise and escape by the
upper openings while cold winter air is drawn in at the lowest
entrance. Hence ice formations in wind tunnels (in case percolating
water and snowmelt can enter) are always found in proximity to
the lowest openings. In the winter of 1921-22, for instance, a mini-
mum of — 10° was recorded in the Eisriesenwelt at a distance of 600
m from the entrance. It is easy to understand that here almost 2 km
of passageways are constantly coated with ice."

H. Oedl has made several hundred observations of temperature
and humidity in the Eisriesenwelt at all seasons and has compared
them with data from other caves. The conclusions stated here are
those given in the summary of H. Oedl (505) and in the other
works mentioned in the literature cited.

SECTION VI

THE INFLUENCE OF PLANT COVER

The living plant in its existence and growth is fitted by climate to
its environment. One of the most important factors of a habitat,
therefore, is its climate. It is a combination of macroclimatic and
microclimatic features.

Plants, as living organisms, possess a peculiar heat and water econ-
omy. Along with this they exert a reaction on the microclimate of
their environment. But as they grow, they change their size and
form. In this way they affect the heat and moisture content of the
soil in which they stand and the air into which they extend. There
is, of course, an interaction between the plant, which depends on the
climate of its habitat, and the climate, which is partially dependent
on the plant.

The influence of plants on the climate of their environment in-
creases with their size and with the number of its fellows. At first
it is exerted in the realm of microclimate exclusively. But it gradu-
ally expands beyond the microclimate to macroclimatic dimensions,
as R. Geiger (599) has pointed out in greater detail in a survey of
the interaction of weather and forest. It is no longer a matter of in-
difference to a country and its macroclimate, whether it be wooded
or un wooded.

The law of interaction of plants on their environmental climate
leads to the term "plant climate" (5^2,4) . It would be more accurate
to speak of a "climate of a planting," or a "vegetation climate" (6).
If general use is made of such designations, they should include all
relations of the plant world and the habitat climate. The word
"plant-climate" cannot be limited, as seems almost the case with E.
Tamm (545), to 2-meter high forms of vegetation which are in-
teresting to agriculture.

The investigation of the interaction between growing plants and
microclimate considered as environmental climate is of great practi-
cal significance. As we gaze over the landscape in our latitude we
see the earth normally covered by plant communities. Fields and
gardens afford us nourishment; the forest, one of the most impor-
tant and versatile of raw materials. In agriculture and forestry, in
gardening and viticulture, the first care of the grower is for the

270 THE CLIMATE NEAR THE GROUND

young plants, which, on account of their tenderness, are particularly
sensitive to weather conditions and yet in their youth are especially
tried by the extreme conditions of the microclimate near the ground.
Consequently increasing attention is being paid in these days to pre-
cautions in the culture of field and forest which will foresee the
habitat climate of the young plants, and to how such care along with
the growing plants may influence their environment.

This sixth section is devoted to a description of the altogether at-
tractive, but not easily fathomed, variable relationships of plants and
microclimate. They will be best appreciated if we first take the
plants by themselves, without reference to the air which bathes
them, and ask the question, how they as living organisms react to
meteorological processes. Let us begin our study with the heat
economy of plants.

CHAPTER 26
THE HEAT ECONOMY OF PLANTS, AND PLANT TEMPERATURES

By day plants undergo heat irradiation from sun and sky; by night
they radiate heat outward. Part of the incoming radiation which
falls on a deciduous leaf is reflected at the leaf surface; part pene-
trates the leaf and is there used to raise its temperature; another, and
usually smaller, part passes entirely through the leaf, emerging from
its shaded side. It is necessary first of all to comprehend the part
played by each of these three processes. A number of botanists,
A. Seybold in particular, and many meteorologists have studied the
radiation economy of leaves and have furnished us a fairly good
idea of the process. Br. Huber (514) is one whom we can thank for
an excellent summary of the whole heat economy of plants. R. Orth
(527) recently has surveyed the work of the Seybold school.

We begin with the reflection of radiation from leaves. It is a func-
tion of wave length. To understand reflectivity we make use, as
before, of the albedo, which is the reflected radiation expressed as
percentage of the incident radiation. In considering reflection from
the bare ground we differentiated three spectral ranges and now do
likewise.

On the short-wave, or ultraviolet, end of the spectrum (wave-
lengths below 0.36 fj, = 360 m/jt) the albedo of living leaves is small;
it is less than 10. K. Biittner and E. Sutter (^07) found a value of
only 2 on a sand heath. Plants behave, accordingly, like sand and
earth.

In the visible spectrum from 0.36 to 0.76 ft, where we recognize
radiation as light, since it is visible to the human eye, the albedo of
green leaves lies between 8 and about 20. On the white surfaces of
panaschich leaves it reaches the exceptional value of 60. In the
table given in Chapter 13, an albedo of from 5 to 18 was given for
the forest while from 15 to 30 was given for fields and meadows.
These figures fit in well. Normally, then, even in the visible portion
of the spectrum, only one fifth, or at the most, one fourth, of the
light falling on a leaf is reflected.

It is otherwise in the long-wave, infra-red portion of the spectrum,
with wavelengths over 0.76 /*. As early as 1925 A. Angstrom (260)
showed that the albedo amounts to 44, which is considerably higher.
The accuracy of this figure is directly ascertainable if one uses differ-

272

THE CLIMATE NEAR THE GROUND

ent filters in photographing a landscape containing trees. Such
filters allow only definite bands of wavelengths to pass, and of
course correspondingly sensitive plates must be used. In 1930 E. von
Angerer (5/0) published such photographs. In the infra-red photo
the trees in a landscape, which normally appear dark, are light —
almost white — a sign that they reflect much radiation.

Living plants, as a consequence of what has been said, have a
reflectivity highly dependent on wave length — in contrast to bare
ground. F. Sauberer (522) carried out comparative measurements of
a meadow with grass 12 cm high, and a concrete pavement. The
result is reproduced in Fig. 120. The solid curve represents the

Meadow, 12 cm high
Concrete, dry

green gro

30
in

—

"""""•

x^~"

""*••-
^>

m 750 TOO 650 6W S50 SCO

Wave length m/i

FIG. 120. The reflection from the surface of living plants (continuous line) and a
dead surface (broken line) in relation to the wave length. (After F. Sauberer)

meadow. It shows a weak maximum of reflectivity at 500 m/z, (in
the green) and a very strong maximum at 800 m/i, which is far into
the infra-red portion of the spectrum. The albedo here is 45, which
is in good agreement with the measurement of A. Angstrom. Con-
crete behaves differently, its reflectivity decreasing gradually as the
wavelengths shorten.

As we pass still further into the infra-red, the albedo of plants
seems to decrease again. K. Egle (5/1) , for the green leaves of five
different plants, found values from 33 to 49 (averaging 42) in the
neighborhood of i.o fji while around 2.4 ft the values were between
5 and 16 (averaging 9). Mention should probably be made here of
the measurements of G. Falckenberg (269) who, for the wavelength

THE HEAT ECONOMY OF PLANTS 273

region Xraax = 10 p, ascertained an albedo of 5. for zonal leaves of
pelargonium, and 4 for pine needles.

Surveying the data up to this point, we can represent it in the
following table:

TABLE 40

Spectral range

Wave lengths in /x

Albedo of leaves
and plants

Ultraviolet .

below 0.36

below 10

Visible light

0.36-0.76

8-20 with maximum

Infra-red

{0.8o
1.0

at 0.51 A*
45 (maximum)

2.4

10.0

9
5

This spectral distribution of reflectivity influences the heat econ-
omy of the plant. The less the reflection, the more radiation the
plant absorbs in the range in question. In the range of wavelengths
in which the sun radiates most of its energy, the plant is susceptible
to heat radiation. In one part of the long waves, however, the re-
flectivity (and, as we shall see later, the transmissivity also) is greater
— the absorption correspondingly less. According to KirchhofFs law,
for a definite wavelength and temperature the ratio of absorption to
emission (outward radiation) is constant. In waves of about 0.80 p
where plants absorb little, they also emit little. Long waves, how-
ever, as already stated, are the range in which nocturnal outward
radiation at low temperatures proceeds — the range which the
ground and plants of the earth use, in comparison with sun tempera-
tures. It is consequently not to be concluded, as A. Angstrom (260)
believes, that a plant cover possesses in selective reflectivity or ab-
sorptivity a certain self-protection against nocturnal loss of heat by
radiation. It will take further measurements to give assurance on
this point.

Plant leaves also possess a certain amount of transmissivity for
radiation. This can be directly observed in the midst of a dense
deciduous forest in so far as the visible spectrum is concerned by the
dim green light. The permeability (or, less aptly, "transparency"),
which physicists and meteorologists call "transmissivity," and which
botanists designate also as "diathermance," varies, like the albedo,
with the wave length. In general a high albedo corresponds to a

274

THE CLIMATE NEAR THE GROUND

high coefficient of permeability. By the latter term we mean the
percentage of incident radiation which the leaf transmits.

In the short wave range permeability is small — less than 10, as is
the albedo. In the visible spectrum it varies from 5 to 20 with a weak
maximum at from 0.55 to 0.58 //,, in the yellows and greens. The eye,
which is most sensitive to green, perceives the light in a forest as
green. There is, however, a very strong maximum in the infra-red,
at about 0.8 /*,. Fig. 121, which is taken from the measurements of
F. Sauberer (522) shows how abrupt the increase of permeability is

FIG.

TOO 600

Wave length in m/x

121. Radiation permeability of three different leaves in relationship to wave
length. (After F. Sauberer)

at this point. The permeability values are given in relation to wave
length, the solid line representing a young leaf from a red beech ; the
dotted line, one from a primrose, and the dot-and-dash line, one
from a hellebore. If our eyes were equally sensitive to all wave
lengths, the depths of the forest would appear infra-red to us, rather
than green. K. Egle (5/1), who measured the wavelengths of i.o
and 2.4 ft, found an average permeability of 47 and 25%, respec-
tively.

While the reflected radiation is partially diffuse (non-directional)
and partially directed, the penetrating radiation is entirely diffuse.
It should be mentioned that, according to F. Sauberer, it makes a
difference whether the radiation strikes the upper or the lower
surface of a leaf. For example the index of permeability of a white
poplar leaf was 22 when radiation fell on the upper side, but only
15 when it fell on the lower side.

Within a forest, radiation is not only weakened but altered in its
spectral composition. We shall have data on this point to offer in
Chapter 30, which deals with radiation relationships in a forest.

THE HEAT ECONOMY OF PLANTS 275

Forest shade, consequently, is different shade from that which is
observed on the north side of a high wall which shuts off the sun.
Diffuse sky light is always particularly rich in short-wave radiation
(blue sky). This latter kind of shade A. Seybold (52^) proposes
calling "blue shade." Considering the maximum transmissivity of
leaves at 0.8 //,, the corresponding name for forest shade would be
"infra-red shade"; it is better to stick to the way it looks to us and
call it "green shade." In botany the distinction is an important one
for the structure of blue-shade plants which in diffuse skylight by
the wall of a house is quite different from that of green-shade plants
which spend their life under the leafy screen of a mixed or deciduous
forest. To follow this further would take us too far into the realm
of botany.

If the albedo (R) and the index of transmissivity (/) are known,
we at once have the percentage of absorbed radiation (A), for
R + D + A must equal 100 units, or the total incident radiation.
We must therefore conclude that in the ultraviolet absorption
amounts to about 90% of the incident radiation, that it diminishes
with increasing wavelength, reaches a minimum of about 25% in
the yellow-green and, after another slight rise, falls off to its chief
minimum of between 5 and 10%. Still further into the infra-red
the value of 10% is again attained at i.o ^, after which the absorp-
tion climbs to 65% at 2.4 //,.

While the radiation relationships of a certain place do constitute
an inescapable climatic factor for bare ground (see Chapt. 36), they
do not do so for the living plant. It has a great many ways of pro-
tecting itself against too strong radiation and several possibilities by
which it may lessen the harmf ulness of too much outgoing radiation.
"Nature," says P. Filzer (5/2) "does not work by a diagram; a living
substance is no stiff physical system but a plastic, and can solve the
same problem in many different ways/'

The possibilities of avoiding excessive irradiation rest with the
structure and position of the leaves. An example of this has been
given by the case of the compass and gnomen plants. The profile
position of their leaves in a sunny location (called the vertical posi-
tion) results in the least possible surface area being offered to mid-
day radiation. A rippled leaf surface hinders the whole leaf from
receiving the maximum radiation no matter what the sun's position.
The albedo varies with changing leaf color. In the case of the
cactus, the often too violent impact of radiation is partially broken
by tufts of thorns which lie parallel over the leaf surface, or felt-like

276 THE CLIMATE NEAR THE GROUND

cushions of a similar nature. The part played by the anatomical
structure of the plant belongs to the province of botany. Bruno
Huber's summary (514) of this subject may be consulted.

Of more interest perhaps to meteorologists is the variable lighting
of leaves. By this we mean the ability of leaves to initiate variations
in the radiation balance by means of movements of their leaf organs.
There are leaves of certain living plants which, after 15 minutes of
irradiation by an electric heater, will assume their daytime sleeping
position, in which, by the action of their peculiar leaf joints, they
are able to crease their surfaces and so lessen their effective heat-
absorbing area. In their nighttime sleeping position too, the leaves
stand almost vertically, perhaps in order to reduce nocturnal radia-
tion losses. O. W. Kessler and H. Schanderl (5/7) have published
some fine photographs of the white melilot (melilotus albus) with
its leaves in different positions, to which we have already referred.
In the dry Mediterranean district and on tropical steppes there is
said to be noticeable "a peculiar change in the appearance of the
landscape according to the hour of the day." It is apparent that this
must react on the microclimate.

In addition to radiation, there are other factors which affect the
temperature of plants. The respiratory heat of plants as a result of
metabolism inclines to a rise of temperature. Normally it may be
disregarded, and only in the sprouting and blooming of the higher
plants does it attain a magnitude worthy of consideration. Even
then it is most limited. Evaporation (transpiration of plants) tends
to cooling. Since for every gram of water given off, there are from
570 to 600 calories required of the plant according to the tempera-
ture, this heat loss may reach considerable proportions. Finally there
is heat exchange with the surrounding air, which, for the plant, may
be either positive or negative.

Taking all the above-mentioned factors into consideration, there
finally results for the plant at any given moment a positive or a
negative remainder, which occasions a rise or a fall of its tempera-
ture. In general, therefore, a plant, a leaf, a needle, a branch, does
not have the same temperature as the surrounding air. This is a
fundamental rule which one must heed carefully. In general it may
be said, that the plant is warmer when the ground surface is warmer
than the air layer resting upon it — when there is a positive radiation
balance. This is the case during the day. Conversely, by night the
plant is, for the most part, cooler than the air.

The differences between plant and air temperatures disappear,

THE HEAT ECONOMY OF PLANTS 277

however, if the plant is not carrying on its own radiation exchange.
This occurs only when there is no such exchange in the atmosphere,
to speak of — i.e., at the times of transition from positive to negative
radiation balance and vice versa — at evening and morning and,
secondly, with completely covered sky, rain, driving snow, fog, etc.
They disappear also in the case of those parts of a plant which are
screened by other parts. The inner and under parts of a tree or
shrub — yes, even a small plant — in this case have an interchange of
radiation only with other parts of the plant and these, in general
have the same temperature. Radiation exchange with the surround-
ings is carried on by only the outer leaves.

This precept finds practical application in estimating frost danger.
A two year old pine seedling, standing under an old-wood screen of
frost-hardy birch, has about the same temperature as the surround-
ing air. The same plant, standing in the open nearby, will be colder
than the surrounding air. The temperature difference between in-
side and outside is, in the case of the plant, greater than that
measured in the air with the aspiration psychrometer. (See Chapter
40.)

The measurement of plant temperatures is, in itself, no easy task,
because the plants to be measured must remain undisturbed in their
life functions. Thin leaves, needles and blossoms present difficulties
on account of their smallness. A small mercury thermometer can be
used with tree trunks, thick branches, fleshy leaves and fruits. In this
way F. D. Young (52$) in California, for example, observed orange
temperatures just beneath the skin, on the side of the fruit turned
away from the tree, in order to let the fruit present their own evi-
dence in the matter of frost danger. The oranges could be super-
cooled to — 4.2°C before they froze.

The method of quickly wrapping a freshly picked leaf about a
mercury thermometer will give a rough approximation of leaf tem-
peratures. The calorimetric method has been tried also. In this, leaves
were dropped into a vessel filled with turpentine and the tempera-
ture change measured. If the specific heat of the leaves has been
determined, the original leaf temperature can be calculated.

The thermoelectric method is one which is today in common use.
One soldered junction of copper and constantan wires is kept at a
fixed temperature by means of a portable thermos flask, while the
other junction is formed into a "thermoneedle" which is inserted in
the plant or pressed against it. An accuracy of 0.1° can be attained
without difficulty.

Ordinary thermoneedles, however, are not fine enough to prevent

278 THE CLIMATE NEAR THE GROUND

radiation errors of 2 or 3° C in sunny leaf surfaces. Furthermore, it
is the portion of the soldered joint nearest to the conducting wires
which is most effective and this in use is often outside the leaf, bud,
etc. A. Made (5/9) who worked this problem out carefully, has
recently successfully adapted the Albrecht resistance thermometer
to the measurement of leaf surface-temperatures. He used a 0.015
mm platinum wire a few centimeters long. Not only can an accuracy
of about 0,2 C be realized, with proper handling, but the apparatus
has the further advantage of being capable of recording. The temp-
erature records shown in Fig. 123 were obtained with this apparatus.
What excellent results can be obtained from its use is attested by the
detection of a pool of cold air on the upper surface of a radiating
castor-oil leaf by H. Ullrich and A. Made (525). The leaf was de-
cidedly arched and formed a little bowl whose rim was i cm high.
The tiny drop of cold air reached just to this height.

Let us now discuss what more can be asserted as to the tempera-
tures of plants, particularly in relation to air temperature.

Minutes 0

~ — leaf temperature
— air temperature
FIG. 122. Reaction of leaf temperature to sudden sunning

Every leaf has a certain thermal lag. If it is exposed to the sun, it
takes some time, perhaps 5 or 10 minutes, before its temperature has
risen to such a point that the heat lost to the unaltered air equals the
heat gained by the absorption of insolation. Fig. 122 shows the
temperature curve of two different leaves which were suddenly ex-
posed to the sun and as suddenly shaded. The observations were

THE HEAT ECONOMY OF PLANTS

279

435.38

14-
16-
18-
20-
22-
24-

14.538
10-

12-
14-

18-
20-
22*
24-

15.5U
4-

15 20 25 30 359C
I I I I I

15 20 25 30 35°C

FIG. 123. Registration of leaf surface temperature and (dotted line) air temperature
for two days. (After A. Made)

280

THE CLIMATE NEAR THE GROUND

made by A. M. Smith (524) in 1906 under the tropical sun in a
garden at Seradeniya, on the island of Ceylon. The current air
temperature is represented by the dot-and-dash line. At the moment
of exposure to the sun, the temperature of the thin leaf rises immedi-
ately; that of the fleshy leaf rises only after a short interval, quickly
at first, then more slowly. The temperature approaches the well
known Newton curve of equilibrium, which has been described
above. The difference between air and plant temperature reaches the
significant amount of 11° in the case of the fleshy leaf and still
higher for the magnolia.

Fig. 123 shows a continuous record of the true leaf surface
temperature of two different plants over a period of two days. It
was published at Muncheberg by A. Made (5/9). The solid line
refers to a fleshy, hard leaf of Bilbergia nutans (a hothouse plant of
the pineapple family) . The broken line refers to the thin, deciduous

9 h 8

FIG. 124. Temperatures of the air, the earth's surface and different plants. (After

Br. Huber)

leaf of Plectranthus fruticosus (cockspur, a small shrub of the
labiaflora). The concurrent course of the true air temperature is
shown by the dotted line. During the middle of the day the leaf
surfaces are 10° or more warmer than the air. The greater lag of
the fleshy leaf is discernible in the delayed rise and fall of tempera-
ture; the greater mass, in the lower maximum. When the radiation
balance is negative, the leaf surfaces are cooler than the air, the
thinner one correspondingly cooler than the thick one. Splendid
temperature records of a peach twig are to be found in another
paper by A. Made (518).
Similar measurements by H. Ullrich and A. Made (525) indi-

THE HEAT ECONOMY OF PLANTS

281

cated a difference of more than i° between sunny and shaded por-
tions of a leaf only 2 cm apart.

At Rathen in the Elbsandstein mountains, Br. Huber (5/5) made
simultaneous records of surface, air and plant temperatures in a
wind-shielded SSW trough. He used thermocouples which regis-
tered on a Hartmann and Braun multiple thermograph. Fig. 124
gives an example. It shows, what Br. Huber found to be a rule of
general application, that all the plant temperatures lay between the
air temperature and that of the ground surface. Projecting parts of
plants attained about 1/3 of the temperature excess of the dry
ground; parts near the ground, from 1/2 to 2/3 of this temperature.
The record of Lactuca in Fig. 124 is an example of the former,

FIG. 125. Course o£ temperature in a green alder twig which penetrated the snow

cover

while that of a Carnegia (barrel cactus), which in Fig. 124 is errone-
ously labeled "Echinocactus," represents the latter. Nearby the
Opuntia surface-sprout, which was growing in a naturally vertical
position, and whose temperature followed a course similar to that
of the Lactuca, an Opuntia sprout was placed artificially perpendi-
cular to the sun. The corresponding record is designated Opuntia -1-
in Fig. 124. The temperature of this sprout closely approached that
of the ground surface.

Br. Huber has found a maximum temperature of 56° for living
plants in a sempervivum with air temperature of 35°. Temperatures
of 50° have been repeatedly obtained by different observers.

282

THE CLIMATE NEAR THE GROUND

Finally it should be pointed out how the radiation and tempera-
ture conditions of the air near the ground determine the temperature
of the plants growing in it. Towards the end of the winter of 1933,
while snow was still on the ground G. & P. Michaelis (520) made
some thermoelectric bark temperature measurements of a green alder
twig at Allgau in the little Walser valley at 1670 m msl. The results
are shown, by means of isopleths, in Fig. 125.

March 16, 1933, was slightly cloudy; the air temperature ranged
between —2 and +4° C. The temperatures in the bark of the green
alder twig fluctuated between —4 and +30° C. The greatest tem-
perature fluctuation would be looked for at the snow surface, on

38
36
3V
32
30
28
26
2V
22
20
U
16

12
10

t,.*

?-•

'^.

*•'/

'•'/

•>N

„•'"'

— .

v. \

ty (

•*

^5^

-0

-9
— p

pen

air

*«-*.

•*>*,

•«^_

—

auze b
archme
ellopha
erforate
I

39
nt bag
ne bag
d cetlo
1

•lie

155.

—

- c
- P

aha

ne bag

5 6 7 8 y 10 11 12 13 fV 15 W 17 18 W 20 21 22 23 t

FIG. 126. Course of temperature on five clear days in bags which are used for biologi-
cal experiments

account of radiation. The cooling effect of the snow through con-
duction, however, raised the point of maximum fluctuation of the
twig temperature somewhat above the snow level; at least as long as
incoming radiation prevailed, the twig was overheated. At 10 A.M.
the temperature maximum was at a height of about 15 cm, but when
the snow began to melt right by the twig, the maximum moved
downward. At night, when the counter-radiating twig had almost
the same temperature as the snow the minimum remained at the
surface.

THE HEAT ECONOMY OF PLANTS 283

The following microclimatic phenomena are presented as a sup-
plement.

In gardening experiments, blossoms are often enclosed in parch-
ment or gauze bags or the like in order to prevent their random
pollination by wind or insects. It is self-evident that such covers
considerably change the radiation and heat balance of the flowers
and thus their temperature. Anyone making biological experiments
with blossoms should not overlook this fact.

N. Weger (527) has studied the relationships involved in this
condition. Fig. 126, which is based on mean values of five radiation
days in May and June, 1937, shows the curves for the temperature in
the open air (heavy line) and in bags of four different materials.
During the time of incoming radiation it is as much as 15° hotter
in the bags than in the open; at the time of outgoing radiation it is
somewhat cooler. The temperature march in the little bags with
their small volume of air follows the course of radiation more
quickly than does the free air and therefore attains a maximum a
good three hours sooner. The air heats up most rapidly in the
transparent cellophane bags. Perforating the bags brings the maxi-
mum temperature down by 2° on account of the improved convec-
tion. The ventilation is best in the gauze bags; their overheating
amounts to 6° or 7° less than in the cellophane bags.

CHAPTER 27

RADIATION AND TEMPERATURE RELATIONSHIPS IN A Low
PLANT COVER

In Chapter 17 we spoke of plants, indeed, but only of such small
ones — by reason of their youth or nature — that they could hardly
alter the characteristics of the ground surface. This kind of a plant
cover we called ground cover and have already said all there is to say
in describing its influence on the microclimate.

Now, however, our attention is directed toward what happens
when the plant stretches up into the sea of air yielded to it from the
sway of the wind, and brings continually higher layers into its sphere
of influence, gradually forming the habitat climate from the ground
up.

The high forest is quite the opposite of the ground cover. Under
the close crown of an old stand there is room for a body of air
several dekameters deep, whose properties are conditioned by the
stand. This is the realm of the trunk climate, peculiar to the forest.
Insofar as there still is a climate near the ground, it is raised from
the solid surface and re-located at the crown level. There is where
radiation is absorbed and sent out, the free-air wind is retarded,
and water is given off to the air as it is from the earth in the open.
In place of the ground, which now lies within the forest depths,
de-activated, the crown surface becomes the outer, active surface.
This conception, introduced to meteorology by A. Woeikof, will
give us the best idea of the interaction between plant and micro-
climate. We must realize, however, that the expression "surface" is
no longer to be used in its old sense, but idealized, for at the top of
the forest it is a space with decided vertical compartments which
has taken over the role of the solid ground surface.

Between ground cover and forest we introduce as an intermediate
stage the low plant cover. To this belong all agricultural crops such
as grain-fields and potatoes as well as forest plantings which have
not yet passed the sapling stage, all bushes, meadows, weed fields,
etc. The low plant cover differs from the ground cover in that it
characterizes the air space which it encloses and on its part interacts
with the outer air; it differs from the forest in that it lacks the
seclusion from the outside which characterizes the space within the
dense crown as an independent body of air. The two following

RADIATION AND TEMPERATURE 285

chapters have to do with this low plant cover. Together their title
should be: "Agricultural Microclimatology," were it not that on the
one hand they also include problems which are important to forestry
while, on the other hand, agriculture has an ardent interest in the
microclimatic questions which have been described in the earlier
sections of this book. The relations of forest and microclimate are
to be treated in Chapters 29 through 35.

In order to depict the microclimatic relationships in and over a
low plant cover, let us begin with the radiation economy.

Fig. 127 represents, side by side, an extent of bare ground and a
meadow with grass i m high. The insolation from above is com-
pletely absorbed in the surface layer of the bare ground. In the

1.0hm

0.5

'108 "*•* 0.19*

Bare ground Meadow

FIG. 127. Gradual absorption of radiation in a meadow. (After A. Angstrom)

grass, a greater portion is caught between the blades. In the dia-
gram, the values of insolation intensity are expressed in calories per
sq cm and min. as A. Angstrom (260) observed them in a field
grown up with meadow grass and Dactylis glomerata. We see that
at 50 cm above the ground the intensity is still scarcely weakened,
but then it decreases rapidly to a fourth of its original value at 10
cm, while at the ground it is only a fifth of the radiation which falls
above.

In a young elm thicket, 3 m high, which was filled with dense
undergrowth and overgrown with clematis, F. Sauberer (522) ob-
served the radiation still to be found in the first meter above the
ground, using Lange resistance cells with a filter, which are espe-
cially sensitive for wavelengths in the orange. It varied greatly with
the season. In June and July there was a minimum of brightness
close to the ground for at this season the low early-growing plants

286

THE CLIMATE NEAR THE GROUND

were fully developed. As these die down, the brightness at the
ground increases. Here are some of the figures:

TABLE 41

BRIGHTNESS DISTRIBUTION IN DENSE ELM THICKET
(in % of outside brightness)

Height above ground

ioo cm

July 5, IQ^6

O.OI

o 06

O.I 3

O 23

2 I

Tulv IQ, iQ^6

o 03

O 17

O 41

~> ~>

Nov. 15, 1936

. . . . 0.50

After the leaves fell in November almost a fourth of the outside
brightness penetrated to within 10 cm of the ground, but only one
half of one percent reached the ground. In the winter condition of
the elm thicket not quite ioc/o reached a point i cm above the
ground, as further measurements showed.

Fig. 128 shows the daily march of visible radiation in different
agricultural crops. The radiation intensities there represented were

340

'in the clear

4* ' •» «*

barley 12-15 cm high rye 80 cm high

20*

clover 30 cm high

FIG. 128. Daily course of brightness on the ground in different fields on a May day.

(After F. Sauberer)

measured (again by F. Sauberer) May 6 and 7, 1935, on the ground in
the fields. They represent the amounts of light which are available
for germinating weeds in the fields. The great difference between
the vertically standing blades of the scanty barley or dense rye, and
the broad clover leaves which intercept much radiation is very
evident. We shall revert to this difference in considering tempera-
ture relationships.

There is also a considerable exchange of radiation within the
lower plant cover, to which F. Sauberer has called attention. As
portions of plants here and there are warmed by absorbed radiation

RADIATION AND TEMPERATURE 287

from sun and sky, this heat cannot not be passed on either by the
poorly conducting plant or by the poorly conducting air. Even
convection is limited on account of the enforced inertia of the air
entrapped among the plants. Heat transport takes place rather by
radiation which passes from the warmer to the cooler parts of the
plants. Furthermore the radiation reflected from the plant surfaces
must not be forgotten. The interchange of radiation, therefore,
within the plant cover is very complicated and becomes still more so
by reason of the selective properties of leaves, which were described
in Chapter 26. As a result, not only the amount but also the nature,
of the radiation within the lower plant cover is subjected to continual
change.

However difficult it may be to comprehend these processes by
themselves, the one fundamental fact is plain, that a plant-covered
plot receives no more and no less heat than a barren plot of like
size, for vegetation does not affect the intensity of irradiation.
Only the portion lost by reflection can differ. Likewise the out-
going radiation from i sq m of plant-covered soil and the like area
of bare soil by night are equal (in contradiction to the false repre-
sentation in the first edition). Here, too, it is only variable ab-
sorptivity and albedo — with consequent altered power of radiation
— which can make a difference, and even this is of scarcely any
practical significance.

What, however, completely alters the effect of plant cover is the
distribution of the given amount of heat gained or lost. While, in
the case of bare ground, the whole exchange is at the border surface
between soil and air, there is available in the plant cover a high
vertical space instead. This distribution of the day's warmth protects
from sudden heat, while the similar spread of nocturnal cold pro-
tects from damagingly low temperatures. Plants modify the temper-
ature fluctuation of the climate near the ground.

With this we turn from a consideration of radiation exchange, to
heat exchange. Let us begin with the temperature distribution in a
low plant cover at the time of incoming radiation.

Fig. 129 represents midday temperatures in a bed of antirrhinum
(snapdragon) in the summer and autumn of 1923 as observed by
R. Geiger (179) in the convent garden of St. Boniface at Munich.
The plant cover is charted schematically according to its measured
height and density. The flowers with their horizontally placed leaves
capture the insolation in the upper layers. The "outer active surface"
and with it the temperature maximum consequently is located near
the top of the plant cover. July is an exception, since in this month

288

THE CLIMATE NEAR THE GROUND

the young low-growing plants are still scattered so that the heating
of the open bit of ground between them determines the vertical
temperature distribution.

A comparison of the July and August curves will make clear why
on microclimatic grounds, young crops often do not begin to grow

July

August

September

October - November December

FIG. 129. The incoming radiation type in a flower bed

luxuriantly until they have "joined hands" — i.e. when the separate
plants touch one another. In July (Fig. 129) this is not yet true;
the sensitive young plants still have to endure the sudden midday
heat of open ground. In August, however, the outer active surface

FIG. 130. The temperature unrest in a ilA m high growth of a young pine showing
external active surface layer

is raised above the ground. This ties in with the necessity of shield-
ing sensitive newly set out garden plants in hot weather.

Fig. 130, which was published by R. Kanitscheider (55^), shows
how all weather phenomena act on the top of the plant cover. He
took temperature readings with a resistance thermometer every 2
seconds in dense growth of ground pine on a southerly slope near
Innsbruck (1600 m msl). Fig. 130 represents the result in relation
to height above ground and time of day on the cloudless 28th of

RADIATION AND TEMPERATURE 289

July, 1931. The figures within the chart are mean differences be-
tween two successive readings in tenths of a degree. At all times the
top surface of the pines is the most turbulent zone. The turbulence
is greatest, not at the time of maximum temperature, but at the time
of maximum radiation.

It is different with the midday temperature distribution in a plant
cover consisting of vertically standing single plants. Fig. 131 gives
measurements which R. Geiger (179) made in 1925 in a field of

IIIIIIIIIHllf

i - 30 April 1 • 10 May if • 20 May '21 - Jf May 1 June • 10 July 26 July - 12 August

FIG. 131. The incoming radiation type in a field of winter rye

winter rye. These measurements were made on the Nederling ex-
perimental plot of the National Institute for Horticulture and Plant
Protection at Munich. Until the 20th of May when the grain had
already reached a height of almost i m, the temperature maximum
remained at the ground surface, so easily could sun and sky radia-
tion penetrate down between the stalks. Then the site of the maxi-
mum rose but still remained far below the top surface of the plants.
After the grain was cut on July 26th, the normal incoming radiation
of an unplanted field re-established itself.

According to the nature of the plant cover, the outer active surface
either coincided with the upper surface of the plants or lay far below
it. We can thank P. Filzer (529) for systematic investigations into
the influence of size and density of vegetation. As an example of a
horizontally distributed plant community he chose the sunflower;
for one which is vertically distributed, maize. Surface area and
density of the plantings were varied. Nine beds were sown with
each plant. The areas of the beds were 90, 64 and 45 cm square. The
density of seeding was so regulated that the distances between plants
amounted to 8.6, 6.0 and 4.2 cm. As the average noon measurement
on four clear days of September, 1934 he found the following temp-
erature differences between the ground surface and a point i m
above it (+ means the ground was warmer, — means it was colder,
than at i m) : —

290 THE CLIMATE NEAR THE GROUND

TABLE 42

TEMPERATURE GRADIENT ABOVE THE GROUND AT TIME OF
INCOMING RADIATION

Structure of Plant Cover

Horizontal (sunflower)

Vertical (maize)

Density: Large Medium Small

Large Medium Small

Large Area

. . -3.0

-1.8

+0.5

-2.5

-0.4

4-3-7

Medium Area . . .

• -i-7

-0.7

+0.9

Small Area

. . --0.6

+0.2

+3-8

— 1.2

4-1.0

+3-2

The closer the plants stand and the greater the area of the bed, so
much cooler is the lowest air layer during the whole day with
consequent greater development of a characteristic microclimate in
the plant cover. On the other hand the highest temperature occurs
at the ground consistently in all cases where the density of stand is
least, but with medium density it occurs there only if the bed area
is small.

FIG. 132. The outgoing radiation type in a flower bed

At night, with outgoing radiation, the relationships are different,
and to these we shall now turn our attention.

Referring again to P. Filzer's investigations they indicated no
clear nocturnal relationship between temperature distribution on
the one hand and the size and density of plant cover on the other.
The air at night was consistently warmer at the ground than at a
height of i m. Radiation was from the top surface of the crop as
was to be expected.

Now let us return to R. Geiger's measurements in the antirrhinum
bed and in the winter rye. The nocturnal curve is given in figures
132 and 133. In both crops the outgoing radiation was greater from
the upper parts of the plants (which radiated freely to the cold night
sky) than from the lower parts which, for the most part, gave off
their heat only to the upper parts. The nocturnal cold air accumu-

RADIATION AND TEMPERATURE

291

lated first in the upper part of the plant cover. That is also where the
lowest temperature would be found, if the cold and consequently
heavy air did not sink down.

This sinking can easily take place in the flowerbed (Fig. 132)
since the parts of the plants stand rather far apart, leaving plenty
of air space clear to the ground. In the rye field, however (Fig. 133)
the stalks below form a thick felt which slows up all air movement.
Thus it comes about that in the flowerbed the daytime maximum
occurs above and the minimum at the ground, while in the grain
field the maximum occurs near the ground and the minimum half-
way up. These conditions are significant in questions of frost
protection.

For the daily march of temperature in a low plant cover, we refer
to H. Berg's description (98). On the 6~7th of October and the
20— 21 st of November, 1934, he recorded the temperature and vapor

2.0m

Night

Apr. 1-30 May 1-10 May 11-20 May 21-31 June 1-July 10 July 26-Aug. 12

FIG. 133. The outgoing radiation type in a field of winter rye

pressure in a 10 to 15 cm calluna cover on the Bissendorf moor
between i m above, and 30 cm below, the ground surface, publish-
ing his results in tables and diagram. R. Fleischmann (550, 537)
carried out various measurements in different kinds of grain fields,
in tobacco and in corn and has been able to show thereby that each
kind of grain has a particular kind of "species" climate. At
Gottingen W. Paeschke (567) made short series of measurements
in fallow land, in low and high grassland, in high wheat and in
turnip fields with excellent physical measuring technique. As for
agriculture, E. Tamm (545—547) at Berlin obtained most comprehen-
sive records of all the important weather elements over a period of
years. His measurements in crops of winter rye, wheat, barley, oats,
potatoes, corn, lupines, hemp, soya beans and flax are unfortunately
worked up according to the method of temperature summation and
averages, exclusively. At the agrarian meteorology research station

292

THE CLIMATE NEAR THE GROUND

of the Imperial Weather Bureau at Giessen, W. Kreutz (556) made
a series of measurements in potatoes, flax, rape, corn, barley and
wheat and has shown the manifold implications of the problem in
an entirely new method of attack.

From the measurement of A. Mades (5jS) at the research station
of the Imperial Weather Bureau at Miincheberg (Mark) we offer a
daily temperature curve from a stand of topinambur. The records,
which were made with a radiation-shielded resistance thermometer,
ran throughout August, 1935. Fig. 134 shows the temperature
march for Aug. 4. (The mean values for the month of August give
practically the same daily march only somewhat smoothed.) At the
time of measurements the topinambur stood 73 cm high. The six
points of measurement are shown, according to their height, by
arrows at the right-hand end of the illustration.

FIG. 134. Diurnal temperature course on a clear August 4, 1935, in a stand of
Topinambur at Miincheberg. (After A. Made)

In the open planting which still allows the sun partial access to the
soil, the noontime temperature maximum lies at the ground. Not
until the latter half of August does it rise from the ground, as the
crop thickens. A fresh wind, A. Made found out, had the same effect
as thinner seeding. On the one case increased convection removed
the excessive heat above so that the location of the temperature
maximum is lowered; in the second case, it is located at the ground
from the beginning, as a result of the permeability of the crop to
radiation. At night the minimum is evidently at the upper surface
of the vegetation. The topinambur thus behaves like the grain
field in figures 131 and 133.

There is still one particular crop to consider, which depends to a
great extent on the peculiarity of the climate near the ground —
grapes. It was R. Kirchner (5^) who carried out the first useful
studies in vineyards of the Palatinate. They have been edited by
K. Sonntag (544) and extended somewhat. More recent measure-
ments have been made by N. Weger (551).

RADIATION AND TEMPERATURE 293

In the Palatinate the vines are customarily supported on wires
so that they reach a height of only 70 to 120 cm and take full advan-
tage of the sunny microclimate near the ground along the slopes.
About noon on Sept. 17, 1933, K. Sonntag found the temperature
depicted at the left in Fig. 135 in a vineyard at Mussbach, where the
rows ran north and south. The graph at the right shows nighttime
conditions. The active surface is doubly present in this vineyard.
The surface of the vines heated up, as well as the ground between

a, ;
J8

At midday At night

FIG. 135. Temperature distribution on September 17, 1933, by day and night in a
vineyard. (After K. Sonntag)

the vines. The temperatures at the ground, however, for reasons
which we have explained, are considerably higher than those of the
vines. By night it is especially cold at the height of the trunk. This,
as K. Sonntag remarks, is very important in the utilization of dew
by the plants. Because dew is always precipitated on the coldest
surfaces, the leaves are thoroughly wet at night, while the branches
and ground remain dry. "Even outside the vineyard an iron bar
standing on the street was dry from the ground up as high as the
first branches, though covered with water drops at the height of the
foliage."

All the observations made thus far on temperature relationships in
the lower plant cover refer to the climatic province of central Europe.
It is granted that in northern countries the utilization of radiated
heat is of still greater importance. A. Wegener (550) pointed out in
reference to Lundager's measurements, which were made in north-
eastern Greenland at almost 77° N, that the temperature in the
midst of plants, as averaged from numerous summer observations,
was 8° to 9° — and, in some cases, even 16° — higher than in the
surrounding air.

294

THE CLIMATE NEAR THE GROUND

On the other hand we have interesting relationships in the tropics,
about which we are quite well informed. It appears that plant tem-
peratures show the same features as they do with us in the summer.
Careful measurements on this subject have been made by L. A.
Ramdas and his collaborators, R. J. Kalamkar and K. M. Gadre
(547, 542) at Poona in India at latitude 18°. Fig. 136 gives an ex-
ample from their work. It represents the average temperature dis-
tribution as to height for the hours of sunrise (temperature mini-
mum) and midday (temperature maximum), at three stations

200\-

midday

10V 22
Air temperature (°C)

FIG. 136. Temperature distribution in a field of 2^2 m high sugarcane, a field of

i l/z m high millet in comparison to open land in Poona. (India) (After L. A. Ramdas,

R. J. Kalamkar, and K. M. Gadre)

closely grouped. The time is the latter half of December, 1932,
a period which in India is characterized by the dry northeast mon-
soon, which blows from the land toward the coast. The open
country (solid line, with measuring points indicated by small circles)
has the midday incoming radiation type developed to a marked
degree. By night the lowest temperature occurs, not at the ground
surface, but 15 cm above — a peculiarity to which we have already
called attention in Chapter 7.

Millet (Rabi jowar) is not an irrigated crop. It stood 150 to 180
cm high at the time of the measurements. Its temperature through-
out the day is lower than in the open. The difference is greatest at
the shaded ground and becomes slight at the top of the grain. As
the millet grows higher and denser, the upper surface of the crop,
as later measurements showed, receives so much radiation that the

RADIATION AND TEMPERATURE 295

temperature there becomes even somewhat higher than in the open.
At night the migration of the origin of outgoing radiation to the
crop surface manifests itself by lower temperatures at heights over
50 cm, while beneath that height the plants gain little heat.

Sugar cane is an irrigated crop which stood about 2^ m high at
the time of measurement. The observations recorded in Fig. 136
were all made within the crop area. It is considerably cooler there
than in the open; at midday the difference near the ground amounts
to i4°C. Even at night it is still i° warmer in the lower layers. Not
until a height of 130 cm is reached does the cold air from the out-
ward-radiating crop surface make itself felt.

The author calculated the following daily temperature fluctuation
at the three places of observation during the period from the 4th to
the 1 6th of December 1932.

TABLE 43
Average daily temperature range in °C

Height above the
ground in cm

Open
country

Millet crop
unwatered

Sugarcane
watered

183

29.8*

30.0

27.5

122

30.0

29.9

27.0

30.6

29.8

26.4

31.0

29.7

25.5

31-6

29.9

24.6

32-5

29.7

23.8

33-6

29.4

23-3

34-8

29.3

23.0

36.7

29.3*

22.7*

While the greatest fluctuation in the open occurs at the ground
surface, this condition is reversed in the crops. At 183 cm which is
still somewhat below the top surface of the crop, the outer active
surface can be recognized in its temperature effect.

K. Wien (549) the German scientist who died at Nanga Parbat,
made test measurements in middle East Africa. On March 31 and
April i, 1934 he carried out some temperature measurements in a
young coffee plantation in the German colony of Oldiani, which is
situated in 3° south latitude, at an elevation of 1730 m. The meas-
urements were obtained at the ground and at a height of 1.5 m.
Measurements were also made in a neighboring forest of fullgrown
evergreens — a forest above the steppe zone. While the daily tem-
perature fluctuation in the open amounted to 11°, it mounted to 20°

296 THE CLIMATE NEAR THE GROUND

on the black one-time forest floor of the coffee plantation, since the
space between the coffee shrubs was sufficient to let the sun reach
the ground. In the high forest the daily range was only 8°.

H. Scaetta (5^) made observations in some of the tropical high-
lands. On June 19, 1929 he recorded the following noon tempera-
tures at Karisimbi, north of Lake Kiwu (2° S) at 4506 m msl.
Temperature, in the free air = 5° C
Inside Alchimilla thickets = 14.6° C
In the top layer of dry soil = 16.2° C
An hour later

Temperature, in the free air = 3.5° C

" in a clump of Poa glacialis = 17.4° C

" in a dry lichen sod on a lava plateau = 19.4° C

It appears that in tropical highlands the plant cover has the same
effect on the temperature as A. Wegener found in Greenland.

CHAPTER 28
HUMIDITY AND WIND RELATIONSHIPS IN A Low PLANT COVER

The first time anyone compares temperature, with a thermometer,
inside and outside a low plant cover, he is struck by the greater
warmth inside. Examples of this were given in Chapter 23. One
reason for this is the retention of ground heat by the protective plant
cover; the other is the direct addition of heat to this air-space among
the plants by means of radiation which leaves, stalks or twigs absorb.

Whoever compares, with a hygrometer, the atmospheric humidity
inside and outside a low plant cover, is likewise struck by the high
humidity which exists inside. Here also there is a two-fold cause.
On one hand, the plant cover (even if dead) retards the removal
of the water vapor given off by the soil, while on the other the living
plant cover gives off water vapor continuously because it must trans-
pire in order to live.

O. Stocker (56^), for example, observed the following atmos-
pheric humidity values in a meadow at Freiburg. These measure-
ments were made on an almost calm day — July 18, 1920 — with an
air temperature of 29°.

At a height of 100 cm in open air 57%

At a height of 13 cm, between clover leaves 78%

At a height of 2* cm, in the grass 96%

The extraordinarily high humidity gradient within the first meter
from the ground appears clearly in these figures. On July 1-6, 1930,
at Farmsen near Hamburg, E. Martini and E. Teubner (702) deter-
mined the following values of relative humidity in grass, on humus-
filled sandy loam : —

TABLE 44

Hour of day

9A.M.

1 2 Noon

3 P-M-

6 P.M.

In open air

... 88

In grass 50 cm high
In grass 20 cm high

89
98

68
85

49
78

80
80

In grass 10 cm high

298 THE CLIMATE NEAR THE GROUND

The comparison between heat and water content does not hold
absolutely, however. The temperature stratification reverses during
the course of the day : the ground which gives heat to the air by day,
receives it back by night. But as to humidity, the stratification, as a
whole, remains constant; a considerable current of water vapor is
continuously passing upward from the ground.

Nevertheless, as we look at it more closely, the humidity stratifica-
tion in a low plant cover becomes really complicated at least when
we are considering relative humidity, as is usually the case. It de-
pends, of course, not only on water-vapor content but also on tem-
perature. Let us first clarify the process of water-vapor enrichment
in the plant-filled air layer near the ground.

The sum of all the transpiring ground and plant surfaces standing
on a square meter of land amounts to between 20 and 40 sq m.
The output of water is greater in proportion since according to the
most recent research of P. Filzer (556) it is proportional to the
density of the crop. One might at first think that the contrary re-
striction of evaporation and the screening of the ground which also
increases with crop density would soon set a limit to the possible
yield of water. Yet such limitation is not noticeable with even a
forty-fold multiplication of evaporating surface.

According to measurements which J. Bartels and W. Friedrich
( £55, ^57) made at the Eberswald lysimeter installation, evaporation
from ground covered with vegetation is about twice that from bare
ground. This value which applies to dry sod may, according to the
studies of P. Filzer, increase to five-fold for other vegetation and —
for a short time only — to a maximum of eight-fold. This plentiful
supply of water vapor is more easily retained within the plant cover,
the denser the latter is. Consequently the relative humidity mounts
in proportion to crop density. For example, P. Filzer (556), as the
average of several readings, obtained the following values in corn
plantings of three different densities : —

TABLE 45

Density of stand

Dense

Medium

Light

Outside

Sq cm leaf area per cc air space .
Relative humidity

. 1.81
. 73

0.82
64

0.38
51

41%

At the crop surface the hygrometer is very erratic as moist air
parcels from the crop mingle with dry air from without.
If the ground is dry and the leaf development at a certain height

WIND RELATIONSHIPS 299

above it is especially rank, this will be evident in humidity stratifica-
tion. On three different days in the summer 1907 Gregor Kraus (72)
observed the following air humidities at a beautifully developed
male fern which was growing in the shade: —

TABLE 46

Forenoon

Noon

Afternoon

At i m (above the fern)
Between the leaves
On the shaded forest floor ...

58
70
60

71
88

The lack of evaporating leaves at the ground and probably also the
reduced amount of evaporation of the soil at lower temperature
make themselves felt.

To be sure the air between the leaves never attains complete
saturation with water vapor. The amount of evaporation depends
on the temperature of the evaporating surface, not on that of the
air. As soon as evaporation begins, the evaporating surface experi-
ences a cooling effect; this, in turn, reduces evaporation, so that the
water vapor given off from the leaf surfaces does not suffice for
complete saturation of the contiguous air. Consequently the air
between plants also remains in general below 100%. R. Wenger
(565) observed 98% between leafy plants on a rainy day. This is
the highest verified value which has been observed. (It is recognized
that determination of relative humidity close to the saturation point
is attended with great difficulties.)

In dry times and dry regions (and here again in light plantings)
the increase of air humidity between plants in comparison with the
surrounding air is no longer noticeable since then the temperature
effect prevails. O. Stocker (564), as a result of his studies of water
balance of Egyptian desert plants, came to the conclusion that: —
"There is no case where a rise of relative humidity within the leafy
framework of a desert plant has been proved; on the contrary, in
several instances the humidity in the neighborhood of transpiring
leaves has shown a diminution. This phenomenon results from the
fact that, on the one hand, the desert wind hinders any enrichment
of the transpired water vapor about the transpiring organs, while,
on the other hand, the insolation reflected from the earth and also
from the plants as heat, favors an increase of temperature and
a consequent lowering of relative humidity in proximity to the
plants."

300 THE CLIMATE NEAR THE GROUND

This temperature effect tends finally to the air in the lower plant
cover being drier than that in the open. The measurements of F.
Firbas (557) on habitat conditions over sandstone and basalt led to
the conclusion that: — "Where the ground in open plant communi-
ties can warm up considerably above the air temperature, the rela-
tive humidity during the day decreases toward the ground. Where,
on account of a close plant cover, the differences between air and
ground temperature become less, or the latter lags behind the
former, the opposite condition prevails, the relative humidity in-
creasing toward the ground."

Returning to the manner of expression employed in Chapter 10,
we may say: "Although the transfer of water vapor in the ground
air is intensified through the plant cover, the vertical distribution of
relative humidity is as a rule of the wet type. In dry regions and
dry times the humidity stratification may reverse itself under the
over-ruling effect of high temperature."

We have only one series of observations from the tropics — that
of L. A. Ramdas, R. J. Kalamkar and K. M. Gadre (542). The re-
suits are shown in Fig. 137 and 138. They refer to the same research
area at Poona and the same period from the i6th through the 31 st
of December, 1932 as does Fig. 136. In southern India a different
humidity stratification in part was found over bare ground from that
prevailing in Europe (see Chapter 10). Consequently it is impossible
to say to just what extent the relationships indicated in Figs. 137
and 138 hold true for us. They do, however, give a good idea of
water-vapor conditions in crops where they were noted.

Let us begin with relationships at the time of incoming radiation.

The vapor pressure (Fig. 137) increases a bit toward the ground
(wet type). In the unwatered millet field, the vapor pressure is
about i mm higher on account of evaporation from the plants, but
the stratification is the same as in the open. In the sugar cane, which
must be irrigated from time to time, the vapor pressure at the wet
ground is very high and decreases greatly with height. This is also
true of the relative humidity in the sugar cane throughout the day
(Fig. 138) for it decreases from 60% at the ground to 30% at a
height of 2 m. In the open, on the contrary, there exists a weak form
of the dry type of relative humidity in the middle of the day. Here
too the millet field is intermediate between sugar cane and the open.

At night the dry type of humidity prevails for both degrees of
moisture, since the black "cotton" soil of India has the property of
absorbing a great amount of water vapor at night. A single excep-
tion exists in the irrigated field of sugar-cane (Fig. 137) where the

WIND RELATIONSHIPS

301

vapor pressure at night is higher in the first few centimeters above
the ground than it is higher up.

200

D
O

O)
-C
0>

At sunrise

midday

sugarcane — *-

millet —• •

open land -— o—

"**«*

56787
Vapor pressure (mm)

FIG. 137. Distribution of vapor pressure in the lower plant levels and in the open
in Poona. (After L. A. Ramdas, R. J. Kalamkar, and K. M. Gadre)

Dew is of great importance in the water economy of plants, both
at times of drought and in places which are prevailingly dry. Dew

200-

±150-

' <

I
Midday At sunrise

\ \

> •

x <
1 sugarcane — — x —

» *

(

\ open land — O— —

X>>>\ 4 ."

V.A ""^-X — x aff m%

UO 60 . 60

Relative humidity (%)

FIG. 138. Distribution of relative humidity in lower plant layers and in the open in

Poona

is a form of precipitation whose frequency is dependent (as are
frost and glaze formation) on the temperature of the wetted surface.
Consequently dew is a microclimatic phenomenon. The dew-plate

302 THE CLIMATE NEAR THE GROUND

developed and introduced to meteorology by E. Leick (575) today
makes possible very comparable dew measurements in various lo-
calities. The more important publications dealing with dew prob-
lems are listed in the references pertaining to the preceding chapter
[(566) to (586)]. Attention is called particularly to the summariz-
ing report of }. Stephan (580) .

Let us now turn to the effect of plant cover on wind movement
near the ground.

The high daytime temperatures and the high humidity which we
have described can exist amongst plants only because it is difficult
for the wind which sweeps over the ground to penetrate the plant
cover. Convection is noticeable in its upper portion only. The wind
therefore merely "wipes away the vapor cap over the crop," as P.
Filzer (555) so strikingly expresses it.

The movement of plants is such that their braking action is differ-
ent from that of the solid ground. Leaves of different plants have
different movements. Stalks of grain wave in the wind. Their sway
is similar to oscillations of a mechanical system represented by the
plant. The mass distribution in a tree determines the period of its
movement to and fro once the impulse has been imparted by the
wind. This is easily observed in a forest. If successive gusts in a
storm accidentally strike a tree in rhythm with its natural period of
oscillation, the danger of breakage or uprooting is much increased,
as noted by A. Schmauss (562). Wind damage therefore need not
result from wind pressure alone but may also result from this reso-
nance phenomenon.

In 1915, G. Hellmann (2/6) in discussing wind research at Nauen,
stated that an anemometer placed at a height of 2 m lost velocity if
the grass beneath it was full grown. The growing grass had the
effect of bringing the ground closer to the anemometer. In its
braking action on wind velocity in the air near the ground the sur-
face of the ground was no longer effective at height z = o, but at
another hypothetical surface at the height z — #0. The value #0
evidently depends on height and the kind of plant cover; it is called
the "roughness height," z0.

Calm prevails within the plant cover. Suppose one lies down on
the storm-swept heath between bushes of calluna. "It seems as
though one had dropped into a sink-hole : above, the elements battle
— but under the callunas hardly a breath is felt" (A. Koelsch). These
conditions have been numerically expressed in the excellent measure-
ments of O. Stocker (563) .

WIND RELATIONSHIPS 303

On the heath near Bremerhaven, for example, on Jan. n, 1921,
he observed the following wind velocities during a storm: —

At a height of 180 cm above the heath 9.3 m per sec.

Between the top calluna branches at 50 cm .... 3.7 " " "

Between the top calluna branches at 30 cm .... 1.4 " " "

Between the callunas at 10 cm i.o " " "

On the sunny, windy i2th of October, 1920: —

At 180 cm above the heath 5.1 m per sec.

At 40 cm — between the calluna tops 1.7 " " "

At 2 cm, in a small open space between the

callunas — less than 0.008 " " "

On the basis of numerous similar measurements, O. Stocker (564)
concluded that most German weedy plants are never subjected to
velocities in excess of i m per sec — their normal amount being, on
the contrary, often under o.i m per sec. The first example given
above (3.7 m per sec between the calluna tops) represents the maxi-
mum which Stocker has ever measured. In a desert climate with
strong winds, conditions are different. It need hardly be said that
this wind protection, afforded by the plants themselves within the
vegetation cover, is of great importance for their water economy.

W. Kreutz (560) by means of measurements at a height of 25 cm
in a wheat field and in two other fields planted with beans and pota-
toes, respectively, has determined the braking effect of the plant
cover on winds within it in percentage of the wind velocity. Sum-
marizing his data, we find the following percentages: —

TABLE 47

Wind Speed

Braking effect in %

(m per sec)

Wheat

Beans

Potatoes

Under i

3°

1—2

2-3 .

Over 3

.11

Accordingly, the retarding effect of a low plant cover is relatively
less, the higher the wind velocity. (We shall see in Chapt. 35 that
it is just the opposite with the screening effect of a spruce wind-
break.)

We now have a good series of measurements covering the in-

3°4

THE CLIMATE NEAR THE GROUND

WIND RELATIONSHIPS

305

fluence of the kind of plant cover on wind retardation. In the first
place let us consider Fig. 139, which represents the variation in wind
structure over different kinds of fields. The method which Wilh.
Schmidt (7/2) used in obtaining these measurements has already
been described in Chapter 4. The scene of the observations was at

soo

•too

300

200

100

45 cm zero point displacement

\l I

Wind speed (cm/sec)'

200 3oo m

FIG. 140. Change in wind speed with altitude over a turnip field. (After W. Pacschke)

Hommelsheim in the Rhine valley, near Diiren. Wire racks (wind-
pressure plates) covered with cloth were placed over a perfectly flat
field of wheat stubble. They were spaced 50 cm apart vertically
and 60 cm horizontally. A similar installation was prepared over a
turnip field whose uniformly dense growth of leaves lay from 40 to
50 cm above the ground. The wind had a sweep of at least 200 m
across the turnip field before reaching the point of measurement. In

306

THE CLIMATE NEAR THE GROUND

the case of the stubble field, the approach was much longer yet. In
both instances the wind had time to adjust itself to roughness of the
ground.

Fig. 139 represents vertical sections reaching to a height of ii m
above the stubble field or the top of the turnip leaves. The lines of

FIG. 141. Average wind velocity profiles over different types of plant cover and ground
surfaces. (After W. Paeschke)

equal wind velocity are drawn for steps of 25 cm per sec, with the
intervening spaces colored alternately black and white. Where the
wind turns for a time into another than the prevailing direction,
the fact is indicated by wide vertical shading. Both tests, as can be
seen by the time scale, lasted only 5 or 6 seconds.
There is in general a normal wind stratification over the stubble

WIND RELATIONSHIPS 307

field and a uniform quiet circulation with a slight variation of speed
with time. Above the turnip field, however, there is great turbu-
lence. The rough, coarse surface at times even causes a reversal of
the wind close above the leaves; at times the air seems to stand
still (say for 5 seconds or so) .

W. Paeschke (561, 224) carried out at Gottingen an experimental
study of roughness, using the most modern research methods. In
Fig. 140 we reproduce a representation of the windspeed distribu-
tion over a turnip field. Curves i to 6 correspond to tests at differ-
ent hours on a clear radiation day. (July 26, 1935). As we leave the
ground the wind speed at first changes only slightly. Only when
we get above the crop surface does it increase — rapidly at first,
then more slowly. The wind distribution with height can also be
considered to apply here, if, instead of the ground surface, a rough-
ness height ZQ equal to 45 cm is taken, and only from there up does
the formerly given equation apply.

Naturally, ZQ depends entirely on the height and kind of the plant
cover. Fig. 141 Paeschke's summary of measurements on different
kinds of fields, even a bracken heath, an airport and a snow-field.
For each type of surface, the roughness height is drawn in as a hori-
zontal line. The wind distribution over the snow-field with its
slight roughness is the same as that over the bare ground. In the
other curves a two-fold division is necessary — the part below z0 and
the normal part above #0. But it is not only the magnitude ZQ which
depends on the height and kind of plant cover, but also the exponent
a in the equation in Chapter u, which represents the variation of
wind speed with height. The measurements of W. Paeschke (224)
gave: —

TABLE 48

Kind of Soil or Roughness Reciprocal

of plant cover height z0 cm. Exponent Value i/a

Smooth snow surface 3 5.0

Gottingen airport 10 4.3

Bracken 10 4.0

Low grassland 20 ' 3.8

High grassland 30 3.6

Turnip field 45 3.0

Wheatfield 130 3.5

These results give us a complete picture of wind relationships
within a low plant cover. In the forest the "roughness height11 in-

308 THE CLIMATE NEAR THE GROUND

creases to quite different magnitudes. The part below z0 belongs to
the calm trunk space, which we shall discuss in Chapter 32.

Besides the single factors thus far mentioned (radiation, tempera-
ture, humidity and wind) it is also useful at times to consider cool-
ing, which depends on the other four. A. Kestermann (559) was
the first to make comparative measurements of garden trees and
shrubs, using two Pfleiderer and Biittner frigorigraphs. They
showed the frigorigraph peculiarly suited to microclimatic research.
We refer here to his easily accessible work.

CHAPTER 29
FOREST METEOROLOGY, FOREST CLIMATOLOGY, AND STAND CLIMATE

From the low plant cover we proceed to the forest. To a certain
extent this means passing from agricultural questions to those of
forestry.

The term forest-meteorology includes all that unites the forester
and the meteorologist. As meteorology is divided into climate and
weather so also does forest meteorology include two rather different
domains. The forester is interested in weather science insofar as the
various weather processes are of significance for his forest. These
are, in most cases, sources of damage — wind, avalanches, sleet, late
frosts, droughts and such.

In forest climatology, macroclimatic problems should be men-
tioned first. A planting grows up in an alternation of favorable
and unfavorable years. The forest manager consequently consults
the climatic data of meteorological stations when he wants to deter-
mine the connection between weather cycles and growth. There is
no forest development without a climatological basis. It is impos-
sible to select kinds of wood and strains for development without a
knowledge of the macroclimate, especially when it is a question of
varieties native to other lands and climatic zones. The great work of
C. A. Schenck (6/5) on foreign forest and park trees consists, in its
first volume, of a macroclimatology of the various forest belts of the
earth. In the long history of forest development it is necessary to
make allowance for climatic fluctuations and changes.

The microclimate is of prime importance for the forester because
it is the habitat climate of the young forest seedlings. The forest is
never more sensitive to climate than in its formative years. The
habitat climate of the plantation is, however, influenced by the culti-
vation measures employed by the manager. Consequently he has a
direct, practical interest in the habitat climate. What has been said
in the first chapters of Section VI as to the relation between the low
plant cover and the microclimate applies also to the fundamentals
of forest-meteorology as a science.

Beyond this the forester must be familiar with climatic relations
in his older plantings. For one thing he will want to know how the
forest responds to weather events; how the heat economy and the

310 THE CLIMATE NEAR THE GROUND

water economy of the forest are maintained in the unity of crown-
space, trunk-space and soil; how these relationships vary with the
season, the type of wood, the age of the planting and its condition.
Then the effect of this climate on the immediate surroundings of
the forest will interest him, for he prefers to start his new plantations
in proximity to the old and thus under the climatic influence of the
latter. The microclimate in the neighborhood of a mature woods is
therefore a habitat climate for the young growth. All these questions
are treated in the following chapters. One might think they could
be combined under the title, "forest climate," but that designation
has come to have a different meaning in the course of the history of
meteorological research.

When in the beginning of the I9th century the leaders of the
French revolution most recklessly wasted the forest of France, the
consequences soon appeared with frightful clearness. The European
public almost as a unit became interested in the necessity of forest
maintenance. Climatologists were given the task of determining the
effect of forests on the macroclimate, its "welfare effect" as it was
called, thus giving forest politics a powerful weapon. Various meth-
ods of attaining the goal were tried.

In the second half of the igth century the newly established
meteorological networks published their first series of measure-
ments. They were first used and tested on the question of whether
in heavily forested countries or sections the climatic relations could
be proved different from those in unforested areas. In this direction,
for example, H, E. Hamberg (602) and A. Woeikof (6^5) pro-
ceeded. With such a loose network of observing stations as existed
at that time the method was inevitably unsuccessful. Latitude, alti-
tude, continentality, topography, location with respect to centers of
action in the atmosphere, and many other factors prevented the
forest influence from being segregated. Soon the idea was suggested
that the sudden deforestation or sudden reforestation of a country
would set the scene for a magnificent experiment to this end. In the
course of the varied history of mankind such cases have occurred.
But there are other obstacles. In a country which neglects its forests
the conditions are scarcely favorable for undertaking through care-
ful scientific research to determine the harmful consequences .of such
wastefulness. Moreover, reforestation takes too long.

An exception to this is found in tropical lands where forest growth
is amazingly rapid. In 1875 a new forestry law initiated a great re-
forestation project in the central part of southern India. In an in-
vestigation covering the decade before and after the reforestation,

FOREST METEOROLOGY 311

H. F. Blanford (587) believed that he established an increase of
precipitation as a result. A. Kaminsky (606) showed, however, that
there had been a great climatic fluctuation in progress, by which the
control stations chosen by Blanford outside the forest had, acci-
dentally, not been affected. This is another proof of how difficult
are such experiments with a widely-spaced network of observation
stations.

In order to demonstrate the influence of forests on precipitation,
J. Schubert (629) used — not the national meteorological network —
but a supplementary network of 28 rain stations which were in
operation for a decade in the forest region of the Letzlinger heath.
In a careful analysis of the resulting observations, he separated the
effects of altitude, latitude and the situation in relation to the sea.
With such a close network this is possible. Moreover, he made
allowance for the wind error in openly situated rain-gauges, and for
the condensation of moist air. Then he was able to show, by calcu-
lating the probable errors, that the relationship between precipitation
and reforestation was closer than the influence of all the other acci-
dentally effective circumstances. The conclusion of this work which
appeared in 1937 was twofold: —

i. Of the year's precipitation on the Letzlinger heath, 6% can be
ascribed to the influence of reforestation, and 2. The influence of
the forest in dry years is demonstrably greater than in the wet years.

In this connection, the first extensive observations are interesting
which we have now-a-days from the tropic virgin forest, namely the
Congo region. In 1934, M. Gusinde has made these measurements
on the Ituri, a tributary of the Congo; F. Lauscher fully worked up
these observations (6oob}. The yearly annual rainfall was remark-
ably greater in the clearings within the virgin forest than at the
stations outside of the huge forest region. In 1934, on the Ituri an
annual precipitation of 1979 mm was measured; for eight surround-
ing stations in N, E, S, and W amounts between 1127 and 1853 mm,
on the average 1491 mm were found. Thus, the region of the virgin
forest received 30 per cent more precipitation. In accordance with
this fact, the relative humidity in the virgin forest was 15 per cent
higher, the temperature of the air i.5°C lower than in the surround-
ings. Although these values ought to be considered with great cau-
tion because of the short time of observation, the big area and the
possibility of local influences (Hole-cuttings! see page 350) the ob-
servations speak more for an increase of rainfall by the forest than
against it.

312

THE CLIMATE NEAR THE GROUND

The classic method which was, in the igth century, prescribed for
determining forest influence on climate, consisted in the erection of
so-called "duplicate forest stations," which were begun in Bavaria
by E. Ebermayer (592), in Prussia by A. Miittrich (6//, 6/_j) and
were, in their general features, carried out in Austria too by
von Lorenz-Liburnau — being imitated in many other countries.
Meteorological stations were operated in pairs with one station in
open country to study the open country climate and the other in a
nearby forest to study forest climate.

It is evident that "forest climate" was considered to be the same
as "trunk-space climate." But this is only one portion of forest

FIG. 142. Magnitude o£ Austausch (left) and temperature distribution (right) in
open country and in the woods on a summer day. (After H. G. Koch)

climate. That it was the first to be noticed and observed is easy to
understand, because a man walking through the forest experiences
the physical and psychical effects of this climate first. Physically
considered, however, it is a very unimportant part. A glance at Fig.
142 will show this at once.

H. G. Koch (133), by attaching electric resistance thermometers
to small rubber balloons, was able to measure temperature and con-
vection in the first 100 m above the ground. He also included in his
measurements a 17 m pine forest in the Luneburg heath. At u
A.M. on August 28, 1936 the temperature distribution in the forest
and in the neighboring open country was that shown at the right in
Fig. 142. The incoming type of radiation was well developed above
the open country. On a heath area surrounded by forest the tern-

FOREST METEOROLOGY

313

peratures near the ground were still higher, as a result of the lesser
convection there. The type of temperature profile remained the
same. It was very different in the forest. In the space above the
sunny crown the air is really warmer than at the same height in the
open, but what is gained there is lost in the trunk space. Fig. 142
makes it clear that no conclusion as to the effect of the forest on the
macroclimate can be drawn from a comparison between open
country climate and trunk-space climate near the ground. Such a
conclusion requires consideration of the whole atmosphere affected
by the forest.

J J a S

N D

F M A M
Month of year

FIG. 143. Influence on the kind of forest on the diminution of diurnal temperature
fluctuation in the trunk space in comparison with open country. (After A.

Miittrich)

If one is aware of these hypotheses, he can proceed on "forest
climate" research of his own with undivided interest for they afford
an excellent insight into the trunJ^-space climate (as it is experienced
by a man walking through the woods) in contrast to that in the
open. The data of duplicate forest stations have been thoroughly
edited by H. Burger (5^9, 590), von Lorenz-Liburnau (608), A.
Miittrich (6//-6/j) and particularly, by J. Schubert (6/7-629). Here
we shall briefly describe the air temperatures.

The effect of shading by the forest crown is to reduce the tempera-
ture range in the trunk-space in comparison with that in the open.
The amount of difference depends to a high degree on the kind of
tree. Fig. 143 shows the small daily temperature fluctuation in the
trunk-space as compared with the open according to a 1 5-year series

314 THE CLIMATE NEAR THE GROUND

of observations made by A. Mihtrich (6/1) at 5 pairs of stations in
a fir forest, 4 in a pine forest and 6 in a beech forest. The observa-
tions were made outside and inside a shelter placed 1.8 m above the
ground.

All three curves show the anticipated annual march with a maxi-
mum in summer when radiation is strongest and a minimum in
early winter. Most striking is the curve of the deciduous forest.
When, in spring, the increasing insolation falls on the bare beech
forest, the difference between field and forest is slight. What the
trunk-space loses to the open in radiant heat on account of the shade
by trunks and branches, it regains because its quiet air retains the
heat. Reference has been made previously to the unusually high
temperatures which F. Firbas (288) found about this time in the
leaf mold. As soon as the leaves come out there is a sudden change.
The dense leafy crown intercepts all radiation. The daily fluctua-
tion in the beech forest is reduced almost 5° on the average and
reaches a value which is attained by no other kind of wood at any
time.

The evergreen forest is much more uniform in its range. The
trunk-space is at no time shielded so little as is the deciduous forest
before the leaves come out, nor so much as is the beech forest in
full leaf. The curves for the two kinds of evergreens run practically
parallel. That of the spruce forest, with its dense, dark crown, is at
all times somewhat higher than that of the lighter pine forest.

If a person wishes to tackle the problem in general of the influ-
ence of forests on the macroclimate it can be done only by first in-
vestigating the heat and water balance of the forest in its entirety
and comparing the result with the heat and water balance of un-
planted ground. In so doing he does not measure the effect but
goes back to the causes on which it is based.

Even those interested chiefly in the practical side of forestry are
eager to understand the forest itself as a meteorological whole.
Since, on grounds earlier mentioned, the word "forest-climate" must
be avoided, we shall by preference speak of a "stand" climate. The
term stand climate consequently is to be understood as including
the microclimate of the crown space together with its sphere of in-
fluence, the trunk-space climate (which Boos (657) has well called
the "climate inside the stand", the climate of the forest floor and
the climate of the air layer next to it insofar as the latter differs
from the trunk -space climate.

As Fig. 142 indicates, the stand climate as a whole can be under-
stood only by fixing the attention mainly on the outer active surface

FOREST METEOROLOGY

315

(Chapter 27). This — in the stand — is the crown surface. There
is where the measuring instruments must be placed, for there is

.15m

Instrument shelter

QSr

Cross section of the 9.2 m platform

FIG. 144. Observation scaffold at the Wondreb Forest station for the investigation of
forest climate. (After R. Geiger)

where the meteorological processes take place. Latest research has
proceeded along this line.

In 1924, the Forest-Meteorology Institute of Munich, under the
direction of A. Schmauss, first erected a strong, high observation

316 THE CLIMATE NEAR THE GROUND

tower in a pine plantation in Ostmark, Bavaria. To illustrate the
method of work, the scaffold is represented in Fig. 144, as R. Geiger
(649) described it. It was built like a hunter's look-out, on a tri-
angular plan, using three tree-trunks (AT, y), to which six platforms
for instruments were attached, and which were united at the top by
cross beams (q). In order to protect the instruments as much as
possible from any influence due to their installation, the six stories
were not built solid, but protecting roofs (j) were erected over a
transverse board so as to protect against rain and hail. Otherwise,
the air had free access to the instruments. Beneath the protecting
roof there was a thermograph, a hygrograph (shown at h in Fig.
144 so as to indicate the general lay-out), and the attendant control
instruments. The anemometers («/) were mounted between the
floors in order to avoid interference by them; iron brackets held
them at the proper distance from the poles. Access to the instru-
ments for the observer who was in constant attendance during the
time of measurement was by means of a ladder on the outside of
the tower, beside which ran the electric wiring for the anemometer,
whose indications were recorded in a forest hut.

This method of investigation has been used many times since.
In 1927 R. Geiger and H. Amann (650) built two 27 m scaffolds of
a similar sort in an old oak wood of the Schweinfurt Forestry De-
partment. About the same time C. Schmid-Curtius (258) erected a
very solid tower in a 20 m fir planting at Inselberg in Thuringia,
which was used principally in studying the health-giving effect of
the forest. Finally in 1931 H. Ungeheuer (654) built an observation
platform equipped with electric thermometers in a 17 m beech wood
in the Taunus. At the four places mentioned, which are situated in
plantings of four different kinds of trees, research has been carried
out on forest climate as a whole.

The meteorologist who has to watch and care for his instruments
on a scaffold continuously, experiences at first hand how the crown
space governs the forest climate. R. Geiger (599) has given us a
description of it.

In the following chapters — 30 to 33 — this stand climate will first
be described in a high, thrifty old planting, ready for cutting. We
shall then indicate, as an example, how far the forest follows the
processes of free air and how it differs from them, thus giving rise
to a special climate. Chapters 33 and 36 will then take up the in-
fluence of stand composition, the microclimate of clearings and
cuttings and that of stand borders.

CHAPTER 30
RADIATION RELATIONSHIPS IN AN OLD STAND

In connection with Fig. 127 we showed how the radiation of sun
and sky on meadow is absorbed throughout a relatively large verti-
cal range. In a forest too, the radiation is caught by leaves and
needles, twigs and branches so that only a little is able to reach the
forest floor. The "outer active surface" in the case of the forest is
the crown surface. In contrast to the meadow, however, the greater
part of the radiation is obstructed by this highest layer of the plant
cover.

Fig. 145 shows the brightness distribution in a 120 to 150 year old
stand of red beech intermixed with occasional spruces which was
located on a 20° southeast slope at Lunz (Austria) about 1000 m
above sea level. The measurements were carried out by E. Trapp
(6^6) in 1937 by means of photocells, which are especially sensitive
to yellow and green light, using an observing tower with several
platforms. The data from sunny and cloudy days are averaged
separately and shown thus.

In general about 80% of the incident radiation is caught in the
crown space. Less than 5% reaches the forest floor. Although the
absolute amount of radiation on sunny and cloudy days is naturally
very different, the relative distribution shown in Fig. 145 indicates
no difference worth mentioning. On sunny days the relative ab-
sorption is greater because the proportion of direct insolation is
greater. But on cloudy days there is only diffuse sky light, which,
because it is not uni-directional, penetrates the interior of the stand
more easily. This applies particularly to the upper part of the trunk
space. On the forest floor the difference doesn't amount to much.

A series of other measurements has proved that, for a definite
place in the forest floor the relative amount of illumination received
is fairly independent of the prevailing weather. A. Angstrom and
C. Chr. Wallen (657) ascribe great practical significance to this
circumstance. It makes it possible, they say, to use the many years
of radiation observations available at meteorological stations in the
open for the determination of the radiation used by plants standing
in forest shade. If one has completed only a short series of measure-
ments at the place in question in any kind of weather, the conver-

318 THE CLIMATE NEAR THE GROUND

sion factor is at hand by which the series of many years' length can
be applied to the place desired.

Fig. 145 shows the distribution of illumination in a single stand.
What fraction of the outside light penetrates in general to the forest
floor depends to a great extent on the kind of woods, the age of
the stand, its closeness and, in the case of deciduous trees, on the

i
I'

Trunk space

120- 150 year old
forest of red beech

20 4ff 60 BO

% of brightness in open country

FIG. 145. Decrease of brightness in the interior of a thick foliage of red beech growth.

(After E. Trapp)

stage of leaf development also. A man walking in the forest enjoys
this dim, colored light. The difference between dark spruce forest
and a light pine stand impresses one. But the habitat factor — light
on the forest floor — is also of direct significance to its utilization by
the undergrowth, by Sprouting seedlings and the ground flora of
the forest, in its growth.

J. Wiesner (647) was the first to undertake a systematic series of
measurements, using Hecht's optical wedge. The optical wedge
gives values for the wave length range between 360 and 440 m^t —
the blue part of the spectrum. R. Geiger and H. Amann (650)
carried out measurements by this same method during 1928 and
1929 in a 115 year old oak forest at Schweinfurt. More recently
barrier layer photocells with different sensitivity have been used, in
some cases with light filters. Experiments in many stands were
made by F. Lauscher and W. Schwabl (642) in 1933 and by F.

RADIATION RELATIONSHIPS 319

Sauberer and E. Trapp (644) in 1935-36. The following table,
arranged according to kinds of trees will give an idea of their
findings : —

TABLE 49

Kind of Trees
(old stand)

Illumination on the forest floor
in % of outside illumination

Leafless

Leafed out

Red Beech

Deciduous
26-66

2-40

3-35
8-60
20-30

647, 642, 646
647, 650, 644
642
642

640
647, 642
642, 644

Oak .

42-60

Ash .

30— 80

Birch
Silver Fir

Evergreen

2— 2O

Spruce

4—40

Scotch Pine

22-40

The figures fluctuate decidedly with the composition of the stand.
They show the general limits which — aside from extreme cases —
have been actually observed.

The following numbers show by an example the variation of
brightness in a stand of timber in dependence on the development
of the vegetation and the character of the stand. The measurements
were executed by W. Nageli (6430) in Adlisberg near Zurich in
1939, in a 70 year old stand. The conifers (A) were pure firs. The
mixed forest (B) consisted of 55 per cent firs, 36 per cent beeches,
and 9 per cent other deciduous trees. The deciduous forest (C)
comprised 73 per cent beeches, 22 per cent ash trees, and 5 per cent
other deciduous trees.

TABLE 50

Brightness in the stand in % of that
above open land

Time of Measurement: Coniferous
trees
(A)

Mixed
trees
(B)

Deciduous
trees
(C)

End of April before sprouting

22
14

51
23

End of May after sprouting
End of September shortly before foliage
changes color

320 THE CLIMATE NEAR THE GROUND

With the deciduous trees the scattering of the individual values
was much greater than with the conifers, especially in the time be-
fore sprouting.

An individual observation, a tropic virgin forest, 30 m high in the
region of the Congo shows, according to M. Gusinde and F.
Lauscher (6oo£), at 2 meters above the forest ground only i per
cent of the outside brightness. Below 2 m the decrease of bright-
ness was again considerably under the influence of the vegetation
near the ground so that just at the ground we must expect about o.i
per cent. This remaining light is entirely diffuse. H. Eidmann
(6400) found in a mountain wood at Fernando Poo 0.4 per cent.

The values are in agreement with the observations which J.
Deinhofer and F. Lauscher (640) made on the shortening of the
duration of twilight in a forest. By "twilight" is meant the period
between sundown and the onset of darkness (when reading is no
longer possible in the open). In a deciduous forest the end of "civil
twilight" occurs 16 minutes earlier than in the open; in an evergreen
forest 20 minutes earlier, and in an old, high forest, 28 minutes
earlier — assuming a cloudless sky. If the sky is cloudy the curtail-
ment amounts to three quarters of an hour — in rainy weather, to
as much as 54 minutes. These facts are recognized as significant in
the settlement of cases at law.

It follows, as a result of the different permeability of deciduous
leaves for various wavelength bands, of which we have spoken in
Chapter 26, that the crown space acts not only to weaken, but also
to filter, the radiation. For example F. Sauberer (522) observed in a
7 to 10 m stand of white beech in the Wienerwald one cloudy day
in May between 9 and 12 A.M., that the orange radiation (at about
0.6 /*) was reduced to about 8% of its value in the open, yet the
total radiation was reduced only to 20% of its original value, for,
in the second case the wavelengths around 0.8 /z, where there is
maximum permeability, were included also.

The filtering of light is very evident if attention is paid to the
kind of radiation which is effective in the stand in spring when the
leaves are coming out. K. Egle (5/7) found the following intensities
of radiation expressed in percentage of radiation of equal wave
lengths falling on the stand: —

RADIATION RELATIONSHIPS 321

TABLE 51

In the band of

. 0.71

0.65

0.57

0.52

0.45

O.36M

Color

red

orange

yellow

green

blue

violet

March 12 (Buds still closed) .
April 15

. 61

5i
36

May 10

. 10

«:

lune 4

• 14

As the leafy roof thickens and the season advances the radiation
is increasingly reduced but in the blue (short wave region much
more than in the red).

G. Mitscherlich (643) has made many measurements of the de-
pendence of light relationships on the age of the stand in numerous
spruce plantings in the Dietzhausen forest district. This district is
in the Prankish Buntsandstein region on the south watershed of
the Thuringian forest. With a "Sixtus" photometer such as is used
in photography as an exposure meter he observed in 87 different
stands the illumination as compared with measurements in the open
just before and after. The result is shown in Fig. 146.

Lumber yield
T - classes

QIV: »

. i i i i i i i i i i i i i

0 10 20 30 W 50 60 70 80 90 100 110 120 130

Age Years

FIG. 146. Dependence of brightness in the interior of pine forests on the age of the
growth. (After G. Mitscherlich)

The first young open stand closes in so that by the time it is 17
years old the dense crown allows scarcely 10% of the outside light
to penetrate. But then as its age increases there is a steady increase
of interior illumination. At the age of 120 years a value of 30 to
35% has been reached. The better yielding classes (I, II in Fig. 146)
with their less numerous but more sturdy trunks let through in
general more light than the poorer ones.

322

THE CLIMATE NEAR THE GROUND

Comparative observations showed that these figures afford a prac-
tical habitat factor for the ground flora. With illumination below
16% the forest floor remains bare. Between 16 and i%% the first
unpretentious mosses appear. Between 22 and 26% scattered
berries are found and at about 30% the first spruce copses. These
values naturally assume favorable soil conditions, otherwise the
limits are higher.

Thus far we have spoken of average conditions in different stands.
What about differences between very limited areas in one and the
same stand ?

One often notices, on a sunny day, how stray sunbeams break
through the tree top canopy — how spots of light appear on the

+0*

FIG. 147. Recording of the brightness on the outer half (I) and inner half (II) of an
oak forest. (After F. Lauscher and W. Schwab!)

forest floor and move on with the course of the sun. At a given
moment there may be the greatest differences in brightness between
adjacent places. Or at a given place on the forest floor, there may be
great fluctuations in brightness from one moment to the next. Fig.
147 is a reproduction of two records made by means of Lange
photocells about midday on the sunny yth of August, 1933. Curve I
is the illumination on a meadow, influenced only by solar elevation
and by cloudiness; Curve II is a similar record within a 40 year old
ash stand, 16 m high. The two areas were close together, at Press-
baum, 25 km west .of Vienna. The records were published by F.
Lauscher and W. Schwabl (642). In addition to the weakening of

RADIATION RELATIONSHIPS

323

the light, they indicate the prevailing irregularity of the light factor
at the forest floor.

This irregularity decreases as the sky becomes clouded and the
ratio of direct to total radiation also decreases. E. Trapp (646) has
determined the distribution of illumination throughout the greater

Lower meadow

FIG. 148. Map of crown density o£ a 150 year old beech growth at Lunz. (After
photograph by E. Trapp)

part of a stand by means of thousands of separate measurements
and has depicted it on maps. We present, as a sample, one of his
"cloudy weather illumination maps." Special significance is attached
to these maps in that they coincide closely with vegetation maps.

Fig. 148 represents the amount of ground coverage by the crowns
of a 150 year old beech forest covering 50 m sq, according to careful
measurements: A meadow borders the stand at the lower left and
extends two arms into the forest. Fig. 149 represents the accompany-

324

THE CLIMATE NEAR THE GROUND

ing average distribution of illumination with a clouded sky. The
brightest parts have more than 80% of the outside illumination; the
darkest, less than 2%. The sky screening by the tree tops makes
itself felt even over the meadow. The distribution on an occasional
cloudy day is uniform compared with that in fair weather and
shows no abrupt changes. Trees standing alone have no practical

[ [Over 80%

[.•.'•;[ 50-80%
I ••.•;.'• I 20-50 %
|%/#[ 10-20%

iHl 2-5%

I Under 2%

FIG. 149. Distribution of brightness in cloudy weather in the beech woods of Fig. 148.

effect on their surroundings, but groups of trees are very effective.

All illumination measurements within a stand which have been
quoted hitherto, have dealt with quantities of light falling on a
horizontal surface — the so-called "overhead light." K. Brocks (639)
has also investigated the lighting of surfaces at various inclinations
within a stand. His findings in an oak, a pine, and a beech stand
may be consulted in the summary published by J. Schubert.

More simple than daytime radiation relationships, are those of
the night. Outgoing radiation proceeds exclusively from the upper
surface of the tree crowns. As P. Seltzer (655) observed in the

RADIATION RELATIONSHIPS 325

Hagenauer forest, leaves in the tops of the trees cooled off 2.5°
below the temperature of the surrounding air, when the wind is
calm and the sky half clouded, while leaves below the crowns
cooled only about 0.4° below. The former radiated heat toward the
night sky; the latter only toward the somewhat cooler tree crowns.
Nocturnal cooling in an old stand, therefore, is entirely a function
of the outer active surface.

CHAPTER 31
TEMPERATURE AND HUMIDITY RELATIONSHIPS IN AN OLD STAND

The radiation relationships depicted determine in general the tem-
perature relationships. First let us try to give a clear picture of the
connection between the two. For this purpose, Fig. 150 and 151 will
give us the daily temperature march in an old oak stand in the
Schweinfurt forest district, according to the observations of R.
Geiger and H. Amann (650). The stand was 24 m high; the 115
year old oaks were interspersed with 40 to 50 year old beech pole
wood.

A series of thermocouples was erected on an observation scaffold-
ing like that shown in Fig. 144, so that a temperature measurement
could be taken at all levels of the stand — even above the crowns.
The thermocouples could be connected in turn, by means of a rotary
switch, with a Zeiss loop galvanometer standing on the ground. A
temperature profile was obtained by measurements at seven points
every 30 seconds. Off and on there were pauses for testing the in-
struments and making comparative readings. The figures give an
excerpt from the record on the calm, sunny i8th of August, 1930. In
order not to overcrowd the sketch the data from two stations in the
crown space have been omitted. The lines indicating the other five
stations are heavy in proportion to their closeness to the ground.

The cross-section of the thermocouple wires was so great that the
temperatures shown are not true air temperatures but are affected by
direct insolation. The amount of this influence cannot be given
exactly, but it may be assumed that the thermo elements do not re-
spond much differently from small twigs or leaves on the tree. Figs.
150 and 151 are therefore excellently suited to make clear the daily
temperature march in a high old stand, in its response to nocturnal
counter-radiation and, in particular, the continuous effective diurnal
radiation from sun and sky.

The record (Fig. 150) begins at the time of sunrise. It is coldest
in the oak crown (23 m) in agreement with the conditions of out-
going radiation as described in foregoing chapters. It is warmest on
the forest floor. As the sun rises, warming-up sets in above the
crown (27 m) as a result of the first level rays stretching out across
the stand. It increases rapidly, so that after an hour the temperature
there is about 5°C higher than in the whole stand, where uniform

TEMPERATURE AND HUMIDITY

327

temperatures prevail at nearly all levels for practically the whole
night. Not until after 7 A.M. (see second line of Fig. 150) does the
crown space, as the sun climbs higher, begin to warm up, while even
yet it is still cool on the forest floor. This is the hour when the whole
insect world arises in this favored warm and light zone of the forest.

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By 8:20 A.M. the temperature in the crown space (23 m) has
equalled that above the crown, and as time goes on, surpasses it, for
the dense crown canopy now absorbs the radiation of the higher-
rising sun. Finally, now — three hours after sunrise! — the lower
layers of the forest at last begin to share in the day's heat. (The

328 THE CLIMATE NEAR THE GROUND

temperature lines at the lower right of Fig. 150 rise and draw apart.)
But from above the cool outer air sinks into the stand. The strong
heating of the crown canopy acts in conjunction with it to produce
a vigorous temperature turbulence in the realm of the tree tops.
There thus results about midday the condition represented in the

o
be

upper half of Fig. 151: — In the crown space is the highest tempera-
ture and the most unsettled temperature condition. Above this in
the free air, and below it in the trunk space, the temperature de-
creases; in the latter direction in particular the temperature disquiet
decreases also. The lowest line, which corresponds to a height of

TEMPERATURE AND HUMIDITY 329

3 m above the forest floor, shows the amazing uniformity of tem-
perature which the forest traveler finds so pleasant of a summer
noontime — usually without thinking what a lively heat exchange
is going on above him in the crown space.

Nevertheless the midday hours show a stable condition. The lines,
with the exception of fortuitous fluctuations, run in a horizontal
direction. This is the time when heat input and output are practi-
cally equal; the forenoon temperature rise is ended; the afternoon
fall has not yet begun.

The reverse temperature movement in the second half of the day
follows the same course as the morning rise. The example, which
is taken from the period about 6 P.M. (in the lower part of Fig. 151)
shows a smoother curve than that of the morning. The cause lies
in the stable stratification of the cold air which is constantly sinking
down from the crown space. The morning heating has to overcome
the stability of the nocturnal temperature stratification; the evening
cooling is furthered by the establishment of this stable stratification.

From the record as reproduced we see the normal temperature
stratification in an old stand. By night, temperature differences are
slight. Either the whole air mass is isothermal or, if the crown
canopy is sufficiently dense, the cold air remains above it. This is the
opinion expressed by von Lorenz-Liburnau (608). R. Geiger (649)
once observed a temperature minimum in the crown of a pine
stand. In connection with Fig. 153 we shall revert to similar results
of H. Ungeheuer (654). But such differences can amount to only a
few tenths of a degree. On the other hand it happens in light
stands that the sinking cold air of the crown space results in a
temperature minimum on the forest floor. P. Seltzer (653) observed
a double minimum — one in the crown, the other on the ground.

While nevertheless such differences have more theoretical interest
than practical significance, the temperature contrast during the day
is very significant. Aloft in the crown space there is a very marked
temperature maximum. This is, as one might guess from the evenly
drawn recording, not simply a radiation effect, but may also be con-
firmed by measurements of the true air temperature. In thin stands
a second weak maximum at the forest floor may sometimes be
demonstrated.

At Leningrad, N. von Obolensky (652)* in May and June 1922,
determined the temperature distribution in a young growth of fir,
using an Assmann aspiration psychrometer, and in July, August and
September, did likewise in a young oak growth. As the mean of
the i P.M. temperature on clear days he found: —

330

THE CLIMATE NEAR THE GROUND
TABLE 52

Month

Forest
floor

In the
crown

Crown
surface

At some distance
above the crown

May
June

. . . . 16.6
IQ.7

16.4
18.0

19.7
23.2

16.8

20.S

July
August
September . . .

, . . . 19.2

. ... 18.1
15.0

20.1

18.3
16.1

22.1
21.0

I8.7

21.6

20.4

17.9

Here the temperature maximum lies at the outer active surface.

The average diurnal march of the air temperature in the stand is
shown in Fig. 152. It is the mean of 12 calm September days of

FIG, 152, Diurnal course o£ temperature in a pine grove in September 1924. (After

R. Geiger)

1924, in a 14 to 1 6 m, 65 year old pine stand, as observed by R.
Geiger (649). The solid curve refers to the record just above the
crown (16 m); the dot and dash curve, to that 0.5 m above the
ground. At the lower edge of the diagram the temperature differ-
ence is shown on an enlarged scale.

It is always warmer above the crown, but the difference at night
is slight and uniform, when the regulation of temperature at the
time of the nocturnal calm is exclusively from the tree crowns.
There is, moreover, a minimum difference about noon, when the
high sun penetrates at least partially into the forest, and when at
the time of maximum wind velocity (which, as is well known,
occurs around midday,) convection is most fully developed. The
difference maxima, however, occur morning and evening, when the
sun is low and convection slight.

TEMPERATURE AND HUMIDITY

331

In 1931 and 1932, H. Ungeheuer (654) made extensive records of
true air temperature in a 17 m, 136 year old beech stand on the
northwest slope of the Taunus. He placed resistance thermometers
in small wood shelters protected from radiation. The data have
been arranged according to average values for hours, months and
years — also according to weather conditions. As an example, we
give in Fig. 153 the daily march on clear calm summer days.

20°

16*
H*

1.59
UP
0.5>
0.0*

-ft

f//

10*

FIG. 153. Diurnal course of temperature in a beech grove on a bright summer day.
(After H. Ungeheuer)

In the upper half of the diagram the temperature march in the
crown space (at 17 m) is represented by a solid line; in the trunk
space (at n m) by a broken line; and at 3 m above the ground by
a dotted line. All the laws which have been described are illustrated.
It is noteworthy how in the evening, the cooling effect of the out-
going radiation from the crown canopy appears in the difference
between the solid and the dotted curves. The curve of differences
between crown and ground shows the same double wave as Fig.

152.

The relative humidity in an old stand is governed principally by
the water output of the leaves of the crown space. E. Ramann has
demonstrated, as J. Schubert (627) reports, the great scarcity of
water at about i m depth in the forest soil, that is, in the root region.
The forest floor evaporates water to an extent dependent on the

332

THE CLIMATE NEAR THE GROUND

degree of development of the ground flora and the openness of the
stand. The lack of air movement in the trunk space retains the
water vapor so that high humidity is the most characteristic feature
of its microclimate. Drying out occurs only from the top, where
the higher daytime temperature is favorable to a lower relative
humidity.

The vertical distribution of relative humidity shows several types
in the course of the day, which are represented in Fig. 154.

Before sunrise there is high humidity in all layers; when dew is
precipitated, complete — or nearly complete — saturation. The ob-
server on foot in a forest is not apt to notice much dew formation,

»J0000^^

FIG. 154. Types of distribution of relative humidity in the grove

since in an old stand his attention is on the forest floor. Most of the
dew, as I have several times been able to observe, is deposited on
the upper surface of the crown, decreasing continuously and de-
cidedly downward into the inside of the stand. Above the crown
the deposition of dew was so great at times that it required several
hours of sunshine to complete its evaporation.

As soon as the sun has risen, the warmed up crown surface begins
to dry up. Through the action of the drier outside air on the upper
part of the crown space, there results the distribution which is char-
acteristic of the "morning type," as shown by curve Tl in Fig. 154.
— dry above, nocturnal moisture still evident below. The atmos-
pheric boundary layer which we recognized in describing the morn-
ing temperature relationships at the top of the crown surface can
also be easily recognized in the relative humidity by the course of the
curve TV

TEMPERATURE AND HUMIDITY 333

As the sun gets high and the wind freshens normally, effecting a
more thorough mixture of outside air and forest air, the drying out
process penetrates the interior. The forest atmosphere now receives
water vapor chiefly from two sides — from the forest floor and from
the crown canopy with its countless transpiring leaves or needles.

So two surfaces appear in the stand climate in reference to the
humidity. While the forest floor surface plays only a subordinate
role in respect to temperature, it is very important for the transfer
of water vapor. Although the temperature maximum at the forest
floor, when there is one, is always slight, the humidity maximum is
well developed, especially when the forest floor has a living plant
cover. As proof of this we offer the measurements of O. Stoker
(563) which he made within a high spruce forest on the Riesenge-
birge at Jannowitz, about 10 A.M. on July 16, 1921. He found the
following values of relative humidity:

At a height of 6 cm in a widespread stand of oxalis $4%

At a height of 30 cm between myosotis &7%

At a height of 100 cm in the open forest 59%

The input of water vapor from the forest floor and from the crown
modifies the drying effect of the outer air which attempts to pene-
trate the interior of the stand. A "midday type" takes form (T2 in
Fig. 154) two maxima of relative humidity, one above the other.
The lower one is caused by water received from the forest floor; the
other, by that from the tree-tops. Since in the upper part of the
crown there is constant intermixture with the outer air, the latter
maximum appears to be displaced downward to the lower edge of
the crown space.

While the curve TI falls within the range of high humidity (dry-
ing begins only at the top), and T2 at the time of midday minimum,
T3 has an intermediate position. It represents the evening type of
humidity distribution. While at this time the air above the crown
is still completely under the dominance of the drying daytime
hours, the steady transfer of water vapor from the ground begins to
be more effective as the temperature decreases in the shady forest
with the more oblique rays of the sun. At this time consequently
there occur the greatest humidity differences at the different heights.
Under such conditions I have observed differences of as much as
25% between the forest floor and the air just above the crown.

The types of humidity distribution described, explain at once the
daily march of humidity which is represented in Fig. 155 in a
manner similar to that used for the temperature in Fig. 152.

334

THE CLIMATE NEAR THE GROUND

The difference in the daily range of relative humidity between
crown space and forest depths (shown at the lower edge of the
chart) is this: From its lowest value at the time of the morning
temperature minimum, when all layers are close to saturation, the
difference rises about 5% at daybreak and until midday remains at
approximately this point. In the late afternoon it again begins to
increase and reaches a point which is, on the average, between 15
and 20%. From then on, the difference decreases steadily until it
again reaches its minimum between midnight and sunrise.

400
90
80
70
60

SO
10
5

I I r

Above forest floor

Above crowns

Difference

122468101224681012

FIG. 155. Diurnal course of relative humidity in a pine grove. (After R. Geiger)

The daily minimum of relative humidity coincides with the tem-
perature maximum, which occurs at about 2 P.M. Because it is at
about this time that the difference between the relative humidities
in the upper and the lower part of the forest first begins its after-
noon ascent to a maximum, observations confined to the daily ex-
treme values of relative humidity in the different layers of the forest
would show only slight differences. The advantage possessed by
the forest plant over that growing in the open, consists only partially
in the higher daily minimum of relative humidity in the forest and
much more in the long duration of the humidity surplus, which in
the evening hours can attain a considerable height.

By using thermoelements which he kept moist, H. Ungeheuer
also obtained the atmospheric humidity. As an average of 126 calm,
clear, summer days the following values of relative humidity at the
different hours of the day were obtained : —

TEMPERATURE AND HUMIDITY 335
TABLE 53

Height above Hour of the day

the forest floor

m i 3 5 7 9 it 13 15 17 19 21 23

17 81 82 83 80 77 73 67 65 69 76 79 81

n 80 80 83 8 1 78 73 69 67 72 77 79 80

3 80 80 83 83 80 77 71 69 73 78 78 79

0.3 82 82 83 82 78 74 68 65 69 76 80 83

The lowest humidity during the 24 hours occurs everywhere at
3 P.M. The dampest measuring places at a given hour are emphasized
by bold type. Day and night differ considerably. Throughout the
day and evening the wettest layer is not at the ground, as shown
in Type T2 and Type T3, but 3 m above. At night the two surfaces
which dispense water vapor become prominent, and it is noteworthy
that the maximum at the ground has almost the same value as that
in the crown. In both cases the air layer next to the forest floor
appears dry in comparison with results hitherto given. The reason
for this is the great scarcity of plants on the floor of the forest. For
the most part the ground was covered with yellow, but not rotting,
leaves. It is also possible that the slope wind, which could sweep
through the forest, had something to do with it.

CHAPTER 32
WIND AND PRECIPITATION IN AN OLD STAND

Just as insolation falls on the surface of the stand from outside, so
does wind movement from outside impinge upon the forest. We
shall for the present disregard the case of a stand where the wind
blows through from the side, and confine our attention at first to
relationships in an old, close stand.

Anyone passing through the forest when the wind is strong and
gusty will notice that first of all the roar of the storm is heard over
the forest; several seconds later the tree tops begin to wave and a
little later still the increased movement of the air is felt, directly.
This lag in storm force from the top downward, which is caused by
the baffle action of the forest, is accompanied by a reduction of its
intensity. A stormy gust above the crown is felt within the stand as
only a slight breeze.

We have a series of measurements of the vertical distribution of
wind speeds made by R. Geiger (649) in a 15 m pine stand. Six
four-cup anemometers, operating for 188 hrs. showed the following
mean air-speeds: —

TABLE 54

Height of the
anemometer
m

Position of anemometer

Average wind
speed
m/sec.

16.85
1370
10.55
7.40
4.25
1. 10

Above the tree tops
Upper limit of tree tops
In the tree tops
Upper part of trunk space
Within trunk space
Over the forest floor

1.61
0.90
0.69
0.67
0.69
0.6o

This shows that the reduction in wind speed is principally in the
crown space. From the lower limit of the crown down to just above
the ground there prevails an astonishingly uniform, gentle air move-
ment. Only below one meter is there another reduction, bringing the
speed on the ground to zero. The greater part of the wind's kinetic
energy is, therefore, like radiant heat energy, consumed at the
crown roof and only a small part at the ground.

WIND AND PRECIPITATION

337

This is seen still more clearly if the above-given wind measure-
ments are arranged according to speed. Fig. 156 shows the variation
of velocity with height for three groups of wind forces. With gentle
winds the braking appears only in the crown space, but with
stronger winds (curve at the right) a freshening of the wind in the
trunk space at a height of about 7 m is noticeable. From thence
downward the speed diminishes until stopped by the ground.

0 1 2 3 4m*

FIG. 156. Distribution of wind speed in a pine grove

Longer and more recent series of measurements have been carried
out by R. Geiger and H. Amann (650), in the previously mentioned
old oak stand at Schweinfurt. The records were made partly in
the springs of 1928 and 1929 before the leaves were out, and partly
then and in the fall of 1928 after the leaves were out on the lofty
oaks and lower beeches. The results demonstrate the effect of leaf
development.

Fig. 157 shows the wind distribution in each case. Before the
leaves are out it is naturally easier for the wind to penetrate the
bare stand. To be sure, there is a braking action evident in the
crown space, since there the twigs and branches are thicker. But
down through the whole trunk space there is a slight decline of
velocity. On the other hand, once the million leaves have unfolded,
the trunk space is virtually stagnant. A noteworthy consequence of
this is that above the crown the wind blows even more strongly, as
Fig. 157 shows.

The plants of the trunk space enjoy great quiet, which protects,
but also spoils them. This is best appreciated if one calculates the
number of calm hours in the Schweinfurt series of measurements.

338 THE CLIMATE NEAR THE GROUND

Expressed in percentage of all hours recorded (206 before leafing,
494 after leafing) it amounts to:

TABLE 55

Number of calm hrs. (%)

Height above the

Position of the

Before

After

forest floor

anemometer

Leafing

Above the crown

In the crown

Lower edge of crown

Above the forest floor

/ Z 3

Average wind speed

'see.

FIG. 157. Influence of the condition of foliage on the distribution of wind speed in an
oak grove with beech under growth. (After R. Geiger and H. Amann)

By calm hours are understood those in which the anemometer,
which has a starting speed of 0.7 m per sec, did not move.

Now what does the forest do with the precipitation that falls
on it?

The rain — which we shall consider first — first wets the crown
with its countless leaves or needles and twigs. If the rain is very

WIND AND PRECIPITATION 339

light, in fine drops and of short duration, this is as far as it gets
into the forest. But as soon as the precipitation gets a little heavier,
the water is passed on when the crown is thoroughly wet. Part is
conducted by twigs and branches to the trunk and runs down. All
the remaining falls to the ground.

It was early recognized that one cannot say of a rain measurement
merely that it was made "in the forest." It depends on where the
rain-gauge stands. E. Hoppe's (657) careful, inclusive research has
shown that with the formerly practiced installation of a single gauge
beneath the crown of a tree, an average error of from 25 to 30% in
the measurements must be expected as a result of chance variations
in exposure. For a single measurement the error may amount to
far more than 100%. With a light rainfall, no relation can be
shown between the amount caught by a single gauge and the "true
rainfall" which E. Hoppe determined by 20 raingauges arranged
within the stand in two rows crossing one another at right angles.

Fig. 158 makes clear the distribution of rain within a stand,
according to the data of E. Hoppe. Let us first consider the ever-
green forest, the observations were made in a 60 year old spruce
stand. With light rainfalls (up to 5 mm) two thirds of the whole
amount of rain is caught by the crown. The heavier, and usually
the longer lasting, the rain, the less (as is easily understood) the
proportion which is used in wetting the tree crown. It is worthy of
note that even with the heaviest rainfall a fifth of it never reaches
the inside of the forest. As to the water that runs down the tree
trunks, it does not amount to much — less than 5% even in a
cloudburst. The amount which drops through the crown and so
reaches the ground, only in rainfalls over 10 mm amounts to half
that which falls on the forest. This portion which drops through, is
unevenly distributed within the forest. It is least close to the trunk
and increases toward the periphery of the tree. This is shown by
the following figures from the same stand, which give — for all
rainfalls, irrespective of strength — the precipitation at specified dis-
tances from the trunk, as a percentage of the rain falling on the
forest : —

Distance from the Near

trunk in meters . o to l/i 1A to i r to i l/2 Over 1 l/2 Openings
Percentage 55 60 63 66 76

Let us now return to the observations made in an 88 year old beech
forest (Lower part of Fig. 158). That part of the rainfall which
clings to the leaves is relatively much less in a deciduous forest than

340

THE CLIMATE NEAR THE GROUND

in an evergreen stand. With the greater density of the leafy canopy
this at first seems surprising, but this is the answer: While the drops
remain hanging on the separate spruce needles, they flow together
on the beech leaves and pass over twigs and branches to the trunks
and from there downward. Consequently the proportion of the rain
dropping through the crown amounts to more than 50%, even with
the weakest rainfall and the quantity running down the trunk

Fir woods

Beech woods

5 10 15 20 mm

Intensity of rainfall
FIG. 158. Distribution of rain in narrow and broad-leafed forests. (After E. Hoppe)

amounts to a fifth of the total. According to a report contained in a
letter from F. Sauberer, still more recent measurements by H.
Friedel at Lunz have shown different relationships in that in a
beech forest in full leaf the downflow of water at the trunk begins
only after quite heavy falls of rain. As long as precipitation in the
open does not exceed 10 mm the portion which runs down the trunk
is said to be negligible. So far as I know this has not been published.
As to the distribution of snow in an old stand, we unfortunately
possess no such conclusive series of measurements as for rain. From
comparative observations of snow depth in a stand and in the open,
such as J. Schubert (624, 627) reported for the duplicate forest sta-
tions of Prussia, it may be concluded that most of the snow falling
on the stand gets to the forest floor. While the ratio of rain outside

WIND AND PRECIPITATION 341

to that inside the forest, as an average of 120 years from many pairs
of stations, was 100 to 73, the ratio for snow was 100 to 90. The
snow measurements of H. Hesselman (669) in Sweden show nearly
the same depth of snow in a pine forest as in cuttings. In any case
snow reaches the floor of the forest more easily than does rain. One
reason is that snow which accumulates on the crown branches
breaks away by its weight and falls to the ground. Moreover, low
temperatures prevent any great loss through evaporation directly
after a snowfall, such as occurs after a summer rain.

G. Priehausser (6520) made some exceptionally fine observations
on snow relationships in spruce stands of all ages in the Bavarian
forest. He showed how frost saucers form under single spruces in
the course of the winter. Ground frost begins where the side
branches are bent down to the ground by the snow, and increases
rapidly in strength toward the bare inside of the protected area.
Open portions of an old spruce stand receive the full depth of snow,
which does not blow away. Under the protection of single spruces
only powder snow reaches the ground. Close spruce stands support
a porous cover of dropping snow. Particulars may be obtained from
the publication mentioned.

CHAPTER 33
THE INFLUENCE OF MAKE-UP OF THE STAND ON ITS CLIMATE

Forest meteorology, as mentioned in Chapter 29, was, in the igth
century, merely an adjunct of forestry management. Today, as forest
microclimatology, it is called on to serve as an auxiliary science in
forest building. As it is now a self evident fact to the forest scientist
and practical man that he should take soil conditions into consid-
eration as a habitat factor in cultivation, so is this increasingly true
as a habitat factor in microclimatology.

In the service of practical forest building the science of microclim-
atology directs its attention not only to the type of stand climate as
described in the three preceding chapters, but far more to the varia-
tions from this type which reforestation projects have occasioned.
Much has already been accomplished in this new field of endeavor.
In this and the two following chapters we can give only a survey
showing in what direction development is proceeding. It should give
an extended hand of encouragement to everyone who has learned to
foresee the significance and wonderful future of this practical science.

First we shall accompany H. G. Koch (670) on a fair-weather
temperature measuring expedition in a motorcar through a forest
district in the neighborhood of Leipzig in order to get a firsthand
impression of the changing temperatures one finds in different kinds
of stands. In Fig. 159, at the top, is presented a cross-section through
the country, showing the various stands traversed. The daily temper-
ature march for July 8th and 9th, 1933 is given below in isopleths.
The times of sunrise and sunset are indicated by broken lines. At
these transition periods between day and night the isotherms crowd
together and lie practically horizontal. This means that at those
times the temperature is undergoing a great, and everywhere similar,
change. The temperature fall at evening and rise in the morning
are meteorological occurrences of such magnitude as to overshadow
differences within the stand.

At the times however when the heat exchange reaches equilib-
rium, local peculiarities become effective. This is much more true
at night than at midday, for Fig. 159 shows that the "islands" in the
isotherm map are more sharply outlined at night than at midday.
The cause of this is the greater quiet of the night air and its thermally
stable stratification. About noon the temperature is above 25°C in

INFLUENCE OF MAKE-UP OF THE STAND

343

three places — they are, as a glance at the upper sketch shows, the
clearings, stretches of open land and nursery areas. On these same
areas it is very cold at night. At several places on the islands it is
below n°C.

R. Geiger (649) was the first to carry out comparative measure-
ments in old stands of the Wondreb forest district, which were
similar as to situation and previous history, but variously treated as

July 8-9, 1933cTw

? &.00 T° Merer

FIG. 159. Diurnal course of temperature within an enclosed forest region at Leipzig.
(After H. G. Koch)

to upbuilding. The problem to be investigated was the numerical
determination of the microclimate difference between a stand with a
uniform crown canopy and a stand with varying height. One por-
tion of a 65 year old pine stand (designated as I) had a loosely
closed, uniform crown; the other portion (II) was thickly under-
grown with spruce so that there were tree tops at all levels. This
latter portion had, as the forester says, "step closure." The observa-

344

THE CLIMATE NEAR THE GROUND

tions in the two adjacent portions of the stand were made with the
aid of two observation scaffolds such as are depicted in Fig. 144.

Although the two places were only 86 m apart, the stand climates
proved to be very different. The air in the trunk space of II, on
account of the numerous crowns, reacted more slowly to the penetra-

In fir stand with

pine undergrowth u.24n\

4CT/J

FIG. 1 60. The penetration of cold thunder storm air in two neighboring but differ-
ently constituted forest stands

tion of the outside air than did that of the pure pine stand I. This
can best be demonstrated by a lag test which Nature herself carried
out.

When a thunderstorm broke on the afternoon of May 21, 1925,
after a hot morning, the temperature and humidity showed the

INFLUENCE OF MAKE-UP OF THE STAND

345

curves reproduced in Fig. 160. The inrush of cold air can be fol-
lowed through its separate phases. First came a weak forerunner
(V) then, in three steps, the squall itself (Hj-Hg), followed by a
small return of warm air (R). The forest air in both stands followed
this sudden change of condition with a certain delay. This is most
evident in the case of the third step (//3) for at the onset of H3 the
temperature above the stand had already dropped to 11°, while in
the stand it had reached only i4°C.

The greater lag in the reaction of stand II appears clearly in the
following particulars:

116.0
I 24

116.0

FIG. 161. Combat of the humid air of the trunk region with the dry open air in
the same thunder storm

1. The temperature minimum above the stands was 9.6°. In the
trunk space of I it was 10.3°; in that of II, it was 10.8°. The temper-
ature maximum which was attained some time after the inrush of
cold, was 22.4° above the crowns (sunshine again prevailed over the
crown canopy). In stand I it was 18.7°; in II, only i4.6°C.

2. The first recoil of dry outer air brought a drop in humidity of
25% above the crowns; within stand I, 13%; while within II it
was hardly noticeable.

3. From the time of the cold air invasion until the late hours of
the evening, stand II held its internal moisture better than did
stand I. (The dot and dash curve at the bottom of Fig. 160 remains

346 THE CLIMATE NEAR THE GROUND

continuously somewhat above the thin, solid line.)

The contest between the dry outer air and the moist interior of
the forest is made still clearer for us by Fig. 161. The ordinate rep-
resents the sum of the hourly humidity variations without regard to
their sign. The two solid lines representing conditions above the
stand have their maximum one to two hours earlier than the
broken-line curves which represent conditions within the forest.

0.0

2.0

2.5

0.5 1.0

Wind speed in m/sec

FIG. 162. Average wind speed in two neighboring but differently constituted forest

stands

The struggle between the different air masses displays itself in the
very unsettled state of the humidity, for now moister and now drier
portions of air stream pass the instrument. It began at the very top
of the forest. Shortly after 6 P.M. the struggle had penetrated the
pure pine stand I; later and with diminished energy it made itself
felt in part II, the lower portion of which was filled with spruce.

The different climatic lag of the two forest plantings which was
clear to us in the records described, now expresses itself in all the
climatic factors — most clearly in the average wind velocity. Fig.
162 reproduces the curves for three different velocity levels in each

INFLUENCE OF MAKE-UP OF THE STAND 347

stand. In stand I the wind is uniform above and below; in II it
blows unhindered above the crown while within the stand it is
much quieter.

The temperature relationships are more complicated.

At the hottest time of day the close stand II remains on the
average as much as 2°C cooler than stand I. As to the vertical temp-
erature distribution, the hottest zone in I extends down into the
crown space on account of the relatively good air mixing, while in
II it remains above the crown canopy. But, since the incident and
absorbed solar energy is the same in the two adjacent stands, the
lower temperature within stand II is balanced by a higher maximum
above the crown.

Just as by day the interior of the spruce-filled stand II is relatively
cool in comparison with I, so by night it is relatively warm, and the
relationship above the crown is again reversed. The lag of the
minimum within II is somewhat greater than within stand I, in
contrast with the appearance of the minimum above the crown.

Evaporation in the two stands, as calculated from temperature,
vapor pressure and wind according to Trabert's formula, was as
follows — the amount in the open being taken as 100:

TABLE 56

Height

Stand I
Pure Pine

Stand II
(Pine and spruce)

1 6.0 m (above the crown)

IO3

IOO

12.6 m (in the crown space)

2.4 m (in the trunk space)

The habitat climates of the two neighboring stands are, therefore,
as appears in all that has been said, very different — showing how
great an influence the forester has through his varying management.

In 1927—30 R. Geiger and H. Amann (650) made similar compari-
sons in a light old oak stand, with and without beech undergrowth,
in the Schweinfurt forest district. In 1927—8, H. Burger (65$) com-
pared the trunk space climates in a spruce stand of uniform age and
a nearby fir-spruce "blended" (intertilled) forest at Thur in the
canton of Bern. A. Angstrom (655) in 1925-34 determined the in-
fluence of different densities of planting on the temperature of the
forest floor at depths of 15, 30 and 45 cm. Several years of observations
at Vindeln in North Sweden showed that in thickly planted stands
the ground temperature was 2 to 3° higher than in the thinner stands

348 THE CLIMATE NEAR THE GROUND

and that, correspondingly, in the spring the ground thawed out 2 to
4 weeks earlier. In 1935-36 Boos (657), at Erdmannshausen, com-
pared the forest floor temperature (as well as humidity, wind and
precipitation in the air near the ground) in two pine-spruce mixed
stands and two beech stands.

In a mixed stand of pine, spruce with a few firs and beeches, in
the Jura region C. von Wrede (684) in 1923-4 investigated the
climatic difference of openings and thinned strips. In a portion of the
forest thickly filled with undergrowth an opening was cut; i.e. a
very small clearing made within the old stand — in the present case
circular with a diameter of 13 to 14 m — on which the young
growth of the next forest generation could grow up under the pro-
tection of the old generation. Quite nearby an east- west thinned
strip, 50 to 60 m wide had been laid out, where so many single
trees had been removed from the close forest that the ground was
only about 43% covered by crowns ("degree of stocking" = 0*43) .
The light which fell on the opened stand permitted the young
growth to rise on the forest floor.

In the midst alike of opening and thinning C. von Wrede erected
an observation shelter 40 cm above the ground, in order to determine
the conditions in the airspace near the ground in which the young
growth had to develop. Temperature measurements indicated a
more moderate climate in the opening than under the screen strips
as the following figures show :

TABLE 57

Difference of absolute Greatest daily temperature

monthly extremes range in month

1923

Opening

Screen

Opening

Screen

June

18.1

20.Q

12.5

ISX

July

. . . 23.6

26.7

I^.O

16.7

August

24.0

27.Q

20.6

24.0

The cause lies in the wind relationships. Cup anemometers were
placed i m above the ground and the wind motion was read off
according to the final observations. The average wind speeds were:

TABLE 58

In the month of June July August September 1923

In the opening 0.26 0.50 0.55 0.35 m per sec.

Under the screen 0.54 0.85 0.72 0.49 m per sec.

INFLUENCE OF MAKE-UP OF THE STAND

349

The wind speed under the thinned strip averaged il/2 times
greater than in the opening. In addition the wind direction ob-
served in the opening was for the most part just the opposite of that
in the open. We must imagine the wind circulation in opening and
thinned strip is as shown in Fig. 163. A whirl forms amid the
surrounding, protective old stand. The upper wind reaches in only
partially and the whirl causes the frequent reversal of wind direc-
tion at the ground. H. Pfeiffer (676) meantime has been investigat-

Opening Thinned strip

FIG. 163. Wind movement in an opening and in a thinned strip

ing this air movement directly by means of smoke experiments (see
Chapter 35). It is of great practical significance where there are
courtyards within great blocks of houses. If in Fig. 163 we imagine
the surrounding forest at the left replaced by houses, we can readily
understand that the chimneys in the houses at the right will not
draw properly, and that in those dwellings the fire will often be
driven out the stove doors. In such cases it helps to plant trees in
the courtyard to hinder the formation of whirls.

If the air cannot penetrate the opening, neither can the sun; so the
air does not warm from the ground upward, but rather from the
stand outward as is proved by temperature measurements near the
ground (179). On the other hand, so much insolation reaches the
forest floor through the loose crown of the thinned strip that the air
warms from the ground up. Beneath the thinned strip the relative
humidity of the air was from 5 to 7% less than in the opening.

H. Amann (795), also, has made meteorological studies of the
influence of a screen stand. We shall come back to this subject in
Chapter 40, in connection with the question of frost.

CHAPTER 34

THE MICROCLIMATE OF CIRCULAR SLASHINGS, CLEARINGS
AND CUTTINGS

The forester likes to rejuvenate his stands by means of hole cuttings
or hole slashings (usually circular areas) in the old stands. The
moderate temperature range, the high atmospheric humidity and
the calm air of the surrounding trunk space, characterize the habitat
climate of the hole cutting as well. From the very beginning the
young growth finds there the climatic conditions favorable to its
development. We have already met such a hole cutting, in the
preceding chapter, in the "opening" of von Wrede's studies.

Young ground plants require both light and sun to flourish, con-
sequently there is an effort to enlarge the openings. Moreover the
necessity to give the future generation sufficient space leads to the
same end. But, as the size of the openings increases, their micro-
climate alters. The greater penetration of insolation by day and the
increased outgoing radiation by night result in extreme ground
temperatures, which, on account of the quiet air, are able to make
their influence felt to the full in the habitat climate over the forest
floor. For this reason the larger clearings are indeed very warm by
day, yet on spring nights they are very much in danger of frost,
which is a real hindrance to their practical usefulness in certain
macroclirnatic areas.

The larger the hole cutting is made, up to the point where it
deserves the name of forest clearing, the more the wind from above
can reach into it, down to the air next to the ground. This signifies
a reduction of the daily temperature range and a lessening of the
frost danger. In the transition from a narrow hole slashing to a
broad clearing it is to be expected that at some certain size the
habitat climate will be particularly extreme. Below this critical size
the climate is milder on account of less radiation; above this size,
on account of less quiet air. It is not the diameter D of the clearing
which is the effective dimension, but its ratio to the height H of the
surrounding stand. This ratio D:H we call farther on the "index
size of the clearing."

MICROCLIMATE OF CLEARINGS 351

The outgoing radiation A in the midst of a circular clearing may
be calculated, according to F. Lauscher (6j), in percentage of radia-
tion in the open from the mean screening angle (h) by means of
the formula

Here r is a function of the observed vapor pressure e which can be
represented with sufficient accuracy by the equation r — o.n +
0.034 e- The screening angle h is to be measured from the ground at
the middle of the clearing. Calculating this from tan h = 2H/D
gives too large values of A, as R. Geiger showed (667) for the horizon
of a hole-cutting is formed not only by the trees at the border, but
(where there are gaps in the front row) by the tops of trees farther
back.

As to the calmness of the air in the clearings, we have good
witness in numerous temperature measurements of earlier days. In
1872 P. la Cour (659) showed that forests are surrounded by a belt
of increased temperature fluctuations. This is chiefly a result of
heightened radiation effect by reason of greater calmness of the air.
According to C. G. Bates (7/5), windbreak strips increase the daily
range by 5°. Somewhat later, H. E. Hamberg (602), in his classic
investigation into the influence of forests on the climate of Sweden,
showed that clearings possess a climate of greater extremes than that
of the open. J. Schubert (626), in connection with observations in
Neumark from 1900 to 1903, found 9.4° as the daily temperature
range in the trunk space during August and September, 9.9° in
nearby open country and 10.8 ° in a shelter within the clearing.

B. Danckelmann (660) discovered an increase of frost danger
with increase in size of clearings, as a result of observations in the
Mark forest. In 1894 clearings up to an index size of i*4 showed
complete or nearly complete freedom from frost; with an index of
il/2 the danger was still within reasonable limits; but with an index
of 2 or more the frost danger was great. This result depends natur-
ally on the accidental frequency of late frosts during the period of
observation.

R. Geiger (667), in 1940, carried out a systematic series of experi-
ments in a 26 m mixed stand of pine and beech at Eberwald. Seven
circular cuttings of different diameters had been made in the stand.
The following table gives the relative sizes and the result of a num-
ber of measurements:

352 THE CLIMATE NEAR THE GROUND

TABLE 59

Diameter D in m ....

Index size D/H
Average screening angle
Outward radiation (%
Open)

h . .
of t

hat in

90°

0.46

72°

0.85

59°

o-93
58°

1.47
48°

1.82
40°

3.36
26°

Rain (% of that in open)
Midday temperature (8
(amount warmer than

June
the

1940)
stand)

87%
0.7

1.6

2.0

105%
5.2

4.1

The index sizes had been selected to extend beyond 3. As a result
the outgoing radiation in the largest clearing amounted to within
13% of open country figures, so that the investigation embraced all
the values likely to be encountered under practical forest conditions.

The result of rainfall measurements during the months of June
through Sept. 1940, was that in each month the least rainfall occurred
in the smallest clearing, for rain which fell slantingly was more or
less caught by the crowns of the surrounding trees. The greatest
amounts were caught by the 38 m cutting; its bordering trees stood
back so far from the center that they did not obstruct the rain. The
effect of the quiet air was, that, on the average, 5% more rain was
caught than in the open. In the 87 m clearing the excess was only
2%. The clearing with the 1.5 index therefore represented a critical
size — with respect to the above mentioned characteristics.

Similar relationships appeared for the temperature measurements,
which were made on a sunny day, using an aspiration thermometer
in the middle of the clearings at a point 10 cm above the ground.
The table indicates how much warmer the clearing was than the
surrounding stand. The values reach a maximum for indices from
1.5 to 2.0 and then decline markedly.

The results of night temperature observations give a different
picture entirely. Fig. 164 shows that the temperature declines uni-
formly as the diameter of the clearing increases. This is as true for
the mean of the 17 coldest nights in the spring and summer of 1940
as for the coldest late frost night of the year, which occurred on
June 6. It is well known that calm, radiation nights are the danger-
ous ones for late frosts, so that there is no noticeable effect of wind
on temperatures. Perhaps the critical size is found only above an
index of 3.4 or — what is more probable — the straight line in Fig.
164, as the index rises, approaches asymptotically the nocturnal temp-
erature minimum of open country. That it is not merely outgoing
radiation which is in control, appears, however, in that the tempera-

MICROCLIMATE OF CLEARINGS

353

ture fall is not proportional to the published radiation values A.
There must be in addition either a warming influence in connection
with the small clearings or a cooling influence in connection with
the large ones. The former is accounted for by the mixing of the
cold air in the clearing with the warmer air from the trunk space;

;
8°

"*>

^^X^^ 17 cold spring nights

» ^S^

5 "X^

6°

^OW

^X^

^S^ o

•\^^

^*^^^

^NSSSN|(^

14'

^X^

^^^N^

^Xo

^v^^^

.June 6, 1940

^X.

^\^

0°

i- .1,1,1.

0 20 40 60 80 Meter

Diameter of clearing (m)

I 1 1 1 J 1 t 1 .

I 1 t

u 0.1 0,2 0,3 0,4 0,5 0.5

Area of clearing in hectares

. 1 1 .

U 1 2 3

Ratio: clearing diameter to height of growth
FIG. 164. Increase of frost danger in clearings o£ increasing size

the latter, by the descent into the clearing of air which has been
cooled above the crowns of the surrounding stand — a process which
in the following chapter we shall designate as a "nocturnal forest
wind."

Outgoing radiation relationships in narrow openings and forest
cuttings (also called lots or spacious glades) can be computed ac-
cording to the suggestion of F. Lauscher (6j). If h be the screening
angle, looking outward from the middle of the cutting toward the

354

THE CLIMATE NEAR THE GROUND

stands (considered as of uniform height on both, sides), then the
outgoing radiation from the cutting S in percentage of radiation in
the open can be calculated from the following figures:

h:
S:

100

5
93

10
86

'5
80

60
30

F. Lauscher and W. Schwabl (642) have studied the diurnal
illumination conditions in such cuttings. Fig. 165 shows measure-
ments made at Lunz in a north and south cutting, 20 m wide, in a
80 to 100 year old mixed stand of spruce, fir and beech. The illumina-

100%
80
60

100%

75 90

Paces

FIG. 165. Illumination conditions in a N-S- directed forest cutting. (After measure-
ments by F. Lauscher and W. Schwabl)

tion values, which were obtained with barrier layer photocells, are
expressed in percentage of those simultaneously observed in the open.
As the sketch indicates, the cutting extended between points 35
and 63 (number of paces) on the ordinate scale. It is seen from the
north, so that east is at the left and west at the right. The broken
line and the dotted one were obtained in sunshine. The maximum
brightness is as great as in the open but its location in the cutting
varies with the movement of the sun. The solid line represents a
measurement with cloudy sky. In that case it is not the direct
insolation but the sky radiation (analogous to nocturnal conditions)
which determines the illumination; it consequently reaches at no
point the brightness of open country conditions, as it does in clear
weather.

E. Schimitschek (680) has made some estimates as to the sunny-
ness of wedge cuttings.

MICROCLIMATE OF CLEARINGS

355

in
by

the
H.

Wind conditions in cuttings have been investigated
Anzing-Ebersberg forest by R. Geiger (664) and later
Pfeiflfer (676) using models in a wind tunnel.

Fig. 1 66 shows schematically the results of Geiger 's measure-
ments. The heavy arrow represents the wind aloft, over the cutting;

FIG. 1 66. The wind motion in a forest cutting in relationship to the upper wind

the small arrows, the winds in the cutting. The stronger the winds,
the closer the lines. Dotted areas indicate dead air spaces. If the
wind is blowing perpendicular to the course of the cutting
the wind gusts above the cutting are indicated by opposing pairs of
arrows. The wind whirl shown in a clearing formation (Fig. 163)
can also be observed in a cutting; in such case the wind at the ground
is blowing opposite to the general direction. There may even be a

356 THE CLIMATE NEAR THE GROUND

double whirl (one above the other) as H. Pfeiffer has determined.
In this case the wind at the ground, just as in the trunk space, blows
in the common direction while at half the height of the surrounding
stands there is a countercurrent.

When the wind blows down aslant into the cutting, the maximum
velocity is displaced from the wind-sheltered side toward the stand
which bears the blast of the wind, just as in the case of light the
maximum brightness is displaced from the shaded side toward the
opposite one. If the wind moves approximately along the axis of
the cutting there ensues a decided maximum speed at the edge of
the stand, which guides the wind.

CHAPTER 35
THE CLIMATE OF THE STAND BORDER

Great significance from the point of view of forest building is
attached to the climate of the border areas for the forester usually
renews his stands at the edge of the old wood. For this purpose he
makes use of the outer edge, that is, the strip of open land beyond
the forest, and also the inner edge, which lies beneath the bordering
trees of the stand.

The climate at the stand border results, as R. Geiger (666) has
stated, from two fundamentally different causes. In the first place
it is a transition climate between that of the trunk space and that of
open country. The contrast between the two leads to an exchange
of their properties. The influence of the trunk space climate pre-
dominates on the outer edge; the open country climate, on the inner
edge. In the second place, the edge of the stand is like a high step in
the land. According to the direction it faces, it catches insolation or
it withholds it from the open country. It catches the wind and
opposes itself to rain or snow. Insofar as the stand border is in the
"shadow" of the wind, it protects the open country and may lessen
or increase its precipitation.

The second list of causes are the more effective in their action.
The most powerful factor among them is the daytime radiation of
heat, which we shall describe first. The diffuse sky radiation is
really ineffective for it acts on stand borders in all directions without
distinction. The greater the ratio of sky radiation to total radiation,
so much the less difference is there between the various stand
borders (compare what was said in Chapt. 22 with regard to slope
climate) . This applies in cloudy weather and in northern countries.
Only direct insolation causes differences.

Fig. 167 shows the duration of sunshine for stand borders in all
directions and at all seasons. It is based on the 1895—1934 series of
observations at Karlsruhe, from the work of J. von Kienle (429).
The number of sunshine hours refers to a month as unity. The
irregularity of the curves reflects the changing weather conditions,
which even in the 40 year mean are not entirely smoothed out.
Looking at the picture as a whole there is symmetry on the one
hand between spring and autumn conditions, and, on the other,

358 THE CLIMATE NEAR THE GROUND

between the relations of east and of west borders to that on the
south. The four black corner areas belong to the borders which
face the north and which have no sun in winter. The longest dura-
tion of sunshine is found in midsummer on the southern exposures
(in contrast to sunshine intensity as discussed in Chapter 21). Along
the stand borders from SW through S to SE the duration of sun-

Hours of sunshine

FIG. 167. Monthly duration of sunshine on the edge of a stand from all directions
in relationship to the time of year. (After calculations by J. U. Kienle)

shine from the beginning of May to the end of August exceeds 150
hours per month. In this region there are two maxima, separated
by the bad weather of June, which as the "European monsoon"
usually brings more clouds and precipitation than does May with
its pure air, and August which already reaches toward the clear
autumn days of "Indian summer."

The figures cited by J. Schubert for intensity of irradiation will
serve to give some information at least for the stand borders which
face in the four main directions. They too are based on measure-
ments in which average conditions of cloudiness are considered.
Reference has already been made to the special position in which
south borders stand at the end of winter.

What parts of the borders gain in radiation, the others lose through
shading. R. Geiger (665) has furnished some information as to
width of shading in front of the stand — assuming level ground.
Fig. 168 applies to the summer solstice at the latitude of Munich
(48°N).

THE CLIMATE OF THE STAND BORDER

359

On the horizontal scale are the directions which each stand margin
faces, while on the vertical scale are shown the hours of the day
(true sun time). In the inner portion of the chart are the lines of
equal width of shade, expressed in units of stand height. The heavy

border lines between sunshine and shade (zero shade width) unite
all the possible conditions under which the sun shines directly along
the edge of the stand. The moments of sunrise and sunset are indi-
cated by the upper and lower broken lines. In the areas where the
lines of equal width of shade are not extended, the values have no
practical significance because the shadows are so long and the sun

360 THE CLIMATE NEAR THE GROUND

so weak. At midday, however, when radiation is strong, the lines
are correspondingly closer. It is quite evident that the chart is
symmetrical with respect to morning and evening.

From the upper right to the lower left, between the heavy zero
lines there stretches the broad white band whose extent indicates
full sunshine on the stand margin. In addition there is also an open
area at the upper left and at the lower right. This is because in
midsummer the sun goes so far north of the east point that it reaches
the stand borders which face NNW. These consequently receive
sunshine in the very early morning and also a second time toward
evening. The same holds for NNE borders.

If it is desired to determine at what time of day shade covers a
cultivated strip 15 m wide in front of a 20 m stand which faces
WSW, we see from Fig. 168 that for x = 15/20 = 0.75, the crop
lies in shade from sunrise till 9 A.M. on June 21 st. Shortly after n,
the whole crop comes into the sun and remains so till sunset.

There is a very surprising special case discovered by J. Schubert
(jp) which we must not omit. The width of shade in front of a
north margin on March 21 st and Sept. 23rd (the equinoxes) is inde-
pendent of the time of day, so from sunrise to sunset it is constant in
amount. The long, slanting shadow of morning and evening is of
the same width as the steeper midday shadow, which falls at right
angles to the stand margin.

Sunlight and skylight pass into the forest between the marginal
trees and there favor the development of the young growth. On the
other hand the stand darkens the open country in front of it. Fig.
169 shows the resulting transition according to measurements of
F. Lauscher and W. Schwabl (642). It depends to a great degree
on the lighting conditions how the transition takes place. If the sky
is clouded, and the forest (a stand of ash at the left in Fig. 169) is
not yet in leaf (curve i) the brightness outside (= to 100) and
inside are not very different. Curve 3 corresponds to the leafless
condition in sunshine. The direct radiation is strongly reflected by
the branches of the forest so that the differences are considerably
more than for curve i. The shading effect extends farthest into the
open when the trees are in full leaf and the sun is unclouded (4).
The dotted line corresponds to a fully leafed condition with cloudy
sky. For other kinds of woods, such as spruce, for instance, the
difference of illumination inside and outside is greater as we already
know. The transition, however, is about the same; only a heavy
stand cover such as spruce has, can cause a particularly darkened
border area.

THE CLIMATE OF THE STAND BORDER

362 THE CLIMATE NEAR THE GROUND

At night the stand affords the neighboring strip of open land some
protection from outgoing radiation according to the research of
R. Geiger (666). For plants within the over-hang of the trees, half
the sky is cut off. The nocturnal net loss by radiation of heat is con-
sequently only half that in the open, for the exchange of radiation
with the stand itself is unimportant since the latter has practically
the same temperature as the radiating soil. This protection from
outgoing radiation, however, decreases very rapidly with distance
from the stand. As shown in Chapter 2, this radiation is greatest
toward the zenith sky, and access to this is open as we get away
from the stand. As a result, at a distance equal to the height of the
trees, the counter radiation has already reached 90% of that in the
open. It must always be remembered that the frost protection of the
border zone near the old wood is caused not only indirectly by
reason of the warmer trunk-space air but also directly by reason of
diminished net outgoing radiation.

Wind relationships at the stand border become clear if we differ-
entiate — according to the excellent proposal of H. Pfeiffer (676) —
between an active and a passive forest influence on the wind field.

A passive forest influence consists in the action of the forest as a
hindrance to air currents. At the edge of the stand which faces the
wind, the currents are forced upward. The consequence is a dead
air zone at the ground, estimated at il/2 stand-heights in breadth.
Above this the wind speed is somewhat greater on account of the
compression of the lifted stream lines. M. Woelfle (683) using an
Albrecht hot-wire anemometer (214), investigated the penetration
of the outside wind into a dense stand of spruce. With weak winds,
20 to 30% of the outer velocity was found in the inner edge. With
brisk winds the protective effect of the mantle increased so that the
percentage value decreased. M. Woelfle attributed this to the screen-
ing effect of the mantle by which he meant the overlapping of the
spruce twigs and the consequently thicker screening of the stand
toward the outside.

The wind distribution in the lee of a forest is considered in
Chapter 39, in connection with a description of windbreaks. In
addition the reader is referred to the rules for wind action in cuttings
as given in the preceding chapter.

The temperature action exerted by the forest is an active influence
bearing on the wind distribution. This is a question of winds which
the forest itself generates.

When during the day the air layer near the ground becomes
heated over the open country but remains cool in the forest under

THE CLIMATE OF THE STAND BORDER 363

the screen of the tree tops, the cooler air of the trunk space may
flow out into the open as a diurnal forest wind. L. Herr (So) and
also K. Dorffel (66j) have demonstrated it by means of the cooling
and moistening of the air which it brings out. In its origin it is
very similar to the sea breeze which during the day blows from the
cool sea in over the hot land. Even in 1920 A. Schmauss (68 1) men-
tioned "Sea Breezes without a Sea."

There have as yet been no observations of a nocturnal country
wind corresponding to the land breeze and filling the counter part
of the diurnal forest wind. The braking action of the stand hardly
lets such a wind develop. On the other hand there arises a nocturnal
forest wind representing an overflow of cold air from the crown
surface out over the surrounding open country. It has been studied
and well described by H. G. Koch (670, 6703) with the help of
rubber balloons. On the level it attains a speed of only i m per sec.
It is noticeably stronger on a mountain slope, where the upper part
is forested. The cold air near the crowns then flows down-grade at
speeds of as much as 3 m per sec and sinks to the open ground at
the border of the stand. The hunter on a lofty post dislikes this noc-
turnal forest wind because it carries his scent to the wild life outside.

Seed distribution at the margin of the stand, which is strongly
influenced by wind and by convection was carefully investigated by
H. Hesselman (669) in the Swedish province of Vasterbotten. In
the midst of a 90 year old 18 m pine stand at Lund, situated in a
bare cutting 100 by 200 m, 262 seed boxes of 54 sq m area were con-
structed flush with the ground. The seed yield of the winter of
1936—37 was distributed as follows over the various zones near the
stand :

TABLE 60

Seeds caught

Distance o£ seed
boxes from edge
of stand

Number

Weight
\g

of seed wings

per sqm

Length
(mm)

Width
(mm)

In inner edge 37 m .
22m .
Both sides of stand
In outer edge 7.5 m

l°5
;. 89
margin 73

144
122
= IOO

71
40

3-8
3.6
3.6
34
34
2.9

12.9
12.7
12.9
I3-I

12.8

13.0

4.2

4-i
4.2
4.0
4.2
4.2

22 m .

T7 m

364 THE CLIMATE NEAR THE GROUND

The decrease in number of seeds with distance from the edge of
the stand was steady and in good agreement with the theory of Wilh.
Schmidt (//j) based on the law of convection. (Chapt. 4.) The
size of the seed wings makes no difference, for the last two columns
of the table show the same average size, but the light seeds are, on
the average, carried further than the heavy ones. The light ones,
however, are also the bad ones. While at the edge of the stand only
7% of the seeds were hollow shells, the percentage at a distance of
37 m was 19%.

A true transition phenomenon which occurs only at the border of
the stand, is fog precipitation. When wind-blown air carries water
droplets, as happens particularly with mist in the higher parts of
the mountains, the droplets are caught by the twigs, leaves and
needles on the side of the forest which is exposed to the wind. They
fall to the ground as additional precipitation. P. Descombes (66 1)
calls this "occult precipitation," R. Suring (according to 674), "hori-
zontal precipitation." We prefer the designation "fog precipitation"
as used by K. Rubner (6j8) .

Table Mountain at Capetown is known for the so-called table-
cloth which results from the driving fog which forms on the wind-
ward side of the mountain and dissipates again on the lee side. When
Marloth (675) placed two rain gauges — the one in normal location,
the other covered with a bundle of twigs, the latter, after two months
of observations had caught 16 times as much as the open gauge.
P. Descombes and others drew some bold conclusions from this as
to the amount of water gained by the forest through fog precipita-
tion. The effect is however limited to heights where fogs are
prevalent and it is always a forest margin phenomenon.

F. Linke (675, 674) has made some measurements in spruce
stands in Taunus at 800 m above sea level. In the years 1915—19 he
found the following excess of the rain gauge located in the forest
over the one in the open.

TABLE 61

Month

Rain gauge

Least
fog
(June)

Most
fog
(Nov.)

Summer

Winter

Year

Directly at forest
Farther into fores
Average number

edge . . . 104

301
259

131
90
14

184
J59

157
123
18

t 87

of fog — days n

THE CLIMATE OF THE STAND BORDER 365

The excess therefore reaches very high values. It decreases con-
siderably as soon as we move into the forest from the margin.

K. Rubner (67^, 679) constructed a special gauge for the measure-
ment of fog precipitation and used it in a six year series of measure-
ments at the Erzgebirge at 745 m. According to his findings, Linke's
figures are to be considered as upper limits for our German condi-
tions. His investigation also shows, however, that fog precipitation
may be of considerable importance in the water economy of the
stand zone near the edge of the forest.

A similar transition phenomenon exists with the dust content of
the air. The forest filters out the dust which exists in abundance
above the open land. Sometimes, we can recognize this effect from
the dust cover of the trees on the forest border in the vicinity of
very dusty roads. M. Rotschke (2560) investigated this process
under normal conditions and found in the case of wind perpendicu-
lar to the stand a strong maximum of dust content at the inner
border, and in the case of wind diagonal to the border of the stand a
great increase at the outer border. For the first case a numerical
example may be mentioned which is chosen for a 12 m high stand
of firs, pruned up to 2 m. The wind above the open land was at the
time of observation (Jan. 29, 1935) 2-3 m/sec. the temperature at
the ground, lightly covered with snow, was — 2°C. At the surface
of the ground, there were:

10100 10200 10300 14000 11800 1500 dust particles

loom 500 25m outside, 251x1 50111 loom inside the stand,

per liter of air. The interior of the stand then becomes more and
more free of dust, a fact considered as one of the advantages of the
air in the interior of the woods.

The subjects thus far considered have given us some idea of how
many factors determine the climate of the forest margin. Both the
inner and outer edges possess a habitat climate with special, well
developed characteristics. The practical forester has long reckoned
with them. C. Wagner (682) especially, in his book "The Funda-
mentals of Spatial Arrangement in the Forest," has based the method
of screen cuttings from the north edge predominantly on the stand
climate. He has derived the climates of the different stand borders
from only two factors — i.e. direct insolation and the rain falling
obliquely from the west — obtaining excellent results by this method.

It is regrettable that there are no measurements, no actual data, on
the climates of stand margins. We do not know today what is the

366 THE CLIMATE NEAR THE GROUND

average amount of precipitation at the different margins nor how
heavy rains and light snow which is easily drifted by wind and
turbulent air, differ in this respect. We do not know the original
distribution of dew as it forms, nor whether the early removal of
dew at the borders which get the early morning sun is alone respon-
sible for the different benefit derived therefrom. Whether the same
stand border is warmest at all seasons or whether there is a seasonal
displacement; whether the driest border always coincides with the
warmest, the wettest with the coolest; whether ground temperatures
and dryness are always parallel with the climate near the ground or
whether this relationship is disturbed by the macroclimate; how all
the elements mentioned vary in the realms of the inner and outer
margins — all this is of direct significance to the living conditions of
the young growth, but we have no observational data thereon.

If it be asked, why this is true, we must answer first that the task
of evaluating a habitat climate in figures is a tedious and difficult
one. All observations are complicated by accidental weather condi-
tions. The constant, significant features of the habitat climate must
be sifted out. The forest manager usually has neither the time nor
opportunity to make observations or interpret them. But this is not
the only reason. Rather, the problem itself is too complicated. Not
only season and weather, but also the kinds of wood, treatment of
the stand, condition of the soil and topography — all result in con-
tinually varying data. No one can expect to achieve more than
purely local results from first measurements of this kind.

R. Geiger has very recently shown (667) that a great circular hole
cutting offers, along its inner margins the best possibility for this
type of research. The uniform air body resting above the cutting
and in no way of different effect, microclimatically, by reason of
outer influences, is altered in respect to habitat only at the edges of
the stand. This alteration, according to observations during the
summer of 1940, is so great that it can be measured without great
technical difficulty. There is thus a possibility of solving the funda-
mental questions concerning the climate of forest margins in a sys-
tematic way.

SECTION VII

THE RELATION OF ANIMATE CREATURES AND MAN TO THE
MICROCLIMATE

CHAPTER 36
THE ANIMATE WORLD AND THE MICROCLIMATE

The microclimate is an effective habitat factor for stationary plants.
This is the information we were able to derive from Section VI.
Now we shall go a step further and take up the relations between
the animate world and the microclimate.

For animals also, the microclimate is very important. Although
they have in general, the ability to change their habitat, which plants
lack, they are nevertheless subject to the influences of the micro-
climate to a large extent. This is particularly true of creatures which
move slowly, if at all — such as larvae, worms, beetles, caterpillars,
etc. But there are many large creatures as well of a "fixed habitat*'
as W. Kiihnelt (697) expresses it. Yes, even the swift whose flights
in the upper airlayers always arouse our wonder, has to return at
night to its home whose habitability depends on the microclimate.

H. Grimm (692) has attempted a general answer to the question
whether microclimatic research is as essential to zoology as it is to
botany. His answer is an emphatic "Yes."

"Animal geography," he says, "should be based, in its form, on
microclimatic considerations. The range of any kind of animal
breaks up along its borders into island-like occurrences. Just as in
the distribution of plants, animals in an unfavorable macroclimate
can exist only in microclimatically favorable places. The egg and
larva of the hook worm, for example, which are adapted to tropical
temperatures, in our country find the conditions of their native
climate in tunnels and mountain gorges where they prosper very
well, to the discomfiture of mountain workers. The rat flea, which
carries the plague, flourishes in the underground heating plants of
Paris, although he is a guest from warmer lands. E. Martini and E.
Teubner (702) proved, through laboratory experiments and observ-
ations in the open, that the true malaria mosquito (Anopheles)
makes different demands on the microclimate from other mosqui-

368 THE CLIMATE NEAR THE GROUND

toes. This has a direct bearing on the danger of malaria for men
in the tropics, for the microclimate of tropical residences, including
stables and other buildings, determines which kind of mosquito is
suited or not for living with man. Many such examples can be cited.

It is necessary, therefore, in a textbook of microclimatology, to
consider also the relations of the microclimate to the animate world.

In describing the relations between the plant world and the
microclimate, we began in Chapter 26 with the heat economy of
plants. The animal has its own heat economy as well, but through
the more numerous and advanced life processes which distinguish
animal from plant, some difficult physiological questions are in-
volved in addition to the outside factors in the heat economy of the
animal body. We need only remember the warm blooded animals.
It is neither possible nor necessary to take up these questions here.
We shall confine ourselves to a study of the microclimate as an
environmental factor for the animals.

Attention is first directed to the dependence of the life and growth
of the animal world on the microclimate.

While describing in Chapter 22 the influence of different sun ex-
posures, we mentioned the investigations of E. Schimitschek (706)
on the bark beetle. The scarcely moving larvae of this kind of ani-
mal developed or died on one and the same tree-trunk according to
the temperature conditions of the bark climate. According to W.
Kiihnelt (697) the physical properties of the soil (compare Chapter
14) — its heat conductivity and water permeability — are responsible
for the appearance of certain kinds of animals. Animals which espe-
cially need heat consequently press farther northward on sandy
soils. For example, the Mediterranean locust (Stenobothius Fischeri)
as far as the sand dunes of lower Austria.

In connection with Fig. 89, we have already described the marked
microclimatic differences in the special observation network at Lunz
in their effect on the plant world. The animal world, too, is condi-
tioned by the same habitat conditions, as E. Schimitschek (706) had
demonstrated.

The pine-bud roller (Evetria turionana Hb.) is often seen as low
as the saddle of the sink-hole, whose significance in the temperature
stratification within the sink hole has been mentioned in Chapter 18,
Ten meters below the saddle it is very rare indeed, while 30 m still
lower it occurs only sporadically, since most of the caterpillars have
died. A true bark beetle (Pityogenes conjunctus Reitt.) occurred
in the dying and dead knee pine twigs of the sink hole. "On July
19, (thus Schimitschek describes the conditions in the cold ground

ANIMATE WORLD AND THE MICROCLIMATE 369

of the sink hole) the eggs of all broods studied were laid; in only a
few rare instances there were some newly hatched larvae. Besides
this freshly laid brood there were, — not counting the brood just
hatched and in some cases still harboring young beetles — also some
broods with larvae three quarters grown. On Sept. 23rd the larvae
of the July brood were half grown — part of them gone. Late laid
eggs do not survive the larval stage here but die off. The generation
here is biennial; in the most favorable cases, of a ii year term. On
the highest slopes of the sink hole, an annual generation could be
proved without exception. The frequency of occurrence of Pityo-
genes conj. increases from below upward! The number in the
brood at the upper edge of the sink hole is greater than that at the
bottom of the sink hole."

H. Franz (697), in a similar instance, showed the distribution of
various kinds of beetles in a valley at Parndorf (southeast of
Vienna). In the meadows of the moist valley bottom there are dif-
ferent kinds from those on the dry slopes or on the higher pastures.

It therefore appears that certain kinds of animals can serve as
identification for a definite microclimate to which they are con-
fined. Insects are the best climate indicators of all. W. Kiihnelt
(697) has proposed the term "bioclimatic index forms." In the realm
of the microclimate they permit a classification on the basis of ani-
mals, much as in the macroclimate W. Koppen's "beech climate" is
identified with a plant. According to H. Grimm (692), J. H. Blake,
in a quantitative study of forest insects, has differentiated four defi-
nite zones between elevations of 0.15 and n.oo m. M. Klemrn sub-
divided the growth of a meadow floor into six zones on the basis of
animate inhabitants — i. e. the geobium, with angleworms, beetle
larvae and butterfly pupae; the herpetobium, with beetles, spiders
and ants; the bryobium with mites and spring tails (collembolae) ;
the phyllobium with orthopters, aphids and caterpillars; the antho-
bium with all flower visitors; and finally the aerobium with the
libellae. Each of these six zones is characterized by a definite micro-
climate and by certain kinds of creature.

As plants and their habitats are closely bpund together, so are ani-
mals and their habitats. It is possible, however, for animals as well
as plants to exist in unfavorable circumstances. As protection against
too great heat they have heightened transpiration; against the
danger of dryness the animal has a number of weapons which W.
Kuhnelt (698) has summarized. But in addition the animal has a
fundamentally different possibility, which we have already men-
tioned— that of motion — change of environment. While an un-

37° THE CLIMATE NEAR THE GROUND

favorable microclimate represents for plants an "inescapable environ-
mental condition," to use an expression of W. Hausmann (7^6), an
animal can, to a greater or less degree, get away from it.

Lizards avoid harmful overheating by seeking cooler, shadier
places. The spotted lizard (Uma notata), according to W. Mosauer
(70^) endures the enormously high noon-time temperatures in
open desert sands by elevating its body from the ground as it runs
rapidly about. Pools which are drying up are forsaken by salaman-
ders and other inhabitants; this usually occurs at night, when
moisture conditions are most favorable. Dancing mosquitoes, as F.
Lauscher (699) accidentally observed, in a brisk wind which blew
part of the time up-valley and part of the time down, always sought
shelter in the lee of a hedge. According to F. S. Bodenheimer (686)
a swarm of African migratory locusts take a position at midday
such that the main axis of their bodies is paralleled to the sun's rays,
thus absorbing as little heat as possible.

But animals do not use their faculty of movement merely to escape
an unfavorable microclimate; they also seek out a favorable one.
These same migratory locusts, which at noon all protect themselves
from the heat, in the morning all present their broadside to the in-
solation in order to enjoy the early morning sun. Anyone who keeps
chickens may from them too learn microclimatology at unpleasant
seasons, for they are wonderfully wise in choosing the most comfort-
able place in all the range accessible to them at any moment and
for any given weather.

I found a fine observation of H. Wiele's (7/7) in the description
of his experiences while hunting animals for Hagenbeck in the
Himalayas. In the month of April, before the trees were in leaf, a
great swarm of locusts appeared in the neighborhood of Rawalpindi.
A heavy thunder storm in the night and the consequent temperature
drop had evidently exhausted the creatures exceedingly, for they re-
mained motionless as soon as they landed. The author thus describes
his journey through the swarm. "When we had traversed the dense
nucleus of the flight and were again walking in the bright sunlight,
we found that the beautifully smooth-rolled country road, covered
with bright grayish blue, crushed granite, was so thickly covered
with locusts that it appeared to be overlain with a thick, loosely-
woven, shrieking bright-green carpet, on which the shadows of the
tree skeletons with their thousand-fold ramifications stood out
sharply as a design in a pale gray color tone. For not a single insect
sat in the shadow pattern"

According to the observations of E. T. Nielsen (705), the leaf

ANIMATE WORLD AND THE MICROCLIMATE 371

locust (Tettigonia viridissima) in Denmark begins its song in the
afternoon, sitting meanwhile on low growths, such as weeds or
reeds. Then in the evening they are heard singing up in the trees.
In order to determine whether the latter were different locusts, or
whether the same ones which had been singing below climbed up as
darkness came on, he tied several meters of thread to some test ani-
mals. The locusts which were first singing on the ground did indeed
climb spirally up the trees when the ground air began to cool. E. T.
Nielsen assumes that they seek more comfortable temperatures at
higher levels.

I have occasionally been able to observe in an old pine stand how
in the early morning hours the insects seek the warm air layer which
the rays of the morning sun spreads over the tree tops while cool,
moist night air still fills the trunk space. Thousands upon thousands
of hovering flies, mosquitoes and butterflies assemble there — a
plethora of life, whose existence in such masses seems scarcely
credible. The living cloud was so sharply defined at its lower boun-
dary by microclimatic limits that to one climbing up the observation
ladder it seemed like sticking his head through a boundary surface.

The examples just cited have shown that animals understand how
to avoid the unfavorable microclimate in their life customs and how
to seek the favorable. They also often exercise astonishing prudence
in the location of their dwellings.

The great nest of the forest ant is really nothing but a miniature
testing ground for demonstration of different exposure climates in
all directions. The construction of the nest of poorly conducting
materials, such as evergreen needles or litter, makes the differences
very distinct. G. Wellenstein (7/0), in September 1927, made nu-
merous temperature measurements within the Trier district on the
nest of the red forest ant (formica rufa L.) As an example there are
observations on a nest 80 cm high and over 12 m in circumference
which stood under young spruces on a steep slope. Although the
weather was cloudy and rainy on the observation days of Sept. 13-15,
1927, the following temperatures were found, here arranged accord-
ing to time of day:

Hour of day 3 A.M. 9 A.M. 5 P.M. 9 P.M.

Air temp, above the nest .... 8 u 10 i2°C

Nest temp. shady 12 14 13 i6°C

at depth of 25 cm. sunny 21 22 18 22 °C

The nest at a point 25 cm below the surface on the shady side was
3° to 4° warmer than the surrounding air, and on the sunny side 5°

372 THE CLIMATE NEAR THE GROUND

to 9° warmer than on the shady side. The design of the nest is of such
a form that the different microclimates can be utilized by the animals.
A. Steiner (70$) describes the design and construction of the nest as
follows (here somewhat abbreviated). "On the southern side, pro-
tected from the wind, there is a dome shaped structure made of earth
and vegetable material, and filled with numerous air spaces. The
form of the dome which changes purposefully from hemisphere to
cone according to insolation and precipitation conditions serves as
heat collector. It reaches its relatively greatest effectiveness in this
respect at lower solar positions; thus a mathematical calculation
shows that for latitude 47° a hemispherical dome at noon on Dec.
2ist receives twice as much insolation as does a horizontal surface,
at the equinoxes ii as much and on June 2ist, 1.05 as much. In
addition to increasing heat absorption there are also means for re-
ducing heat loss, in particular the thick dome roof of vegetable
material — a poor heat conductor — the inner insulating air cham-
bers, and the nightly closing of the nest openings. By these means,
the temperature in the center of the nest, which is at an average
depth of 30 cm, often remains for a long time in summer between 23
and 29°, which is 10° above the corresponding ground temperature.
The temperatures in the upper part of the dome where conditions
vary from place to place with the position of the sun, are used to
best advantage by tireless shifting of the brood. In a similar manner
the brood is protected from overheating, by moving it into lower
portions of the nest."

Tropical termites, on the other hand, have to protect themselves
against excessive insolation. Fig. 170 shows, according to R. Hesse
(694) a termite nest from Arnhemsland in North Australia. As the
right hand half of the illustration shows, the structure as viewed
from the noon side is extraordinarily narrow and pointed. If, how-
ever, it is seen from the west or east (left hand view), it appears
extended. These compass nests of termites are the counterpart of
the compass plants mentioned in Chapter 22. They are the termites'
method of protecting themselves against insolation which at lat 11°
is all too strong. The earthen galleries which the termites build along
their roads are also, according to W. Kiihnelt (69$), to be considered
as protective measures against too great evaporation.

The entrances to rabbit burrows in the sand dunes along the
North Seas coast are often placed very efficiently. On the island of
Sylt, for example, I saw such an entrance situated on the mid slope
of a sand dune so that the accumulated water of a heavy rain could
not enter. Overhanging clumps of heath weeds protected it like an

ANIMATE WORLD AND THE MICROCLIMATE

373

awning from rain and dropping water. The exposure was southerly
so that the entrance received plenty of sun and no north wind could
blow into the burrow. A huge thicket on the west afforded addi-
tional protection against a storm from that direction.

H. Lohrl (700) reports concerning the bats (nyctalus noctula
Schreb) that in their wide range they are astonishingly skillful
in selecting a place with the warmest microclimate for hiberna-

FIG. 170. Compass nest of a species of termite in North Australia. (After R. Hesse)

tion. The animals observed in Munich selected (as they often do)
a plaza in the great city where in winter it is warmer than in the
country, choosing moreover in the warmest part of the city an inside
facing house corner which opened toward the southeast. 12 m above
the street — high above the cold ground air layer — the animals took
over two holes, some 50 cm deep, in the wall behind the eaves where
they were protected from rain. The inside of the house was heated
and one of the main steam-pipes leading to the bedrooms ran past
the nest. The following simultaneous temperature measurements
were made: at the outside meteorological station, —14°; on the roof
of the building, —5°; at the hibernation quarters, about o°!

In the choice of favorable microclimate by communal animals,
warming by bodily heat is a factor. A. Himmer (695) has shown

374 THE CLIMATE NEAR THE GROUND

that the heat regulating mechanism of the community is better the
higher and narrower is the optimum temperature range for the
development of the brood. A nest of Vespa vulgaris between July 24
and October 5 was, on the average, 16.4° warmer than the outside
temperature, a beehive, 12.3° warmer. E, T. Nielsen (704), in the
summer of 1937, made comparative thermoelectric temperature
measurements in an empty nesting box, in a second in which bumble
bees had made their nest, and in the open air nearby. For the 14
hour term he found the following average temperatures :

TABLE 62

Time of Observation

Free Air

Empty

With bumblebees

July 9th-a6th
August I2th-27th ....
Aug. 29~Sept. i ....

. . . 18.6
. . . . 19.6
. . . . 17.3

20.2

20.9
17.9

30.6
24.2

1 8.6

In the first period, since the nest was fully occupied, the bumble-
bees produced 10° excess temperature; in the evening 9 P.M., as
much as 13°. This excess gradually diminished; toward the end of
August there were only a few bees left and they were killed on the
29th. At once the temperature differential fell to less than i°.

In conclusion special reference should be made to forest entomol-
ogy. It is known that injurious forest insects are ever present. They
become a real danger to the stand only when the normal biological
equilibrium is disturbed and a great multiplication of the harmful
insects takes place. The weather plays a great part, for the develop-
ment of egg, larva, pupa, and butterfly depends at every stage on
temperature and moisture conditions which can be studied in the
laboratory. Mass increase, as is known today, presupposes the acci-
dental coincidence of favorable meteorological conditions in succes-
sive years. Naturally it is not the macroclimate which makes the
difference, but the microclimate which the caterpillars experience on
the twigs and needles in the treetops. In the more recent battles
against such outbreaks of forest insects, habitat measurements have
been carried out in the tree tops of the stand in question. More along
this line than can be discussed here, will be found in the writings of
H. Eidmann (688), K. Escherich (689) and W. Zwolfer (7/5). In
dusting poisons by airplane, microclimatic considerations have been
necessary to assure success. This was mentioned at the end of
Chapter 4.

CHAPTER 37
THE UNINTENTIONAL EFFECT OF MAN ON THE MICROCLIMATE

When we attempt to survey the relations of man to the microclimate
the first thing that strikes us is that man like the other animals
avoids unfavorable habitat and seeks the favorable. This is, as with
beasts, an instinctive procedure at first.

Anyone who has to wait in the street in a cutting winter east
wind, forsakes the stormy corner and seeks a calm microclimate.
When the first warm days of spring arrive in the large cities the
mothers with their baby-carriages instinctively find the sunniest,
warmest, and most sheltered microclimate in the city. In summer
we find those, both on the beaches and in the mountains, who are
true artists in the discovery of comfortable places to lie and sit. I
once read of the homeless of London, who spend their nights on
Victoria quay, that they learn accurately the temperature of every
house wall and seek particularly the outer walls of hotel kitchens.

With increasing civilization man loses such sensitivity to the
microclimate. Only at a later stage of development is there a con-
scious process of a rational search for the best microclimate, as we
may say in imitation of W. Hellpach's (757) "rational selection of
climate."

Obviously it is the consciousness of purpose which differentiates
the relations between man and his microclimate from those between
other animals and theirs. But we shall not speak of that at present.
Long before the idea of a microclimate had taken form, and before
there was any research into microclimatic laws, man as master of
nature exerted a powerful influence on the formation and dissolu-
tion of microclimates.

These unpremeditated effects we shall consider in this chapter.
The first fact to meet the eye is that man is a great disturber of
microclimates.

Unmolested nature, which reveals the ri'ch diversity of creation,
possesses an enormous number of microclimates. They exist close
together in harmonious contrast. Man's measures of culture, how-
ever, show the monotony and poverty of purposeful, reasoned action.
This appears in almost all the factors which determine the micro-
climate.

Agriculture has shown a preference for some few plants which

376 THE CLIMATE NEAR THE GROUND

have proved most profitable. With the reduction in number of
varieties there has been a standardization in structure of the plant
communities. It is only necessary to look at the uniform fields sur-
rounding a city. The climate near the ground has become a unified
climate.

In forestry the most profitable kind of wood — in Germany,
spruce — has been planted exclusively to an increasing extent, often
in militarily directed plantations. In place of the natural mixed
forest with its variegated mixture of different microclimates has
come the more profitable but monotonous artificial forest. Only very
recently have great calamities and a new understanding of biological
harmony in the forest paved the way for a change.

A similar development is observable in a few advanced countries.
The destruction of forests as a result of excessive lumbering has
worked out in a roundabout way, through changed soil and micro-
climatic relationships, to affect the macroclimatic condition of the
country in question. There is no lack of warning examples. H.
Scaetta (767) on the basis of his experiences in central Africa, says
that the burning of undergrowth practiced by the natives is "the
great destroyer of the original microclimate." The plant cover used
to modify in a thousand different ways the transition from the
ground climate, which, under a tropical sun is extreme and inimical
to vegetation, to the climate of the free atmosphere — thus giving a
chance for life to very different biological communities. When the
plant cover fell a victim to fire, the unfavorable microclimate alone
returned to control over the bare ground.

How this impoverishment of vegetation and microclimate go
hand in hand may be seen by a single example from a botanical
microclimatic study by K. Hummel (739). In the Rotach valley in
Allgau, which, at a height of over 500 m, possesses a harsh climate,
decidedly heatloving plants of a predominantly southern range occur
on the uncultivated south and southwest slopes. Among these are
Cotoneaster tomentosa, Epipactis rubiginosa, Cephalenthera rubra,
Orchis purpurea, etc. Temperature measurements of these habitats
showed an extremely warm microclimate with summer tempera-
tures up to 70° on the ground and up to 45° in the air close to the
ground.

The increasing opening of the Rotach valley has the effect of the
establishment of pasture areas in place of the original mixed forest
in this neighborhood as a concession to the needs of the dairy in-
dustry. There are also cultivated forests with uniform close crowns.
Heatloving plants will soon find no place of refuge afforded them

UNINTENTIONAL EFFECT 377

by that microclimate in which alone they are able to bloom and bear
fruit consistently. The time is in sight when they must disappear
entirely.

In addition to the impoverishment of the plant world there is an
equalization of ground conditions. It was pointed out in Section IV
how kind and condition of soil work out in the ground climate. The
increasingly technical cultivation of the soil tends to its increasingly
more perfect mixture. Moist meadows are dried up; waste land is
turned into meadows, useless thickets and woods are removed.
From south Mahren, for example, F. Kolacek (742) reports that
about 1700 A.D. a total of 85 square kilometers (i. e. 3.4%) of the
country was occupied by ponds; today that area has been reduced to
0.1%.

But the labor of man does not always lead to destruction of the
microclimate. He also establishes new microclimates, especially
through his building activities.

Every newly built dwelling makes a number of separate climates
out of the single one preexisting near the ground above the buildiftg
site. On the south wall the microclimate will be so favorable that
good fruit, perhaps even grapes, can be grown. This gain is at the
expense of the north side, which is dark, cold, damp and raw. Still
different are the east and west sides. The climates of the various
rooms are modifications of these four outdoor climates. In addition
there is the cellar climate and the attic climate.

Where a nucleus of buildings is formed, there will be in time a
special city climate. It differs so decidedly from open country climate
and has such great significance for the civilized man of today that we
must devote the next chapter to its treatment.

In industrial regions, finally, there ensues a landscape where the
slope and drainage of the ground are the only remaining features in
common with the original natural conditions. But in its new form
it too is rich in microclimates of the most varied sorts. Refuse
dumps afford new slope climates. The lifeless underground of huge
track areas creates a very hot microclimate near the ground. Where
a road embankment intersects the country slope climates are
formed; they can make cold air dams which may cause floods of cold
air. Interference with soil drainage has its reaction on the micro-
climate. The reader can fill in the rest of the picture from his own
experiences.

The unintentional disturbance of manifold microclimates on the
one hand, and the establishment of new ones on the other, has been

378 THE CLIMATE NEAR THE GROUND

recognized by man only by the real, practical damage resulting
therefrom.

Leveling off the landscape lessens the inequalities of the surface.
(See Chapter 28) . The wind, which is always blowing strongly in the
upper air, can therefore more quickly and more strongly affect the
ground surface. Increasing the velocity of the wind close to the
ground may raise dust if the soil be light and the weather dry.
There will be dust storms. The soil which is borne away takes
sown seed with it. Plant roots are laid bare so that they die, or they
may be buried and choked. The sum of small effects is great
damage.

In the dry areas of the western United States the wide open spaces
have favored extensive use of tractor plows. They are most efficient
on broad flat plains. As a result of the combination of topography
and method of cultivation came dust storms. The reaction today
threatens to become catastrophic since a diminution of rainfall in con-
sequence of a change in the macroclimate still further strengthens
die disturbance of the microclimate.

The danger of erosion is not absent from us either. According to
H. Schwarz (762) there is at Vienna, in northern Marchfeld, about
5,000 hectares, in the southern Vienna basin about 11,000 hectares, of
erosional land. The drainage of sour meadows and clearing of
forests is said to be the cause of dust formation here.

The establishment of new climates has proved especially advan-
tageous in cities and industrial areas. Here are possibilities for im-
proved health which command attention. We return to this in the
following chapter.

CHAPTER 38
THE CITY CLIMATE

There are two methods of determining the influence of a city on its
climate. Plenty of material is afforded by the records of many ob-
servation stations which are situated partly inside and partly outside
the city. From a comparison of a series of observations the charac-
teristics of the city climate can be determined in relation to time of
day, season and weather. In order to estimate climatic changes in
large, growing cities it is necessary to have many years* series of
undisturbed observations from such stations as a basis.

Since such comparable stations are rare and since experimental
observations often give valid conclusions, Wilh. Schmidt in Vienna
and A. Peppier in Karlsruhe, almost simultaneously, in the year
1929, made use of a new method which was soon accepted with
general approval and found application in most great cities. It is the
method of temperature-measuring journeys. Temperature measur-
ing equipment, usually electrical, is installed in a motor car, free
from influence of the motor, and records are made during the
journey. By traversing, allowance is made for frequent return to the
same point of the field in order to screen out the influence of tem-
porary temperature changes. A bicycle with a mercury thermometer
on the steering post is useful when needed.

Recently, in addition to temperature, similar measurements have
been made of atmospheric humidity, solar and sky radiation, the dust
content of the air, etc. The motor car must have the necessary
equipment and as a "research auto" becomes a moving laboratory.
Wilh. Schmidt in Vienna was the first to use such an outfit. It
naturally finds application in microclimatological research far be-
yond the limits of city climates.

In 1937 the results of city climate research were assembled and
presented by A. Kratzer (jSi). His comprehensive book should be
in the hands of everyone who wishes to go into the question
thoroughly. In the following survey I have tried first of all to men-
tion publications which have recently appeared; in the literature on
the present chapter, only those works not appearing in Kratzer's 250
titles have been listed.

In Germany a third of the inhabitants live in large cities; two
thirds, in places of over 2000 inhabitants. In the whole world almost

380 THE CLIMATE NEAR THE GROUND

10% of all the people are included in 540 large cities. There is prob-
ably no other microclimate which has so far-reaching an effect on
mankind, therefore, as that of the city.

In cities great quantities of coal are burned by industry and in
household heating. This means an artificial input of heat and a
pollution of the air. The influx of heat is the easiest of all causes to
understand in its influence on the city climate; it effects a rise of
the city temperature in comparison with that of the surroundings.
The question is whether the increase of heat is important enough
on the whole to play a part in the heat balance.

According to A. Kratzer we can assume that in large German
cities there is received an average of 15 to 30 calories per day per
sq cm throughout the year, according to the known consumption
of coal. With this we compare the results of the Karlsruhe radiation
records. The addition of heat from direct insolation and sky radia-
tion amounted to 52 cal per day per sq cm of level ground as a
December average; in June this figure was 518 cal per day. The
amount of artificial heat is therefore by no means negligible. In
winter, while it is above the yearly average, it helps out the natural
heat furnished by the sun. It is somewhat different with the effect of
this additional heat on temperature, for while the irradiation from
sun and sky affects not only the city areas but the surroundings as
well, the artificial heat is limited to the city. Thence it is carried
away upward and outward with a speed proportional to the amount
of air movement.

The pollution of city air is very important. In London there is an
average deposit of 12 g per day per sq meter. For the industrial
area of Rochdale (near Manchester) the amount is twice as much.
The total amount of soot which falls on the county of London in
one minute can scarcely be carried away by a strong man. To these
excreta of industry there is to be added the train smoke, insofar as
coal is burned, and the dust which street traffic continually stirs up.

In measuring the dust content of air, the best means we have today
is the Zeiss conimeter. H. Herrig (776) carried out some measure-
ments in 1936 at Marburg on the Lahn. The city of Leipzig was
carefully studied by A. Lobner (784). According to him, three dust
layers, one above the other, can be recognized in a great city. The
lowest, which lies between houses and on open spaces is caused by
street traffic and railway smoke. A second layer which is fed by
chimneys lies above the houses, about 20 m from the ground.
Above this, at a height of from 50 to 60 m, is a third, which is caused

THE CITY CLIMATE 381

principally by factory chimneys. The two upper dust layers increase
the dust in the street air only when there is rain and fog.

The study of dust distribution with different wind directions
permits the location of centers of dust distribution and recognition
of the purifying action which narrow green areas already have evi-
denced. Fig. 171 shows the lines of equal dust content per liter of

7r Explanation of symbols

'«* N^

I 1 Not built up

Thickly built

Factory region

• Stations among
railroad tracks
[: :"J grassy plains
woods in leaf
and pork area
Cemeteries

FIG. 171. Dust distribution in the city of Leipzig with east north east wind. (After

A. LSbner)

air on a day with ENE wind, the recorded figures to be multiplied
by 100. The air extending above land without buildings has a small
dust content and is immediately enriched with dust as we enter the
city of Leipzig. In the depot district in the NE part of the city
this enrichment is very sudden and strong. The green areas of

382 THE CLIMATE NEAR THE GROUND

Rosental as sketched on the map filter the dust out again just as
quickly. As we pass through the air of the industrial district to the
west, its dust content increases only slightly, which probably indi-
cates that the high chimneys throw out only the highest dust layer
— not that near the ground, in which the measurements were made.

As a whole, Fig. 171 shows clearly the growth of air pollution with
the enlargement of built-up town areas. How important such dust
content measurements are in the estimation of state hygiene scarcely
needs emphasis. According to A. Lobner's proposal, the hygienic
status of a city in reference to its dustiness should be defined and
determined by such measurements.

To the stranger the great dome of haze which hovers over a
large city and covers it like a flat black bowl in fine calm winter
weather, appears as remarkably characteristic. "Outside," as A.
Kratzer clearly expresses it, "the blue sky laughs over the landscape,
while in the city all is covered with gray and the sun shines only
with a weak yellowish-red light. Outside it is possible to see church
towers several kilometers away; inside, the houses on long streets
soon disappear in impenetrable gray. The larger the city, the
denser, heavier and more resistant is its haze hood."

This haze hood absorbs a notable amount of sun and sky radia-
tion; when incoming radiation prevails, as is the case at noontime
and in the summer, it intercepts part of the heat. Consequently the
haze hood attains a temperature higher than that of the surround-
ing air at the same level. The result is that the ground air of the
city, which stands to lose this part of the insolation, is cooler than
the surrounding ground air. The midday temperature maximum in
the city, as shown by measurements, is as much as 0.5° lower than
outside the city.

In our climate (Germany), however, where outgoing radiation pre-
vails the greater part of the time, the protection against net loss of
radiation afforded by the haze dome is much more effective. At
night, and especially in winter, therefore, a large city is warmer
than the country. The effect is intensified by the already mentioned
artificial heating by numerous fires which are more numerous than
ever at times of prevailingly outward radiation. The lowest temper-
ature of the day in the city is consequently i to 2° higher than out-
side the city. As the city grows, the daily temperature minimum
considered absolutely, grows also. Recently H. Arakawa (772) de-
termined for Osaka a rise of 2.6° in a century; for Tokio, 1.5°.

From these premises it follows that the diurnal range of the city
temperature is restricted in comparison with the temperature of the

THE CITY CLIMATE 383

surrounding country, the higher minimum having more to do with
this than does the lower maximum. The great amount of masonry
in a large city acts in the same direction, warming up slowly and
also cooling off slowly. As a result the city lags behind in the general
morning warm-up. On the other hand the streets hold the heat in
the evening, especially in midsummer. Fig. 172 shows the tempera-
ture distribution of a July evening in Karlsruhe. In the center of

FIG. 172. Temperature distribution in the urban area of the city of Karlsruhe on a hot
summer evening. (After A. Peppier (From A. Kratzer: Das Stadtklima))

the city it is as much as 7° warmer than in the open country, as is
clearly shown by the course of the isotherms and by the temperature
cross-section shown at the side. During the first half of the night
the evening cooling process proceeds very gradually from the outly-
ing portions of the city toward the middle.

H. K. Metzler (786) was able to establish this phenomenon for
humidity as well, by a series of measurements in Hannover. On the
clear nights of Sept. 18-19, I934» tne maximum relative humidity at
the airport near the city occurred at 10 P.M. In the suburbs the time
of maximum was 2 A.M., and, in the interior of the city, not until
6 A.M. Moreover, these succeeding maxima were about 10% lower
respectively. Hence, the city area is dry in comparison with open
country. This is especially true of summer evenings, when a differ-
ence of 30% has been measured in Munich between the center of
the city and the English Garden. It holds However throughout the
day. Consequently cities are about 5% drier than the country. This
is readily explained by the lack of evaporating surfaces (with the
exception of grass plots) and the speedy removal of precipitation
into the sewers.

When, with relatively calm weather at midday in the summer,
the city is warmer than its surroundings, it is able to set up its own

384 THE CLIMATE NEAR THE GROUND

peculiar circulation system. Just as the air streams into an open fire
from all sides,1 a light wind blows toward the center of the city from
all sides. It brings fresh air from the outer areas, at the same time
raising and dividing the haze hood. Cumulus clouds can form in
the rising airstream, rich in condensation nuclei, just as they often
do above great fires.

H. Mrose (788) has recently pointed out the need for paying more
attention than has hitherto been given, to the influence of winds
having their origin in microclimatic conditions such as have been
described in Part V.

In spite of its drier air, the city has more fog than the country.
Dust and the combustion products of coal furnish such a rich supply
of condensation nuclei that, for a correspondingly similar state of
readiness of the atmosphere for condensation, the formation of fog
droplets begins first in city areas. Fog is often observed first, or at
its densest, in the neighborhood of smoke-enveloped railway stations.
The growth of cities has consequently led to a noticeable increase in
fogginess. B. Hrudicka (777) has recently published the number
of days on which fog occurred at Prague, according to many years of
homogeneous observations at the astronomical observatory in that
city. For 20-year intervals since 1800 the average annual number of
foggy days is

1800 1820 1840 1860 1880 1900 1920
83 80 87 79 158 217

The effect of industrialization since the middle of the past century
is clearly evident. Here and in other places this increase has recently
ceased. On the contrary the relationship is becoming more favorable,
in spite of continued city growth. The explanation lies in more per-
fect combustion of coal through better designed furnaces and in the
introduction of electric railway equipment.

The tendency of city air toward condensation, together with the
upward movement of the air over the center of the city can have an
influence on precipitation. Fig. 173 shows lines of equal precipita-

1The following little experience may be mentioned here: — On Saturday, June 6,
1931, I was awakened about 3:45 A.M. at my home, n Arcis St., Munich, by the
howling of the wind. The weather forecast had been for a clear sky with no winds;
so I hurried to the window to have a look at this surprising turn in the weather,
and to take in the flower-pots from the window sill as a precaution. To my astonish-
ment I found the sky free of clouds and filled with stars, yet the lofty trees were
tossing violently to and fro. Only when I hastened to the other side of the house did
I perceive the fiery column of the burning Glass Palace directly before me. What had
wakened me was the inrush of air into the conflagration, which ceased when the
fire was extinguished.

THE CITY CLIMATE 385

tion over the urban area of Pasing and Munich. This cloudburst is,
of course, not caused by the city. But it is no accident that the culmi-
nation of the process occurred right here. The location of the two

FIG. 173. The cloud burst type of rain is released over large urban area. (After
J. Haeuser (from Kratzer: Das Stadtklima))

precipitation maxima in Pasing and Munich, as well as several
similar instances, make it very probable that there exists here a
microclimatic effect of the city, which expresses itself in the forma-
tion of weather of macroclimatic magnitude.

CHAPTER 39
THE CONSCIOUS MODIFICATION OF THE MICROCLIMATE BY MAN

As man discovers his relation to microclimatic phenomena, he first
gives conscious consideration to them. There follows later a willful
attempt to influence and modify the microclimate.

W. Hellpach (737), in his work "Geopsyche," speaks of the ra-
tional climatic search which men today can and should carry on.
So far as he has the opportunity he should find the climate best
suited to the preservation and development of his bodily and spir-
itual powers. In time of sickness, when such questions are of prime
importance, every good physician has always advised his patient in
the choice of a suitable health resort. In such advice, as K. Bihtner
(720) has only very recently observed, microclimatic questions must
receive suitable consideration. It is desirable in a climatic health
resort or air "cure" that, in addition to certain healing waters or
other curative means, it should possess a climate favorable to re-
covery. Whether this requirement is fulfilled or not in a given case,
depends, in places situated in the middle or high mountains, or sur-
rounded by forests, largely on microclimatic conditions, such as have
been treated in parts V and VI of this book. Attempts have recently
been made to standardize the concept of health resort in the public
interest, according to verifiable climatic conditions, so as to restrain
sordid advertising. K. Knoch (740, 74/)> F. Linke (750), A. Gregor
(7^2, 7jj) and W. Morikofer (757) have expressed themselves on
this, and have submitted recommendations.

But even a healthy person can and should choose his climate in-
telligently, and this becomes a search for the best every-day micro-
climate, since the abode of man is always connected with a definite
microclimate. Whoever gives some attention to this in his spare
moments, even for very short intervals of time, will be astonished
at the great number of previously unrealized possibilities.

These considerations apply in even greater degree to the choice of
a climate for communities. E. Flach (725) has given suggestions for
selecting summer camp locations for the Hitler youth, which are
filled with microclimatic facts. In these camps the youth who in
their city houses are deprived of proper light, air, sun, wind and
rain, are toughened without suffering any harm to their health.
The tents are set up in the air layer near the ground. The relations

CONSCIOUS MODIFICATION 387

of the ground and of the surroundings are consequently of great
effect on the demands which day and night are placed on the young
people. In the first place, a dry foundation is sought for the camp;
sand is preferred. The air should be free from dust. Hence the loca-
tion should not be close to settlements or main roads but near to
patches of woods which filter out the dust. Places with periodically
blowing winds and collecting basins of nocturnal cold air are to be
avoided. The slope of the land and the surroundings should permit
the free access of sunshine, without being too exposed to precipita-
tion. If these requirements are to be fulfilled, careful attention must
be paid to the microclimate of the locations.

In close connection with the conscious search for a suitable micro-
climate is the conscious modification of the microclimate.

It has been shown in Chapter 37 what strong repercussions the
occupation of the earth by man has on the microclimate. In particu-
lar, all that has hitherto been said as to the influence of soil types,
of condition of the ground and of plant cover, can serve as proof of
how dependent the microclimate is on man. While the weather,
and especially the macroclimate, is free from regulation by man, the
microclimate is relatively easily affected and molded to his will. In
this lies the far-reaching practical significance which microclimatol-
ogy has for human life. Man can consciously control climatic condi-
tions for himself and also for the plants and animals on whose wel-
fare his own depends. The regulation partakes of the character of
an adjustment of macroclimatic conditions within the range of the
microclimate, where, in the final analysis, the whole life of plants,
animals and man is spent.

How far man can influence the microclimate directly to his per-
sonal advantage, we can learn from the book, "Artificial Climate in
Human Environment," by E. Brezina and Wilhelm Schmidt (7/8),
which appeared in 1937. The microclimatic picture begins with
clothing, which alters the natural heat capacity of man. The amount
of material as well as its permeability to heat and to wind is so
chosen that the most favorable microclimate possible is produced be-
tween skin and clothing. In "Physical Bio-climatology," by K.
Buttner (7/9) we have a new book about the natural heat economy
of man and how it is modified by clothing.

L. Weickmann (765) has constructed a thermohygrograph the
size of a watch, which can be worn directly on the skin. This per-
mits making a record of temperature and humidity in the micro-
climate over the skin and furnishes the data necessary to its proper
regulation.

388 THE CLIMATE NEAR THE GROUND

Fig. 174 represents an experimental record made on Feb. 21, 1938
by a gunner in an anti-aircraft regiment during gunnery practice. To
avoid interference between the two recording pens, they have been
displaced 90°. The time scale farthest from the center corresponds
to the dotted humidity record, which we shall examine first. It

Artillery exercises
Cannoneer Weickmann Feb. 21, 1938

FIG. 174. Temperature and humidity recordings of the microclimate over the skin
with L. Weickmann's pocket thermohygrograph

begins at the top of ^Fig. 174. After breakfast, the observer went out-
doors at about 7 o'clock. There was a light frost, so the humidity
above his skin dropped to 40%. A short but steady run causes it to
rise again at once. The succeeding gunnery practice in the sun per-
mits the humidity to fall again at first. Increased exertions, however,

CONSCIOUS MODIFICATION 389

bring about a steady rise, culminating in an outbreak of perspiration
at about 10 o'clock, after which the humidity holds for some time at
100%. Only with removal of the outer clothing at about n A.M. is
the desired drying of? accomplished.

The temperature, the time scale for which is indicated by the fine
inner figures, varies between 33° and 34° after the first rise. It is
lowest, following the outbreak of perspiration between 10 and n
o'clock. This is the sensation of "feeling chilly" one experiences
when standing for a time in perspiration soaked clothes.

The next step in regulation of the microclimate we may call the
"bed-climate." According to recent measurements by H. Landsberg
(745), the temperature under the bed covering is decidedly depend-
ent on the room temperature and, indirectly, on the outdoor temper-
ature. The average maximum attained in the course of the night is
30°, but in a cold room it may be as low as 25°. The microclimate in
bed is therefore not always a true protective climate as has been pre-
viously assumed. Adjustment to the latter condition is, however,
readily accomplished by a healthy organism.

The room climate or climate of the living space has been
thoroughly studied by K. Egloff (722) at Davos, and also by A.
Amelung and H. Landsberg (7130) and by F. Linke (757). Physi-
cians have also concerned themselves in this investigation. A com-
pilation may be found in the book by E. Brezina and Wilhelm
Schmidt (718).

The regulation of this microclimate, which is of such great signifi-
cance for the life and activities of man, takes place in various steps.

In the first place the location of the room in the building, as to
compass direction, height above ground, and surroundings, deter-
mines its microclimate so that construction represents one of the first
stages in climatic control. The necessity of utilizing whole build-
ings and the question of cost set certain limits to this. The skill of
the architect must get the best out of the location he has, using the
building material at hand, and utilizing the number, form and
arrangement of windows to the best advantage.

Within the limitations set by construction, further regulation
proceeds by means of window ventilation or special ventilating
equipment. In this way, the disadvantage is modified that in a closed
room "dead inside air" exists in opposition to the "fresh outside air"
(C. Dorno, 72/0). Less of the air is confined in the motionless
"ground layer" near the floor. In midsummer the maximum temper-
atures are lowered by screening out radiation. In winter time, the

390 THE CLIMATE NEAR THE GROUND

heating of the rooms improves the temperature conditions but
normally is combined with a significant drying of the air.

A third step, finally, is the production of an artificial climate which
is entirely independent of the surrounding weather processes. This
manufactured climate is today merely a question of cost, since there
are no fundamental technical obstacles to maintaining even the
largest rooms at constant temperature and humidity with the ad-
mission of purified air exclusively.

This artificial climate in a limited space is unquestionably a
necessity where the natural climatic conditions make human life
and work impossible — for example, in the 1800 meter deep gold
mine in South Africa, where the temperature of the surrounding
rock is at 50°C with the air at complete saturation. It should be
considered in shipping, for in certain parts of a ship, particularly
the boiler-room and engine-room, a voyage in the tropics makes ex-
traordinary demands on the personnel. These have been considered
recently. Anyone who is interested in this aspect should look into
the books on marine sanitation by H. Ruge (760) and, on the
meteorological side, by T. Berke and G. Castens (7/6) H. D.
Harries (755), H. Michler (756) and F. Wagner (764).

Furthermore, the artificial regulation of climate is desirable for
houses in hot countries. In our macroclimate, the technical indus-
tries come to mind, such as tobacco processing, where the manu-
facture is considerably influenced by temperature and humidity
conditions.

Our acclimation to the indoor-climate causes an entirely wrong
idea of the macroclimate in which we are living. This is valid not
only for winter time, though in this season we become aware of the
cold despite the protecting clothing when we go outdoors; but also
in spring and fall and even in summer we generally estimate the
climate as too warm, since we escape the nocturnal portion of the
daily temperature course by flight into the bed-climate. In the first
World War, I made this striking discovery during the war of
movement which compelled us to camp in the fields during the
night.

Besides the dwelling place also the storerooms have to be taken
into consideration where often goods sensitive to weather, as pota-
toes, milk, preserves, seed goods, flower bulbs, etc. are stored. We
should not forget the air raid shelters which protect the people in
the hour of peril and should not be too damp and cold. Under cer-
tain conditions, whole houses can serve to store goods and must be
planned microclimatically for these goods, as is the case with refrig-

CONSCIOUS MODIFICATION 391

crated buildings, graneries, warehouses, places for storing overseas
goods, etc. For marine shipping a special storing-meteorology exists.

Starting from the climate of the rooms we proceed to the climate
of the house. On the basis of his personal experiences in many
climates in many parts of the earth from Greenland to the South Sea,
Kurt Wegener (764^) has given a sketch, worth reading, of how
men in building their houses avoid or at least moderate the incon-
venient features of the general climates except in cases with which
other points of view are paramount. H. Amende (^14) in 1938 in-
vestigated light conditions in the clinics at Jena in comparison with
those of the neighboring mountain estates. It appeared that the clinics
have a very unfavorable microclimatic situation, so that there is no
possibility of therapy with natural light. Today, before building a
clinic in a hilly country, the microclimatic conditions would be in-
vestigated with a sunshine recorder (495). Wilhelm Schmidt and
W. Schwabl (496) have used this instrument in testing the suita-
bility of different neighboring pastures for cattle. V. Conrad and
W. Hausmann (727) attempted to find the physiography most fav-
orable, in regard to wind conditions for a sanitarium, and recom-
mended a gentle slope of a "carpback" shape. Such a location is
free from the drafty air of passes and deep valley, it does not
have the strong winds of peaks and domes, and by the shape
mentioned above it avoids lee eddies, which might bring up dust
from the lower ground. Furthermore, protection against wind and
dust can be gained by the establishment or maintenance of forest
windbreak belts. F. W. P. Gotz (7^/) has praised a mountain
cirque as favorable in respect to light conditions.

It is probably undisputed today that in the establishment of hos-
pitals, sanitaria, convalescent homes, etc., microclimatic viewpoints
are recognized and thoroughly considered. To teach what these
considerations should be is one of the chief aims of this whole book.
Suggestions on this subject are to be found in Chapter 25. The con-
sideration should not be postponed too long. W. Hausmann (7^6)
once expressed this in the following words: "It is essential to seek
the advice of a climatologically inclined physician and a medically
interested climatologist in regard to all buildings of a public nature,
especially hospitals, convalescent homes, sports plazas, bathing
resorts, etc., and such advice should have weight in city planning
wherever possible. But, if this advice is to be of use, it must be had
before contracts are let, for the best "medical-climatic" ideas will be
wasted if the foundations have been laid according to preestablished
plans.

392 THE CLIMATE NEAR THE GROUND

To the initial concept of "building climate" we must append
therefore, those of settlements, blocks of houses and cities. This
brings us into the realm of hygiene, and here we must refer to
A. Kratzer (7#/), as well as E. Brezina and W. Schmidt (718) in
whose books further material is to be found.

In addition to the intentional modification of the microclimate in
the direct interest of man, there is a similar modification for ani-
mals. Man seeks to ameliorate the living conditions of the useful
domestic animals, insofar as they depend on the microclimate, and
to make those of harmful animals as difficult as possible.

P. Lehmann (747) was probably the first to call attention to the
significance of the climate in a stable and to the necessity for its
systematic observation and regulation. Recently, A. Mehner and
A. Linz (755) have published a series of temperature measurements.
According to them, temperature fluctuations in an empty stable are
half as great as those of the outdoors, while in an occupied stable
they are only one eighth as great. On the floor they are greater than
near the roof. The correlation coefficient between stable temperature
and outside temperature will serve as a measure of excellence of the
stable. P. A. Buxton (50/0) gives the daily course of the temperature
in a cow-stable in Palestine. Veterinarians and builders can work
together in finding and producing the most favorable microclimate.

Man will deprive harmful animals of all microclimatic conditions
favorable to their growth and reproduction. Several examples of
this are to be found in Chapter 36. Microclimatology can also aid in
the war of extermination, for only he who understands all phases in
the life history of an animal can succeed in mastering it even under
unfavorable circumstances.

What applies to animals in the service of man, holds true also for
the plants which furnish his nourishment. At the best he furnishes
the plants their own house with an artificial climate. This of course
is possible only for special experimental and breeding purposes.
A. Made and W. Rudorf (755) have very recently described the
microclimate in a modern air-conditioned greenhouse at the Kaiser
Wilhelm Institute for breeding research in Miincheberg. But even
the garden breeding establishments which are not air-conditioned,
such as the ordinary greenhouses and hot-beds, serve to modify the
microclimate, artificially, in favor of the plants. During 1940,
A. Made (752) published several series of measurements of the
temperature march in such establishments, which are of basic interest
in regard to observational technique.

CONSCIOUS MODIFICATION 393

In Section VI of this book (on the influence of plants on the
microclimate) the reader has long since inferred what effect human
activities in the culture of plants in the open exert on their micro-
climatic living conditions. Herein is the field of activity in which
man is able to mold the microclimate most effectvely and adapt it to
his uses. Here is the most important aim of modern microclimatol-
ogy. P. Lehmann (746) has given a fine presentation of the possi-
bilities which present themselves from the standpoint of purely
practical agriculture.

Reference has been made repeatedly here to such practical applica-
tions. For the sake of completeness, however, two problems must
be raised in particular, to which we have been unable to give suffi-
cient attention thus far. These are the questions of artificial wind
protection and artificial frost protection.

The great damage to agriculture and forestry which accompany
excessive wind speeds near the ground, especially in combination
with soil dryness, has been mentioned already in Chapter 37. The
danger is obviated by the use of strips of shrubbery, copses and
forests, which are most effective when they run at right angles to the
direction of the prevailing wind. Experience in such windbreak
strips has been gained on the Russian black-earth steppe between
the Dnieper and the Volga, on the prairies of the United States,
along the North Sea coasts of Germany and Denmark, and in sev-
eral other localities.

The effect of a wind break hedge extends not only down wind
but to a smaller degree also against the wind. The wind velocities,
measured to establish the protecting effect, are given in per cent of
the undisturbed open land speed, which is observed far outside the
protected region. The range of the protecting effect is not indicated
simply in meters but generally the height of the protecting hedge is
used as the unit; a hedge two times as high offers protection for
double the distance. But there is no agreement about the height at
which the wind should be measured and with what reduction of the
open land speed the protecting effect is still considered as sufficient.
The wind speed, diminished in the protected zone, changes continu-
ously to that over the open land. The statements, therefore, fluctuate
within widest limits. To give an idea of the order of magnitude it
may be said on the basis of the measurements of M. Woelfle (767-
769) in Germany, and W. Nageli in Switzerland (7570) that to the
front the protecting effect extends against the wind to from 5 to 8
times the height of the protecting hedge, behind it to 25 to 35 times

394

THE CLIMATE NEAR THE GROUND

its height. The extensive measurements of the Danish Heath Society
and C. E. Flensborg (726) resulted in:

At the distance of (m) . . 5 10 20 40 60

Wind speed (%) 30-40 45~55 60-70 70-80 80-90

It goes without saying that the width and the arrangement of the
protecting hedges are of importance. A hedge which can be a little
blown through by wind seems to be even more advantageous than a
solid wall; this can be justified by aerodynamic considerations.
Further, a level area is always assumed. Naturally, in a territory
slanting away from the wind, the effect reaches farther.

Prevailing wind
direction

Meadow
Woods
Permanent hedges

Hedges for the first
40 to 50 years

Scafe
0 100 200 300 400

„ !

:
~ 1

.......I

:
:
1
1

*i
:

: ^™"

...... 4

—

FIG. 175. Wind sheltered area of wood and hedge strips as postulated by M. Woelfle

Windbreaks according to the design of Dr. Hellmuth have been
installed on the Rhone heights for the agricultural development of
the plateau region. They are said to create at altitudes of from 700
to 900 m above sea level a calm microclimate for future settlements.
A special study in the summer of 1937 was initated by M. Woelfle
(769) to recommend the type of windbreak shown in Fig. 175.

The 50-meter shaded windbreak strips enclose the sheltered plots
measuring 250 by 1000 meters. The windbreak, which consists of
half of evergreens (spruce) and half of deciduous trees and which

CONSCIOUS MODIFICATION 395

when fully grown will be 15 m high is relatively broad, because on
its windward side allowance has to be made for the adverse action
of the wind, snow and frost. If at least 30 to 40% of the free wind
is to be screened from the whole inner field, rows of hedges must be
constructed one to two meters wide and 4 to 5 meters high. These
hedges are of additional use as cow-fences for the pastures, as nesting
sites for birds and as suitable repositories for the stones collected
from the arable land, as well as a place to get hazelnuts. The hedge-
rows most sheltered from the wind, which are indicated by the
dotted lines in Fig. 175, can be removed later, when the forest strips
have grown high enough. The scheme of Fig. 175 will in individual
cases be adapted to the particular lay of the land, soil conditions and
traffic requirements. It furnishes, however, a fine example of a
planned microclimate such as is possible with modern large scale
planning.

The problem of artificial windbreaks belongs in general to for-
estry, rather than to meteorology. The difficulty consists in the use
of suitable kinds of trees in the proper mixture, in the correct style
and manner of planting and in the care of the windbreaks.

Of a particularly meteorological nature, however, are the questions
posed by the problem of artificial frost protection. We shall speak
of it as a second special practical application of microclimatology.
Before we take up the discussion of the specific protective measures,
some comments must be made on the origin of destructive frosts.

CHAPTER 40
DESTRUCTIVE FROST AS A MICROCLIMATIC PHENOMENON

When, in the spring, the plant world has awakened from its winter
rest, night frosts continue at intervals for some time. We call them
"late frosts." In a similar manner "early frosts" come before the end
of the growing season. In our German macroclimate, nights with
temperatures below freezing occur in certain places even in July and
August. These are described as "summer night-frosts." We shall
group late frosts, early frosts and summer night-frosts under the
heading "destructive frosts."

The destructive frost is typically a phenomenon of the micro-
climate. There probably are spring nights on which, over the whole
country, the blossoms freeze and the young plant growth is killed.
But the general rule is, that on cold nights, cold places are visited
particularly. The farmer knows in his experience, and the forester
in his, of just such endangered places.

E. Munch and F. Liske (799) in 1926, in a study of frost danger
to the spruces of Saxony, proved from the macroclimatic observa-
tions of the meteorological stations, the influence of physiography on
susceptibility to late frosts. They separated the many years of ob-
servational data according to cold and warm locations and thus
obtained the correlation shown in Fig. 176 between the number of
May and June frosts and the altitude above sea level. In both in-
stances the frequency of late frosts increases in accelerated ratio with
increase of altitude. Although the nature of the macroclimatic ob-
servations made in shelters suppresses the differences found there,
they are clear enough in Fig. 176. In the air space close to the ground
in a given climate the number of frost nights listed on the abscissa
are to be multiplied many fold.

R. Geiger, M. Woefle and L. Ph. Seip (455) have published com-
prehensive microclimatic observations on this question from the
Bavarian forest. In Fig. 116 (ch. 24) the scattering of minimum
night temperatures in the ground air on the slopes of the Great
Arber was shown. The law of temperature decrease with height
was recognizable only by statistical summation of the several obser-
vations and even then only above the great temperature inversions in
the valleys. The influence of the microclimatic conditions was on

DESTRUCTIVE FROST

397

the contrary so decisive that a station at 800 m above sea level might
be even colder on the average, and another at the same altitude
warmer, than a station at 1400 m. It is obvious that this has a great
influence on the relative frequency of late frosts. The predominance
of the microclimatic influence over a recognized macroclimatic law,
as here demonstrated, is the best possible proof that the destruction
of plants by frost is a microclimatic affair.

woo

V01234567Q91011
Average number of frosts in May and June

FIG. 176. Increase of frost danger with altitude in the Erz mountains. (After
E. Munch and F. Liske)

Essential to the occurrence of a killing frost is the cold wave
which first upsets the general temperature level. It is caused by the
transport of cold air from source regions near the pole (in the case
of advective frost). In addition to this there are the orographic con-
ditions which intensify the nocturnal cold to the freezing point
(radiation frost). The microclimatic laws which are in effect here
are none other than those we have already enunciated throughout
this whole book. It is only necessary to recapitulate them briefly
with a view to practical frost protection and to refer to earlier ex-
planations.

Local damaging frost is intensified by the following conditions:

1. by a clear sky, since this favors heat radiation outward (see
Ch.2andCh.8).

2. by dry air, since water vapor increases the counter radiation of
the atmosphere at night (see Ch. 2).

398 THE CLIMATE NEAR THE GROUND

3. by absence of wind, since this leaves the temperature stratifica-
tion by outgoing radiation undisturbed, with the coldest air next to
the ground (see Ch. n).

4. by the poor heat conductivity of the earth, which, in the first
place, lessens the nocturnal movement of heat out of the ground —
a movement which would reduce the temperature drop of the out-
ward-radiating surface. In the second place, the heat supply of the
ground is, on the whole, slight, for even during the day, little heat
is stored up. In the third place, the high daytime temperatures asso-
ciated with the poor heat conductivity excite the plants to premature
spring growth on these very frost-threatened soils. (See Ch. 14.) In
this class of soils belong the moors which, in spite of their high water
content, are, by virtue of their structure, theoretically poor heat
conductors (see Chs. 13 and 14).

5. by strong evaporation, which occurs after rainfall and in the
presence of a plant cover that gives off much water because of the
amount and nature of its surface area (cooling by evaporation).

6. by the local advection of cold air, especially by cold air floods,
as described in detail in Chapter 18.

7. by the lack of natural protection from outward radiation, such
as is afforded by every shrub and tree. This follows of course from
what has been said about radiation screens at the edge of plantings.
(SeeCh.35.)

It is a matter of common experience that the air over meadows or
weedy crops is colder at night than that over bare soil. Differences
of as much as 9°C have been measured. This can be directly ob-
served at times by reason of the formation of hoar frost. Moreover,
the fog which lies over meadows and not elsewhere usually owes its
occurrence to this temperature relationship. On account of the sig-
nificance of this fact for grape and fruit culture and for forestry at
the time of late frosts, the designation "grass frost" has been adopted.

The most erroneous explanations for this phenomenon are current.
The assumption, that the meadow is a greater nocturnal radiator of
heat outward, is false. Also, the manifold multiplication of surface
area by means of stalks and leaves is unimportant. (See Ch. 27.)
Numerous measurements show consistently that a meadow gives off
less heat by radiation at night than does the solid earth. In a funda-
mental work on the grass-frost, F. Sauberer (800) mentions, for ex-
ample, an evening observation at Lunz, in March, 1937, in which,
by the use of an Albrecht radiation-meter (372) a radiation of 0.079
cal per sq cm per min. was measured over a meadow, as compared
with o.i 10 cal over nearby solid ground. In agreement with the law

DESTRUCTIVE FROST 399

of the dependence of radiation on temperature, therefore, the out-
going radiation of the colder meadow is also less than that of the
warmer, bare ground. Why, then, is the meadow prevailingly
colder ?

In the first place, there is the circumstance that there are about
20 to 50 sq meters of living leaf surface to one sq meter of ground,
which are effective so far as evaporation is concerned even if not for
radiation. Consequently the heat loss through evaporation is con-
siderably higher in the case of the meadow than in that of bare
ground. In the second place, the plants are on a poorly conducting
foundation of decayed vegetation; this is particularly true for un-
cared for mossy meadows and still more so for weedy forest plant-
ings. That there is a very large range of temperature above such a
poorly conducting substratum, with especially cold nights, has been
stated before (Ch. 14). Finally, when the two causes mentioned
have lowered the meadow temperature, the temperature contrast in
respect to the adjacent air layer becomes greater. Consequently the
grass, either by convection or by radiative pseudo-conduction, with-
draws more heat from the air just above it than does the solid
ground. In this way is the "grass frost" to be explained.

The laws stated above as governing the occurrence of a destructive
frost, make it possible to plan protective measures against it. By this
we mean, not the steps to be taken when first the danger is seen to
threaten, which we shall treat in the following chapter, but such as
in the long run will lead to a lessening of the frost hazard.

W. J. Humphreys (797) says in one place: "The best time to
protect fruit from frost is when the orchard is being laid out." The
nature of the plants and the microclimate must be adapted to one
another. This demands knowledge of microclimatic relationships
and selection of the most suitable varieties.

For low-growing plants, protection by a screen of high trees is a
decidedly effective measure. In forest practice it is common to pro-
tect the young growth with a "fore-planting" to ward off late frosts.
On the Upper Bavarian plateau, for example, spruce plantations are
screened by a protective growth of fast-growing birches. In northern
Germany old pine slashings are used for this purpose, to allow the
tender Douglas firs to reach maturity. The old stand is heavily
pruned in order not to deprive the lower plants of essential sunshine.
Even a heavily pruned stand of trees makes a serviceable frost
screen.

H. Amann (793) has carried out microclimatic temperature meas-

400

THE CLIMATE NEAR THE GROUND

urements in a stand of 32 year old birches, averaging n meters high,
in the Anzing-Ebersberg forest. The birches covered an area of
0.88 hectares. On one side of this was a bare space in the perfectly
level country, which was left for comparison. On the opposite side
was an old spruce forest. The young spruces under the birches had
attained a height of between 0.8 and 1.5 meters and an age of 14
years. In May, 1927, H. Amann observed the following nocturnal
minima at a height of 25 cm above the ground : —

TABLE 63

Barren Area

Under the Birch foreforest

Measuring Point I
Point

Point II

In the In the
border Interior
of the

Near
An Old
Stand

barren
area Pointl

Point II

May n, 12 —
May 14, 15 —

1 1.0

8.0
3-8
2.9

4.1

— 10.9
- 7-7
- 3-5

— 2.1

- 3-7

— 1.2

-7.3 -6.2
—4.1 —2.6
-1.7 +0.4

+0.2 +1.5
— 1.3 —0.2

—0.4 +0.1

-5.6

-1.9

+0.6

+2.3

+0.4

+0.2

-5.2
— 2.0
+ 1.0

+0.5
+O.O

May 15, 16 —

May 25, 26 —
Mean on n May
nights —
Temperature difference
between 25-100 cm
height —

The protection of the open stand of birches resulted in a tempera-
ture gain of 4° for the average of the n coldest May nights. In indi-
vidual cases this was as much as 6°. At the edge of the birch forest
next to the bare area the excess of heat was less, while next to the
old stand it was greater — an indication that a partial air exchange
takes place along the borders. The difference in temperature be-
tween the heights of 25 cm and 100 cm above the ground shows
clearly that the effectiveness of the fore-planting consists in the
cutting down of outward radiation, for above the barren area this
slight difference in height results in a temperature difference of
from 1.2 to 1.4°; this is normal for the outgoing type of radiation.
In the birch forest the difference is practically zero, as it is also in the
old stand, a proof that it is only the advection of cold air, whether
from the outward-radiating crown-space of the birches, or from the
colder side areas, that determines the temperatures at the ground,
and not a process of outward radiation.

It is possible to obtain further effective frost protection by con-

DESTRUCTIVE FROST 401

trolling the flow of cold air. Staudacher (804) was in 1924 probably
the first to call attention to the fact that it is not the lay of the land
alone which accounts for the accumulation of masses of cold air in a
hollow but also the plant cover, which permits or hinders the move-
ment of the air. He calls the area from which cold air masses can
flow in freely to a certain point, the "source region" of the frost. He
has shown that the size of this source may be subject to great varia-
tions in the course of time and that the liability of the basin to
damaging frost varies with the size of the source.

In Fig. 177, AMB represents a cross-section of such a physiographic
basin. Considering the form of the land only, the source region is
bordered on each side by the elevations A and B. At night all the
air between A and B will flow toward M as it cools. But if a circle

FIG. 177. The conception of the frost source region.

of forest W lies half way up the slope, the downflow of the air lying
above W is practically stopped by the braking effect of the air move-
ment in the forest, so that the cold lake is divided into two parts as
indicated in Fig. 177. Under certain conditions this may be advan-
tageous. The sudden increase of destructive frosts in places which
previously had suffered little, is, according to Staudacher, attributable
in most cases to an enlargement of the source area by artificial
means. O. W. Kessler and W. Kaempfert (813) published a diagram
of an ideal landscape which could be altered, by artificial guidance
of cold-air movement and by control of water conditions, from a
frost-visited area to a frost-free one.

In conclusion it may be mentioned, that security against a surprise
attack by a destructive frost is a matter for preventive measures.
Some may trust to alarm thermometers. There are various types
which have been tested in practice ($/_?). They are placed in the
garden which is to be protected. When the temperature falls below
a previously determined critical point, a bell is set ringing which
calls those responsible in the emergency from their warm beds.

This method, however, gives much too short notice. It is better to
make use of frost forecasts at the same time. First one should consult
the weather forecast and if necessary the special frost warning of the

402 THE CLIMATE NEAR THE GROUND

nearest weather bureau office. This must be modified by experience,
according to the favorable or unfavorable microclimatic conditions
of the garden in question. Finally one will make his own frost fore-
cast based on his own instrumental observations.1 There are a num-
ber of rules for this, which cannot be mentioned here. Information
on the subject may be found if necessary in the work of O. W.
Kessler and W. Kaempfert (813).

1In the first edition I dealt with frost forecasts in chapters 22 and 23. In dis-
cussing the work of J. Schubert on pages 196-198, I erroneously interpreted the
condition which J. Schubert advanced as necessary for the advent of a night frost, as
being a sufficient one. This error I gladly correct here.

CHAPTER 41
THE BATTLE AGAINST DESTRUCTIVE FROST

Frost prevention belongs almost exclusively to the time of late frosts
and to the month of May in particular. The possibility of combat-
ting destructive frosts with artificial means depends on the rarity of
its occurrence, for every battle against frost demands a considerable
outlay of capital and energy — in preparation, in readiness and in
strenuous night work. All this can be absorbed the more easily in
the conduct of a business, the more rarely a late frost occurs in the
given locality.

For this reason the first successful development of artificial frost
protection was in the fruit-growing regions of the United States of
America. The valuable orange industry of California lies in a geo-
graphical latitude comparable in Europe to that of southernmost
Italy or the northern coast of Africa. The fact that, in spite of this
location, the winter frosts can do so much harm there, is based on
macroclimatic conditions. In the western part of the continent the
ranges of the Rocky Mts, and in the eastern part, the Alleghenies,
run from north to south and lead far southward the cold-air masses
which in Winter stream down intermittently from the great Can-
adian reservoir of cold. The cold waves under certain weather con-
ditions are able to penetrate clear to the sub-tropical fruit belt and to
induce there such low temperatures that the hope of a harvest may
be dashed in a single night. This is, however, such a rare occurrence
that quite an expensive outlay for combatting frost can be made to
pay for itself.

Here enters another consideration. There is cheap material at
hand for the oil heaters which are there used, a million of which
were already in service in 1914. It is a by-product of the oil refineries
in this land which is richer than all others in oil. This fact, together
with the rarity of frost and the great value of the crop, makes the
method practicable. In the first edition of this book the descriptions
in the chapter on artificial frost protection were based almost ex-
clusively on the experience of the United States.

In the meantime the problem has been pressed forward forcefully
in Germany. Our noblest German crop, the winegrape, bears such a
crop in favorable situations that artificial protection of the frost-
endangered lower vineyards has seemed profitable in spite of the

404 THE CLIMATE NEAR THE GROUND

great expense involved. Renewed attempts have been stimulated.
During the years 1926-1928, O. W. Kessler applied the newly for-
mulated laws of microclimatology so successfully in the Oppenheim
wine district that in the month of May, 1928, alone, several hundred
thousand marks worth of produce was saved by artificial methods of
frost protection.

This result naturally helped the further expansion of the experi-
ment. An "Imperial Committee on frost protection in the German
wine industry" undertook the organization of research — again
under the direction of O. W. Kessler. The weather service center in
Hamburg established a microclimatic frost-observational network
in the "four counties" under K. Bender. Wilhelm Schmidt con-
tributed his talents for finding the proper research technique for a
given problem to this practical task in the wine-producing area of
Gumpoldskirchen near Vienna. After the overthrow of the govern-
ment the newly founded Imperial weather service took the place of
all the preceding organizations. Under the leadership of K. Knoch
this research was joined with his agricultural meteorological project
and was furthered by the guarantee of substantial support. A special
institute at Trier, in the vineyard district of the Mosel, directed by
O. W. Kessler is now the center of this research.

In 1940, O. W. Kessler and W. Kaempfert (#13), in quite a large
volume on "Prevention of Frost Damage," published the results of
all the research up to that time both in Germany and elsewhere. In
this book can be found a description of the more recent studies at
Oppenheim in 1928 and 1929, in the Ahr valley during 1930, at
Saarstein in 1931, and since 1933 in the district around Trier in
particular. It is to be understood that the discussion which follows
is based for the most part on this work and the results derived there-
from.

In a small business, such as a single orchard, different kinds of
protective measures against frost may be successful and satisfactory.
But for a large establishment only one of the many possibilities has
stood up under a long trial; that is, direct heating. Nevertheless we
shall have a look at all the more important methods, since the ex-
periments connected with them furnish a fine, comprehensive en-
richment of our microclimatological knowledge.

There are two fundamentally different possibilities in artificial
frost fighting. Either the attempt is made to retain the heat already
present and in some way prevent further decline of temperature
during the critical night, or heat is artificially added. First let us
consider the former possibility.

THE BATTLE AGAINST DESTRUCTIVE FROST 405

In the main it is the radiation of heat outward which accounts for
the nocturnal temperature fall, as we learned in Chapter 2. Protec-
tion against radiation is therefore protection against frost, and for
this there are three methods. First, the endangered plants may be
covered, either singly or as a whole, with cardboard screens, caps,
braided mats, boards or the like. Second, an artificial smoke screen
may be laid down over the area in danger. Third, the plants may
be covered with water by flooding.

As to coverings, O. W. Kessler makes a distinction between
screens and caps. The screens, which are usually set up in a hori-
zontal position, come between the plants and the night sky and
absorb the radiated heat themselves, so that the plants do not cool
below the air temperature and even serve as protection to the sur-
rounding ground. Screens are most effective when the sky is clear
and radiation outward is strong. By and large the gain in tempera-
ture for the plants is seldom more than i.5°C.

While in the case of screens as here defined it is assumed that
there is a free exchange of air on all sides, the cap encloses a definite
air space, depending on its form and size. Fig. 178 shows, according
to Wilhelm Schmidt's measurements (#77) the temperature distribu-
tion about a cone-shaped cap such as is used in the Gumpoldskirchen
area. Outward radiation from the cap cools its upper surface to (in
this case) — 3°C. The ground beneath the cap has the advantage of
the radiation shield and remains at from +4° to +6°, while the un-
covered ground nearby cools off to —2°. With the cap, the move-
ment of heat from the soil is made available for the enclosed air, so
that caps accomplish more than screens. The frost protection of the
plants under the cap now depends on whether the cold-air skin of
the cap surface, or the warm-air skin on the ground, controls the
temperature of the inner space. In order to diminish the influence
of the cold-air skin as much as possible the cap should be con-
structed of non-conducting material in order that the cooling of the
outer surface may be carried through as little as possible to the inner
side. It is furthermore desirable that the cap should have an opening
near the ground, as shown in Fig. 178. The cold air inside seems to
leak out through this hole, while experience shows that cold air
from the outside does not force its way in through (perhaps an effect
of the cold air sliding down the steep outer surface of the cap) .

We must give careful attention to these rather involved tempera-
ture relations in a very small space since, otherwise, very erroneous
conclusions may be drawn. The owner of a certain garden, for in-
stance, sought to protect part of his plants by covering them with

406 THE CLIMATE NEAR THE GROUND

empty conserve cans. The supposedly protected plants froze while
the others did not. The metal was a good radiator and conductor of
heat, while the airspace between the tin cans and the plants was too
small, and there was no outlet for the cold air along the inner wall.
With suitable form and location for the caps, a temperature gain
of about 2° can be counted on. In many cases this is not enough.
On account of the great labor involved in repeated coverings and
uncoverings, the method is not suited to large scale installations.

FIG. 178. Temperature distribution about a Gumpolclskirchen frost protection
shelter. (After Wilh. Schmidt)

The large fixed screens which we find here and there in small busi-
ness do not fall under this condemnation; they are like an oversized
cap, for the air circulation under them is restricted. Properly made
and applied, they furnish excellent protection but are not much
used in fruit and grape culture.

Frost smoking consists in the production of smoke or fog by means
of smoke cartridges or fog apparatus. The desired turbidity of the
air results either from incompletely burned carbon (soot) such as
the residue from the burning of large quantities of raw naphthalene,

THE BATTLE AGAINST DESTRUCTIVE FROST ' 407

or from chemical fog, which may be produced through the use of
ammonium chloride, phosphorus pentoxide, as zinc fog, from acid
fog, or in any one of many other ways. The protective action really
consists in the continuous lowering of outgoing radiation in the
smoke-covered district. Experiments by O. W. Kessler showed a
reduction from 0.12 to 0.06 calories per sq cm per minute. For this
action it is assumed that no cold air from without flows into the
smoked area, and that no wind to speak of sets the artificial fog in
motion during the night. This uncertainty, which is much greater
in uneven country, has caused frost-smoking to be quite generally
displaced in these days by frost-heating.

As the third measure for the conservation of the heat already
present, we have mentioned "flooding." If the plants to be protected
are entirely submerged in water, they are removed from the cold air
layer near the ground and enveloped in the warm ground climate.
Their frost protection is complete, since even in the most severe cold
wave the most that can happen is a sheet of ice on the water surface.
But only in the rare instances where the plants will tolerate such
submergence and where water is quickly available in sufficient
quantity, is this method practicable — in the case of cranberry cul-
ture in the U. S. A., for example.

Now let us return to the second possibility for artificial frost
protection : the production of heat.

Here, too, three different paths may be followed. Apropos of the
flooding we have just mentioned, let us first take up artificial water-
ing as a frost protection.

The heat afforded by this method is the freezing heat of water.
If i g of water at o°C becomes ice at o°C, 80 calories are released —
the same amount of heat which is consumed by the melting process
when passing from the solid to the liquid phase. When it has begun
to freeze, and the endangered plants are then sprinkled (not by any
means sooner!), an ice coating is at once formed about the wet
leaves, twigs and stalks. The heat of freezing which is thereby
released, hinders a further temperature drop as long as the water
supply is sufficient (about 2 liters per sq m per hour). It is evident
that the sprinkling cannot be halted but must be continued — not
only till the frost is over but until the temperature rises considerably
above zero, for as soon as sprinkling ceases, evaporation sets in,
with a consequent cooling which must not again reduce the plant
temperature below freezing.

400 • THE CLIMATE NEAR THE GROUND

Experience teaches that even the most sensitive vegetation, such
as tomatoes, for example, will survive unharmed when the outside
temperatures are several degrees below zero (in one case which came
to our attention, as much as —7°). The plant temperature held
steadily at — 0.5 °C.

From the nocturnal stratification of the ground air, we know (see
Ch. 2) that warmer air is always to be found at some distance above
the ground. We can therefore consider the possibility of producing
a vertical mixture of the air by ventilation, thereby bringing warmth
down to the neighborhood of the ground.

Artificial convection should be a means of artificial frost control.
We could upset the air stratification by means of machines, as the
sun does naturally when it rises. Such a process is probably possible,
by using great electric fans, but insufficient for the needs of the
whole country. The gain in temperature is only about i° and would
extend only about 30 meters from the fan, at most. This method,
consequently, would be expensive in application, though it is used
in the United States in combination with heaters.

For practical protection against frost damage — especially on a
large scale — there remains only the last method, still to be dis-
cussed— the direct production of warmth by heating. At first
thought it may seem ridiculous to "heat the outdoors," like a room,
on cold nights. We might think the outlay in fuel could never be
justified by the savings to be made. Yet all experiments in Ger-
many and the United States show that this method is the only one
which is both possible and practical.

There are three means of heating. Where oil is cheap, as it is in
the United States, oil-heaters are employed. Fig. 179 shows a Cali-
fornia fruit orchard on level ground. The heaters were built in
from the first, 200 or 300 of them per hectare. At the edge of the
orchard they are somewhat closer together than in the middle. At
the critical time in the winter they are kept filled from an oil truck.
When frost comes they are lighted as rapidly as possible. Sequence
and number to be lighted depends on weather conditions (wind
direction and temperature).

In Germany, hard coal, brown coal, briquettes or wood is burned
in the heaters. There are a number of types of burners, easily
portable, easy to service and very economical in fuel consumption.
Heating is most effective when there is about one heater to every
50 sq meters, and when each heater produces at least 10,000 calories
per hour. Fig. 180 shows a vineyard in a tributary valley of the
Saar on the occasion of one of Kessler's experiments during the

THE BATTLE AGAINST DESTRUCTIVE FROST

409

410 THE CLIMATE NEAR THE GROUND

night of May 2-3, 1935. Seven rows of briquette burners had been
installed and had just been lighted when this picture was taken. The
first row can be clearly seen on the lower boundary wall of the
vineyard. These heaters were 41/z meters apart. Five lines above
stood the second row of heaters, whose smoke can just be discerned.
The spacing of these heaters was somewhat greater (5^ meters).
The higher up, the farther the rows were apart and the greater the
spacing between heaters, for the heat produced of course moved up
the slope.

FIG. 1 80. Briquet heating ovens in a vineyard will be lighted with the onset of night

frosts

The simplest method is the third one, in which there is no cost for
equipment. Briquettes were placed out in the open — say four
briquettes, at distances of 1.4 meters apart. The difficulty of quick
kindling was met by introducing a mass of raw napthalene and
sawdust between the briquettes. When the frost began, it was
possible for two men to accomplish the kindling of 160 heaps of
briquettes in 25 minutes, in spite of preceding rainy weather — the
first pouring petroleum over the kindling, while the second set it
aflame with a soldering torch.

Electrical heating by the use of the support wires of the vineyard
has been attempted. It is, however, too expensive to install and
operate.

The heating process is, more than the others, independent of top-
ography and can be easily suited to the threatening danger. A
temperature rise of from 3° to 4° is commonly attained with proper
distribution of the fires. Fig. 181 shows the temperature measure-

THE BATTLE AGAINST DESTRUCTIVE FROST

411

ments 50 cm above the ground on an experimental area 63 meters
square (outlined surface), on which 100 oil-heaters (Maurer pattern)
were used. O. W. Kessler conducted the experiment at Oppenheim
on April 26, 1929. Within the experimental plot, 17 thermometers
were distributed; outside, there were 28. The initial temperatures,
before heating began, lay between 6.0° and 6.5 °C (Upper left-hand
plan). After lighting the first heaters at 10:15 P.M. the temperature
in the midst of the experimental field rose to 8°, while at the south

22 °° Hour

lighting

After lighting

223° Hour

All heaters burning

V
\

I
I

23™ Hour

FIG. 181. Lines of equal temperature near the ground in a heating experiment with
oil heaters. (After O. W. Kessler)

edge where the heaters were not yet burning there was a drop to 5°.
Was this a sucking in of cold air by the first fire? A quarter of an
hour later, when all the heaters were burning (lower left-hand
plan), we find a good, uniform distribution of the warm zone, with
the temperature 3° warmer in the middle. At this reading the
temperature holds for some time, as shown by the distribution of the

412 THE CLIMATE NEAR THE GROUND

isotherms an hour later (lower right). The only change is that a
light wind has displaced the warm center somewhat to the side.

According to the data of O. W. Kessler and W. Kaempfert, the
cost of heat production amounts to only from i% to 2% of the
damage which might be expected. It is, therefore, thoroughly prac-
ticable in vineyards and valuable orchards.

LITERATURE
ABBREVIATIONS

Ann. d. Hydr. = Annalen der Hyclrographie und Maritimen Meteor-
ologie, published by the Deutsche Seewarte. Mittler & Sohn, Berlin.

Abh. Pr. Met. 1. = Abhandlungen des Preussischen Meteorologischen
Instituts, Berlin.

Ar\. /. Mat. = Arkiv for Matematik, Astronomi och Fysik, Stockholm.

Beth, z. Botan. Centralbl. — Beihefte zum Botanischen Centralblatt,
G. Fischer, Jena.

Beitr. Phys. d. jr. Atm. — Beitrage zur Physik der freien Atmosphare.
Akademische Verlagsgesellschaft, Leipzig.

Ber. D. Bot. G. — Berichte der Deutschen Botanischen Gesellschaft.
G. Fischer, Jena.

Bio\l. B. — Bioklimatische Beiblatter der Meteorologischen Zeitschrift.
Fried. Vieweg & Sohn, Braunschweig.

C. R. Paris — Comptes Rendus des seances de F Academic des Sciences,
Paris.

Forstw. C. — Forstwissenschaftliches Centralblatt. P. Parey, Berlin.

Geograf. Ann. = Geografiska Annaler, Stockholm.

Gerl. B. = Gerlands Beitrage zur Geophysik. Akademische Verlags-
gesellschaft, Leipzig.

]ahrb. f. wiss. Bot. = Jahrbiicher fiir wissenschaftliche Botanik. Gebr.
Borntraeger, Berlin.

La Met. = La Meteorologie, Paris.

Met. Mag. — The Meteorological Magazine, London.

Met. Z. = Meteorologische Zeitschrift. Friedr. Vieweg & Sohn, Braun-
schweig.

M. W. Rev. = Monthly Weather Review, United States Department of
Agriculture, Washington.

Naturw. = Die Naturwissenschaften. Jul. Springer, Berlin.

Planta — Planta. Archiv fiir wissenschaftliche Botanik. Jul. Springer,
Berlin.

Quart. J. = The Quarterly Journal of the Royal Meteorological Society,
London.

R. /. W. Wiss. Abh. = Wissenschaftliche Abhandlungen, Reichsamt fiir
Wetterdienst, Berlin.

Sitz-B. Berlin. Atyd. — Sitzungsberichte der Preussischen Akademie der
Wissenschaften zu Berlin.

Sitz.B. Wien. A\ad. = Sitzungsberichte der Akademie der Wissen-
schaften in Wien. Mathematisch-naturwissenschaftl. Klasse.

414 THE CLIMATE NEAR THE GROUND

Tdt-B. Pr. Met. 1. — Tatigkeitsbericht des Preussischen Meteorologischen

Instituts, Berlin.

Thar. Forstl. Jahrb. = Tharandter Forstliches Jahrbuch. P. Parey, Berlin.
Veroff. Gcoph. I. Leipzig = Zweite Serie der Veroffentlichungen des

Geophysikalischen Instituts der Universitat Leipzig.
Wetter — Das Wetter, Monatsschrift fur Witterungskunde. O. Salle,

Berlin.
Z. /. angew. Met. = Zeitschrift fiir angewandte Meteorologie. Aka-

demische Verlagsgesellschaft, Leipzig.
Z. /. F. u. Jagdw. = Zeitschrift fiir Forst- und Jagdwesen. Jul. Springer,

Berlin.

To THE INTRODUCTORY CHAPTER

(General works, bibliographies, histories of microclimatology)
j. Filzer, P., Aus. d. Fruhzeit bioklimat. Forschung. * D. Biologe 5,
168-172, 1936.

2. Geiger, R., Mikroklimatologie. * Z. f. angew. Met. 45, 74-85, 1928.

3. — , Die vier Stufen d. Klimatalogie. * Met. Z. 46, 7-10, 1929.

33. — , Uber selbstandige u. unselbst. Mikrokl. * Met. Z. 46, 539-544,

4. — , Mikroklima u. Pflanzenklima. * Handb. d. Klimat. I D. Berlin,

Borntraeger, 1930.

5. — , D. Mikrokl. u. s. Bedeutung f. d. belebte Natur. * Z. f. angew.
Met. 48, 137-146, 1931.

53. — , Mikroklimatologie: Ruckblick und Ausschau * Met. Runds-
chau /, 140-144, 1947.

6. Geiger, R. & Schmidt, W., Einheitl. Bezeichn. in kleinklimat. u.
mikroklimat. Forschung. * Biokl. B, /, 153-156, 1934.

7. Grimm, H., Justus v. Liebig u. d. Mikroklimatol. * Z. f. angew.
Met. 48, 30, 1931.

73. Hartel, O., Mikroklimaforschung und Pflanzenphysiologie *
Z.f.d. ges. Naturwiss. 1944, 19-27.

8. Kassner, C., Z. Geschichte d. Mikroklimat. * Met. Z. 51, 393-394,
1934.

9. Knoch, K., Allgemeine Klimalehre. * Handb. d. Klimatol. v. J. v.
Hann. 4. Aufl. I. Bd. Engelhorn, 1932.

10. — , Angew. Klimatol. als. Forderer wirtschaftl. Probleme. * Met.
Z. 54, 470-47^ !937-

loa. Knoch, K. Weltklimatologie und Heimatkunde * Met. Z. 59,
245-249, 1942.

11. Korotkewitsch, V. N., E. Obersicht der d. Mikroklima umfass.
Arbeiten. * Transact. Centr. Geophys. Obs. Leningrad 6, 1936.

12. Kraus, Gregor, Boden u. Klima auf kleinstem Raum. Jena, Fischer,
1911.

13. Ramdas, L. A., Agricultural Met. * Current Science, /, 191-192,

LITERATURE 415

14. — , Micro-Climatology. * Current Science, 2, 445-447, 1934.

15. Reichsamt fur Wetterdienst, Bibliograph. Ber. a. d. Gebiet d.
Agrarmet. Seit 1934.

1 6. Sanson, J., Result, gener. d'une enquete etc. * La Met. 6, 69-97,

17. Scaetta, H., Terminologie climat., bioclimat. et microclimat. * La

Met. //, 342-347, 1935.
1 8. Schmidt, Wilh. and others, Bioklimat. Unters. im Lunzer Gebiet. *

Naturw. 77, 176-179, 1929.

19. Schmidt, Wilh., Neue Wege met. Forsch. u. i. Bedeut. f. Praxis u.
Leben. * Deutsche Forschung 18, 79-114, 1933.

20. — , D. Biokima als Kleinkl. u. Mikrokl. * Biokl. B. 7, 3-6, 1934.

21. Schubert, J., D. Klima d. Bodenoberfl. u. d. unteren Luftschicht in
Mitteleuropa. * Blanck, Handb. d. Bodenlehre II. Jul. Springer,
1929.

22. — , Grundl. d. allgem. u. forstl. Klimakunde. * Z. f. F. u. Jagdw.
62, 689-705, 1930; 64, 7I5-734. 1932; 72> 257-273, 1940.

23. Tagungsberichte der Kommission f. Agrarmeteorolog. in d. Org.
Met. Internat. Tagungen: Utrecht 1923, Zurich 1926, Kopenhagen
1929, Miinchen 1932, Salzburg 1937.

25. Erste landwirt.-meteorolog. Tag., veranstalt. v. d. osterr. Gesellsch.
f. Meteorolog. Wien 1930.

26. Conf. Empire Met. 1929, Agricult. Sect. London 1929.

27. Ann. Report Agric. Branch. India Met. Dep. Since 1933.

BIBLIOGRAPHY FOR PART I

CHAPTER i. MIDDAY HEAT EXCHANGE AT THE GROUND SURFACE.

THE INCOMING RADIATIONAL TYPE.

28. Alt, E., D. Stand d. met. Strahlungsprobl. * Met. Z. 46, 504-513,
1929.

29. Baur, F. & Philipps, H., D. Warmehaush. d. Lufthulle d. Nord-
halbkugel im Jan. u. Juli. * Gerls. B. 42, 160-207, T934-

30. Flower, W. D., Sand Devils. * Met. Office London, Profess. Notes
77, 1936.

31. Hartel, O., D. Alpenlabor. d. Botan. Staatsanst. d. Univ. Miinchen
auf d. Schachen. * Biokl. B. 4, 65, 1937.

32. v. Holzhausen, H., Beob. e. Kleintrombe. * Met. Z. 54, 307, 1937.
323. Ives, R. L. Behavior of Dust Devils. * Bull. Am. Met. Soc. 28,

168-174, 1947.

33. Lauscher, F., D. Zunahme d. Intensitat d. Sonnenstr. mit d. Hohe.
* Gerl. B. 50, 202-215, J937'

34. Marten, W., D. Strahlungsklima v. Potsdam. * Abh. Pr. Met. I. 8,
Nr. 4, 1926.

35. Morikofer, W., Klimatolog. Einfliisse d. Hochgebirges. * Verh. d.
Deutsch. Ges. f. innere Med. Wiesbaden 1935, S. 501.

416 THE CLIMATE NEAR THE GROUND

36. Rossmann, F., D. Bewegungsgesetz d. Kleintromben. * R. f. W.
Wiss. Abh. 5, Nr. 4, 1937.

37. Schlichting, H., Kleintrombe. * Ann. d. Hydr. 62, 347-348, 1934.

38. Schober, H., Beob. d. Ablb'sung e. Luftwirbels bei starken Temp.
Untersch. in d. bodenn. Luftschicht. * Met. Z. 57, 193-194, 1934.

39. Schubert, J., D. Sonnenstrahl. im mittl. Norddeutschl. nach d.
Messungen in Potsdam. * Met. Z. 45, 1-16, 1928.

40. Sinclair, J. G., Temp, of the soil and air in a desert. * M. W. Rev.
50, 142, 1922.

41. Smoliakow, P. T., Zur Theorie d. Gleichgew. d. bodenn. Luft-
schicht. * Gerl. B. 44, 321-336, 1935.

42. Wegener, A., Staubwirbel auf Island. * Met. Z. 37, 199-200, 1914.

43. — , Wind- u. Wasserhosen in Europa. * Braunschweig, Friedr.
Vieweg & Sohn, 1917.

CHAPTER 2. NOCTURNAL HEAT EXCHANGE AT THE GROUND SURFACE.
THE OUTGOING RADIATIONAL TYPE.

44. Angstrom, A., Stud, of the nocturnal radiation to space. * Astro-
phys. J. Chicago 37, 305-321, 1913 and 39, 95-104, 1914.

45. — , A study of the radiation of the atmosphere. * Smithsonian
Miscell. Coll. 65, N. 3, 1915.

46. — , t). d. Gegenstrahlung d. Atmosph. * Met. Z. jj, 529-538, 1916
and 34, 14-24, 1917.

47. — , Record, nocturn. rad. * Medd. Stat. Met. Hydr. Anst. Stock-
holm 3, Nr. 12, 1927.

48. — , Messung d. nachtl. Ausstrahlung im Ballon. * Beitr. Phys. d.
fr. Atm. i^j 8-20, 1928.

49. — , (Jber Variationen d. atmosph. Temp, strahlg u. ihren Zu-
sammenhang mit d. Zusammensetz. d. Atmosph. * Gerl. B. 27,

145-161, 1929.

50. — , Effect, rad. during the second internat. Polar Year. * Medd.
Stat. Met. Hydrogr. Anst. Stockholm Nr. 8, 1936.

51. Asklof, S., t). d. Zusammenhang zw. d. nachtl. Warmeausstrahl.,
der Bewolk. u. d. Wolkenart. * Geograf. Ann. 2, 253-259, 1920.

52. Brocks, K., Nachtl. Temp, minima in Furchen m. verschied.
Boschungswinkel. * Met. Z. 56, 378-383, 1939.

53. Defant, A., Ausstrahlung, nachtl. Abkiihlung u. Bewolkung. *
Geograf. Ann. 4, 99-108, 1922.

54. Dubois, P., Nachtl. effekt. Ausstrahlung. * Gerl. B. 22, 41-99,
1929.

55. Eckel, O., Mess. d. Ausstrahl. u. Gegenstrahl. auf d. Kanzelhohe.
* Met. Z. 57, 234-235, 1934.

56. Ertel, H., Methoden u. Probl. d. dynamischen Met. Berlin, J.
Springer, 1938.

LITERATURE 417

57. Falckenberg, G., Muldenfrost u. Frostflachen in Waldlichtungen.

* Met. Z. 48, 22-25, 1931.

58. Hasche, E., Z. Mess. d. langwell. Himmels- u. Erdstrahlung. *
Gerl. B. 42, 228-231, 1934.

59. Hellmann, G., 0. d. nachtl. Abkiihlung d. bodenn. Luftschicht.

* Sitz-B. Berlin. Akad. 38, 806-813, 1918.

60. Kimball, H. H., Nocturnal radiation, measur. * M. W. Rev. 46,
57-70, 1918.

61. Kriigler, F., Nachtl. Warmehaush. messungen an d. Oberflache e.
grasbewachsenen Ebene. * R. f. W. Wiss. Abh. j, Nr. 10, 1937.

62. Lauscher, F., Bericht ii. Mess. d. nachtl. Ausstrahlung auf d.
Stolzalpe. * Met. Z. 45, 371-375, 1928.

63. — , Warmeausstr. u. Horizon teinengung, I. Teil. * Sitz-B. Wien.
Akad. 143, 503-519, 1934.

64. — , Dampfdruck u. Ausstrahl. in e. Gebirgsland. * Gerl. B. 51, 234-
249, 1937.

65. Linke, F., D. nachtl. effekt. Ausstrahl. unter verschied. Zenit-
distanzen. * Met. Z. 48, 25-31, 1931.

66. Meinander, R., U. d. nachtl. Warmeausstrahl. in Helsingfors. *
Soc. scient. fennica. Comment. Phys.-Mathem. 4, Nr. 16, 1928.

67. Moller, F., Bemerk. z. Warmebilanz d. Atmosph. u. d. Erdober-
flache. * Gerl. B. 47, 215-217, 1936.

68. Petterssen, S., Ein typ. Beispiel v. Ausstrahl. inversionen in e. heit.
Sommernacht. * Met. Z. 45, 72-74, 1928.

68a. Philipps, H., Zur Theorie der Warmestrahlung in Bodennahe.

* Gerl. B. 56, 229-319, 1940.

69. Raman, P. K., Heat radiation from the clear atmosph. at night. *
Proc. Indian Acad. Sciences /, 815-821, 1935.

70. — , Studies in atmosph. radiation. * Ibid. 4, 243-253, 1936.

71. Ramdas, L. A., Sreenivasiah, B. N. u. Raman, P. K., Variation in
the nocturnal rad. from the sky with zenith dist. a. with time
during the night. * Ibid. 5, 45-55, 1937.

7 1 a. Seemann }. & Low. K. Die Auswirkung der Taubildung auf den
Temperaturverlauf in der bodennahen Luftschicht * Met. Z. 6/,
158-161, 1944.

72. Siissenberger, E., D. nachtl. effekt. Ausstrahl. unter verschied.
Zenitdistanzen. * Met. Z. 52, 129-132, 1935.

73. — , Neue Unters. u. d. nachtl. effekt. Ausstrahl. * Gerl. B. 45,
63-81, 1935.

73a. — , Die Bedeutung des Ozons und der Kohlensaure in der
Atmosphare fur die nachtliche effektive Ausstrahlung am Erd-
boden * Ann. d. Hydr. 77, 222-226 1943.

74. Trojer, H., Temp.-Strahl.Mess. mit d. Parabolspiegel. * Met. Z.
57>

418 THE CLIMATE NEAR THE GROUND

75. Wegener, K., D. Strahlung d. Bodens. * Met. Z. 55, 133-137, 1938.

76. Wegener, K. & Trojer, H., D. Temp.strahl. d. Erde u. ihre Mes-
sung. * Ann. d. Hydr. 67, 424-432, 1939.

CHAPTER 3. TRUE HEAT CONDUCTION. THE NORMAL COURSE
OF GROUND TEMPERATURE.

(See also bibliography to Chapter 13)

77. Angstrom, A. & Petri, E., En ny jordtermometer och nagra observ.
over jordtemp. i. Stockholmstrakten. * Tekn. Tidskr. 1928, Heft

23-

78. Biittner, K., D. Warmeubertragung d. Leitung u. Konvektion,

Verdunstung u. Strahl. in Bioklimat. u. Met. * Abh. Pr. Met. I. /o,
Nr. 5, 1934.
783. Geiger, R., Warmehaushaltskonstanten fur den Agrarnieteorologen

* Met. Rundschau /, Nr. 11-12, 322-329, Mai-Juni 1948.

79. Hecht, W., Bioklim. Vers. z. Erforsch. d. Ursachen d. Gehalts-
schwank. d. Arzneipflanzen III. * Heil- u. Gewurzpflanzen
(Freising-Miinchen) /6, 1-57, 1934/35.

793. — , Bioklimatische Versuche zur Erforschung der Ursachen der
Gehaltsschwankungen der Arzneipflanzen * Heil- und Gewurz-
pflanzen 1942 Heft 2.

80. Herr, L., Bodentemp. unter besond. Beriicksichtigung d. ausseren
met. Faktoren. * Diss. Leipzig 1936.

81. Homen, Th., 0. d. Bodentemp. in Mustiala. Helsingfors 1896.

82. — , D. ta'gl. Warmeumsatz im Boden u. d. Warmsestrahl. zw.
Himmel u. Erde. * Leipzig 1897.

83. Keranen, J., Warme- u. Temp.-verhaltn. d. obersten Boden-
schichten. * (Einfiihrung in d. Geophysik II.) Berlin, J. Springer,
1929.

84. Kuhl, W., D. jahrl. Gang d. Bodentemp. in verschied. Klimaten.

* Gerl. B. 8, 499-564, 1907.

85. Leyst, E., (J. d. Bodentemp. in Pawlowsk. * Rep. f. Meteorol. 73,
Nr. 7, 1890.

86. — , Unters. ii. d. Bodentemp. in Konigsberg. * Schr. d. phys.-
okonom. Ges. Konigsberg 33, 1-67, 1892.

87. Maurer, J., Bodentemp. u. Sonnenstr. in d. Schweiz. Alpen. * Met.

Z. 33> I93-i99? i9l6-

88. Meinardus, W., Bodentemp. in d. Wiiste bei Schellal, Oberagypten.

* Gotting. Nachr., mathem.-physik. KL, neue Folge, i, Nr. i, 1935.

89. Rambaut, Undergr. temp, at Oxford as determined by ... therm.

* Radcliffe Obs. Met. Obs., Oxford, 51, 101-204, 1911-15.

89a. Ravet, J., La temp, du sol a Tahiti. * Ann. Phys. Globe France
d'outre-mer 6, 134-135, 1939.

90. Schmidt, A., Theoret. Verwert. d. Konigsberger Bodentemp.
beob. * Schr. d. phys.-okonom. Ges. Konigsberg 32, 97-168, 1891.

LITERATURE 419

91. Schubert, J. D. jahrl. Gang d. Luft- u. Bodentemp. u. d. Warme-
austausch im Erdboden. Berlin, J. Springer, 1900.

92. — , D. Verhalten d. Bodens gegen Wa'rme. * Hdb. d. Bodenlehre,
her. v. E. Blanck, 6, 342-375, 1930.

93. Siegenthaler, J., Bodentemp. in Abhang. v. auss. met. Faktoren. *
Gerl. B. 40, 305-332, 1933.

94. Siiring, R., D. ta'gl. Temp.gang in gering. Bodentiefen. * Abh. Pr.
Met. I. 5, Nr. 6, 1919.

95. Wild, H., U. d. Bodentemp. in St. Petersburg u. Nukuss. * Rep. f.
Meteorol. 6, Nr. 4, 1878.

96. Woeikof, A., Probleme d. Bodentemp. Typen ihrer vertikalen
Verbeit. Verhaltn. z. Lufttemp. * Met. Z. 21, 50-62, 399-408, 1904.

963. Yakuwa, R., tlber die Bodentemperatur von Kobe. * Mem. Imp.
Marine Obs. Kobe 3, 81-90, 1928.

CHAPTER 4. EDDY DIFFUSION AND ITS SIGNIFICANCE.

(Complete bibliography on the question of mass exchange up to year

1939 in the book by H. Lettau (108). Here only the later publications

or those not included by Lettau are listed, and those to which specific

reference is made in the text.)

96b. Albrecht, F. Turbulenzuntersuchungen * Met. Z. 60, 109-121,

J943-

97. Angstrom, A., D. Konvektion der Luft. * Met. Z. 36, 348, 1919.

98. Berg, H., Mess. d. Austauschgrosse d. bodenn. Luftschichten. *
Beitr. Phys. d. fr. Atm. 23, 143-164, 1936.

99. Best, A. C., Horizontal temp. difT. over small distances. * Quart. J.
57, 169-175, 1931.

100. Biidel, A., E. photogramm. Methode z. Stud. d. Strom.- u. Aus-
tauschvorgange. * Beitr. Phys. d. fr. Atm. 20, 9-17, 1933.

101. — , Individuelle Beweg. kleiner Luftmassen. * Ibid 20, 214-219,

102. Geiger, R., Temp.struktur u. Mikroklima. * Met. Z. 47, 425-430,

103. — , Met. Beob. b. d. Mittelfrank. Kieferneulenbekampfung m.
Flugzeug u. Motor i. Fruhjahr 1931. * Z. f. angew. Entomol. 79,
207-222, 1932.

104. Godecke, K., Mess. d. atmosph. Turbulenz in Bodennahe m. e.
Hitzdrahtmethode. * Ann. d. Hydr. 63, 400-410, 1935.

105. Grunow, J., Vers. m. pendelnden Druckplatten. * Tat-B. Pr. Met.
I. 1933, S. 92-95.

1 06. Hornberger, Studien u. Luft- u. Bodentemp. * Forstw. C. 24,

479-498, 1902.
io6a. Koch, H.-G., Ober Temperatur u. Austausch innerhalb der

Bodeninversion * Gerl. Beitr. 49, 407-426, 1937.

420 THE CLIMATE NEAR THE GROUND

107. Lettau, H., Turbul. Schwank. v. Wind u. Temp. in d. bodenn.
Luftsch. als Austauschproblem. * Ann. d. Hydr. 62, 469-473, 1934.

108. Lettau, H., Atmospharische Turbulenz. * Akad. Verl. Ges. Leip-

zig J939-
io8a. Raethjen, P., Zum Warmestrom der Turbulenz. * Ann. d. Hydr.

72, 129-132, 1944.

109. Rempe, H., Unters. ii. d. Verbreitung d. Bliitenstaubes durch d.
Luftstromungen. * Planta 27, 93-147, 1937.

1093. Rombakis, S., Ober die Verbreitung von Pflanzensamen und
Sporen durch turbulente Luftstromungen. * Z. f. Met. /, 359-363,
1947.

no. Rossmann, F., Stromung in d. Streichholzschachtel. * Met. Z. 52,

77* J935-
in. Schmauss, A., Schichtenbild. in Fliissigkeiten. * Met. Z. 49, 203-

204,, 1932.

112. Schmidt, Wilh., Die Struktur d. Windes. * Sitz-B. Wien. Akad.
/3#, 85-116, 1929.

113. — , D. Massenaustausch in freier Luft u. verwandte Erscheinungen.
Probl. d. kosm. Physik 7. Hamburg, H. Grand, 1925.

114. Wegener, A., C. turbul. Beweg. in d. Atmosph. * Met. Z. 29, 49-
59, 1912.

CHAPTER 5. LONG WAVE RADIATION.

115. Albrecht, F., D. Warmeumsatz durch d. Warmestrahl. d. Wasser-
dampfs in d. Atmosph. * Z. f. Geophysik 6, 421-435, 1930.

1153. Brunner, B. H.-Ch., Kiisteneinfluss auf Temperatur u. Feuchte

der bodennahen Luftschichten. * Gerl B. 56, 113-154, 1940.
1 1 6. Falckenberg, G., Neue Unters. ii. d. Bildung v. Bodeninversionen.

* Met. Z. 44, 108-109, 1927.
n6a. — , Aerol. Stud.reise d. Drachenboots d. Rostocker Luftwarte. *

Met. Z. 45, 55-60, 1928.
117. — , Experimentelles zur Absorpt. u. Emm. d. atmosph. Eigenstrahl.

diinner Luftsch. * Met. Z. 48, 135-139, 1931.
1 1 8. — , D. Einfluss d. Wellenlangentransformation auf d. Klima

bodenn. Luftsch. u. d. Temp. d. fr. Atm. * Met. Z. 48, 341-346,

1931.

119. — , Exp. z. Absorpt. diinner Luftsch. f. infrarote Strahlung. * Met.
Z. 5J, 172-175, 1936.

120. — , Exp. z. Druckabhangigk. d. Absorpt. d. Wasserdampfes u. d.
Kohlensaure f. d. infrarote Schwarzstrahl. * Met. Z. 55, 174-177,

121. — , Exp. z. Eigenstrahl. diinner wasserdampfhalt. Luftsch. * Met.
Z. 56, 72-75, 1939.

122. — , Exp. z. Temp.abhangigk. d. infraroten Absorpt. wasserdampf-
halt. Luft. * Met. Z. 56, 415-417, 1939.

LITERATURE 421

i22a. • — , Exp. z. Temperaturabhangigkeit der infraroten Absorption
wasserdampfhaltiger Luft. * Met. Z. 56, 415-417, 1939.

123. Falckenberg, G. & Stoecker, E., Bodeninversion u. atmosph. Ener-
gieleitung durch Strahl. * Beitr. Phys. d. fr. Atm. /j, 246-269, 1927.

124. Fowle, F. E., Water-vapor transparency to low-temp, radiation. *
Smithsonian Miscell. Coll. 68, Na. 8, 1917.

125. Hettner, D. d. ultrarote Absorpt.-spektrum d. Wasserdampfs. *
Diss. Berlin 1918.

1253. Moller, F. Grundlagen eines Diagramms zur Berechnung
langwelliger Strahlungsstrome. * Met. Z. 6/, 37-45, 1944.

126. Miigge, R., Warmestrahl. zw. Himmel u. Erde. * Met. Z. 46, 514-
520, 1929.

127. Schnaidt, F., Z. Absorption infraroter Strahl. in diinnen Luft-
schichten. * Met. Z. 54, 234-242, 1937.

128. — , t). d. Absorption v. Wasserdampf u. Kohlens. m. besond.
Beriicks. d. Druck- u. Temp.abhangigk. * Gerl. B. 54, 203-234,

1939-

129. Simpson, G. C., Further stud, in terrestrial rad. * Mem. Royal
Met. Soc. London j, No. 21 1928-30.

130. Steiner, O., Z. Entstehung v. Bodeninvers. bei wolkenlos. Himmel
u. Landwind. * Wiss. Abh. d. Rostocker Luftwarte 1926.

CHAPTER 6. THE WARMING PROCESS.

1303. Buttner, K. Die Warmeiibertragung durch Leitung und Kon-
vektion, Verdunstung und Strahlung in Bioklimatologie und
Meteorologie. * Abh. Pr. Met. I. /o, Nr. 5, 1934.

i3ob. Fritsche, G. & Strange, R. Vertikaler Temperaturverlauf iiber
einer Grosstadt. * Beitr, Phys. d. fr. Atm, 23, 95-110, 136.

131. Geiger, R., Gibt es e. Lufttemp. d. bodenn. Luftschicht? * Biokl.
B. /, 115-120, 1934.

132. Haude, W., Temp. u. Austausch d. bodenn. Luft iiber e. Wiiste.
* Beitr. Phys. d. fr. Atm. 21, 129-142, 1934.

133. Koch, H. G., t). Temp. u. Austausch innerhalb d. Bodeninver-
sion. * Gerl. B. 49, 407-426, 1937.

134. Malurkar, S. L. & Ramdas, L. A., Theory of extremely high lapse-
rates of temp, very near the ground. * Indian J. of Physics 6, 495-
508(1931?).

135. Raman, P. K., The measurement of the transmission of heat by
convect. from isolated ground to the atmosph. * Proc. Indian
Acad. Sciences j, 98-106, 1936.

136. Ramdas, L. A., The dust-free or dark layer surrounding a hot
body in relat. to the convect. movem. in its neighbourhood. * J.
Univers. Bombay 6, 18-22, 1937.

137. Ramdas, L. A. & Malurkar, S. L., Surface convect. and var. of
temp, near a hot surface. * Indian J. of Physics 7, 1-13 (1932?).

422 THE CLIMATE NEAR THE GROUND

138. Ramdas, L. A. & Paranjpe, M. K., An interferometric method of
meas. temp, and temp.-gradients very close to a hot surface. *
Current Sc. 4, 642—644, 1936.

139. Robitzsch, M., Einige Bezieh. zw. d. Temp. d. Erdoberfl., d. In-
solation u. and. met. Fakt. * Beitr. Phys. d. fr. Atm. 9, i-n, 1921.

140. Schmidt, E. & Beckmann, W., D. Temp.-u. Geschwindigk.feld
vor. e. Warme abgeb. senkrecht. Platte bei natiirl. Konvektion. *
Techn. Mech. u. Thermodynamik /, Heft 10/11, 1930.

141. Yakotani, S., On the small fluctuat. of the temp, in the lower
atmosph. occurring in the daytime. * J. Met. Soc. Japan 14, Nr. 2,

142. Zedler, P., Temp.dauermess. mit e. Aspirationspsychrometer. *
Z. f. angew. Met. 55, 350-353, 1938.

CHAPTER 7. THE COOLING PROCESS.

143. Defant, A., D. nachtl. Abkiihl. d. unteren Luftsch. u. d. Erdoberfl.
in Abhangigk. v. Wasserdampfgehalt d. Atm. * Sitz-B. Wien.
Akad. 725, 1537—1623, 1916.

144. — , U. d. nachtl. Abkiihl. d. untersten staubbelad. Luftsch. * Ann.
d. Hydr. 47, 93-105, 1919.

145. — , D. nachtl. Abkiihl. d. unt. Luftsch. bei bewegter Luft. * Ann.
d. Hydr. 47, 224-227, 1919.

146. Falckenberg, G., Apparatur z. Best. d. momentanen nachtl.
Warmeaustauschs zw. Erde u. Luft. * Met. Z. 47, 154—156, 1930.

147. — , D. nachtl. Warmehaushalt bodenn. Luftsch. * Met. Z. 49,
369-371, 1932.

148. Kriigler, F., U. d. Anteil des Massenaustauschs am nachtl. Warme-
haush. d. Erdoberfl. * Met. Z. 49, 372—376, 1932.

149. Kiihnert, W., E. Boeb. d. Temp.grad. beim Auftreten v. StrahL-
nebel; d. Entwickl. d. Bodenin version. * Beitr. Phys. d. fr. Atm.
1 8, 219-224, 1932.

1493. Meyer, E. G., U. d. Strahl.haushalt horizont. Flachen. * Gerl. B.

53* 352~353> J938-

150. Ramanathan, K. R. & Ramdas, L. A., Derivation of Angstrom's
formula f. atm. radiation and some general consid. regarding
nocturn. cooling of air-layers near the ground. * Proc. Indian.
Acad. Sciences /, 822-829, 1935.

151. Ramdas, L. A., Frost Hazard in India. * Current Sc. ^, 325-333,

1935-

152. Ramdas, L. A. & Atmanathan, S., The vert, distrib. of air temp.
near the ground during night. * Gerl. B. 37, 116-117, 1932.

153. Schmauss, A., D. nachtl. Abkiihlung d. untersten Luftsch. * Ann.
d. Hydr. 47, 235-236, 1919.

154. Schmidt, Wilh., Stud. z. nachtl. Temp.gang. * Sitz-B. Wien.
Akad. ii89 293—319, 1909.

LITERATURE 423

155. Siegel, S., Mess. d. nachtl. thermischen Gefiiges in d. bodenn.
Luftsch. * Gerl. B. 47, 369-399, 1936.

156. Sutton, J. R., On some met. conditions controlling nocturnal radia-
tion. * Transact. Roy. Soc. South Africa 2, 381, 1912.

INSTRUMENTAL TECHNIQUE FOR THE MEASUREMENT OF AIR
TEMPERATURE IN THE MICROCLIMATE

157. Albrecht, F., Thermometer z. Mess. d. wahren Lufttemp. * Met.
Z. 44, 420-424, 1927.

158. — , t). d. Einwirk. d. Strahl. auf frei aufgestellte elektr. Thermo-
meter. * Tat.B. Pr. Met. I. 1933, S. 76-82.

159. Angstrom, A., I. Prinzipielles z. Mess. d. Temp. d. Luft. II. Minim,
therm, z. Bestimm, d. Min.temp. d. Luft. * Comm. Met. Agricole,
Tag-Ber. Miinchen 1932, S. 122-127, Utrecht 1933.

1 60. Bartels, J., Temp.messung in Bodennahe u. Aspiration. * Met. Z.

47> y6-??* *93°'

161. — , Temp, stabformiger Versuchskorper. * Z. f. F. u. Jagdw. 65,

319-327, 1936.

162. Bartels, J. & Kohn, M., Standortsklimat. Unters. in Eberswalde. *
Deutsche Forschung 74, 73-79, 1930.

1623. Brazier, C. E. & Eble, L., Introduct. a 1'etude des temp, de 1'air et
du sol au voisinage de la surf, terrestre. * La Met. /o, 97—110, 1934.

163. Budig, W., Beschirmung v. Bodentherm. gegen nachtl. Ausstrahl.

* Erg. d. Met Beob. Potsdam 1915, XI-XVI.

164. Budel, A., D., Strahl.schutz am Therm. * Z. f. angew. Met. 50,
225-230, 1933.

i64a. Duckert, P., Z. Methode d. Temp.mess. in d. bodenn. Luftschicht.

* Comm. Met. Agricole, Tag.Ber. Salzburg 1937, S. 46-53,
Ley den 1938.

165. Forster, H., U. Fehler, die b. Lufttemp.mess. infolge Warmeleitung
auftreten. * Met. Z. 57, 334-341, 1940.

1 66. Gehlhoff, K., Thermoelektr. Mess. d. nachtl. Temp, verlaufs in d.
unteren Luftsch. * Met. Z. 39, 137-141, 1922.

167. Geiger, R., Ein Messgerat z. Dauerbeob. d. Temp.schichtung am
Boden. * Z. f. angew. Met. 52, 205—213, 1935.

1 68. Geiger, R. & Budel, A., 0. ein tragbares Messgerat f. Temp.-
bestimm. in d. bodenn. Luftsch. * Z. f. angew. Met. 46, 265-270,
1929.

169. Griindl, G., Erfahr. mit Sechsfarben-Punktschreibern b. wider-
standselektr. Temp.mess. * Met. Z. 56, 230—239, 1939.

170. Grundmann, W. & Kassner, L., E. vereinf. elektr. Temp.-mess-
anordnung f. beliebig viele Messstellen u. mehr. Temp, bereiche. *
Z. f. angew. Met. 57, 205-210, 1934.

171. Linke, F., E. transport. Therm-hutte f. lokalklimat. u. mikroklimat.
Unters. * Bioklim. B. 5, no, 1938.

424 THE CLIMATE NEAR THE GROUND

172. — , t). d. Genauigk. d. Temp.mess. mit d. Sechsfach-Punkt-
schreibern d. Fa. Hartmann & Braun. * Met. Z. 57, 263-265, 1940.

173. Made, A., E. Beitrag z. Frage: Wahre Lufttemp. oder Versuchs-
korpertemp. * Biokl. B. 4, 35-36, 1937.

174. — , E. Schutzkorb fur d. Platinwiderst.therm. d. Reichswetter-
dienstes. * Met. Z. 55, 415-417, 1938.

1743. Ramdas, L. A., Rep. on simple methods of measur. in agricult.
met. * Comm. Met. Agricole, Tag.Ber. Salzburg 1937, S. 53-57,
Leyden 1938.

175. Schmidt, Wilh., Ventilation b. Temp.mess. schadlich? * Met. Z.

51, 431-432, 1934-

1753. Wertheimer, E. D., Temp.begrifl in d. Thermodynamik u. in d.
Met. * Met. Z. 45, 457-465, 1928.

CHAPTERS 8 AND 9. THE DIURNAL AND ANNUAL COURSE OF TEMPERATURE

IN THE AIR LAYER NEAR THE GROUND AND THE TEMPERATURE

GRADIENT IN THE AIR LAYER NEAR THE GROUND.

176. Best, A. C., Transfer of heat and momentum in the lowest layers
of the atmosphere. * Geophys. Mem. Nr. 65, London 1935.

177. Favrot, C., Sur les min. de temp, au-dessus du sol a Lyon-Bron. *
La Met. 6, 206-209, 1930.

178. Flower, W. D., An invest, into the variation of the lapse rate of
temp, in the atm. near the ground at Ismailia, Egypt. * Geophys.
Mem. Nr. 71, London 1937.

179. Geiger, R., Das Stationsnetz zur Unters. d. bodenn. Luftsch. Teil
I— III. * Deutsch. Met. Jahrb. f. Bayern 1923-1925.

1 80. Geiger, R. &. Amann, H., dasselbe, Teil IV u. V. * Ibid. 1926-

181. Heyer, E., 0. Frostwechselzahlen in Luft u. Boden. * Gerl. B. 52,
68-122, 1938.

182. Johnson, N. K., A Study of the vertical gradient of temp, in the
atm. near the ground. * Geophys. Mem. Nr. 46, London 1929.

183. Johnson, N. K. & Roberts, O.F.T., The measurement of the lapse
rate of temp, by an optical method. * Quart. J. 57, 131-138, 1925.

184. Kahler, K., Mess. d. Lufttemp. in verschied. Hohe auf der
Schneekoppe. * Tat-B. Pr. Met. I. 1910, S. 123-128.

1843. Karsten, H., Beitr. z. Kenntnis d. Temp.verhaltn. in d. untersten
Luftsch. * Arb. d. finn. landw.-okonom. Vers.Anst. Helsingfors
1921.

185. Knoch, K., E. Beitrag z. Kenntn. d. Temp.- u. Feucht.verhaltn.
in verschied. Hohe ii. d. Erdboden. * Abh. Pr. Met. I. 5, Nr. 2,
1909.

1 86. Made, A., Mess. mit. Widerstandsthermom. an Funkturmen. *

Beitr. Phys. d. fr. Atm. 2/, 309-312, 1934.
187. — , Widerstandselektr. Temp.beob. an e. mikroklimat. Basisstation.

* R. f. W. Wiss. Abh. 4, Nr. 3, 1938.

LITERATURE 425

1873. Mai, S., Desai, B. N., and Sircar, S. P. An investigation into the
variation of the lapse rate of temperature in the atmosphere near
the ground at Drigh Road, Karachi. * Memoirs India Met. Dep.
XXIX, Part i, Calcutta, 1942.

1 88. Morikofer, W., La temp, de Fair dans la couche d'un metre
d'epaisseur au-dessus du sol. * C. R. de la seance Soc. Suisse de
Geophys., Met. et Astr. 1921.

189. Novak, V., D. vertikale Verteilung d. Temp.extreme bei ungesch.
Extremtherm. i. d. bodenn. Luftsch. * Sbornik csl. Akad. zemed.

Prag. /o, 537~545> 1935-

190. de Quervain, F. & Gschwind, M., D. nutzbaren Gesteine d.
Schweiz. * Bern, Verlag H. Huber, 1934.

191. Ramdas, L. A., Kalamkar, R. J. & Gadre, K. M., Agricultural Met.
Stud, in micro-climat. * Indian J. of Agric. Sc. 4, 451-467, 1934.

192. , the same, Pt II. * Ibid. 5, i-n, 1935.

193. Schmidt, Wilh., tJ. d. tagl. Temp .gang in d. unteren Luftsch. *
Met. Z. 37, 49-59, 1920.

194. Seltzer, P., Sur la repartition verticale de la temp, de Fair dans les
2 prem. metres au-dessus du sol. * C. R. Paris 796 (II), 1626—1628,

J933-

195. Slanar, H., Klimabeob. aus Zentral-Island. * Met. Z. 50, 379—383,

1933-

196. Steinhauser, F., Temp.schichtung u. Windstruktur in Bodennahe.

* Met. Z. 52, 439~443> 1935-
1963. Troll, C., Die Frostwechselha'ufigkeit in den Luft- und Boden-

klimaten der Erde. * Met. Z. 60, 161-171, 1943.
i96b. — , Die Formen der Solifluktion und die periglaziale Bodenab-

tragung. * Erdkunde /, 162-175, 1947.

197. Vujevic, P., D. Temp .verbal tn. d, untersten Luftsch. * Sitz-B.
Wien. Akad. 118, 971-1018, 1909.

198. Woeikof, A., Temp. d. untersten Luftsch. * Met. Z. 21, 49-50,
1904.

CHAPTER 10. HUMIDITY RELATIONSHIPS.

(See also the bibliography for Chapters 8 and 9 and for the

Supplement to Part i)

199. Biidel, A., D. Feucht.messung in d. bodennahen Luftschicht. *
Z. f. angew. Met. 48, 289—293, 1931.

200. Buxton, P. A., The measurement and control of atm. humidity in
relation to entomolog. problem. * Bull. Entomolog. Res., London
22, 431-447, 1931.

201. Buxton, P. A. & Mellanby, K., The measurement and control of
humidity. * Ibid. 25, 171-175, 1934.

202. Defant, A., Zum tagl. Gang d. relat. Feucht. * Met. Z. 52, 61-69,
1915.

426 THE CLIMATE NEAR THE GROUND

202a. Dorffel, K. & Lettau, H., Der Wasserdampfiibergang von einer
nassen Platte an stromende Luft. * Ann. d. Hydr. 64, 342-352,
504-510, 1936.

203. Hamberg, H. E., La temp, et 1'humid. de 1'air a diff. hauteurs,
observees a Upsal pendant 1'ete de 1875. Upsala 1876.

204. Hill, S. A., On temp, and humidity observ. made at Allahabad at
various heights above the ground. * Indian Met. Mem. Calcutta 4,
361-394, 1889.

205. Howell, D. E. & Craig, R., A small hygrometer. * Science #9,

544» J939-

2053. Huber, Br., Versuche zur Messung des Wasserdampf-und Koh-
lendioxyd-Austausches iiber Pflanzenbestanden. * Sitz- B. Wien.
Akad. 755, 97-145, 1947.

206. Koch, W., Mess. d. Luftfeucht. mit Thermoelementen ohne
kunstl. Beliiftung. * Gesundheits-Ingenieur 59, 504-505, 1936.

207. Moller, F., tJ. d. tagl. Gang d. Dampfdrucks u. s. interdiurnen
Veranderlichk. * Met. Z. 54, 124-133, 1937.

208. Nielsen, E. T. & Thamdrup, H. M., E. Hygrometer f. mikroklimat.
Untersuch. * Biokl. B. 6, 180-184, 1939.

2o8a. Priigel, H., Zum Problem der Nebelverstarkung und -auflosung
nach Sonnenaufgang. * Ann. d. Hydr. 77, 420-422, 1943.

209. Ramdas, L. A., The variation with height of the water vapour
content of the air layers near the ground at Poona. * Biokl. B. 5,
30-34, 1938.

210. Ramdas, L. A. & Katti, M. S., Preliminary stud, on soil-moisture
in relation to moisture in the surface layers of the atm. during the
clear season at Poona. * Indian J. of Agric. Sc. 4, 923-937, 1934.

211. Rossi, V.j tJ. mikroklimat. Temp. u. Feucht.beob. mit Thermo-
elementpsychrometern. * Soc. Scient. Fennica; Comm. Phys. Math.
6, Nr. 25, 1-22, 1933.

212. Smolik, L., D. rel. Feucht. d. Luft nachst d. Bodenoberfl. * Sbornik
csl. akad. zemed. Prag /o, 98-103, 1935.

213. Szymkiewicz, D., Okolog. Unters. im Torfmoor Czerme I.
Brzesc 1931.

2i3x. Thornthwaite, C. W. & Holzman, B., The determination of
evaporation from land and water surfaces. * M. W. Rev. 67, 4-11,

I939-

2133. Wald, H., E. Psychrometer ohne kunstl. Beliiftung. * Z. f. d. ges.
Kalte-Industrie 39, Heft 6, 1932.

CHAPTER ii. WIND RELATIONSHIPS

214. Albrecht, F., E. Messgerat z. Mess. u. Registr. kleiner Windgeschw.
u. s. Anwendung auf d. Unters. d. Warmeumsatzes an d. Erdo-
berfl. * Met. Z. 47, 465-474, 1930.

LITERATURE 427

215. All, B., Variation of wind with height. * Quart. J. 5$, 285-288,
1932.

2i5a. Bagnold, R. A., The measurement of sand storms. * Proc. Roy.

Soc. London 167, 282-291, 1938.
2150. Carruthers, N., Variations in wind velocity near the ground. *

Quart. J. 69, 289-301, 1943.

216. Hellmann, G., t). d. Bewegung d. Luft in d. untersten Schichten
d. Atm. * Met. Z. 52, 1-16, 1915.

217. — , tJ. d. Beweg. d. Luft in d. untersten Sch. d. Atm. * Sitz-B.
Berlin. * Akad. 1919, 404-416.

218. Hey wood, G. S. P., Wind structure near the ground and its rela-
tion to temp, gradient. * Quart. J. 57, 433—452, 1931.

219. Katheder, F., Auflosung e. Bodennebeldecke d. e. startendes Flug-
zeug. * Z. f. angew. Met. 54, 61-63, J937-

220. Knoch, K., Lebhafte Schwank. d. Temp, an d. Grenzflache d.
untersten Bodeninversion. * Tat-B. Pr. Met. I. 1909, S. 113-124.

221. Kohler, H., E. kurzes Studium d. Austauschs auf Grund d. Potenz-
gesetzes. * Beitr. Phys. d. fr. Atm. 79, 91-104, 1932.

222. McAdie, A. G., Studies in frost protection — effect of mixing the
air. * M. W. Rev. 40, 122—123, 779> I912'

223. Mierdel, F., 0. nachtl. Temp.anstiege an d. Mohne-Talsperre. *
Met. Z. 40, 178, 1923.

224. Paeschke, W., Experimentelle Unters. z. Rauhigkeits- u. Stabil.-
problem in d. bodenn. Luftsch. * Beitr. Phys. d. fr. Atm. 24, 163-
189, 1937.

225. Peppier, A., Windmess. auf d. Eilveser Funkenturm. * Beitr. Phys.
d. fr. Atm. 9, 114-129, 1921.

226. Prandtl, L., Met. Anwendung d. Stromungslehre. * Beitr. Phys.
d. fr. Atm. 79, 188-202, 1932.

227. Prandtl, L. & Tollmien, W., D. Windverteilung iiber d. Erdboden,
errechnet a. d. Gesetz d. Rohrstromung. * Z. f. Geophysik /, 47-
55, 1924/25.

228. Schmidt, Wilh., D. Windgeschw. in Bodennahe. * Met. Z. 36,
88-90, 1919.

229. — , Unters. ii. d. Feinbau d. Windes. * Deutsche Forschung 74,
54-66, 1930.

230. Stevenson, Th., Rep. on simultaneous observ. of the force of wind
at diff. heights above the ground. * }. Scottish Met. Soc. 5, 348-
351, 1880.

231. Sutton, O. G., Note on the variation of the wind with height. *
Quart. J. 5^, 74-76, 1932.

232. Sverdrup, H. U., Second note on the logar. law of wind structure
near the ground. * Quart. J. 65, 57—60, 1939.

233. Viereck, W., Reg.gerat f. geringe Windgeschw. * Met. Z. 50, 426-
428, 1933.

428 THE CLIMATE NEAR THE GROUND

234. Wagner, A., Zur Theorie d. tagl. Ganges d. Windverhaltn. * Gerl.
B. 47, 172-202, 1936.

235. Young, F. D., Notes on the 1922 freeze in southern California. *
M. W. Rev. 57, 581-585, 1923.

CHAPTER 12. OPTICAL AND ACOUSTICAL PHENOMENA. CONTENT OF
DUST, CARBON DIOXIDE AND EMANATION.

236. Auer, R., O. d. tagl. Gang d. Ozongehalts d. bodenn. Luft. * Gerl.

B- 54> I37~I45> I939-

237. Aujeszky, L., Kleinklima u. Schallklima. * Forsch. u. Fortschr. 74,

413-415, 1938.

238. Becker, F., Mess. d. Emanationsgeh. d. Luft in Frankf. a. M. u.
am Taunusobserv. * Gerl. B. 42, 365-384, 1934.

239. Bielich, F. H., Einfluss d. Grosstadttriibung a. Sicht. u. Sonnen-
strahl. * Veroff. Geophys. I. Leipzig 6", Heft 2, 1933.

2393. Bigg, W. H., Road mirages. * Met. Mag. 63, 138, 1928.

240. Braak, C., Luchtspieg. en verwante versch. in ons Polderland. *
Tijdschr. K. Nederl. Aardr. Gen. ^9, 587, 1922.

24ox. Buch, K., Kohlensaure in Atmosphare und Meer. * Ann. d. Hydr.

70, 193-205, 1942.
24oy. Cauer, H., Ol und olhaltige Bestandteile in der Luft. * Angew.

Chemie 5^, 171-172, 1940.
2403. Dufour, L., Des temps, de 1'air et des mirages a la surface du lac

Leman. * Bull. Soc. vand. sc. nat. Lausanne 5, 26, 1858.
24ob. Effenberger, E. F., Kern- und Staubuntersuchungen am Collm-

berg. * Veroff. Geoph. I. Leipzig 12, Heft 5, 1940.

241. Findeisen, W., t). Beob. v. Luftspiegel. auf d. Neuwerker Watt. *
Ann. d. Hydr. 62, 423-426, 1934.

242. Futi, H., On road mirage. * Geophys. Mag. Tokio 4, 387, 1931.
2423. Gish, O. H., The distrib. of electric elements in the atm. near

the earth's surface. * Transact. Americ. Geophys. Union 1940,
S. 314—316.

243. Goldschmidt, H., Mess. d. atmosph. Triibung mit e. Scheinwerfer.
* Met. Z. 55, 170-174, 1938.

243a. Hartmann, W., Erdbodennahe Haloerscheinungen. * Met. Z. 46,
269-270, 1929.

244. Hofmann, A., Fata Morgana in d. Rheinprovinz. * Met. Z. 52,
29-30, 1935.

245. Israel-Kohler, H., Aufg. u. Ziele. d. Boden-Emanationsforschung. *
Balneologe 5, 248-260, 1938.

246. Jones, T. W. V., Road mirages. * Met. Mag. 62, 261—262, 1927.

247. Koppen, W., D. Eigenart d. untersten Luftschicht. * Beitr. Phys.
d. fr. Atm. 15, 205-209, 1929.

2473. Kreutz, W., Kohlensauregehalt der unteren Luftschichten in
Abhangigkeit von Witter ungsfaktoren. * Angew. Bot. 2^, 89-117,
1941.

LITERATURE 429

248. Kohlhorster, W., D. Erdstrahl. auf d. Gelande d. Met.-Magnet.
Observ. Potsdam. * Met. Z. 56, 35-38, 1939.

249. Lehmann, G., Blitze, Wasseradern u. Wiinschelrute. * Met. Z. 49,
284—285, 1932.

250. Meissner, O., D. Gripeepidemie 1939 u. d. Wetter. * Biokl. B. 7,
42-43, 1940.

251. Miyanisi, M., On the mysterious sea fire "Siranui" in Japan. * Sc.
Pap. Inst. Phys. and Chem. Res. 36, 198-243, 1939.

252. Musso, J. O., Spann.erhohung d. elektr. Feldes mittels Mulchierung
d. Bodens. * Biokl. B. /, 21-25, 1934.

253. Musso, J. O., E. mogl. Zus.hang zw. d. Grad. d. elektr. Feldes, d.
Agrotechnik u. d. Ernte. * Biokl. B. 5, 30-35, 1936.

254. Pernter, J. M. & Exner, F. M., Meteorolog. Optik. Wien & Leipzig,
W. Braunmiiller, 1922.

2543. Portig, W., Halo im Eisnebel. * Met. Z. 59, 207-208, 1942.

255. Priebsch, J., D. Hohenverteilung radioakt. Stoffe in d. fr. Luft. *
Met. Z. 49, 80— 81, 1932.

2553. Romage, A. G., Mirage on the Queensferry road. * Proc. Roy.
Soc. Edinburgh 38, 166-168, 1917/18.

256. Ramdas, L. A. & Malurkar, S. L., Theory of extremely high lapse
rates of temp, very near the ground. * Indian J. of Physics 6, 495-
508, 1932.

2563. Rotschke, M., Untersuchungen iiber die Meteorologie der Staub-
atmosphare. * Veroff. Geophys. I. Leipzig //, Heft i, 1937.

257. Schiele, W. E., Z. Theorie d. Luftspiegelungen. * Veroff. Geoph. I.
Leipzig 7, Heft 3, 1935.

258. Schmid-Curtius, C., Heilklimat. Untergrunds- u. Waldluftforsch. i.
nordwestl. Thiiringer W. * Z. f. angew. Met. 46, 161-175, 194-
201, 233-241, 257-262, 1929.

2583. Scott, Captain, Letzte Fahrt I. * Brockhaus Leipzig 1913.

259. Vedy, L. G., Sand mirages. * Met. Mag. 6j, 249-253, 1928.
259a. Wegener, A., Optik der Atmosphare. * Miiller-Pouillets Lehrbuch

der Physik V, i. Halfte (n. Aufl.) 1928.

CHAPTER 13. THE TEMPERATURE OF THE GROUND SURFACE.

260. Angstrom, A., The albedo of various surfaces of ground. * Geo-
graf. Ann. 7, 323-342, 1925.

2603. Bac, S., Schwankungen der Bodenschichten infolge Gefrierens und
Auftauens. * 6. Bait. Hydr. Konf. Berlin 1938.

261. Bartels, J., D. Strahlung u. ihre Bedeut. f. d. Klima. * Z. f. F. u.
Jagdw. 62, 537~563> i93°-

262. Brooks, C. F., Parade-ground temp, at College Stat, Tex. * M. W.
Rev. 47, 80 1, 1919.

263. Bruckmann, W., tX Vers. d. Registr. d. Oberfl.ternp. d. Bodens
mit elektr. Therm. * Tat-B. Pr. Met. I. 1917-1919, S. 111-116.

430 THE CLIMATE NEAR THE GROUND

264. Biittner, K., Mess. d. Sonnen- u. Himmelsstrahl. i. Flugzeug. *
Met. Z. 46, 525-527, 1929.

265. Diem, M., Bodenatmung. Messtechnik u. Ergebn. * Gerl. B. 57,
146-166, 1937.

266. Dorno, C., U. d. Erwarmung v. Holz unter verschied. Anstrichen.

* Gerl. B. 32, 15-24, 1931.

266a. Diicker, A., Der Bodenfrost im Strassenbau. * "Der Verkehr,"
Schulz-Wittuhn Bd. 2. F. Schmidt- Verlag, Berlin and Detmold
1947.

267. Dufton, A. F. & Beckett, H. E., Terrestrial temp. * Met. Mag. 67,
252-253, 1932.

268. Eaton, G. S., High relat. temp, of pavement surfaces. * M. W.
Rev. 47, 801-802, 1919.

269. Falckenberg, G., Absorptionskonst. einig. met. wichtiger Korper
f. infrarote Wellen. * Met. Z. 45, 334-337, 1928.

270. — , Apparat z. Mess. d. Himmelsstrahl. u. Bodentemp. * Met. Z.

45> 422~-425> J928-

271. Fleischmann, R., Beob. u. d. Auffrieren d. Bodens. * Fortschr. d.

Landwirtsch. 6, 673-685, 1931.

272. — , Vom Auffrieren d. Bodens. * Biokl. B. 2, 88-90, 1935.

273. Hausmann, W. & Kuen, F. M., t). d. biolog. Wirkung der von
Oberflachen verschied. Natur reflekt. ultravioletten Strahlung. *
Wiener klin. Wochenschr. 1934, Nr. 24.

2733. Penman, H. L., Daily and seasonal changes in the surface tempera-
ture of fallow soil at Rothamsted. * Quart. J. 69, 1-16, 1943.

274. Ramanathan, K. R., On temp, of exposed rails at Agra. * India
Met. Dep. Scientific Notes /, Nr. 4, 1929.

2743. Ramdas, L. A. & Dravid, R. K., Soil temp. * Current Sc. j, 266—

267, 1934.
274b. Reeder, G., Ground temp, compared with air temp, in a shelter.

* M. W. Rev. 48, 637-639, 1920.

275. Richardson, L. F., The reflectivity of woodland, fields and suburbs
between London a. St. Albans. * Quart. J. 56, 31-38, 1930.

276. Ritscher, A., D. Deutsche Antarkt. Exped. * Ann. d. Hydr. 67,
Beiheft zu VIII, 9—19, 1939.

277. Riicker, F., U. d. Ultrarotreflexion tierischer Korperoberfl. * Z. f.

vergl. Physiologic 2/, 275-280, 1935.

2773. Schanderl, H. & Weger, N., Stud, iiber d. Mikrokl. vor verschie-
denfarb. Mauerflachen u. d. Einfl. auf Wachstum u. Ertrag v.
Tomaten. * Biokl. B. 7, 134—142, 1940.

278. Schmauss, A., Ein d. Erdwurf ahnl. Schneewurf. * Met. Z. 55,
380, 1938.

279. Schmidt, Wilh., E. neues Verfahren z. Messung d. Bodentemp. *
Z. f. Instrum.kde 46, 431-433, 1926.

280. Schmidt, Wilh. & Lehmann, P., Vers. zur Bodenatmung. * Sitz-B.
Wien. Akad. 138, 823-852, 1929.

LITERATURE 431

281. Schropp, K., D. Temp, techn. Oberflachen unter d. Einfluss d.
Sonnenbestrahl. u. d. na'chtl. Ausstrahl. * Gesundh.-Ing. 1931,
S. 729-736.

282. Voigts, H., Strahl.-u. Riickstrahl.mess. mit Hilfe d. photograph.
App. in Travemunde. * Biokl. B. /, 128-133, 1934.

283. — , Das UV-Klima d. Liibecker Bucht. * Biokl. B. 4, 72-77, 1937
und 5, 20-22, 1938.

284. Vujevic, P., D. Temp, verschiedenart. Bodenoberfl. * Met. Z. 29,
570-576, 1912.

285. Wollny, E., Unters. u. d. Einfluss d. Farbe des Bodens auf dessen
Erwarmung. * Forsch. a. d. Geb. d. Agrik.physik /, 43-69, 1878.

CHAPTER 14. THE INFLUENCE OF THE TYPE AND CONDITION OF THE SOIL.
(See also the bibliography for Chapter 3)

286. Balanica*, T., Beitr. z. e. Met. d. Bodens. Diss. Miinchen, Bukarest
1940.

287. Becker, F., D. Erdbodentemp. als Indikator d. Versickerung. * Met.
Z. 54, 372-377, 1937.

288. Firbas, F., U. d. Bedeutung d. therm. Verhaltens d. Laubstreu fur
d. Friihjahrs vegetation d. sommergrunen Laubwaldes. * Beih. z.
Botan. Centralbl. 44, Abt. II, 179-198, 1927.

289. Fuchs, O., Bodenwasser u. therm. Konvektion. * Beitr. Phys. d. fr.
Atm. 20, 174-213, 1933.

290. Geiger, R. & Fritzsche, G., Spatfrost u. Vollumbruch. * Forst-
archiv /6, 141-156, 1940.

291. Homen, T., Bodenphysikal. u. met. Beob. mit besond. Berikksicht.

d. Nachtfrostphanomens. * Berlin 1894.

292. Johnson, N. K. & Davies, E. L., Some measurements of temp, near
the surface in various kinds of soils. * Quart. J. 5J, 45—59, 1927.

293. Keen, B. A., Soil physics in relation to meteorology. * Quart. J. 5$,
229-250, 1932.

294. Keil, K., Temp.erhohung d. Erdbodens bei Branden. * Z. f. an-
gew. Met. 57, 26-27, I94°-

295. Keranen, }., t). d. Bodenfrost in Finnland. * Mitt. d. Met. Zentral-
anst. d. finn. Staats Nr. 12, 1923.

296. Krauss, G. Miiller, K. & Gartner, G., Standortsgemasse, Durch-
fiihr. d. Abkehr von d. Fichtenwirtschaft im nordwestsachsischen
Niederland. * Thar. Forstl. Jahrb. 90, 481-715, 1939.

296a. Kreutz, W., Das Eindringen des Frostes in Boden unter gleichen
und verschiedenen Witterungsbedingungen wahrend des sehr
kalten Winters 1939/40. * R. W. Wiss. Abh. 9, Nr. 2, 1942.
. — , Beitrag zur Erforschung des Boden- und bodennahen Klimas
im Emslandmoor etc. * Z. f, Landwirtsch. #/, 81-112, 1943.
. — , Der Jahresgang der Temperatur in verschiedenen Boden unter
gleichen Witter ungsverhaltnissen. * Z. f. angew. M. 60, 65-76,
1943.

432 THE CLIMATE NEAR THE GROUND

297. Kreutz, W. & Rohweder, M., Korrel. -analyse d. Temp.- u. Feucht.
verlaufs in extrem verschied. Boden u. in d. bodennahen Luft. *
R. F. W. Wiss. Abh. /, Nr. 9, 1936.

298. Lauscher, F., Mikroklimat. Temp.beob. an e. Wintertag im Ge-
birge. * Biokl. B. 5, 65-66, 1938.

2983. Lehmann, P., E. Vorschlag z. Kontrolle d. Bodenegalitat beim
Veg.versuch. * Fortschr. d. Landwirtsch. 7, 247—253, 1932.

299. Mayer, H., Beob. ii. d. Warmeleitfahigkeit. * Synopt. Bearb. d.
Frankfurter Wetterd.st. Linke-Sonderheft, 1933, S. 67.

300. Ramdas, L. A. & Dravid, R. K., Soil temperatures. * Current Sc. j,
266-267, 1934.

301. Ramdas, L. A. & Katti, M. S., Stud, on soil-moisture in relation to
moisture in the surface layers of the atm. during the clear season
at Poona. * Indian J. of Agric. Sc. 6, 1163—1200, 1937.

3013. Schmauss, A., Kleinklimabeob. ohne Instr. * Z. f. angew. Met.

57, 401-402, 1940.
30 ib. — , Kleinklimabeobachtungen ohne Instrumente. * Wetter u.

Klima /, 27—36, 1948.

302. Schmidt, Wilh., 0. kleinklimat. Forsch. * Met. Z. 48, 487-491,

303. Scultetus, H. R., D. Beob. d. Erdbodentemp. im Beob.netze d. Pr.
Met. I. wiihrend d. Jahre 1912—1927. * Abh. Pr. Met. I, 9, Nr. 5,
1930.

3033. Slanar, H., Schneeabschmelzungen im bewachsenen Gelande. *
Met. Z. 59, 413-416, 1942.

304. Vujevic, P., 1). d. Bodentemp. in Belgrad. * Met. Z. 28, 289-301,
1911.

304a. Wild, H., t). d. Diff. d. Bodentemp. mit u. ohne Veget.- bzw.
Schneedecke. * Mem. Akad. Petersburg 8, 1897.

305. Wollny, E., Unters. ii. d. Einfluss d. oberfl. Abtrocknung d. Bodens
auf d. Temp.- u. Feucht.verhaltn. * Forsch. a. d. Geb. d. Agrik.
physik 3, 325-348, 1880.

3O5a. Yakuwa, R., U. d. Bodentemp. in d. verschied. Bodenarten. *
Geophys. Mag. Tokio 6, 179—187, 1932.

CHAPTER 15. THE Am LAYER OVER WATER.

306. Angstrom, A. & Jacobson, S., Temp, matningar i Vanern och
Gotaalv. * Medd. Stat. Met. Hydr. Anst. Stockholm 7, Nr. 6, 1940.

3063. Bruch, H., Die vertikale Verteilung von Windgeschwindigkeit
und Ternperatur in den untersten Metern uber der Wasserober-
flache. * Veroff. I. f. Meereskunde Berlin Neue F. A. H. 38. 1940.

307. Buttner, K. & Sutter, E., D. Abkuhl.grosse in d. Dunen. Riickstrahl.
verschied. Bodenbedeck. f. uv. u. gesamte Sonnenstrahl. * Strahlen-
therapie 54, 156-173, 1935.

LITERATURE 433

308. Conrad, V., Oberfl.-Temp. in Alpenseen. * Gerl. B. 46, 44—61,

309. — , Z. Wasserklima einiger alpiner Seen Osterr. * Beih. z. Jahrb. d.
Zentralanst. f. Met. Wien Jahrg. 1930, Wien 1936.

3093. Defant, A., Der Einfluss des Reflexionsvermogens von Wasser
und Eis auf dtn Warmeumsatz der Polargebiete. * VerofT. D. Wiss.
Inst. zu Kopenhagen, Reihe I (Arktis) Nr. 5, 1942.

310. Dietrich, G., D. Absorption d. Strahl. im reinen Wasser u. im r.
Meerwasser. * Ann. d. Hydr. 67, 411-417, 1939.

311. Exner, F. M., Mess. d. tagl. Temp .sch wank, in verschied. Tiefen
d. Wolfgangsees. * Sitz-B. Wien. Akad. 709, 905-922, 1900.

312. Findeisen, W., t). Beob. auffall. Wellenbildung auf diinner Wasser-
schicht auf d. Neuwerker Watt. * Ann. d. Hydr. 63, 186-189,

1935-

313. Frey, H., D. Friihlingseinzug am Ziirichersee. * Neujahrsbl. 1931,
her. . d. Naturf. Ges. Zurich 133, 1-48, 1931.

314. Herzog, J., Thermische Unters. in Waldteichen. * Veroff. Geoph.
I. Leipzig 8, Heft 2, 1936.

315. Kleinschmidt, E., Beitr. z. Limnologie d. Bodensees. * Schr. d.
Ver. f. Geschichte d. Bodensees 49, 1921.

316. Kuhlbrodt, E. & Reger, J., Die met. Beob. in "Wissensch. Erg. d.
Deutsch. Atl. Exped. a. d. Meteor 1925-27" 14, Berlin 1938.

317. Marquardt, R., Unters. d. Warme- u. Wasserdampfaustauschs
iiber d. Bodensee. * Gerl. B. 36, 78-132, 1932.

318. Merz, A., D. Oberflachentemp. d. Gewasser. * Veroff. Inst. f.
Meereskde Berlin, neue Folge 5, 1920.

3183. Model, F., Die Rauhigkeit der Meeresoberflache. * Gerl. B. 59,
102-142, 1942.

319. Peppier, W., Langjahr. Mittelwerte d. Temp. d. Luft u. d. Wassers
am Bodensee in d. fruhen Morgenstunden, * Wetter 43, 205-207,
1926.

320. — , Beitr. z. Kenntn. d. Oberfl.temp. d. Bodensees. * Z. f. angew.
Met. 44, 250-256, 1927 and 45, 14-20, 99-105, 1928.

321. — , Temp. d. Wassers u. d. Luft auf d. Bodensee. * R. f. W. Wiss.
Abh. 3, Nr. 7, 1937.

322. Pichler, W., Temp.mess. an e. Tumpel. * Biokl. B. 4, 25-27, 1937.

323. — , Sind d. Verlandungszonen d. Seen in ihrer Thermik Klein-
gewassern v. entsprech. Tiefe okologisch gleichwertig? * Biokl. B.
5, 107-109, 1938,

324. — , D. Almtumpel als Lebensstatte. * Biokl. B. 6, 85-89, 1939.
3243. Roll, U., Zur Frage des taglichen Temperaturgangs und des

Warmeaustausches in den unteren Luftschichten iiber dem Meere.
* Aus d. Archiv d. Seewarte 59, Nr. 9 Hamburg 1939.

325. Sauberer, F., t). d. Lichtverhaltn. d. Binnenseen. * Biokl. B. 6, 33-

434 THE CLIMATE NEAR THE GROUND

326. Schmidt, Wilh., O. d. Reflexion d. Sonnenstrahlung an Wasser-
flachen. * Sitz-B. Wien. Akad. 777, 75-89, 1908.

3263. — , Absorption d. Sonnenstr. im Wasser. * Ibid. 7/7, 237-253,
1908.

327. — , tJber Boden- u. Wassertemp. * Met. Z. 44, 406-411, 1927.

328. — , Ein Jahr Temp.mess. in 17 osterr. Alpenseen. * Sitz-B. Wien.

Akad. 143, 43I~452J J934-

328b. Volk, O. H., Ein neuer fur botanische Zwecke geeigneter Licht-
messer. * Ber. D. Bot.-G. 52, 195-202, 1934.

329. Wiist, G., Temp. u. Dampfdruckgefalle in d. untersten Metern ii.
d. Meeresoberflache. * Met. Z. 54, 4-9, 1937.

CHAPTER 16. THE AIR LAYER NEAR SNOW.

330. Angstrom, A., On the radiation and temp, of snow and the con-
vection of the air at its surface. * Ark. f. Mat. 7j, Nr. 21, 1919.

331. — , D. Einfluss d. Bodenoberflache auf d. Lichtklima. * Gerl. B.
J4, 123-130, 1931.

3313. Diem, M., Schneeforschung. * NaturWiss. 32, 12—20, 1944.
33 ib. — , Messungen an einer Scheedecke. * Z. f. angew. Met. 67,
37-50, 1944.

332. Eckel, O. & Thams, Ch., Unters. ii. Dichte, Temp. u. Strahl.-
verhaltn. d. Schneedecke in Davos. * Geologic d. Schweiz, Hydrol-
ogie Lief. 3, 275-340, 1939.

333. Gabran, O., D. Luftdurchlassigk. e. Schneedecke u. deren Einfluss
auf d. Dberwinterung d. Pflanzen. * Met. Z. 56, 354-356, 1939.

333a. Georgi, J., Die bodennahe Luftschicht viber dem gronlandischen
Eis. * Veroff. Deutsch. Wiss. Inst. Kopenhagen I, n, 1943.

334. Gotz, P., D, Strahl.klima von Arosa. Berlin 1926.

335. Geiger, R., D. Schutz d. Kulturen durch e. Schneedecke. * Forstw.
C. 5#, 105-114, 1936.

336. Horton, R. E. & Leach, H. R., Snow-surface temp. * M. W. Rev.
62, 128-130, 1934.

337. Juhlin, J., Sur la temp, nocturne de 1'air a diff. hauteurs. Upsala
1890,

338. Kalitin, N. N., D. Strahl.eigensch. d. Schneedecke. * Gerl. B. 34,
354-366, 1931.

339. Keranen, J., U. d. Temp. d. Bodens u. d. Schneedecke in Sodan-
kyla. Helsinki 1920.

340. Korhonen, W. W., U. d. lokale Veranderlichk. d. Schneedecke. *
Met. Z. 49, 72-76, 1932.

3403. Kreutz, W., Schutzwirkung einer Schneedecke. * Z. f. angew.
Met. 5#, 305-314, 1941.

341. Lauscher, F. & Eckel, O., Z. Kenntnis d. Winterklimas d. Kanzel-
hohe. * Mitt. d. Volksgesundh.Amts Wien 1931, Heft 6/7.

341 a. Levi, F. & Chorus, U., Wintertemp. in u. unter d. Schneedecke. *
Verh. d. Schweiz. Naturf. Ges. /7j, 319, 1932.

LITERATURE 435

342. Lindholm, F., Beitrag z. polaren Lichtklima. * Biokl. B. 5, 26-30,

1938.

342a. Lohle, F., Absorptionsmessungen an Neuschnee und Firnschnee.

* Gerl. B. 59, 283-298, 1943.

343. Michaelis, P., Okolog. Studien an d. alpinen Baumgrenze: I. D.
Klima u. d. Temp.verhaltn. d. Vegetationsorgane im Hochwinter.

* Ber. D. Bot. G. 52, 31-42, 1932.

3433. — , , II. D. Schichtung d. Windgeschw., Lufttemp. u. Evap-
oration iiber e. Schneeflache. * Beih. z. Botan. Centralbl. 52, 310-

332, 1934-

344. — , , V. Osmot. Wert u. Wassergehalt wahrend d. Winters

in d. verschied. Hohenlagen. * Jahrb. f. wiss. Bot. 80, 337-362,
1934.

345. Nyberg, A., Temp, measurements in an air layer very close to a
snow surface. * Geograf. Ann. 20, 234-275, 1938.

346. Olsson, H., Radiation measur. on Isachsen's Plateau. * Geograf.
Ann. 1 8, 225-244, 1936.

347. Paulcke, W., Prakt. Schnee- u. Lawinenkunde. * Berlin, Julius
Springer, 1938.

348. Rossmann, F., Beob. u. Schneerauchen u. Seerauchen. * Z. f.
angew. Met. 57, 309-317, 1934.

349. — , D. Schnee d. Schilaufers. * Z. f. angew. Met. 57, 382-391, 1934.

350. Sauberer, F., Vers. u. spektrale Mess. d. Strahl.eigenschaften v.
Schnee u. Eis mit Photoelementen. * Met. Z. 55, 250-255, 1938.

351. Sverdrup, H. U., Diurnal variation of temp, at polar stations in
spring. * Gerl. B. 32, 1-14, 1931.

352. Thams, Ch., 0. d. Strahl.eigenschaften d. Schneedecke. * Gerl. B.

53> 371-388, 1938-

353. Tolsky, A., U. d. Temp. d. Schneedecke. * Geophys. u. Met. Z. in
Russland 2, 137-164, 1926,

3533. Troll, C., Busserschnee in den Hochgebirgen der Erde. * Peterm.
Geogr. Mitt. Erganz. H. 240, 1942.

354. Wild, H., t). d. Differenzen d. Bodentemp. mit u. ohne Schnee-
decke nach d. Beob. im Konst. Gbserv. in Pawlowsk. * Mem.
Petersburg. Akad. VIII. Ser. 5, Nr. 8, 1897.

CHAPTER 17. THE AIR LAYER ABOVE A SOD COVER.

355. Bartels, J., Verdunstung, Bodenfeucht. u. Sickerwasser. * Z. f.
F. u. Jadgw. 65, 204-219, 1933.

356. Berg, H. & Metzler, H. K., D. Temp. u. Feuchtefeld in 1.5 m
Hohe iiber d. Flugplatz Hannover. * Erf. Ber. d. Deutsch. Flug-
wetterd. 9. Folge, Nr. 2, 1934.

357. Friedrich, W., Mess. d. Verdunstung vom Erdboden. * Deutsche
Forschung 27, 40—61, 1934.

358. Geiger, R., Mikroklimat. Beschreibung d. Warmeschichtung am
Boden. I. * Met. Z. 53, 357-360, 1936.

436 THE CLIMATE NEAR THE GROUND

359. — , — II. * Met. Z. 54, 133-138, 1937-

360. — , — - III. * Met. Z. 54, 278-284, 1937.

361. Knochenhauer, W., Mikroklimat. Erganz. zu: Inwieweit sind d.
Temp. u. Feuchtigk.mess. unserer Flughafen reprasentativ? * Erf.
Ber. d. Deutsch. Flugwetterd. 9. Folge, Nr. 2, 1934.

362. Leick, E. & Propp, G., Bodentemp. u. Pflanzenwuchs in ihren
wechselseit. Bez. auf d. Insel Hiddensee. * Mitt. a. d. Naturw.
Ver. f. Neuvorpommern u. Riigen 57/5$, 79—113, 1930/31.

363. Munch, E., Nochmals Hitzeschaden an Waldpflanzen. * Naturw.
Z. f. Land.- u. Forstw. 72, 169-188, 1914.

364. — , Beob. u. Erhitzung d. Bodenoberflache. * Ibid, /j, 249-260,
1915.

365. Newnham, E. V., Observ. of temp, close to the ground on clear
calm nights. * Met. Mag. 65, 59, 1930.

3653. Rethly, A., Istambul-Erenkoi homersekleti megfigyeleseibol
(Deutsche Zus.fassung). * Az Idoyaras 1930, Nov./Dez.-Heft.

366. Rouschal, E., D. kiihlende Wirkung d. Transpirationsstroms in
Baumen. * Ber. d. D. Botan. Ges. 57, 53-66, 1939.

367. Runge, H., Entstehung v. Bodennebel durch Auspuffgase. * Z. f.
angew. Met. 54, 307-308, 1937.

368. Schwalbe, G., t). d. Temp.minima in 5 cm iiber d. Erdboden. *
Met. Z. jp, 41-46, 1922.

SUPPLEMENT TO PART I. HEAT TRANSFERS.

369. Albrecht, F., D. kalorimetr. Strahl-unters. u. d. met. Beob. in
Lappland im Juni u. Juli 1927. * Abh. Pr. Met. I. /o, Nr. 4, 1934.

370. — , tJ. d. Zusammenhang zw. tagl. Temp.gang u. Strahl.haushalt.
* Gerl. B. 25, i-35> 193°-

371. — , E. Messgerat f. d. Mess. d. Warmeumsatzes im Erdboden. *
Met. Z. 49, 294-299, 1932,

372. — , E. Strahlungsbilanzmesser z. Mess. d. Strahl.haushaltes v.
Oberflachen. * Met. Z. 50, 62-65, X933-

373. — , D. Messgarate d. Warmeumatzes d. pflanzenbestandenen Erd-
oberfl. unter bes. Beriicks. v. Mess, im Walde. * Z. f. angew. Met.
54, 105-115, 137-146, 1937.

374. — , Messgerate d. Warmehaushaltes an d. Erdoberfl. als Mittel d.
bioklimat. Forschung. * Met. Z. 54, 471—475, 1937.

3743. — , Untersuchungen iiber den Warmehaushalt der Erdoberrlache

in verschiedenen Klimagebieten. * R. f. w., Wiss. Abh. 8, Nr. 2,

1940.
374b. — , Der gegenwartige Stand und die Aufgaben der Warmehaus-

haltforschung. * Met. Z. 60, 43—56, 1943.
374C. — , Untersuchungen am Strahlungsumsatzmesser mit Quecksilber-

thermometern. * Z. f. Met. /, 41—46, 1946.

375. v. Bezold, W., D. Warmeaustausch an d. Erdoberfl. u. in der
Atm. * Sitz-Ber. Berlin. Akad. 1892, S. 1139-1178.

LITERATURE 437

376. Falckenberg, G., D. nacht. Warmehaushalt bodennaher Luft-
schichten. * Met. Z. 49, 369-371, 1932.

377. Franssila, M., Mikroklimat. Unters. d. Warmehaushalts. * Hel-
sinki 1936.

378. Homen, Th., D. tagl. Warmeumsatz im Boden u. d. Warmestrahl.
zw. Himmel u. Erde. * Leipzig 1897.

379. Kriigler, F., Nachtl. Warmehaushaltsmess. an d. Oberfl. e. gras-
bewachsenen Ebene. * R. f. W. Wiss. Abh. 3, Nr. 10, 1937.

380. Niederdorfer, E., Mess. d. Warmeumsatzes iiber schneebedecktem
Boden. * Met. Z. 50, 201-208, 1933.

381. Okada, T., t). d. tagl. Warmeaustausch in e. Schneedecke. * J. Met.
Soc. Japan 1907, Nr. 4.

382. Sauberer, F., Mess. d. nacht. Strahl.haushalts d. Erdoberflache. *
Met. Z. 53, 296-302, 1936.

383. — , Mess. d. Strahl.haushalts horizont. Oberfl. bei heiterem Wetter.

* Met. Z. 54, 273-278, 1937.

384. — , bei Bewolkung 4-10. * Met. Z. 54, 273-278, 1937.

385. — , Einiges ii. Erfahr. mit d. Strahl.bilanzmeser nach F. Albrecht.

* Met. Z. 54, 329-333, 1937.

386. Schmidt, Wilh., D. Warmeumsatze an d. Erdoberfl. mit bes.
Rikksicht a. d. Nachtfroste. * Fortschr. d. Landwirtsch. 3, 385-
396, 1928.

387. Schubert, J., D. Warmeaustausch im festen Erdboden, in Gewassern
u. in d. Atm. * Berlin 1904.

BIBLIOGRAPHY FOR PART II

CHAPTERS 18-20. COLD-AIR FLOODS AND CoLD-Am DAMS. NOCTURNAL

TEMPERATURE RELATIONSHIPS IN VALLEYS. CoLD-Am WINDS

DOWN-SLOPE, DOWN-VALLEY AND GLACIER WINDS.

388. Cornford, C. E., Katabatic winds and the prevention of frost
damage. * Quart. J. 64, 553~587» I938-

389. Cox, H. J., Thermal belts and fruit growing in North Carolina. *
M. W. Rev., Suppl. Nr. 19, 1923.

390. Defant, A., D. Abfluss schwerer Luftmassen auf geneigtem Boden.

* Sitz-B. Berlin. Akad. 1933, S. 624-635.

391. Dobson, G. M. B., Frost hollows and fruit trees. * Quart. }. 64,
588-591, 1938.

392. Ekhart, E., Neuere Unters. z. Aerologie d. Talwinde: D. period.
Tageswinde in e. Quertale d. Alpen. * Beitr. Phys. d. fr. Atm. 2/,
245-268, 1934.

393. Fenner, G., D. belgische "Nebelkatastrophe" vom 3. u. 4. Nov.
1930. * Medizin. Welt 1935, S. 1860.

394. Flury, F., D. Todesursache bei d. Nebelkatastrophe im Maastal. *
Arch. f. Gewerbepatholog. 7, 117-125, 1936.

394a. Guminski, R. D. lokale Klima d. Dniestertalabhangs bei Szu-
tromince. * Bull. Met. et Hydr. 15, Nr. 10—12, 1935.

438 THE CLIMATE NEAR THE GROUND

395. Hallenbeck, C., Night-temp, studies in the Roswell fruit district. *
M. W. Rev. 46, 364-373, 1918.

396. Henry, A. J., Cox on thermal belts and fruit growing in North
Carolina. * M. W. Rev. 57, 199-207, 1923.

397. Heywood, G. S. P., Katabatic winds in a valley. * Quart. J. 59,

47~57> 1933-

398. Hoffrogge, Ch., Experim. Unters. d. bodennahen Luftstromungen

am Hang u. im ebenen Gelande. * Z. f. Geophysik 75, 184—213,

1939-

3983. Hough, A. F., Frost pocket and other microclimates in forests of
the northern Allegheny Plateau. * Ecology 26, 235-250, 1945.

399. Jaumotte, }., Sur le brouillard meurtrier de la vallee de la Meuse. *
Ciel et Terre 47, 100—106, 1931.

400. Koch, H. G., Die Saaletalnebel bei Jena. * Met. Z. 52, 10-15, 1935.
4003. — , Uber den Temperaturverlauf bei Saaletalnebel. * Z. f. Met. /,

122—128, 1947.

401. van Leeuwen, St., D. Nebelkatastrophe im Industriegebiet siidl. v.
Liittich. * Miinchn. Med. Wochenschr. 1931, S. 49.

402. Luft, R., D. Klima v. Bonn-Beuel. * B. f. angew. Met. 55, 155-158,
191-197, 234-239, 1938.

403. Malsch, W., E. seltener Fall v. Vereisung. * Z. f. angew. Met. 55,

3I~32> J938-

404. Manig, M., Nachweis d. Kaltluft d. erfrorene Dahlien. * Biokl. B.

6, 22-23, X939-

405. Marvin, C. F., Air drainage explained. * M. W. Rev. 42, 583-
585, 1914.

406. McDonald, W. F., Night radiation and unusual minim, temp, near
New Orleans. * M. W. Rev. 68, 181-185, 1940.

4063. Nagler, W., Thermische Eigentumlichkeit an Talstationen. *
Z. f. angew. Met. 60, 374-375, 1943.

407. Nitze, F. W., Unters. d. nachtl. Zirkulationsstromung am Berg-
hang d. stereophotogramm. vermess. Ballonbahnen. * Biokl. B. j,
125-127, 1936.

408. Obrutschew, S., D. neue Kaltepol in d. Jakut. Republ. * Met. Z.

4*> 359-36o> i93i- .

409. Pierce, L. T., Temp, variations along a forested slope in the Bent
Creek Exper. Forest. * M. W. Rev. 62, 8-12, 1934.

410. Predescou, C., Contrib. a 1'etude des climats locaux a Cluj. * Min.
de FAgricult. et des Domaines Nr. 176, Bucarest 1929.

411. Reiher, M., Nachtl. Kaltluftfluss an Hindernissen. * Biokl. B. j,
152-163, 1936.

412. Scaetta, H., Le climate ecolog. de la dorsale Congo-Nil. * Inst. R.
Col. Beige, Mem. Coll. in 4°. Bd. j, Briissel 1934.

413. — , Les avalanches d'air (Luftlawinen) dans les Alpes et dans les
hautes montagnes de I'Afrique centrale. * Ciel et Terre 57, 79—80,
1935.

LITERATURE 439

414. Schmauss, A., Luftlawinen in Alpentalern. * D. Met. Jahrb. f.
Bayern 1926, Anhang F.

415. Schmidt, Wilh., D. tiefsten Min.temp. in Mitteleuropa. * Naturw.

416. Schultz, H., tX Klimaeigentumlichk. im unt. Rheingau, unter bes.
Berucks. d. Wisperwindes. * Frankf. Geogr. Hefte 7, Heft, i, 1933.

417. Schulz, L., Lokalklimat. Unters. im Oberharz. * Biokl. B. 3, 25-29,
1936.

418. Smolik, L., Beitr. z. Entstehung d. Kaltluftseen in d. Talern. *
Sbornik csl. Akad. zemed. Prag //, 200-204, 1936.

419. Tollner, H., Gletscherwinde in d. Ostalpen. * Met. Z. 4$, 414—421,

I931-

420. Wagner, A., Theorie u. Beob. d. periodischen Gebirgswinde. *
Gerl. B. 52, 408-449, 1948. Includes extensive bibliography.

421. Witterstein, F., Kleinklimat. Unters. im Rheingau. * Jahrb. d.
Nass. Cer. f. Naturk. 83, 59-105, 1936.

422. Young, F. D., Effect of topography on the temp, distrib. in southern
California. * M. W. Rev. 4$, 462-463, 1920.

423. — , Nocturnal temp, inversions in Oregon and Calif. * M. W. Rev.
49, 138-148, 1921.

CHAPTER 21. THE SUNNINESS OF DIFFERENT SLOPES.

4233. Albrecht, F., Ergebnisse von Dr. Haudes Beobachtungen usw.
Rep. Scient. Exped. to the NW Prov. China under the leadersh.
Dr. Sv. Hedin IX. * Met. 2. Stockholm 1941.

424. Biihler, A., Einfluss d. Exposition u. d. Neigung gegen d. Horiz.
auf d. Temp. d. Bodens. * Mitt. d. Schweiz. Centr. Anst. f. d.
forstl. Vers.Wesen 4, 257—314, 1895.

4243. Eser, C., Berechn. d. Bestrahl.intensitat gegen d. Horiz. verschieden
geneigter Flachen. * Forsch. a. d. Geb. d. Agrik.physik. 7, 100-121,
1884.

425. Fritsch, E., Beitr. z. Erklarung d. Wanderungen d. Max. d. Boden-
temp. * Met. Z. 6, 151-153, 1871.

426. Gessler, R., D. Starke d. unmittelb. Sonnenbestrahl. d. Erde in
ihrer Abhangigk. v. d. Auslage unter verschied. Breiten u. zu
versch. Jahreszeiten. * Abh. Pr. Met. I, 8, Nr. i, 1925.

4263. Haude, W., Ergebnisse der allgemeinen meteorologischen Beo-
bachtungen nach den Drachenaufstiegen an den beiden Stand-
lagern bei Ikengiing und am Edsen-gol 1931/32. * Rep. Scient.
Exped. to the NW Prov. China under the leadersh. Dr. Sv. Hedin
IX. Met. i. Stockholm 1940.

426b. Kaempfert, W., Sonnenstrahlung auf Ebene, Wand und Hang. *
Wiss. Abh. R. f. W. p, Nr. 3, 1942.

426c. — , Zur Besonnung sudseitiger Spaliermauern. * Gartenbauwis.
I943-

440 THE CLIMATE NEAR THE GROUND

427. Kerner, A., t). Wanderungen d. Max. d. Bodentemp. * Met. Z. 6,
65-71, 1871.

428. — , D. Anderung d. Bodentemp. mit d. Exposition. * Sitz-B. Wien.
Akad. JOG, 704-729, 1891.

429. v. Kienle, J., D. tatsachl. u. d. astronom. mogl. Sonnenscheindauer
auf verschieden expon. Flachen. * D. Met. Jahrb. f. Baden 1933,
Anhang.

430. Kimball, H. H. & Hand, I. F., Daylight illumination on horizon-
tal, vertical, and sloping surfaces. * M. W. Rev. 50, 615-628, 1922.

431. Perl, G., D. Komponenten d. Intensitat d. Sonnenstrahl. in ver-
schied. geograph. Breiten. * Met. Z. 53, 467-472, 1936.

432. Pers, M. R., Calcul du flux d'insolation sur une facade en pente. *
La Met. //, 429-435, 1935.

433. Schmidt, Wilh., Auswertung d. Wiener Sonnenstrahl.mess. f. prakt.
Zwecke. * Fortschr. d. Landwirtsch. /, Heft 19, 1926.

434. Schoy, C., Probleme d. Besonnungsdauer. * Tat-B. Pr. Met. I. 1915,

(«H44).

4343. Schutte, K,, Die Berechnung der Sonnenhohen fur beliebig ge-

neigte Ebenen. * Ann. d. Hydr. 77, 325-328, 1943.

435. Wollny, E., Unters. ii. d. Einfluss d. Exposition auf d. Erwarm. d.
Bodens. * Forsch. a. d. Geb. d. Agrik.physik /, 263-294, 1878.

436. — , Unters. ii. d. Einfl. d. Expos, d. Bodens auf dessen Feucht.-
verhaltnisse. * Ibid. 6, 377-388, 1883.

437. — , Unters. ii. d. Feucht.- u. Temp.verhaltn. d. Bodens bei ver-
schied. Neigung des Terrains gegen d. Horizont. * Ibid. 9, 1-70,
1886.

438. — , Unters. ii. d. Feucht.- u. Temp-verhaltn. d. Bodens bei ver-
schied. Neigung d. Terrains gegen d. Himmelsrichtung u. gegen
d. Horizont. * Ibid, /o, 1-54, 1888.

CHAPTER 22. MICROCLIMATIC EFFECT OF DIFFERENT EXPOSURES
TO SUNSHINE.

439. Filzer, P., D. Mikroklima v. Bestandsrandern u. Baumkronen u. s.
physiolog. Ruckwirkungen. * Jahrb. f. wiss. Bot. 56, 228—314, 1938.

440. Gerlach, E., Unters. ii. d. Warme vernal tn. d. Baume. * Diss. Leip-
zig 1929.

441. Hartel, O., Mikroklima u. Wachstum in Tulpenbeeten. * Biokl. B.
6, 134-137, 1939-

442. Huber, Br., Aster linosyris, ein neuer Typus der Kompasspflanzen
(Gnomonpflanzen). * Flora 29, 113-119, 1934.

443. — , Notiz ii. Kompasskriimmungen bei Agaven-Bliitenstanden. *
Ber. D. Bot. G. 57, 182-184, J939-

4433. Kaempfert, W., Einfluss der Pflanzrichtung,- weite und -hohe auf
die Besonnungszeit und -dauer. * Biokl. Beibl. /o, 148—153, 1943.

444. Krenn, K., D. Bestrahlungsverhaltn. stehender u. liegender
Stamme. * Wien. Allg. F. u. Jagdztg. 57, 50-51, 53-54, 1933.

LITERATURE 441

445. Scamoni, A., U. Eintritt u. Verlauf d. mannlichen Kiefernbliite. *
Z. f. F. u. Jagdw. 70, 289-315, 1938.

446. Schade, A., U. d. mittl. jahrl. Warmegenuss v. Webera nutans u.
Leptoscyphus Taylori im Elbsandsteingebirge. * Ber. D. Bot. G.

35> 490-505. 1917-

447. Schanderl, H., D. derzeitige Stand d. Kompasspflanzenproblems. *
Biokl. B. 4, 49-54, 1937. Includes wide bibliography.

448. Seeholzer, M., Rindenschale u. Rindenriss an Rotbuche im Winter
1928/29. * Forstw. C. 57, 237—246, 1935.

CHAPTERS 23 AND 24. THE SKIN OF Am ON MOUNTAIN SLOPES, AND
MORE ON THE INFLUENCE OF TOPOGRAPHY.

449. Brocks, K., Lokale Unterschiede u. zeitl. Anderungen d. Dichte-
schichtung in d. Gebirgsatm. * Met. Z. 57, 62-73, I94°-

450. Biidel, A., D. Zugspitzbahnversuche. Teil I: D. Met. Jahrb. f.
Bayern 1929, Anhang E; Teil II: ditto 1930; Teil III: ditto 1931.

451. Bujorean, G., Zwei extreme Standorte bei Cluj. * Veroff. d. Geo-
botam. Inst. Riibel /o, 145-151, 1933.

452. Burckhardt, H. u. Flohn, H., D. atmosph. Kondensationskerne. *
Abb. a. d. Geb. d. Bader- u. Klimaheilkunde ^, Berlin 1939.

4523. Ekhart, E., Neuere Untersuchungen der Aerologie der Talwinde:
Die periodischen Tageswinde in einem Quertal der Alpen. * Beitr.
z. Physik d. fr. Atm. 2/, 245-268, 1934.

453. Firbas, F., Vegetationsstudien auf d. Donnersberg im Bohm.
Mittelgebirge. * Lotos (Prag) 76, 113-172, 1928.

454. Geiger, R., Mess. d. Expositionsklimas (9 Teile). * Forstw. C. 49,
665-675, 853-859, 9I4~923> 1927; 5°> 73-85> 437-448, 633-644,
1928; 57, 37-51, 305-315, 637-656, 1929.

455. Geiger, R., Woelfle, M. & Seip, L. Ph., Hohenlage u. Spatfrostge-
fahrdung (7 Teile). * Forstw. C. 55, 579-592, 737-746, 1933; 56,
141-151, 221-230, 253-260, 357-364, 465~484» I934-

4553. Held, }. R., Temperatur u. relative Feuchtigkeit auf Sonnen- u.
Schattenseite in einem Alpenlangstal. * Met. Z. 5$, 398-404, 1941.

456. Huttenlocher, F., Sonnen- u. Schattenlage. * Erdgesch. u. landes-
kundl. Abh. a. Schwaben u. Franken 7, 1923.

457. Innerebner, F., U. d. Einfluss d. Exposition auf d. Temp.-verhaltn.
im Gebirge. * Met. Z. 50, 337-346, 1933.

4573. Jelinek, A., Messung der Abkiihlungsgrosse in einem Alpental. *
Biokl, B. 9, 145-150, 1942.

458. v. Kerner, F., Kleinklirnatisches aus d. tirol. Gschnitztale. * Biokl.

->
459. Kinzl, H. & Wagner, A., Pilotaufstiege in d. peruanisch. Anden. *

Gerl. B. 54, 29-55, I938-

4593. Kreutz, W., & Wehrheim, H., Kleinklimaforscbungen im Glock-
nergebiet in Anlehniing an praktische Bediirfnisse. * Biokl, B. 9,

442 THE CLIMATE NEAR THE GROUND

460. Kiinkele, Th., Spatfrost u. Hohenlage. * Forstw. C. 55, 577~579>
1933.

461. Kiinkele, Th. & Geiger, R., Hangrichtung (Exposition) u. Pflan-
zenklima. * Forstw. C, 47, 597-606, 1925.

462. Lauscher-Wittmann, A., Temp.verhaltn. am Ostabhang d. Wiener-
waldberge. * Biokl. B. 4, 170-174, 1937.

463. Lautenbach, F., Expositionsklima oder Boden? * Allg. F. u.
Jagdztg. 105, 216-228, 1929.

464. Potzger, J. A., Microclimate and a notable case of its influence on
a ridge in Central Indiana. * Ecology 20, 29-37, 1939.

465. Reidat, R., 0. unperiod. Anomalien d. tagl. Gangs d. Lufttemp. zu
Erfurt u. auf d. Inselberge. * Mitt. d. Thiiring. Landeswetterwarte,
Heft i, Weimar 1930.

4653. Schmauss, A., Absinken einer Inversion. * Z. f. angew. Met. 59,
260-263, 1942.

466. Schmidt, Wilh., Einige Ergebnisse v. Temp.beob. an e. Hang. *
Z. f. angew. Met. 47, 204-211, 1930.

467. — , Hochgebirgsklima u. Technik. * Naturw. 22, 381-384, 1934.

468. Sonntag, K., Klimaforschung im Weinbaugebiet. * Pfalz. Museum
48, Heft 1/2, 1931.

469. — , D. Klima d. Kalmit. * Nicht veroff. Ber. ii. d. Kalmit-Observ.

470. Trankevitch, N. N., Some every-hour observ. on hygrometric air
conditions, made in the points of a relief. * Rec. of the Far East
Geophys. Inst. II (IX), 240—249, 1932.

471. Wagner, A., D. d. Feinstruktur d. Temp.gradienten an Berg-
hiingen. * Z. f. Geophysik 6, 310-318, 1930.

472. Woeikof, A., Temp. u. Feucht. in Berg u. Tal im Amurland. *
Met. Z. j/, 140—143, 1914.

CHAPTER 25. CONCERNING THE RANGE OF VALIDITY OF METEOROLOGICAL

STATIONS.

473. v. Basco, F. & Zolyomi, B., Kleinklima u. Vegetation auf d. Hoche-
bene d. Biikkgebirges. # Biokl. B. 2, 74-78, 1935.

474. Brooks, Ch. F., Einige Probl. kleinklimat. Unters. aus Neu-
England. * Met. Z. 48, 493, 1931.

475. — , An early morning weather profile from Cape Cod to central
Massachusetts. * Bull. Americ. Met. Soc. /6, 93-94, 1935.

4753. Burchard-Dostal, E., t)ber Differential-Klimogramme. * Biokl. B.
8, 102-109, 1941.

476. Diesner, P,, D. Geltungsbereich klimat. Stationen. * Wetter 43,
21-24, J926.

477. — , Weitere Bern. ii. d. Geltungsbereich klimat. Stat. * Wetter 44,
93-95, 1927.

478. Dostal, E., Wie Temp. u. Feuchteverlauf sich innerh. v. 24 Stunden
an benachbarten, aber versch. angelegten Stat. entwickelt. * Z. f.
angew. Met. 57, 178-182, 1940.

LITERATURE 443

479. Geiger, R., Zum Geltungsbereich met. Stat. * Wetter £?, 134-135,
1926.

480. Hartmann, W., Temp.mess. langs d. Arlbergbahn. * Z. f. angew.
Met. 50, 286-289, 1933.

481. Hettner, A., D. Wege d. Klimaforschung. * Geogr. Z. 30, 117-120,
1924.

4813. Kaempfert, W., Bestimmungen der moglichen Sonnenscheindauer
mit Hilfe eines einfachen Hohensuchers und der Tagbogenver-
kiirzung. * RfW. Wiss. Abh. 9 Nr. i, 1941.

482. Kassner, C., 0. d. Einfluss d. Zahl d. Messstellen auf d. Darst. d.
Niederschl.verteilung in Karten. * Tat-B. Pr. Met. I. 1916, (19)-

483. Kern, H., D. Untersch. d. Extremtemp. zweier benachbarter
Klimastat. * Z. f. angew. Met. 55, 283-288, 1938.

484. Knoch, K., D. Einfluss geringer Gelandeversch. auf d. met. Ele-
mente im norddeutsch. Flachlande. * Abh. Pr. Met. I. 4, Nr. 3.
1911.

485. — , Z. Methodik klimatolog. Forschung. * Tat-B. Pr. Met. I. 1924,

49-59-

486. Lauscher, F., Weitere Stud. u. d. Sonnenstrahl-intens. in d. steir.-
niederosterr. Kalkalpen. # Met. Z. 57, 336-341, 1934.

487. — , Grundlagen d. Strahl.klimas d. Lunzer Kleinkl.stationen. *
Beih. z. Jahrb. d. Zentralanst. f. Met. Wien Jahrg. 1931, Wien 1937.

488. Lauscher, F., Steinhauser, F. & Toperczer, M., E. Profil d. Sonnen-
strahl.intens. durch d. steir.-niederb'sterr. Kalkalpen. * Met. Z. 49,
300-306, 1932.

488a. Lautensach, H., Klirnakunde als Zweig landerkundl. Forsch. *
Geogr. Z. 46, 393-408, 1940.

489. Ludwig, G., Gleichzeit. Mess. v. Kondensat.kernen an zwei
benachb. Orten. * Met. Z. 53, 106-108, 1936.

490. Schmauss, A., Zwei zeitgemasse Fragen) * Met. Z. 34, 380-381,
1917.

491. Schmidt, Wilh., Biokl. Unters. im Lunzer Gebiet. * Naturw. 77,
176-179, 1929.

492. — , Kleinklimat. Beob. in Osterreich. *. Geogr. Jahresber, aus
Osterr. 76, 42-72, 1933.

493- — > D. Tagbogenmesser, e. Gerat z. Verfolgen d. Bahn d. Sonne
am Himmel. * Met. Z. 50, 328-331, 1933.

494. — , Observ. on local climatology in Austrian mountains. * Quart. J.

fo» 345~352> I934-

495. — , 0. neuere Messungs- u. Berechn.methoden d. StrahLgenusses
f. bioklimat. Zwecke. * Strahlentherapie 67, 689-696, 1938.

496. Schmidt, Wilh. & Schwabl, W., Strahl.genuss u. Ertrag im Ge-
birgsland. * Biokl. B. 2, 78-83, 1935.

497. Schulz, L., D. Einfluss d. Harzes auf Wetter u. Witterung im
Fruhjahr 1936. * R. f. W. Wiss. Abh. 6, Nr. i, 1939.

444 THE CLIMATE NEAR THE GROUND

498. Stepanowa, N., D. Genauigk. d. Beob. u. d. Mikrostruktur des
Elementes im Raum. * Met. i Hydrol. Moskau 1936, Nr. 8.

499. Tinn, A. B., Local temp, variations in the Nottingham district.
* Quart. J. 64, 391-401, 1938.

500. Topolansky, M., D. Geltungsbereich klimatolog. Stationen. *
Wetter 41, 125—128, 1924.

SUPPLEMENT: THE MICROCLIMATE OF CAVES.

501. Buxton, P. A., Climate in caves and similar places in Palestine. *
J. of Animal Ecology /, 152-159, 1932.

50 la. — , The climate in which the rat-flea lives. * Ind. Journ. Med.
Res. 20, 281—297, 1932.

502. Fugger, Beob. in d. Eishohlen d. Untersberges bei Salzburg. *
Mitt. Ges. f. Salzburger Landesk. 28, 1888.

503. Hauser, E. & Oedl, R., Eishohlen. E. Beitrag zu ihrer physikal.-
met. Erklarung. * Naturw. 9, 721-725, 1921.

503X. Hess, H., Leo Handl's Temperatur-Messungen des Eises und der
Luft in den Stollen des Marmolata-Gletschers und denen des
Ortlergebietes 1917-1918. * Zeitschr. f. Gletscherkunde 27, 168-
171, 1940.

. Kreutz, W., & Wehrheim, H., Klimastudien diesseits und jenseits
des Tauernkamms. * Z. f. angew. Met. 59, 369-390, 1942.

. Lautensach, H., Unterirdischer Kaltluftstau in Korea. * Peterm.
Geogr. Mitt. £5, 353-355, 1939.

504. Mrose, H., E. seltsame Hohlenvereisung. * Z. f. angew. Met. 56,

35°-353> 1939-

505. Oedl, R., 1). Hohlenmeteorologie, m. bes. Rucks, a. d. grosse Eis-
hohle im Tennengebirge. * Met. Z. 40, 33-37, 1923.

506. Paulcke, W., Prakt. Schnee- u. Lawinenkunde (Verstandl. Wiss.
j#). Berlin, J. Springer, 1938.

507. Penck, A., D. Temp.-verhaltn. d. Grotten v. St. Canzian b. Triest.
* Met. Z. 6, 161-164, 1889.

508. Roschkott, A., t). Temp.verhaltn. in Hohlen. * Met. Z. 38, 33-38,
1921.

509. Steiner, L., D. Temp.-verhaltn. d. Eishohle v. Dobsina. * Met. Z.

39,193-199,1922.

CHAPTER 26. THE HEAT ECONOMY OF PLANTS, AND PLANT TEMPERATURES.

(The bibliography is here only so far extended as is not already

covered in the collected works of Bruno Huber (5/4)).

510. von Angerer, E., Landschaftsphotographien in ultrarotem u. ultra-
violettem Licht. * Naturw. /#, 361—364, 1930.

511. Egle, K., Z. Kenntnis d. Lichtfeldes u. d. Blattfarbstoffe. * Planta
26, 546-5^3J I937-

512. Filzer, P., Nordafrikan. Wiiste u. suddeutsche Steppenheide, e.
okolog. Parallele. * Festschr. f. C. Uhlig, Ohringen 1932.

LITERATURE 445

513. Graininger, J., The internal temp, of fruit tree buds II. * Ann.
appl. Biolog. 36, 1-13, 1939.

514. Huber, Br., D. Warmehaushalt d. Pflanzen (Naturw. u. Land-
wirtsch., her. v. Boas 77). * Freising-Miinchen, Datterer, 1935.

515. — , Mikroklimat. u. Pflanzentemp.registrierungen mit d. Multi-
thermograph v. Hartm. & Braun. * Jahrb. f. wiss. Bot. 84, 671-709,

5153. — , Physiologische Rhythmen in Baum. * Met. Rundschau /,
144-147, 1947.

516. Hummel, K., U. Temp, in d. Sojabliite. * Biokl. B. 6, 13-17, 1939.
5i6a. — , Uber Temperaturen in Winterknospen bei Frostwitterung. *

Met. Rundschau /, 147-150, 1947.

517. Kessler, O. W. & Schanderl, H., Pflanzen under d. Einfluss ver-
schied. Strahl.intensitaten. * Strahlentherapie 39, 283-302, 1931.

518. Made, A., E. Beitrag z. Mikroklima e. Obstbaumes. * Gartenbau-
wiss. 72, 127-137, 1938.

519. — , D, Einfadenwiderstandsthermometer als Messgerat z. Best. d.
Oberfl.temp. v. Blattern. * Biokl. B. 6, 11-13, 1939.

5193. — , Temperaturuntersuchungen an Obstbaumen. * Gerl. B. 59,
201-213, I942»

520. Michaelis, G. & P., Okolog. Stud, an d. alpinen Baumgrenze III:

0. d. winterl. Temp. d. pflanzl. Organe, insbes. d. Fichte. * Beih.
z. Botan. Centralbl. 52, 333-377, 1934.

521. Orth, R., Strahlung, Lichtfeld u. Pflanze. * Biokl. B. 5, 68-75,
1938.

52ia. Schanderl, H., & Kaempfert, W., Uber die Strahlungsdurch-
lassigkeit von Blattern und Blattgeweben. * Planta 18, 700-750,

1933-

522. Sauberer, F., Z. Kenntnis d. Strahl.verhaltn. in Pflanzenbestanden.
* Biokl. B. ^,145-155, 1937.

523. Seybold, A., t). d. Lichtfaktor photophysiolog. Prozesse. * Jahrb. f.
wiss. Bot. 52, 741-795, 1936.

524. Smith, A. M., On the internal temp, of leaves in tropical insola-
tion. * Ann. Royal Botan. Gard. Peradeniya 4, 1909.

525. Ullrich, H. & Made, A., Stud. ii. d. Ursachen d. Frostresistenz.

1. Unters. d. Temp.austauschs an Rizinusblattern durch Mess. d.
Oberfl.temp. * Planta 28, 344-351, 1938.

526. -- , II. Unters. ii. d. Temp.verlauf b. Gefrieren v. Blattern u.
Vergleichsobjekten. # Planta J7, 251—262, 1940.

5263. Wegener, K., Die Meteorologie im Leben der Pflanze. * Z. f.
angew. Met. 59, 321-338, 1942.

527. Weger, N., Ober Tiitentemperaturen. * Biokl. B. 5, 16-19, 1938.
5273. Weg;er, N., Herbst, W. & Rudloff, C. F., Witterung u. Phan.

d. Bluhphase d. Birnbaums. * R. f. W. Wiss. Abh. 7, Nr. i, 1940.

528. Young, F. D., Substitution of fruit temp, for air temp, in regu-
lating orchard heating for oranges. * M. W. Rev. 52, 381-387, 1924.

446 THE CLIMATE NEAR THE GROUND

CHAPTER 27. RADIATION AND TEMPERATURE RELATIONSHIPS IN A
Low PLANT COVER.

529. Filzer, P., Unters. ii. d. Mikroklima in niederwiichsigen Pflanzen-
gesellsch. * Beih. z. Botan. Centralbl. 55, 301-346, 1936.

530. Fleischmann, R., Temp.mess. in reifenden Getreidefeldern u. and.
Kulturen. * Fortschr. d. Landwirtsch. j, 1928.

53 10 — ? Beitr. z. Kenntnis d. Mikrokl. in Getreidefeldern vor Ausbruch
d. Rostes. * Az Idojaras 34, VII/VIII, 1930.

532. Geiger, R., Unters. ii. d. Pflanzenklima. * Mitt. d. Staatsforst-
verwaltung Bayerns 77, 1926.

533. Kanitscheider, R., Temp.mess. in e. Bestande v. Legfohren. *
Biokl. B. 4, 22-25, *937-

534. Kirchner, R., Beob. ii. d. Mikroklima d. Weinberge. * Mitt. d.
Pfalz. Ver. f. Naturkunde u. Naturschutz 5, 1936.

535. Klecka, A., Mikroklimat. Beob. in Wiesenbestanden. * Sbornik
csl. Akad. zemed Prag //, 2-10, 1936.

536. Kreutz, W., Agrarmet. Stud. ii. Bestandsklima, ii. Windschutz u.
ii. Transpirat.verhaltn. im Gewachshaus. * R. f. W. Wiss. Abh. 2,
Nr. 7, 1937.

537. Lundegardh, H., Klima u. Boden in ihrer Wirkung auf d. Pflan-
zenleben. * Jena 1930.

538. Made, A., Widerstandselektr. Temp.mess. in e. Topinambur-
bestand. * R. f. W. Wiss. Abh. 2, Nr. 6, 1936.

539. — , t). d. Temp.verlauf in Bestanden. * Gartenbauwiss. 15, 312-

333,1940.

5393. — , Die Agrarmeteorologie in der Pflanzenziichtung. * R. f. W.
Wiss. Abh. 9, Nr. 6, 1942.

540. Putod, R., Action de Fenherbement sur les reboisements en
Algerie. * Rev. des Eaux et Forets 75, 412-426, 1937.

541. Ramdas, L. A., Kalamkar, R. J. & Gadre, K. M., Agricultural
Stud, in Microclimatology I. * Indian J. of Agric. Sc. 4, 451-467,
1934.

542. , Ditto II. * Ibid. 5, i-n, 1935.

543. Scaetta, H., Bioclimats; climats des associations et microcl. de
haute montagne en Afrique Central Equatoriale. * Journ. d'agron.
colon. Juni 1933 (Belgien).

5433. Schmauss, A., D. Klimaraum der Jungpflanze. * Mitt. d. Herm.
Goring- Akad. d. D. Forstwiss. /, 173-180, 1941.

544. Sonntag, K., Ber. ii. d. Arb. d. Kalmit-Observ. * D. Met. Jahrb. f.
Bayern 1934, Anhang D.

545. Tamm, E., Vergl. Temp.mess. in d. Zone d. Pflanzenklimas. *
Landw. Jahrb. #j, 457-554, 1936.

546. _>9 __ ii. * Ditto 88, 479-548, 1939.

547. — , — III. * Ditto «p, 259-318, 1939.

LITERATURE 447

548. Trankevitch, N. N., On the study of the phytoclimate of a wheat
field. * Rec. Far East Geophys. Inst. Wladiwostok /, 271-283,

549. Troll, C. & Wien, K., Oldeani-Ngorongoro. * Wiss. Veroff. d.
Mus. f. Landerk. z. Leipzig, N. F. 3, 95-116, 1935.

550. Wegener, A., D. Wesen d. Baumgrenze. * Met. Z. 40, 371-372,

551. Weger, N., Mikroklimat. Stud, in Weinbergen. * Biokl. B. 6,
169-179, 1939.

552. Wild, H., Diff. d. Bodentemp. mit u. ohne Vegetat. oder Schnee-
decke. * Mem. Petersburger Akad. 8, Ser. T, 1897.

CHAPTER 28. HUMIDITY AND WIND RELATIONSHIPS IN A Low PLANT

COVER.
(Insofar as not already covered by Chapter 27.)

553. Biidel, A., Unters. d. Warmeschutzwirkung v. Cittern u. Pflanzen.

* R. f. W. Wiss. Abh. 4, Nr. i, 1938.

554. Durst, C. S., Notes on the variations in the structure of wind over
different surfaces. * Quart. J. 59, 361-371, 1933.

555. Filzer, P., Experiment. Beitr. z. Synokologie d. Pflanzen I. * Jahrb.
f. Wiss. Bot. 79, 9-130, 1933.

556. — , Unters. ii. d. Wasserumsatz kiinstl. Pflanzenbestande. * Planta
30, 205-223, 1939.

557. Firbas, F., Stud. ii. d. Standortscharakter auf Sandstein u. Basalt.

* Beih. z. Botan. Centralbl. 40, 253-409, 1924.

558. Fleischmann, R., Windschaden in Maisfeldern. * Biokl. B. j, 123-
125, 1936.

559. Kestermann, A., Abkuhl.studien mit bes. Beriicksicht. d. Frigori-
graphen n. Buttner u. Pfleiderer. * Biokl. B. 7, i— 16, 1940.

560. Kreutz, W., D. Windschutzproblem. * Biokl. B. 5, 10-16, 1938.

561. Paeschke, W., Mikroklimat. Unters inerhalb u. dicht iiber ver-
schiedenart. Bestand. * Biokl. B. 4, 155-163, 1937.

562. Schmauss, A., Ober Sturmgefahrdungen. * Forstw. C. 42, 189-
194, 1920.

563. Stocker, O., Klimamess. auf kleinstem Raum an Wiesen-, Wald-
u. Heidepflanzen. * Ber. D. Bot. G. 41, 145—150, 1923.

564. — , D. Wasserhaushalt agypt. Wiisten- u. Salzpflanzen. * Botan.
Abh., Goebel, Heft 13, Jena 1928.

565. Wenger, R., D. wolkenfreie Raum an d. Erdoberflache. * Met. Z.
52, 20-22, 1915.

CERTAIN REFERENCES ON DEW AND HOAR FROST QUESTIONS AS

MlCROCLIMATIC PROBLEMS.

566. AndrianofT, P., Tauregistrier-Araometer. * Met. Z. 44, 425-429,
1927.

448 THE CLIMATE NEAR THE GROUND

567. Bernick, W., Unters. u. d. Taufall auf d. Insel Hiddensee u. s.
Bedeutung als Pfl.faktor. * Mitt. Naturwiss. Ver. f. Neuvorpom-
mern u. Riigen 65/66, 1938.

568. Fritzsche, G., Vergl. Mess. mit. Leickschen Tauplatten. * Biokl.
B. /, 66-73, 1934.

569. Hiltner, E., D. Tau u. s. Bedeutung f. d. Pflanzenbau. * Wiss.
Arch. f. Landwirtsch. j, 1-70, 1930.

570. — , D. Tau u. s. Bedeutung f. d. Wasserhaushalt d. Kultur-
pflanzen. * Prakt. Bl. f. Pfl.bau u. Pfl.schutz 8, 223, 1931.

571. Keller, H., 0. Taumessungen im ariden Hochland Transvaals. *
Met. Z. 50, 321-324, 1933.

572. Kessler, O., D. Tauschreiber Kessler-Fuess. * Biokl. B. 6, 23-26,

1939-

573. Knoche, W., E. Bemerk. ii. d. Taufall. * Met. Z. 56, 322, 1939.

5733. Kyriazopoulos, B., Drosographe: Instr. pour 1'etude de la rosee,
la gelee blanche et la pluie. * Comm. Met. Agricole, Tag. Ber.
Salzburg 1937, S. 97-100, Leyden 1938.

573b. Lehmann, P., and Schanderl, H., Tau und Reif. * R.f.W. Wiss.
Abh. 9, Nr. 4, 1942.

574. Leick, E., Z. Methodik d. relat. Taumessung. * Beih. z. Botan.
Centralbl. 49, Erg.-B., 160-189, I932-

575. — , D. Tau als Standortsfaktor. * Ber. D. Bot. G. 57, 409-442, 1933.

576. Mrose, H., E. volumenometr. Taumessgeriit. * Z. f. angew. Met.
56, I37-M9. 1939-

577. Rubenson, R., V. d. Temp. u. Feucht.verhaltn. in d. untersten
Luftschichten bei d. Bild. d. Taus. * Met. Z. //, 65-75, l876~.

578. Rudel, K., Natiirl. Psychrometer. * Met. Z. 20, 33-35, 1903.

579. Schubert, J., t). Niederschl.formen an d. Erdoberfl. * Z. f. F. u.
Jagdw. 49, 380^393, 1917.

580. Stephen, }., D. Tauproblem (Sammelreferat). * Biokl. B. 5, 75-81,
1938.

5803. — , Zum Tauproblem. * Biologic generalis 17, 204-229, 1943.

581. Stephen, J. & Mildebrath, E., Registr. Taumessung. * Biokl. B. 5,
34-39,1938.

582. Stiive, G., Z. Kenntnis d. Kristallisation d. Wasserdampfes aus
d. Luft. * Gerl. B. 52, 326-335, 1931.

583. Sutton, J. R., On some met. conditions controlling nocturnal radia-
tion. # Transact. Roy. Soc. of South Africa 2, Part 5, 1912.

584. Visser, S, W., E. neuer Tau-Registrierapparat. * Met. Z. 57, 388-

39<>> 1934-

585. Zattler, F., Agrarmet. Beitr. z. Tauproblem auf Grund v. Mess, im
Hopfengarten. * Wiss. Arch. f. Landwirtsch. 8, 371-404, 1932.

LITERATURE 449

CHAPTER 29. FOREST METEOROLOGY, FOREST CLIMATOLOGY AND STAND

CLIMATE.

5853. Amelung, W., & Pfeiffer, C. A., Die Einwirkungen des Wald-
klimas auf den Menschen. * Klin. Wochenschr. 24/25, 563-566,
1947.

586. Bates, C. G. & Henry, A. J., Forest and stream-flow experiment at
Wagon Wheel Gap, Colo. * M. W. Rev., Suppl. jo, 1928.

587. Blanford, H. F., On the influence of Indian forests on the rainfall.

* J. Asiatic Soc. of Bengal 56, Part II, i, 1887.

588. Brooks, C. E. P., The influence of forests on rainfall and runoff. *
Quart. J. 54, 1-13, 1928.

589. Burger, H., Waldklimafragen I: Met. Beob. i. Freien u. in e.
Buchenbestand. * Mitt. Schweiz. Centr.Anst. f. d. forstl. Vers.w. 77,
92-149, 1931.

590. — , — II: Met. Beob. i. Freien, in e. Buchen- u. e. Fichtenbestand.

* Ibid. 18, 1-54, 1933.

591. Deines, G. D. forstl. Standortslehre. * Mitt. a. Forstwirtsch. u.
F. wiss. 9, 387-352, 1938.

592. Ebermayer, E., D. physikal. Einwirkungen d. Waldes auf Luft u.
Boden. * Aschaffenburg 1873.

593. — , Klimat. Wirk. d. Waldes auf s. Umgebung. * Met. Z. /o,
201-214, J^93-

594. Engler, A., Unters. u. d. Einfluss d. Waldes auf d. Stand d. Ge-
wasser. # Burich, Verlag Beer, 1919.

595. Eredia, F., Ricerche sull'influenza delle litoranee e del bosco medi-
terraneo sul clima. * Comm. Met. Agricole, Tag.-Ber. Miinchen
1932, S. 138-139, Utrecht 1933.

596. Ernst, F., D. Bedeut. d. Klimaextreme f. d. Waldbau in Mittel-
europa. * Forstw. C. 56, 86-102, 1934.

597. Faust, R., D. Abkiihlungsklima im Walde u. auf freiem Felde nach
Frigorimeterregistr. * Veroff. Geoph. I. Leipzig 8, Heft i, 1936.

598. Geiger, R., Wald u. Klirna. * Mitt. d. Reichsforstwirtschaftsrates
Berlin 1932.

599. — , U. d. Wechselwirkung v. Wetter u. Wald. * Fostarchiv 75,
195-200, 1939.

600. — , Wetter u. Klima als Standortsfaktor. * Neudammer Forstl.
Lehrb., 10. Aufl. 1949.

6ooa. — , Wider die Gefahrdung des Landes durch Waldzerstorung. *

Allg. Forstz. 2, 137—140, 1947.
6oob. Gusinde, M., and Lauscher, F., Meteorologische Beobachtungen

im Kongo-Urwald. * Sitz.-B. Wien Akad. 750, 281-347, I94I-

60 1. Hall, R. C., Climatic diff. between forested and cut-over areas in
New Hampshire etc. * Biokl. B. 6, 185-186, 1939.

602. Hamberg, H. E., De Finfluence des forets sur le climat de la
Suede. 5 Teile. Stockholm 1885-96.

450 THE CLIMATE NEAR THE GROUND

603. Hirata, T., Contrib. to the problem of the relation between the
forest and water in Japan. * Imper. Forestry Exper. Stat. Meguro
1929.

604. Hoppe, E., Sind d. forstl.-met. Beob. in d. bisher. Weise fortzu-
setzen oder sollte eine Anderung d. bish. Systems eingefiihrt
werden? * Ref. v. 2. Kongr. d. intern. Verb, forstl. Vers.A. 1896.

605. Ijjasz, E., D. Geschichte, Organisation u. Forsch.richtung d. forstl.
Met. Un^arn. * 9. Kongr. d. intern. Verb, forstl. Forsch.-Anst. 1936.

606. Kaminsky, A., Beitrag z. Frage ii. d. Einfluss d. Aufiforstung d.
Waldlichtungen in Indien auf d. Niederschl. * Nachr. d. Geophys.
Central. I. Leningrad Nr. 4 (Year?).

6o6a. Kirwald, E., Bekampfung des Bodenabtrags und Regelung des
Wasserhaushalts in Gebirgen. * Forstwiss. C. u. Thar. Forstl.
Jahrb. (Kriegsgerneinschaftsausgabe) 1944, 37-40.

607. Koloskoff, P. J., Air humidity in the forest and outside it. * Rec.
Far East Geophys. Inst. Wladiwostock /, 255-270, 1931.

608. von Lorenz-Liburnau, Resultate forstl.-met. Beob. * Mitt. a. d.
forstl. Vers.wesen Osterr. 12 und 13, Wien 1890.

609. Lossnitzer, H., Neuere Erkenntnisse in d. forstl. Met. * D. Deutsche
Forstwirt 2/, 337-338, 1939.

610. Meyer, A. F., D. Wald in s. Wirkung auf d. Menge d. fiir d.
Trinkwasserversorgung erschliessbaren Wassers. * D. Gas- u.
Wasserfach 7#, 253-258, 273-277, 293-296, 312-318, 1935.

6n. Miittrich, A., t). d. Einfluss d. Waldes auf d. period. Verand. d.
Lufttemp. * Z. f. F. u. Jagdw. 22, 385-400, 449-458, 513-526,
1890.

612. — , U. d. Einfluss d. Waldes auf d. Grosse d. atmosph. Nieder-
schlage. * Z. f. F. u. Jagdw. 24, 27-42, 1892.

613. — , I), d. Einrichtung v. met. Stat. z. Erforsch. d. Einwirkung d.
Waldes auf d. Klima. * Z. f. F. u. Jagdw. 32, 297-304, 1900.

6133. Paffen, K. H., Waldverwiistung und Wasserhaushalt. * Erdkunde
/, 209-212, 1947.

614. Rubner, K., D. pflanzengeogr.-okolog. Grundlagen d. Waldbaus.
3. Aufl. Neudamm, Neumann, 1934.

6143. Sachsse, H. F., Walderhaltung und Aufforstungen in den Erzge-
birgshochlagen. * Thar. Forstl. Jahrb. pj, 374-395, 1942.

615. Schenck, C. A., Fremdland. Wald- u. Parkbaume. Bd. i: Klima-
sektionen u. Urwaldbilder. Berlin, P. Parey, 1939.

616. Schreiber, P., D. Einwirkung d. Waldes auf Klima u. Wittemng.
* Thar. Forstl. Jahrb. 49, 85-204, 1899.

617. Schubert, J., D. jahrl. Temp.extreme im Felde u. im Walde. * Z.
f. F. u. Jagdw 25, 28-36, 1893

618 — , t). d. Ermittl. d. Temp. u. Feucht.Unterschiede zw. Wald u.

Feld. * Z. f. F. u. Jagdw. 25, 441-456, 1893.
619. — , Temp. u. Feucht. d. Luft auf d. Felde u. im Kiefernwalde. *

Z. f. F. u. Jagdw. 27, 509-525, 1895,

LITERATURE 451

620. — , D. jahrl. Gang d. Luft- u. Bodentemp. im Freien u. in Wal-
dungen. Berlin, }. Springer, 1900.

621. — , Vergl. Temp. u. Feucht.Bestimmungen. * Abh. Pr. Met. i. /,
Nr. 7, 1901.

622. — , D. Niederschlag in d. Annaburger Heide 1901 bis 1905. *
Z. f. F. u. Jagdw. 40, 622-633, I9°^-

623. — , Temp. u. Feucht. zu Eberswalde im Freien u. in e. Buchen-
schonung. * Z. f. F. u. Jagdw. 45, 764-775, 1913.

624. — , D. Hohe d. Schneedecke im Walde u. im Freien. * Z. f. F. u.

Jagdw. 46, 567-572> i9T4-

625. — , D. Einwirkung d. griinen Buchenwaldes auf d. Temp. u.
Feucht. d. Luft. # Wetter, Sonderheft f. Assmanns 70. Geb. 1915.

626. — , Stud, iiber See- u. Waldklima. * Z. f. Balneologie, Klimat. u.
Kurorthyg. /o, 6-n, 99-105, 112-115, 1917/18.

627. — , Niederschl., Verdunst., Bodenfeucht., Schneedecke in Wald-
bestanden u. i. Freien. * Met. Z. 34, 145-153, 1917.

628. — , tj. d. Windstarke in d. unteren Luftsch. u. d. Windschutz d.
Waldes. * Silva 1922, S. 377-381.

629. — , U. d. Einfluss d. Waldes auf d. Niederschlage im Geb. d.
Letzlinger Heide. * Z. f. F. u. Jagdw. 69, 604-615, 1937.

630. Seltzer, P., Influence d'une foret sur la temp, de 1'air. * C. R.
Paris 799, II, 435-438, 1934.

631. Walter, A., t). d. Einfluss d. Waldes auf d. Regenfall in Mauritius.

* Met. Z. 26, 87-88, 1909.

632. — , Forest and climate. * Comm. Met. Agricole, Tag-Ber. Salzburg
1937, S. 63-64, Leyden 1938.

633. Weber, R., D. Bedeutung d. Waldes u. d. Aufgaben d. Forst-
wirtschaft (neu bearb. v. H. Weber). * Loreys Handb. d. Forst-
wiss. 4. Aufl., Band I, S. 43-187. Tubingen, H. Laupp, 1926.

634. Wlissidis, T., U. d. Einwirk. d. Waldes auf d. Klima. * C. f. d.
ges. Forstwesen 44, 94-99, 1918.

635. Woeikof, A., D. Klimate d. Erde (Kap. 12 u. 13). Jena 1887.

636. WoeLfie, M., Waldbau u. Forstmeteorologie. 2 Aufl., Landw.
Verlag Munchen 1950.

CHAPTER 30. RADIATION RELATIONSHIPS IN AN OLD STAND.

637. Angstrom, A. & Wallen, C. Chr., On the illumination in stands of
diff. character and density. * Comm. Met. Agricole, Tag.-Ber.
Salzburg 1937, S. 81-82, 1938.

638. Bartels, J., D. Strahlung u. ihre Bedeut. f. d. Klima. * Z. f. F.
u. Jagdw. 62, 537-563, 1930.

639. Brocks, K., D. rauml. Verteilung d. Beleucht.starke im Walde.

* Z. f. F. u. Jagdw. 77, 47-53, 1939.

640. Deinhofer, J. & Lauscher, F., Dammerungshelligkeit (Beob. u.
Uberlegungen). * Met. Z. 56, 153-159, 1939.

452 THE CLIMATE NEAR THE GROUND

6403. Eidmann, H., Meine Forschungsreise nach Spanisch-Guinea. *
D. Biologe jo, 1-13, 1941.

641. Knuchel, H., Spektrophotometr. Unters. im Walde. * Mitt.
Schweiz. Centr. Anst. f. d. forstl. Vers.w. //, 1-91, 1914.

642. Lauscher, F. & Schwabl, W., Unters. ii. d. Helligk. im Wald u. am
Waldrand. * Biokl. B. i, 60-65, 1934.

643. Mitscherlich, G., D. Forstamt Dietzhausen. * Z. f. F. u. Jagdw. 72,
149-188, 1940.

6433. Nageli, W., Lichtmessungen im Freiland und im geschlossenen
Altholzbestand. * Mitt. Schweiz. Centr. Anst. f. d. forstl. Ver-
suchswesen 21, 250-306, 1940.

644. Sauberer, F. & Trapp, E., Helligk.mess. in e. Flaumeichen-
buschwald. * Biokl. B. 4, 28-32, 1937.

645. Schmidt, Wilh., D. Lichtgenuss unter e. Obstbaum; Mess, nach
neuer Methode. * Fortschr. d. Landwirtsch. 5, 29-33, 1933.

646. Trapp, E., Unters. ii. d. Verteilung d. Heiligk. in e. Buchenbestand.
* Biokl. B. 5, 153-158, 1938.

647. Wiesner, J., D. Lichtgenuss d. Pflanzen. Leipzig, W. Engelmann,
1907.

648. Zederbauer, E., D. Lichtbediirfnis d. Waldbaume u. d. Lichtmess-
methoden. * C. f. d. ges. Forstwesen jj, 325-330, 1907.

CHAPTERS 31 AND 32. TEMPERATURE AND HUMIDITY RELATIONSHIPS IN AN
OLD STAND. WIND AND PRECIPITATION IN AN OLD STAND.

649. Geiger, R., Unters. ii. d. Bestandsklima (6 Teile). * Forstw. C.
47, 629-644, 848-854, 1925; 48, 337~349> 495~5°5> 523~532> 749~
758, 1926.

6493. — , Die Witter ungsbedingungen fur Waldgrossbrande. * Mitt.
Reichsinst. f. Forst. u. Holzwirtsch. Hamburg-Reinbek, Nr. 5,
1948.

650. Geiger, R. & Amann, H., Forstmet. Mess, in e. Eichenbestand
(5 Teile). * Forstw. C. 5^, 237-250, 341-351, 705-714, 809-819,

651. Hoppe, E., Regenmessungen unter Baumkronen. * Mitt. a. d.
forstl. Vers.wesen Osterr. 2/, Wien 1896.

652. von Obolensky, N., Effect of arborous vegetation on the temp, of
the soil and the temp, and humidity of the air. * J. of Geophys.
and Met. _?, 113—139, Moskau 1926.

6523, Priehausser, G., Bodenfrost, Bodenentwicklung u. Flachwurzelig-
keit d. Fichte. * Forstw. C. 6/, 329-342, 381-389, 1939.
. Sauberer, F., & Trapp, E., Temperatur und Feuchtemessungen in
Bergwaldern. * Centralbl. f. d. ges. Forstwesen 67, 233-244, 257-
276, 1941.

653. Seltzer, P., Sur la repartition verticale de la temp, en foret. * Siehe
Ref. in Biokl. B. 2, 55, 1935.

LITERATURE 453

654. Ungeheuer, H., Mikroklima in e. Buchenhochwald am Hang. *
Biokl. B. /, 75-88, 1934.

CHAPTERS 33, 34 AND 35. THE INFLUENCE OF THE MAKE-UP OF THE STAND

ON ITS CLIMATE.

THE MICROCLIMATE OF CIRCULAR SLASHINGS, CLEARINGS AND CUTTINGS.
THE CLIMATE OF THE STAND BORDER.

655. Angstrom, A., Jordtemp. i. bestand av olika tathet. * Medd. Stat.
Met. Hydr. Anst. Stockholm 29, 187-218, 1936.

656. Becker, R., Unters. z. Struktur d. Luftstromung nach synopt.
Methoden. * Deutsche Forschung 14, 35—40, 1930.

657. Boos, Unters. ii. d. Bestandsinnenklima im Pr. Forstamt Erd-
mannshausen. * Mit. a. Forstwirtsch. u. F.wiss. /o, 254-259, 1939.

658. Burger, H., Waldklimafragen III: Met. Beob. im Freien, in e.
gleichaltrigen Fichtenbestand u. im Tannen-Fichten-Plenterwald
b. Oppligen. * Mitt. Schweiz. Centr.Anst. f. d. forstl. Vers.w. /#,
153-192, 1933.

659. Cour, P. la, Skovenes Indflydelse paa Varmen. * Ref.: Z. d. osterr.
Ges. f. Met. 7, 254-256, 1872.

660. Danckelmann, B., Spatfrostbeschad. im mark. Walde. * Z. f. F. u.
Jagdw. 30, 389-411, 1898.

66 1. Descombes, P., Les forets, les pluies et les condensations occultes. *
Annuaire Soc. Met. de France 66, 38-46, 1922/23.

662. Dieckmann, A., Vers. z. Niederschl.mess. aus treib. Nebel. * Met.
Z. 48, 400-402, 1931.

6623. Dienert, F., Contrib. a 1'etude des condensations occultes. * C. R.
Paris /p5, 1261-1263, 1934.

663. Dorffel, K., D. physik. Arbeitsweise d. Gallenkampschen Ver-
dunst.mess. u. s. Anwendung auf mikrokl. Fragen. * Veroff.
Geophys. I. Leipzig 6, Heft 9, 1935.

6633. Eggler, J., Kleinklimatische Untersuchungen in den Flaumeichen-
bestanden bei Graz. * Biokl. B. 9, 94-110, 1942.

664. Geiger, R., D. Windbewegung auf Waldschneisen. * Biokl. B. /,

*34-*37> 1934-

665. — , D. Beschattung am Bestandsrand. * Forstw. C. 57, 789—794,

1935.

666. — , Weitere Bern. z. Klima am Bestandsrand. * Forstw. C. 5$,
262-266, 1936.

667. — , D. Standortsklima in Altholznahe. * Mitt. d. Herm. Goring-
Akad. d. D. Forstwiss. i, 148-169, 1941.

668. Hesselman, H., Einige Beob. ii. d. Beziehung zw. d. Samenverbreit.
v. Fichte u. Kiefer u. d. Besamung d. Kahlhiebe. * Meddel. Fran.
Stat. Skogforsoksanst. 27, 145-182, Stockholm 1934.

669. — , Weitere Studien usw. * Ibid, ji, 1-64, 1938.

670. Koch, H. G., Temp.verhaltn. u. Windsystem e. geschloss. Wald-
gebiets. * Veroff. Geophys. I. Leipzig 6, Heft 3, 1934.

454 THE CLIMATE NEAR THE GROUND

6703. — , D. Wald-Feldwind, e. mikro-aerolog. Studie. * Beitr. Phys.
d. fr. Atm. 22, 71-75, 1934.

671. — , D. mikroklim. Temp.feld bei Bewolkimg u. Wind. * Biokl. B.
2, 121-124, J935-

6713. — , Der Waldwind. * Eine forstmeteorologische Eigenart von

Waldgebieten. * Forstwiss. C. 64, 97-111,1942.
67ib. — , Bestandstemperaturen eines bewaldeten Seitentales bei Jena.

* Mitt. d. Thiiring. Landeswetterwarte Heft 7, 69-98, Weimar
1948.

6710. Lampadius, G., Nebelfrostablagerungen sowie Tau- und Nebel-
niederschlag. * Thar. Forstl. Jahrb. -92, 545-584, 1941.

672. Lehmann, H., Mikroklimat. Unters. d. Abkuhlungsgrosse in e.
Waldgebiet. * Veroff. Geophys. I. Leipzig 7, Heft 4, 1936.

673. Linke, F., Niederschlagsmess. unter Baumen. * Met. Z. •?•?, 140-
141, 1916.

674. — , — . * Met. Z. 38, 277, 1921.

675. Marloth, I), d. Wassermengen, welche Straucher u. Baume aus
treib. Nebel u. Wolken auffangen. * Met. Z. 23, 547-553, 1906.

676. Pfeiffer, H., Kleinaerolog. Unters. am Collmberg. * Veroff. Geoph.
I. Leipzig //, Heft 5, 1938.

677. Rubner, K., Bestandsklima u. Verjungungsverfahren. * Sudetend.
F. u. Jagdz. 30, 353-360, 1930.

678. Rubner, K., D. Nebelniederschlag im Wald u. s. Messung. * Thar.
Forstl. Jahrb. 83, 121-149, *932-

679. — , — II. * Ibid. 86, 330-342, 1935.

680. Schimitschek, E., Forstschutzl. u. forstentomolog. Stud, aus d.
Demonstrationsrevier Pressbaum d. Hochsch. f. Bodenkultur. *
Wien. Allg. F. u. Jagdz. 1932, Nr. 47 & 48.

681. Schmauss, A., Seewinde ohne See. * Met. Z. 37, 154-155, 1920.
68ia. Sigmond, H., Einige Bern. u. d. Sonderklima d. Lochhiebs. *

Sudetend. F. u. Jagdz. 29, 227-228, 1929.

682. Wagner, C., D. Grundlagen d. rauml. Ordnung im Walde. 4. Aufl.

* Berlin, P. Parey, 1923.

683. Woefle, M., Windverhaltnisse im Walde. * Forstw. C. 6/, 65-75,
1939, * 6/, 461-475, 1939, and 6^, 169-182, 1942.

6833. — , Bemerkung zu "Der Waldwind" von H. G. Koch. * Forstwiss.
C. u. Thar. Forstl. Jahrb. (Kriegsgemeinschaftsausgabe) 1944, 131-

136.

684. v. Wrede, C., D. Bestandsklimat. u. ihr Einfluss uf d. Biologic d.
Verjiingung unter Schirrn u. in d. Gruppe. * Forstw. C. 47, 441—
451, 491-505, 570-582, 1925.

CHAPTER 36. THE ANIMATE WORLD AND THE MICROCLIMATE.

685. Blake, J. H., Further stud, on deciduous forest animals commu-
nities. * Ecology 12, 508-527, 1931.

LITERATURE 455

686. Bodenheimer, F. S., Stud. z. Epidemiologie, Okol. u. Phys. d.
afrik. Wanderheuschrecke. * Z. f. angew. Entomol. 75, 435-557,
1929.

687. Buxton, P. A., Insects of Samoa. Part. IX, Fasc. i, * Brit. Mus.
London 1930.

688. Eidmann, H., D. Flugzeugbestaubung d. Forstschadl. u. ihre
Organis. im Lichte neuzeitl. Erfahr. u. Forsch. * Z. f. F. u. Jagdw.
65, 24-48, 65-82, 1933.

689. Escherich, K., D. Forstinsekten Mitteleuropas. Bd. 3. * Berlin,
P. Parey, 1931.

690. Frankel, G., Unters. ii. d. Lebensgewohnh., Sinnesphys. u. Sozial-
psych. d. wandernden Larven d. afrik. Wanderheuschrecke. *
Biolog. Zentralbl. 49, 657-680, 1929.

691. Franz, H., t). d. Bedeutung d. Mikrokl. f. d. Faunenzus.setzung
auf kleinem Raum. * Z. Morphol. u. Okol. d. Tiere 22, 587-628,

69 1 a. Gb'sswald, K., Rassenstudien an der grossen Waldameise auf sys-
tematischer okologischer, physiologischer und biologischer Grund-
lage. * Z. f. angew. Entomolog. 28, 62—124, I94I<

692. Grimm, H., Kleintierwelt, Kleinklima u. Mikroklima. * Z. f.
angew. Met. 54, 25-31, 1937.

694. Hesse, R., Tiergeographie auf okolog. Grundlage. * G. Fischer,
Jena, 1924.

695. Himmer, A., E. Beitr. z. Kenntn. d. Warmehaushalts im Nestbau
sozialer Hautfliigler. * Z. f. vergl. Physiolog. 5, Nr. 2, 1927.

696. Klemm, M., Neue Wege in d. biolog. Biotopforschung. * Naturf.
6, 254-260, 1929.

697. Kuhnelt, W., D. Bedeutung d. Klimas fur d. Tierwelt. * Biokl. B.
/, 120-125, 1934.

698. — , D. Einfluss d. Klimas auf d. Wasserhaushalt d. Tiere. * Biokl.
6.5,11-15,1936.

699. Lauscher, F., Miickentanz u. Windschutz. * Biokl. B. 6, 186, 1939.

700. Lohrl, H., D. Winterschlaf v. Nyctalus noctula Schreb. auf Grund
v. Beob. am Winterschlafplatz. * Z. Morphol. u. Okol. d. Tiere
52, 47-66, 1936.

701. Martini, E. & Hundertmark, A., 0. d. Bedeutung kleinklimat.
Feststell. in Haus u. Stall u. im Freien fur d. Schadlingsbiologie.
* Anz. f. Schadlingskunde 26, 97—101, 1940.

702. Martini, E. & Teubner, E., I), d. Verhalten v. Stechmiicken bei
verschied. Temp. u. Luftfeucht. * Beih. z. Archiv f. Schiffs- u.
Tropenhyg. 57, Beiheft i, 1933.

703. Mosauer, W., The toleration of solar heat in desert reptiles. * Eco-
logy 17, 56-66, 1936.

704. Nielsen, E. T., Temp, in a nest of Bombus hypnorum L. * Vi-
densk. Medd. fra Dansk naturh. Foren 102, 1938.

456 THE CLIMATE NEAR THE GROUND

705. — , Z. Okologie d. Laubheuschrecken. * Saertryk af Ent. Medd.
20, 121—164, 193%-

706. Schimitschek, E., Forstentomolog. Unters. a. d. Gebiet v. Lunz I:
Standortskl. u. Kleinkl. in i. Bez. zum Entwicklungsablauf u. z.
Mortalitat v. Insekten. * Z. f. angew. Entomol. 18, 460-491, 1931.

707. — , — II: * C. f. d. ges. Forstwesen 5$, 225—267, 1932.

708. Steiner, A., Neuere Ergebn. ii. d. sozialen Warmehaushalt d. ein-
heim. Hautfliiger. * Naturw. /#, 595-600, 1930.

7083. Uvarov, B. P., Wetter u. Klima in ihr. Bez. zu d. Insekten. *
Z. f. angew. Entomol. 77, 157-177, 1930.

709. Warnecke, G., Mikroklima u. Verbreit. d. Lepidopteren. * Ento-
mol. Bern., Berlin-Dahlem /, 120-130, 1934.

710. Wellenstein, G., Beitr. z. Biologic d. roten Waldameise m. bes.
Berikks. klimat. u. forstl. Verhaltn. * Z. f. angew. Entomol. 14,
1-68, 1929.

711. Wiele, H., Fur Hagenbeck im Himalaja. * Deutsche Buchwerk-
statten Dresden 1925.

712. Zwolfer, W., Beitr. z. Kenntn. d. Schadlingsfauna Kleinasiens I.

* Z. f. angew. Entomol. 77, 227-252, 1930.

713. — , Stud. z. Okologie u. Epidemiologie d. Insekten. * Z. f. angew.
Entomol. 77, 475-562, 1931.

CHAPTERS 37 AND 39. THE UNINTENTIONAL EFFECT OF MAN ON THE

MICROCLIMATE. THE CONSCIOUS MODIFICATION OF THE

MICROCLIMATE BY MAN.

7133. Amelung, W. & Landsberg, H., Kernzahlungen in Freiluft u.

Zimmerluft. * Biokl. B. 7, 49-53, 1934.
7i3b. Amelung, W., Kunstliches Klima. * Lehrbuch der Bader und

Klimaheilkunde, H. Vogt. J. Springer, Berlin 1940.

714. Amende, H., Exposition, photochem. Ortshelligk., Heliotherapie u.
Platzwahl v. Krankenanst. * Strahlentherapie 6^, 115—128, 1938.

715. Bates, C. G., Windbreaks, their influence and value. * Forest Serv.
Bull. Nr. 86, U. S. A. (1912?).

7153. Bauer, E., D. Wohlfahrtsaufforst. im Flugsandgeb. d. Marchfelds.

* Osterr. Viertelj-schr. f. Forstwesen 54, 103-126, 175-199, 1936.

716. Berke, T. & Castens, G., Z. Kenntn. d. Temp. u. Feucht. d.
Schiffsluft. * Ann. d. Hydr. 57, 169-185, 1929.

717. Bradtke, F. & Liese, W., Hilfsbuch £. raum- u. aussenklimat. Mess.

* J. Springer, Berlin, 1937.

718. Brezina, E. & Schmidt, Wilh., D. kiinstliche Klima. * Stuttgart,
F. Enke, 1937.

719. Buttner, K., Physik. Bioklimatologie. Probleme u. Methoden. *
Akad. Verl. Ges., Leipzig, 1938.

720. — , D. Bedeutung d. Mikrokl. f. d. Klimadosierung. * Strahlen-
therapie 67, 705-710, 1938.

LITERATURE 457

721. Conrad, V. & Hausmann, W., Gesichtspunkte d. medizin. Klimatol.
m. bes. Beriicks. d. med.-klimat. Akt. d. osterr. San. Verwaltung.
M. Perles, Wien, 1930.

72 1 a. Dorno, C., Zur Entwicklungsgeschichte der Bioklimatologie *
Biokl. B. 9, 4-11, 1942.

722. EglofT, K., U. d. Klima im Zimmer u. s. Beziehungen z! Aussen-
klima. * Diss. Zurich Nr. 766. (1934).

7223. Ehrenberg, P., Landwirtschaftlich beachtliche Windwirkungen
und Windschutz in der Landwirtschaft. * Der Kulturtechniker
46, 19-41, 1943.

723. Pels, E., D. Mensch als Gestalter d. Erde. * Leipzig, Bibliogr. Inst.,

724. Flach, E., D. Bedeutung d. lokalklimat. Forsch. f. d. Meteoropath.
d. Erkalt.krankh. * Biokl. B. 5, 22-26, 1938.

725. — , D. klimat. Verhaltn. in Deutschl. u. d. Abhaltung v. Sommer-
lagern. * Miinchn. Mediz. Wochenschr. 1938, S. 947.

726. Flensborg, C. E., D. danische Heidegesellschaft. * Viborg 1939.
7263. Flohn, H., Die Tatigkeit des Menschen als Klimafaktor (Sam-

melreferat). * Z. f. Erdkunde 9, 13-22, 1941.

727. Franssila, M., t). d. Windschutzproblem. * Maatal. Aiakakaus-
kirja, Helsinki //, 168-182, 1939.

728. Geiger, R., Wald und Windschutz. * Forstw. C. 55, 760-762, 1931.

729. — , Mikroklimatologie. * Naturw. 21, 132-137, 1933.

730. — , Forstmeteorolog. Beding. d. Ertragssteigerung. * Raumforsch.
u. Raumordnung 2, 591-592, 1938.

731. Gotz, F. W. P., Strahl.klima v. Arosa.. Berlin 1926.

732. Gregor, A., E. Beitrag z. Klassifikation d. Mittelgebirgsklimas f.
Heilzwecke. * Biokl. B. 6, 187-191, 1939.

733* — j ^X met- Voraussetzungen f. d. Charakter Luitkurort (klima-
tischer Kurort). ^ Biokl. B. 7, 125-127, 1940.

734. Grunow, J., Wetter- u. Klimabeeinflussung. * Geogr. Wochenschr.
1933, Heft. 9.

735. Harries, H. D., Neue schiffsraum-met. Mess. * Ann. d. Hydr. 6/,
13-18, 1933.

736. Hausmann, W., Grundl. u. Organisation d. lichtklimat. Forsch. in
i. Beziehung z. offentl. Gesundh.pflege. * Mitt. d. Volksgesundh.
Amts Wien 1932, Heft 10.

737. Helipach, W., Geopsyche. D. Menschenseele unterm Einfluss v.
Wetter u. Klima, Boden u. Landschaft. 5. Aufl. * Engelmann,
Leipzig, 1939.

738. Hottinger, M., D. Raumklima u. s. Regelung. * Gesundh.Ing. 62,
605-609, 617-622, 1939,

739. Hummel, K., Zum Mikrokl. isolierter Standorte. * D. Met. Jahrb.
f. Bayern 1929, Anhang B.

458 THE CLIMATE NEAR THE GROUND

740. Knoch, K., D. Kurortklimadienst d. deutsch. Reichswetterdienstes.
* Biokl. B. 5, 1-3, 1938.

741. — , li. d. klimat. Anforderungen an e. Kurort. * Biokl. B. 5, 103-
106, 1938.

742. Kolacek, F., D. Einfluss menschl. Tatigk. auf d. klimat. Verhaltn.
in Mahren usw. * Met. Z. 52, 114, 1935.

7423. Kreutz, W., Das Windschutzproblem. * Biokl. B. 5, 10-16, 1938.
742b. — , Agrarmeteorologische Forschungen auf der Hohen Rhon. *

Ber. d. Oberhess. Ges. f. Natur-u. Heilkunde zu Giessen 20,

26-104, I94°-

743. Kroh, A., A micro-climate recorder. * Ecology 2/, 275-278, 1940.

744. Kiister, E. & Meixner, H., Berechn. u. Tab. zur Frage d. Raum-
klimas. * Arch. f. Hyg. 7/7, 158-178, 1936.

7443. Landsberg, H., Werkraumluft u. Gewerbekrankh. (Sammel-
referat). * Biokl. B. 2, 35-37, 1935.

745. — , E. Bett.Temp.-Studie. * Biokl. B. 5, 66-68, 1938.

746. Lehmann, P., Landbautechn. Massnahmen in agrarrnet. Betrach-
tung. * Fortschr. d. Landwirtsch. 5, 797-806, 1930.

747. — , D. Sonderklima d. Stalles. * Ibid. 6, 642-647, 1931.

7473. — , Inwieweit berikks. d. Landwirt d. Klimafaktor? * Biokl. B.
7, 77-85, 1940.

748. Leontiewski, N. P., D. Rolle d. Waldschutzanlagen in bezug auf
Steigerung d. Ernte. * J. of Geophysics Leningrad 4, 139, 1934.

749. Linden, A. J. ter-, D. Winterklima in Gebauden. * Gesundheits-
Ingenieur 6/, 480-483, 1938; D. Sommerklima i. G. Ibid. 6/,
522-526, 1938.

750. Linke, F., Klimat. Anforder. an e. Kurort i. Flachland oder Mittel-
geb. Mittdeuropas. * Biokl. B. 5, 7-9, 1938,

751. — , Z. Physik d. kimstl. Klimas. * Der Balneologe 6, 241—249,

1939-

752. Made, A., t). d. Temp.gang in Gewachshausern, Dunkelkasten u.
Mistbeetanlagen. * Gartenbauwiss. 14, 626-641, 1940.

753. Made, A. & Rudorf, W., Zweck u. Aufbau modern, bewett. Ge-
wachshauser u. ihr Temp.gang i. Vergl. m. d. Freiland. * Biokl.
B. 5, 145-153, 1938.

754. Mayer, A., 0. d. Vermeidung v. Schwitzwasser in Kasekellern. *
Milchwirtschaftl. Forsch. //, 201—210, 1930.

755. Mehner, A. & Linz, A., Unters. ii. d. Verlauf d. Stalltemp. *
Forschungsdienst 8, 525-543, 1939.

7553. Meyer, F. G., Zimmerklimatische Studien. * Warme-und Kalte-

technik ^j, 41-46 und 173-177, 1941.
755b. — , Zimmerklimatische Studien. * Strahlungstherapie 72, 347-

348, 1943-

756. Michler, H., Z. Kenntn. d. Luftbeweg. in Schiflsraumen. * Ann.
d. Hydr. 62, 457-461, 1934.

LITERATURE 459

757. Morikofer, W., Klimatolog. Gesichtspunkte f. d. Erricht. v. Kran-
kenhausern, Sanatorien etc. * Manuscript Davos 1934.

757a. Nageli, W., Untersuchung iiber die Windverhaltnisse im Bereich

von Windschutzstreifen. * Mitt. d. Schweiz. Anst. f. d. forst.

Versuchswesen 2j, 223-276, 1943.
757b. Nageli, W., Weitere Untersuchungen iiber die Windverhaltnisse

im Bereich von Windschutzstreifen. * Mitt. Schweiz. Anst. f. d.

forstl. Versuchswesen 24, 659 ff. 1946.

758. Robitzsch, M., Klima u. Organismus. * R. f. W. Wiss. Abh. /,
Nr. i, 1935. Wide bibliog. included.

759. Roose, H., E. neue Methode z. Best. d. Wandtemp. im Raum-
klima. * Schweiz. Bl. f. Heiz. u. Liiftung 1938.

760. Ruge, H., D. Verhalten d. Lufttemp. u. Luftfeucht. auf e. modernen
Kreuzer in d. Tropen. * Veroff. a. d. Geb. d. Marine-Sanit.-Wesens
Heft 22. Berlin, Mittler & Sohn, 1932.

761. Scaetta, H., Bioclimats; climats des associations et microclimats de
haute montagne en Afrique Centrale Equat. * J. d'agronomie col.
Briissel, June 1933.

7613. Schnelle, F., D. Einsatz d. Met. bei d. Odlandkultivierung. *

Z. f. angew. Met. 54, 221-224, X937'
76ib. Schoenichen, W., Lebende Windschutzanlagen. * Pet. Geogr.

Mitt. 90, 273-278, 1944.

762. Schwarz, H., D. Entstehung d. Flugerde in d. Gauen Niederdonau
u. Wien. * Manuscript Wien 1940.

7623. Seem^nn, J., t)ber die Temperaturverhaltnisse in einem bewetterten
Tiefkuhlgewachshaus. * D. Gartenbauwiss. 77, 186-192, 1942.

762b. — , Die Temperaturverhaltnisse in Gewachshausern mit warme-
absorbierenden Glas. * Biokl. B. /o, 73-76, 1943.

763. Talman, C. F., tJberwachtes Innenklima. * Z. f. angew. Met. 48 ',
346-347, 1931.

7633. Tichy, H., Grundlagen einer Klimabeschreibung von Kurorten. *
Z. f. Met. /, 84-87, 1946.

764. Wagner, F., 0. Temp, an u. im Schiff. * Ann. d. Hydr. 63, 38-40,

1935-
7643. Wegener, K., D.Temp. im Glas-(Treib-)haus. * Biokl. B. 7, 109—

112, 1940.
764^ — , Haus und Klima. * Z. f. angew. Met. 59, 1-6, 1942.

765. Weickmann, L., E. Taschenthermohygrograph als bioklim. Forsch.
mittel. * Sitz-B. d. Sachs. Akad. 90, 47-54, Leipzig 1938.

766. — , Klima u. Wetter im Lebensraum d. Menschen. * Naturw. 27,
22-28, 1939.

767. WoelHe, M., Wald u. Windschutz (3 Teile). * Forstw. C. 57,
349-362, 1935; 58, 325-338, 429-448, 1936.

768. — , Hecken als Windschutzanlagen. * Forstw. C. 60, 15-28, 1938.

769. — , Windschutzanlagen. * Forstw. C. 60, 52-63, 73-86, 1938.

460 THE CLIMATE NEAR THE GROUND

770. Woltereck, H., Klima . Wetter . Mensch. * Leipzig, Quelle &
Meyer, 1938.

771. Wysotzky, Shelterbelts in the steppes of Russia. * J. of Forestry
33, No. 9, 1935.

CHAPTER 38. THE CITY CLIMATE.

(A complete bibliography up to year 1936 will be found in the book by

A. Kratzer (781). Here only later works or those not

mentioned there are listed.)

772. Arakawa, H., Increasing daily min. temp, in large, developing
cities. * Gerl. B. 54, 177-178, 1939.

7723. Arnberger, F., Einige Temperatur- und Feuchtigkeitsmesungen

in Tripolitanien. * Mitt. Geogr. Ges. Wien 83, 249-257, 1940.
772b. Berg, H., Die Bewolkungsverhaltnisse iiber der Grosstadt Koln

und ihrer Umgebung (eine vergleichende Untersuchung) * Z. f.

angew. Met. 60, 108—117, 1943.
772C. — , Der Einfluss einer Grosstadt auf Bewolkung, Niederschlag

und Wind. * Biokl. B. /o, 65-70, 1943.

773. Bider, M., Temp.untersch. zw. Stadt. u. Freilandstationen. * Helv.
Phys. Acta /j, 5-7, 1940.

774. Brazier, C. E. & Perdereau, L., Exemple d'une alteration du climat
resultant de 1'activite humaine. * La. Met. //, 313—324, 1935.

775. Dorffel, K., D. Stadtklima v. Marburg a. d. L. * Z. f . angew. Met.
55, 173-180, 1936.

776. Herrig, H., D. Staubverteilung in Marburg a. d. L. * Biokl. B. 5,

49~57>vI938-

777. Hrudicka, B., Zu d. opt. u. akust. Eigenschaften d. Klimas e.

Grosstadt. * Gerl. B. 53, 337-344, 1938.

778. Kahler, K. & Brandtner, G., Mess. d. Staubgehalts d. Luft in Bad
Tolz. * Biokl. B. 5, 58-62, 1938.

779. Kassner, C., Schneefall in u. ausserh. d. Grosstadt. * Z. f. angew.

M^. 56> 337-339. *939-

780. Keil, K., Windrichtung u. Sicht. * Beitr. Phys. d. fr. Atm. 75, 87-
89, 1929.

781. Kratzer, A., Das Stadtklima * (Die Wissenschaft, Bd. 90). Braun-
schweig, Friedr. Vieweg & Sohn, 1937.

7813. Lammert, W., Beispiel einer extremen Strahlungskalte. * Z. f.
Met. /, 145—146, 1947.

782. Lauscher, F. & Steinhauser, F., Weitere Unters. in Wien u. Umgeb.
* Sitz-B. Wien Akad. 143, 175-196, 1934.

783. Lessmann, H. & Zedler, P., E. Beitr. z. Berliner Stadtklima. *
Biokl. B. 3, 163-165, 1936.

784. Lobner, A., Horiz. u. vertik. Staubverteilung in e. Grosstadt. *
Veroff. Geoph. I. Leipzig 7, Heft 2, 1935.

785. Lossnitzer, H. & Freudenberg, H., Temp. messfahr ten im Gebiete
d. Stadt Freiburg i. B. * Biokl. B. 7, 30—39, 1940.

LITERATURE 461

786. Metzler, H. K., D. Gang d. rel. Luftfeucht. zwischen Freiland u.
Aussenstadt in Hannover. * Biokl. B. 2, 120-121, 1935.

787. Meyer, E. G., Sonnen- u. Himmelsstrahl. in d. Grosstadt u. im
deutsch. Mittelgebirge. * Strahlentherapie 49, 161-165, 1934.

788. Mrose, H., D. Miihltalwind als Frischluftspender f. d. Jenaer West-
viertel. * Z. f. angew. Met. 56, 377—383, 1939.

7883. Otfe, H., Stadtehygiene und Mikroklima. * Gesundh.-Ing. 65,
248-251, 1943.

789. Root, C. J., Airport and city temp, at Detroit, Mich. * M. W. Rev.

67> 99> J939-

790. Schmidt, K., Windverhaltn. in Freiburg an heit. Sommertagen.

* D. Met. Jahrb. f. Baden 1932.

791. Spangenberg, W. W., D. Niederschl.verhaltn. in d. Stadt Schwerin
i. M. * Z. f. angew. Met. 56, 205-209, 1939.

792. Voigts, H., Ergebn. v. Kernzahlungen in Lubeck-Travemunde.

* Biokl. B. 3, 170-174, 1936.

7923. Weinlander, A., Grosstadt und Klima. * Z. f. angew. Met. 59,
39°-393»

CHAPTER 40. DESTRUCTIVE FROST AS A MICROCLIMATIC PHENOMENON.

793. Amann, H., Birkenvorwald als Schutz gegen Spatfroste. * Forstw.
C. 52, 493-502, 581-592, 1930.

794. Batchelor, L. D. & West, F. L., Variation in minimum temp, due
to the topography of a mountain valley in its relation to fruit
growing. * Utah Agric. Coll. Expl St. Bull. 141, Utah 1915.

795. Firbas, F., Unters. u. d. Wasserhaushalt d. Hochmoorpflanzen. *
Jahrb. f. wiss. Bot. 74, 455-696, 1931.

796. Geiger, R., Spatfroste auf d. Frostflachen bei Miinchen. * Forstw.
C. 48, 279-293, 1926.

797. Humphreys, W. J., Frost protection. * M. W. Rev. 42, 562-569,
1914.

798. Lautensach-Loffler, E., D. Sonderklima d. Pfalzer Gebriichs. * Mitt.

d. Pollichia 8, 90-124, 1940.

799. Munch, E. & Liske, F., D. Frostgefahrdung d. Fichte in Sachsen. *
Thar. Forstl. Jahrb. 77, 97-115, 129-148, 161-176, 197-221, 1926.

800. Sauberer, F., t). d. Entstehung d. Grasfrostes. * Biokl. B- </, 174-

. .

80 1. Schmauss, A., Gerichtete Frostschaden. ^ Biokl. B. 6, 187, 1939.

802. Schubert, J., Kalte Juninachte in Norddeutschl. u. d. Frostschutz
im Walde. * Forstarchiv 8, 225—228, 1932.

803. — , Kalteriickfalle u. Nachtfroste. Wind-, Wasserdampf- u. Wald-
einfluss. * Met. Z. 57, 406-410, 1940,

804. Staudacher, D. Frostschaden im Forstbetriebe, deren Ursachen u.
Bekampfung. * Forstw. C. 46, 1-13, 54-66, 98-111, 1924.

805. Tacke, B., Frosterschein. auf Moorboden. * Biokl. B. 2, 86-88,
1935-

462 THE CLIMATE NEAR THE GROUND

806. Ziobrowski, S., U. d. Einfluss d. harten Winters 1928/29 auf d.
Holzgewachse im Rabaflusstale. * Act. Soc. Bot. Poloniae 10,
49-1 1 1, 1933.

CHAPTER 41. THE BATTLE AGAINST DESTRUCTIVE FROST.

(A rich bibliography up to 1939 will be found in the book by O. W.

Kessler and W. Kaempfert (#73). Here only later publications

or those not there listed are included.)

807. Amann, H., Unters. ii. d. thermische Wirkung v. Schutzgittern in
Pflanzgarten. * Forstw. C. 57, 249-251, 1929.

808. Bender, K., D. Friihjahrsfroste an d. Unterelbe u. ihre Bekamp-
fung. * Z. f. angew. Met. 56, 273-289, 1939.

809. Brooks, F. A., Engineering factors involved in orchard heating.

* Mechanical Engineering 1938, S. 677-681.

810. Foss, H., Nattefrost. * Landbrukdirekt. Arsberetning 1928, Oslo
1929.

811. Huber, H., Diesjahr. Erfahrungen m. wasserdichten Papier-Frost-
schirmen. * Schweiz. Z. f. Obst- u. Weinbau 45, 320-321, 1936.

812. Kadner, T.> Nachtfroste, ihre Entstehung, Voraussage u. Abwehr.

* Z. f. angew. Met. 52, 164-167, 1935.

813. Kessler, O. W. & Kaempfert, W., D. Frostschadenverhikung. *
R. f. W. Wiss. Abh. 6, Nr. 2, 1940.

814. Papaioannou, J., D. mikroklimat. Verhaltn. unter Pflanzendecken
aus StofT. * Forstw. C. 54, 666-671, 1932.

815. — , D. Temp.verhaltn. unter Pflanzenschutzvorricht. in Forstgarten.

* Forstw. C. 56, 769-782, 1934.

8 1 6. Sauberer, F., Einige Unters. ii. Nachtfrost u. Frostschutz in Wein-
garten. * Biokl. B. 4, 19-22, 1937.

817. Schmidt, Wilh., Met. Feldversuche ii. Frostabwehrmittel. * An-
hang Jahrb. d. Zentralanst. f. Met. Wien 1927, Wien 1929.

8173. Schonnopp, G., Frostschutz durch Beregnung. * Die Technik in
der Landwirtschaft 22, 64-67, 1941.

818. Schoonover, W. R. & Brooks, F. A., The smokiness of oil-burning
orchard heaters. * Univ. of California Bull. 536, Berkeley, Aug.

819. Schoonover, W. R., Hodgson, R. W. & Young, F. D., Frost pro-
tection in California orchards. * Calif. Agric. Extens. Serv. Circ.
40, 1930.

820. Schoonover, W. R., Brooks, F. A. & Walker, H. B., Protection of
orchards against frost. * Ditto Circ. in, 1939.

821. Sherouse, R. T., Frost protection of ferns by sprinkler irrigation. *
M. W. Rev. 67, 61-62, 1939.

82ia. Weger, N., Die Frostschadenverhiitung in der Landwirtschaft. *
Met. Rundschau /, 29-38, 1947.

SOURCES OF ILLUSTRATIONS

FIG. 22. Beitr. z. Physik d. freien Atmosphare 21 (1933), Fig. i on

p. 130. Akad. Verl.-Ges. Leipzig.

FIG. 23. , Fig. 3 on p. 132.

FIG. 26. Gerl. Beitr. z. Geophysik 49 (1937), Fig. 6 on p. 418. Akad.

Verl.-Ges., Leipzig.

FIG. 29. 47 (1936), Fig. 5 on p. 382.

FIG. 30. , , Fig. 1 8 on p. 398.

FIG. 31. , , Fig. 1 6 on p. 397.

FIG. 52. Anh. z d. Jahrb. d. Zentralanstalt f. Meteorologie Vienna

1927 (Wien 1929), Fig. 12 on p. 25. In commission with

Gerold & Co.
FIG. 55. Jahrb. f. wissensch. Botanik 80 (1934), Fig. 5 on p. 343.

Verl. Gebr. Borntraeger, Berlin.
FIG. 63. Ann. d. Hydrographie 62 (1934), Fig. 3 and 4 from Table

46. Verl. d. Deutschen Seewarte, Hamburg.
FIG. 64. Gerl. Beitr. z. Geophysik 42 (1934), Fig. 3 on p. 374. Akad.

Verl.-Ges., Leipzig.
FIG. 73. Strahlentherapie 54 (1935), Fig. 3 on p. 169. Verl. Urban &

Schwarzenberg, Berlin.
FIG. 74. Veroff . d. Geophysikal. Inst. d. Univ. Leipzig. Band VIII,

Heft 2, Fig. 8 on p. 91. Published by the author.
FIG. 82. Forstwissensch. Centralblatt 58 (1936), Fig. i on p. 108.

Verl. Paul Parcy, Berlin.
FIG. 89. Die Naturwissenschaften 18 (1930), Fig. 2 on p. 368. Verl.

J. Springer, Berlin.
FIG, 94. Forstwissensch. Centralblatt 56 (1934), Fig. 2 on p. 359.

Verl. Paul Parey, Berlin.
FIG. 97. Gerl. Beitr. z. Geophysik 52 (1938), Fig. 4b on p. 439.

Akad. Verl.-Ges., Leipzig.
FIG. 98. Beitr. z. Physik d. freien Atmosphare 21 (1934), Fig. 16 on

p. 266. Akad. Verl.-Ges., Leipzig.
FIG. 99. Forstwissensch. Centralblatt 49 (1927), Fig. n on p. 919.

Verl. Paul Parey, Berlin.
FIG. 104. 57 (1935), Fig. i on p. 238.

464 SOURCES OF ILLUSTRATIONS

FIG. 105. Die Naturwissenschaften 21 (1933), Fig. 5 on p. 135. Verl.

J. Springer, Berlin.
FIG. no. Handb. d. Klimatologie, edited by W. Koppen and R.

Geiger, Band I, Fig. 13 on p. 26. Verl. Gebr. Borntraeger,

Berlin.
FIG. ii2. Forstwissensch. Centralblatt 55 (1933), Fig. i on p. 583.

Verl. Paul Parey, Berlin.

FIG. 115. 55 (1933), Fig. 3 on p. 744.

FIG. 116. 56 (1934), Fig. i on p. 466.

FIG. 117. Gerl. Beitr. z. Geophysik 52 (1938), Fig. 4a on p. 439.

Akad. Verl. Ges., Leipzig.
FIG. 124. Jahrb. f. wissensch. Botanik 84 (1937), Fig. 9 on p. 697.

Verl. Gebr. Borntraeger, Berlin.
FIG. 125. Beihefte z. Botan. Centralblatt 52 (1934), Part B, Fig. 10

on p. 359. Verl. C. Heinrich, Dresden.
FIG. 142. Gerl. Beitr. z. Geophysik 49 (1937), Fig. 8 on p. 422. Akad.

Verl.-Ges., Leipzig.
FIG. 146. Zeitschr. f. Forst- imd Jagdwesen 72 (1940), Fig. 4 on

p. 157. Verl. J. Springer, Berlin.
FIG. 150. Forstwissensch. Centralblatt 54 (1932), Fig. 28 on p. 382.

Verl. Paul Parey, Berlin.

FIG. 151. , Fig. 29 on p. 383.

FIG. 159. Veroff. d. Geophysikal. Inst. d. Univ. Leipzig. Band VI,

Heft 3, Fig. 5 in Appendix. Published by the author.
FIG. 168. Forstwissensch. Centralblatt 57 (1935), Fig. 2 on p. 792.

Verl. Paul Parey, Berlin.
FIG. 170. K. Escherich, Die Termiten oder weissen Ameisen, 1909,

Fig. 27 on p. 82. Verl. Dr. Werner Klinkhardt, Leipzig.
FIG. 171. Verofl. d. Geophysikal. Inst. d. Univ. Leipzig, Band VII,

Heft 2, Fig. 4 in the Table. Published by the author.

FIG. 174. , Band X, Fig. 4 on p. 129.

FIG. 175. Forstwissensch. Centrallbatt 60 (1938), Fig. 10 on p. 82.

Verl. Paul Parey, Berlin.
FIG. 178. Reichsamt fur Wetterdienst. Wissensch. Abh., Band VI,

Nr. 2 (1940), Fig. 55 on p. 84. Verl. J. Springer, Berlin.

FIG. 180. Reichsamt fur Wetterdienst. Wissensch. Abh., Band VI,

Nr. 2 (1940), Fig. 198 on Plate 7.
FIG. 181. Fig. 100 on p. 178.

INDEX OF AUTHORS

ALBRECHT,. F-> 42, 53, 69, 107, 148, 149,
169, 183, 185, 1 86, 187, 189, 227, 278,
362, 398, 419, 420, 423, 426, 436, 439

ALI, B., 105, 106, 427

ALT, E., 415

AMANN, H., 316, 318, 326, 327, 337,

338, 347. 349> 399> 4°o> 4M> 452, 461,

462

AMENDE, H., 391, 456
AMELUNG, A., 389, 449, 456
ANDRIANOFF, P., 447
ANGERER, E. VON, 272, 444
ANGSTROM, A., 15, 16, 17, 18, 21, 40,

41, 113, 129, 130, 164, 184, 272, 273,

285, 317* 347* 4I6, 4!8, 4*9> 423» 429>

432> 434. 451, 453
ARAKAWA, H., 382, 460
ARNBERGER, F., 460
ASKLOF, S., 15, 17, 18, 416
ATMANATHAN, S., 63, 422
AUER, R., 428
AUJESZKY, L., 123, 124, 428

BAC, S., 429

BAGNOLD, R. A., 427

BALANICA, T., 148, 431

BARTELS, J., 33, 71, 102, 129, 177, 178,

264, 285, 298, 423, 429, 435, 451
BASCO, F. VON, 265, 442
BATCHELOR, L., 461
BATES, C. G., 351, 449, 456
BAUER, E., 456
BAUR, F., 415

BECKER, F., 127, 147, 428, 431
BECKER, R., 147, 453
BECKETT, H. E., 134, 430
BECKMANN, W., 422
BENDER, K., 146, 404, 462
BERG, H., 40, 94, 291, 419, 435, 460
BERKE, T., 390, 450, 456
BERNICK, W., 448
BEST, A. C., 69, 71, 73, 75, 83, 84, 105,

175, 419, 424
BEZOLD, W. VON, 436
BIDER, M., 460

BIELICH, F. H., 117, 428
BIGELOW, F. H., xiv
BIGG, W., 428
BLAKE, J. H., 369, 454
BLANFORD, H. F., 311, 449

BODENHEIMER, F. S., 370, 455

Boos, 314, 348, 453

BRAAK, C., 428

BRADTKE, F., 456

BRANDTNERS, G., 460

BARZIER, C. E., 423, 460

BREZINA, E., 387, 389, 392, 456

BROCKS, K., 20, 324, 416, 441, 451

BROOKS, C. F., xiv, 210, 264, 429, 442

BROOKS, C. E. P., 449

BROOKS, F. A., xiv, 462

BRUCH, H., 432

BRUCKMANN, W., 429

BRUNNER, B. H. CH., 50, 420

BUCH, K., 126, 428

BUDIG, W., 423

BUDEL, A., 42, 98, 100, 254, 419, 423,

425, 441, 447
BUHLER, A., 224, 439

BlJTTNER, K., 129, I3O, 154, 155, 271,

386, 387, 418, 421, 430, 432, 456
BUJOREAN, G., 441
BURCHARD-DOSTAL, E., 442
BURCKHARDT, H., 44!

BURGER, H. H., 313, 347, 449, 453
BUXTON, P. A., 100, 266, 392, 425, 444,
455

CARRUTHERS, N., 427
CASTENS, G., 390, 456
CAUER, H., 428
CHATTERJI, G., 132
CHORUS, U., 434
CONRAD, V., 156, 391, 433, 457
CORNFORD, C. E., 437
COUR, P. LA, 351, 453
Cox, H. J., 437
CRAIG, R., 100, 426

DANCKELMANN, B., 351, 453

466

INDEX OF AUTHORS

DAVIES, E. L., 141, 431

DEFANT, A., 18, 62, 203, 410, 412, 416,

422, 425, 433, 437
DEINES, G., 449
DEINHOFER, J., 320, 451
DESAI, B. N., 15, 425
DESCOMBES, P., 364, 453
DIECKMANN, A., 453
DIEM, M., 137, 430, 434
DIENERT, F., 453

DlESNER, P., 442

DIETRICH, G., 153, 433
DOBSON, G. M. B., 437
DORFFEL, K., 363, 426, 453, 460
DORNO, C., 135, 164, 389, 430, 457

DOSTAL, E., 442

DRAVID, R. K., 135, 149, 430, 432
DUBOIS, P., 19, 416

DUCKERT, P., 423
DUCKER, A., 430

DUFOUR, L., 428
DUFTON, A. F., 134, 430
DURST, C. S., 447

EATON, G. S., 132, 133, 430

EBLE, 'L.,. 423

EBERMAYER, E., 312, 449

ECKEL, O., 15, 16, 129, 167, 168, 185,

4T6, 434

EFFENBERGER, F. F., 124, 420
EGGLER, J., 453
EGLE, K., 272, 274, 321, 444
EGLOFF, K., 389, 457
EIDMANN, H., 320, 374, 452, 455
EHRENBERG, P., 457
EKHART, E., 213, 214, 437, 441
ENGLER, A., 449
EREDIA, F., 449
ERNST, F., 449
ERTEL, H., 21, 416
ESCHERICH, K., 374, 455
ESER, C., 439
EXNER, F. M., 121, 156, 429, 433

FALCKENBERG, G., 46, 48, 130, 132, 164,
184, 272, 417, 420, 421, 422, 430, 437
FAUST, R., 449
FAVROT, C., 424
PELS, E., 457
FENNER, G., 437

FILZER, P., 237, 275, 289, 290, 298, 302,
414, 440, 444, 446, 447

FlNDEISEN, W., 121, 122, l6l, 428, 433,
446

FIRBAS, F., 144, 300, 314, 431, 441, 443,

447, 461

FLACH, E., 386, 457
FLEISCHMANN, R., 137, 291, 430, 446,

447

FLENSBERG, C. E., 394, 457
FLOHN, H., 441, 457
FLOWER, W. D., n, 12, 69, 71, 72, 79,

80, 83, 85, 87, 88, 114, 115, 415, 424
FLURY, F., 437

FoRSTER, H., 423

Foss, H., 462

FOSTER, H., 71

FOWLE, F. E., 421

FRANKEL, G., 455

FRANSSILA, M.,, 93, 102, 171, 183, 185,

437, 457

FRANZ, H., 369, 455
FREUDENBERG, H., 460
FREY, H., 155, 4^?3
FRIEDEL, H., 340
FRIEDRICH, W., 177, 298, 435
FRITSCH, E., 226, 439
FRITSCHE, G., 144, 198, 421, 431, 448
FUCHS, O., 150, 431
FUGGER, 444

FtJRLANI, J., 265

FUTI, H., 428

GABRAN, O., 166, 294, 434

GADRE, K. M., 63, 294, 300, 301, 425,
446

GARTNER, G., 431

GAMS, H., 265

GEHLHOFF, K., 423

GEIGER, xi, xiii, xiv, xvii, 27, 41, 43,
45, 57, 58, 71, 75, 90, 98, TOO, 144,
170, 171, 180, 196, 198, 208, 218, 241,
242, 246, 248, 265, 269, 287, 289, 290,
315, 316, 318, 326, 327, 329, 330, 334,

336, 337, 338, 343, 347, 35*, 355, 357,
358, 362, 366, 396, 414, 418, 419, 421,
423, 424, 431, 434, 436, 441, 442, 443,
446, 449, 452, 453, 457, 461

GEORGI, J., 434

GERLACH, E., 233, 440

GESSLER, R., 215, 216, 440

INDEX OF AUTHORS

467

GISH, O. H., 428

GoDECKE, K., 419

GOSSWALD, K., 455

GOTZ, P., 129, 164, 391, 434, 457

GOLDSCHMIDT, H., 1 17, 428

GRAININGER, }., 445

GREGOR, A., 386, 457

GRIMM, B. H., xviii, 367, 369, 414, 455

GRUNDL, G., 71, 423

GRUNDMANN, W., 423

GRUNOW, J., 419, 457

GSCHWIND, M., 75, 76, 425

GUMINSKI, R., 437

GUSINDE, M., 311, 320, 449

HAERTEL, O., 237, 245, 414, 415, 440

HAEUSER, }., 385

HALL, R. C., 449

HALLENBECK, C., 116, 438

HAMBERG, H. E., 92, 310, 351, 426, 449

HAND, I. F., 217, 218, 440

HANDL, L., 444

HANN, J. VON, 226, 414

HARRIES, H. D., 390, 457

HARTMANN, W., 264, 428, 443

HASCHE, E., 14, 23, 417

HAUDE, W., 40, 53, 54, 56, 69, 109,

187, 1 88, 227, 421, 439
HAUSER, E., 444

HAUSMANN, W., 129, 370, 391, 430, 457
HECHT, W., 35, 418

HELD, J. R., 441

HELLMANN, G., 23, 24, 62, 103, 105,
1 06, 1 08, 1 80, 302, 417, 427

HELLMUTH, (DR.), 394

HELLPACH, W., 375, 386, 457

HENRY, A. J., 438, 449

HERBST, W., 445

HERR, L., 34, 167, 363, 418

HERRIG, H., 380, 460

HERZOG, J., 156, 157, 433

HESS, H., 444

HESSE, R., 372, 373, 455

HESSELMANN, H., 341, 363, 453

HETTNER, A., 259, 421, 443

HEYER, E., 76, 424

HEYWOOD, G. S. P., 105, 205, 427, 438

HILL, S. A., 95, 426

HILTNER, E., 448

HIMMER, A., 373, 455

HlRATA, T., 450

HODGSON, R. W., 462

HOFMANN, A., 428
HOFMEISTER, A., 428
HOFFROGGE, CH., 438
HOLZHAUSEN, H. VON, 415
HOLZMAN, B., 426

HOMEN, TH., xviii, 29, 139, 140, 143,

182, 183, 418, 431, 437, 441
HOPPE, E., 339, 340, 450, 452

HORNBERGER, 41, 419

HORTON, R. E., 434

HOTTINGER, M., 457

HOUGH, A. F., 438

HOWELL, D. E., 100, 426

HOWELL, W. E., xiv

HRUDICKA, B., 124, 384, 460

HUBER, B., (BR.), xiii, 133, 239, 271,

276, 280, 281, 426, 440, 445
HUBER, H., 462
HUMMEL, K., 376, 445, 457
HUMPHREYS, W. J., 399, 461

HUNDERTMARK, A., 455
HUTTENLOCHER, H., 256, 44!

IJJASZ, E., 450

INNEREBNER, F., 201, 252, 441

ISRAEL-KOHLER, H., 126, 428

IVES, R. L., 415

JACOBSON, S., 432

JAUMOTTE, J., 438

JELINEK, A., 255, 441

JOHNSON, N. K., 9, 69, 71, 73, 75, 78,

80, 83, 84, 141, 175, 424, 431
JONES, T. W. V., 428
JUHLIN, J., 434

KADNER, T., 462

KAHLER, K., 424, 460

KAEMPFERT, W., xii, xiii, 220, 221, 263,

401, 402, 404, 412, 438, 439, 440, 443,

445, 462
KALAMKAR, R. J., 63, 294, 300, 301,

425, 446

KALITIN, N. N., 164, 166, 434
KAMINSKY, A., 450
KANITSCHEIDER, R., 288, 446
KARSTEN, H., 424
KASSNER, C., 414, 423, 443, 460
KATHEDER, F., in, 427

INDEX OF AUTHORS

KATTI, M. S., 73, 74, 94, 95, 97, 149,

426, 432

KEEN, B. A., 431
KEIL, K., 431, 460
KELLER, H., 448
KERANEN, J., 167, 418, 431, 434
KERN, H., 443
KERNER, A., 225, 226, 440
KERNER, F. VON, 256, 441
KESSLER, O. W., xii, xiii, 276, 401, 402,

404, 405, 407, 408, 411, 412, 445, 448,

462

KESTERMANN, A., 308, 447
KIENLE, J. VON, 215, 216, 357, 358, 440

KlMBALL, H. H., 15, 217, 2l8, 417,
440

KINZL, H., 255, 441

KlRCHNER, H., 255
KlRCHNER, R., 292, 446
KlRWALD, E., 450

KLECKA, A., 446

KLEINSCHMIDT, E., 156, 433

KLEMM, M., 369, 455

KNIEP, H., xviii

KNOCH, K., 69, 81, 82, 96, 259, 386,

414, 424, 427, 443, 458
KNOCHE, W., 448

KNOCHENHAUSER, W., 178, 179, 436
KNUCHEL, H., 452
KOCH, H. G., 58, 59, 312, 342, 343,

363* 4*9> 42I» 438» 453> 454
KOCH, W., 100, 426
KOHLER, H., 427
KOHN, M., 71, 423

KOELSCH, A., 302
KOPPEN, W., 117, 369, 428
KOHLHORSTER, W., 429

KOLACEK, F., 377, 458

KOLOSKOFF, P. J., 450

KORHONEN, W. W., 434

KOROTKEWITSCH, V. N., 414

KRATZER, A., xiii, 379, 380, 382, 383,

385, 392, 460

KRAUS, G., xvii, xviii, 264, 299, 414
KRAUSS, G., 150, 431
KRENN, K. D., 440
KRENN, V., 230, 231, 232, 234
KREUTZ, W., 125, 139, 141, 142, 145,

292, 303, 428, 431, 432, 434, 441, 444,

446, 447, 458
KROH, A., 458

KRUGLER, F., 16, 17, 184, 417, 422, 437
KUEN, F. M., 129, 430
KUHL, W., 418

KtJHNELT, W., 265, 367, 368, 369, 372,

455

KUHNERT, W., 66, 87, 265, 422

KUNKELE, TH., 241, 248, 255, 442

KtJSTER, E., 458

KUHLBRODT, E., 156, 159, 433

KYRIAZOPOULOS, B., 448

LAMMERT, W., 460

LAMPADIUS, G., 454

LANDSBERG, H., 389, 456, 458

LAUSCHER, F., 6, 18, 20, 21, 129, 251,
261, 262, 263, 264, 311, 318, 320, 321,
322, 351, 353, 354, 360, 361, 370, 415,

4i7> 432, 434> 442, 443> 449> 451* 452>

455, 460

LAUTENBACH, F., 442
LAUTENSACH, H., 443, 444, 461

LAUTENSACH-L6FFLER, E., 461

LEACH, H. R., 434

LEEUWEN, ST. VAN, 438

LEHMANN, G., 429

LEHMANN, H., 454

LEHMANN, P., 137, 392, 393, 430, 432,

448, 458

LEICK, E., 176, 302, 436, 448
LEONTIEWSKI, N. P., 458
LESSMANN, H., 460
LETTAU, H., xiii, 37, 40, 41, 42, 420,

426

LEVI, F., 434

LEYST, E., 30, 31, 32, 418
LIEBIG, J. VON, xviii, 414
LIESE, W., 456

LlESEGANG, R. E., 41

LINDEN, A. J., -ter-, 458
LINDHOLM, F., 165, 435

LlNKE, F., 19, 71, 192, 364, 386, 389,

397, 417, 423, 424, 458
LINZ, A., 392, 458
LISKE, F., 396, 397, 461

LOBNER, A., 380, 381, 382, 460

LOHLE, F., 435
LOHRL, H., 373, 455
Low, K., 417

LoRENZ-LlBURNAU, J. R. VON, 312, 313,

450
LOSSNITZER, H., 450, 460

INDEX OF AUTHORS

469

LUDWIG, G., 443

LUFT, R., 212, 438
LUNDEGARDH, H., 446

McAoiE, A. G., 112, 427

MCDONALD, W. F., 438

MADE, A., 69, 278, 279, 280, 292, 392,

424, 445, 446, 458
MAL, S., 425
MALSCH, W., 210, 438
MALURKAR, S. L., 52, 56, 57, 59, 60,

118, 121, 421, 429
MANIG, M., 438
MARLOTH, 364, 454
MARQUARDT, R., 161, 433
MARTEN, W., 4, 415
MARTINI, E., 297, 367, 455
MARVIN, C. F., 204, 438
MAURER, J., 35, 411, 418
MAYER, A., 458
MAYER, H., 150, 151, 432
MEHNER, A., 392, 458
MEINANDER, R., 18, 417
MEINARDUS, W., 35, 418
MEISSNER, O., 429
MEIXNER, H., 458
MELLAMBY, K., 100, 425
MERZ, A., 157, 433
METZLER, H. K., 383, 435, 461
MEYER, A. F., 450
MEYER, E. G., 62, 422, 461
MEYER, F. G., 458
MICHAELIS, G., 282, 445

MlCHAELIS, P., 1 06, IIO, III, 169, 282,

435> 445

MlCHLER, H., 390, 458
MlERDEL, F., 427
MlLDEBRATH, E., 448
MlTSCHERLICH, G., 32O, 322, 452
MlYANISI, M., 429

MODEL, F., 433

MOLLER, F., 14, 417, 421, 426

MORIKOFER, W., 4, 386, 415, 425, 459

MOSAUER, W., 370, 455

MROSE, H., 267, 384, 444, 448, 461

MUGGE, R., 421
MiJHLER, H., 265
MULLER, K., 431

MUNCH, E., 176, 396, 397, 436, 461

MUTTRICH, A., 312, 313, 314, 450

Musso, J. O., 429

NAGELI, W., 319, 393, 452, 459
NAGLER, W., 438
NEWNHAM, E. V., 436

NlEDERDORFER, E., 167, 1 68, 169, 184,

437
NIELSEN, E. T., 100, 370, 371, 374,

426, 455, 456
NITZE, F. W., 205, 438
NOVAK, V., 425

NYBERG, A., 51, 115, 132, 169, 171, 185,
285, 435

OBOLENSKY, N. VON, 329, 452

OBRUTSCHEW, S., 201, 438

OEDL, R., 267, 268, 444

OFFE, H., 461

OKADA, T., 437

OLSSON, H., 164, 165, 435

ORTH, R., 271, 445

PAESCHKE, W., 106, 291, 305, 306, 307,

427. 447

PAFFEN, K. H., 450
PAPAIOANNOU, J., 462
PARANJPE, M. K., 51, 422
PAULCKE, W., 167, 267, 435, 444
PENCK, A., 444

PENMAN, H. L., 430

PEPPLER, A., 106, 379, 383, 427

PEPPLER, W., 156, 161, 433

PERDEREAU, L., 460

PERL, G., 217, 218, 440

PERNTER, J. M., 121, 429

PERS, M. R., 215, 216, 440

PETRI, E., 418

PETTERSSEN, S., 24, 25, 94, 417

PFEIFFER, H., 349, 355, 356, 362, 449,

454
PHILIPPS, H., 16, 18, 23, 48, 138, 415,

4^7

PICHLER, W., 156, 158, 433
PIERCE, L. T., 438

PORTIG, W., 122, 429
POTZGER/J. A., 442

PRANDTL, L., 427
PREDESCOU, C., 438
PRIEBSCH, J., 126, 429
PRIEHAUSER, G., 341, 452
PROPP, J., 176, 436
PRUGEL, H., 426
PUTOD, R., 446

470

INDEX OF AUTHORS

QUERVAIN, H. DE, 75, 76, 425

RAETHJEN, P., 420

RAMAN, P. K., 15, 417, 421

RAMANATHAN, K. R., 15, 63, 64, 132,
136, 176, 422, 430

RAMBAUT, 418

RAMDAS, L. A., 19, 51, 52, 56, 57, 59,
60, 63, 64, 73, 74, 94, 95, 97, 99, 118,
121, 135, 149, 294, 300, 301, 315, 414,
415, 417, 421, 422, 424, 425, 426, 429,
430, 432, 446

RAVET, J., 418

REEDER, G., 430

REGER, J., 156, 159, 433

REIDAT, R., 249, 442

REIHER, M., 202, 203, 204, 205, 438

REMPE, H., 44, 420

RETHLY, A., 436

RICHARDSON, L. F., 430

RITSCHER, A., 134, 430

ROBERTS, O. F. T., 424

ROBITZSCH, M., 422, 459

ROTSCHKE, M., 124, 365, 429
ROHWEDER, M., 139,, 432

ROLL, U., 433

ROMAGE, A. G., 429
ROMBAKOS, S., 42O

ROOSE, H., 459
ROOT, C., 461
ROSCHKOTT, A., 444
Rossi, V., xviii, 91, 100, 426
ROSSMANN, F., 12, 44, 173, 416, 420,
435

ROUSCHAL, E., 176, 436
RUBENSON, R., 448

RUBNER, K., 364, 365, 450, 454

RUDEL, K., 448

RUDLOFF, C. F., 445

RUDORF, W., 392, 458
ROCKER, F., 131, 430

RUGE, H., 390, 459

RUNGE, H., 1 80, 436

SACHSE, H. F., 450

SANSON, J., 415

SAUBERER, E., 153, 164, 165, 174, 183,
272, 274, 285, 286, 319, 321, 340, 398,
433. 435. 437» 445> 452> 461, 462

SCAETTA, H., Xvli, Il6, 192, 212, 296,

376, 4i5> 438» 446, 459

SCAMONI, A., 235, 236, 441

SCHADE, A., 228, 441

SCHANDERL, H., 136, 237, 238, 239, 240,

276, 430, 441, 445, 448

SCHENK, C. A., 309, 450
SCHIELE, W. E., 121, 429
SCHIMITSCHEK, E., 235, 354, 368, 454,

456
ScHLICHTING, 10, 416

SCHMAUSS, A., xi, 41, 43, 44, 63, 116,

123, 133, 152, 212, 302, 315, 363, 420,

422, 430, 432, 439, 442, 443, 446, 447,
454, 461

SCHMID-CURTIUS, C., 127, 316, 326, 429

SCHMIDT, A., 32, 418

SCHMIDT, E., 422

SCHMIDT, K., 461

SCHMIDT, W., xvii, 32, 33, 37, 39, 42,
44, 106, 107, 131, 137, 142, 144, 146,
156, 161, 162, 193, 199, 200, 203, 206,
207, 217, 218, 252, 263, 265, 304, 305,
364, 379, 387, 389, 391, 392, 404, 405,
406, 414, 415, 420, 422, 424, 425, 427,
430, 432, 434, 437, 439, 440, 442, 443,
452, 456, 462

SCHNAIDT, F., 46, 47, 48, 421
SCHNELLE, F., 459
ScHOBER, H., II, 416
SCHOENICHEN, F., 459
ScHONNOPP, G., 462
ScHOONOVER, W. R., 462
SCHOY, C., 2 1 8, 440
SCHREIBER, P., 450
SCHROPP, 136, 431

SCHUBERT, J., 4, 71, 149, 217, 218, 222,
223, 224, 226, 311, 313, 324, 340, 351,
358, 360, 402, 415, 416, 419, 437, 448,
450, 451, 461

SCHUTTE, K., 440

SCHULTZ, H., 212, 439
SCHULZ, L., 212, 439, 443
SCHWABL, W., 118, 321, 322, 354, 360,
361, 391, 443, 452

SCHWALBE, G., 436

SCHWARZ, H., 378, 459
SCOTT, R. F., 123, 429

SCULTETUS, H. R., 432

SEEHOLZER, M., 232, 441
SEEMANN, J., 417, 459
SEILKOPF, H., 122
SEIP, L. PH., 208, 248, 396, 441

INDEX OF AUTHORS

471

SELTZER, P., 324, 329, 425, 451, 452
SEYBOLD, A., 271, 275, 445
SHEROUSE, R. T., 462
SIEGEL, S., 65, 66, 67, 95, 114, 423

SlEGENTHALER, J., 27, 419

SIGMOND, H., 454
SIMPSON, G. C., 421
SINCLAIR, J. G., 7, 416
SIRCAR, S. P., 425
SLANAR, H., 77, 125, 425, 432
SMITH, A. M., 280, 445
SMOLIAKOW, P. T., 8, 416
SMOLIK, L., 426, 439

SONNTAG, K., 256, 292, 2Q3, 442, 446

SPANGENBERG, W. W., 461

SREENIVASIAH, B. N., 417

STAUDACHER, D., 401, 461

STEINER, O., 49, 372, 421, 444, 456

STEINHAUSER, F., 264, 425, 443, 460

STEPANOWA, N., 444

STEPHAN, J., 302, 448

STEPHEN, J., 448

STEVENSON, TH., 106, 427

STEWART, M. N,, xiv

STOCKER, O., 297, 299, 302, 303, 333,

447

STOECKER, E., 48, 421
STRANGE, R., 421
STUVE, G., 448
SORING, R., 364, 419

SUSSENBERGER, E., 15, 19, 417
SUTTER, E., 129, 130, 154, 155, 271, 432
SUTTON, J. R., 423
SUTTON, O. G., IO5, 427, 448

SVERDRUP, H. U., 427, 435
SzYMKiEwicz, D., 97, 177, 426

TACKE, B., 461

TALMAN, C. F., 459

TAMM, K., 269, 291, 446

TEUBNER, E., 297,, 367, 455

THAMDRUP, H. M., 100, 426

THAMS, CH., 164, 165, 167, 168, 434,

435

THORNTHWAITE, C. W., 426
TICHY, H., 459
TINN, A. B., 265, 444

TOLLMIEN, W., 427

TOLLNER, H., 213, 439
TOLSKY, A., 435
TOPERCZER, M. E., 264, 443

TOPOLANSKY, M., 259, 444
TRANKEVITCH, N. N., 251, 442, 447
TRABERT, W., 347
TRAPP, E., 317, 318, 319, 323, 452
TROJER, H., 132, 417, 418
TROLL, C., 77, 425, 435, 447

ULLRICH, H., 278, 280, 445
UNGEHEUER, H., 316, 329, 331, 334,

453
UVAROV, B. P., 456

VEDY, L. G., 429
VIERECK, W., 107, 427
VISSER, S. W., 448
VOIGTS, H., 129, 431, 461
VOLK, O. H., 154, 434

VUJEVIC, P., 41, 71, 425, 431, 432

WAGEMANN, H., 8, 123

WAGNER, A., 108, 211, 255, 428, 439,

441, 442

WAGNER, C., 365, 454
WAGNER, F., 390, 459
WALD, H., 100, 426
WALKER, H. B., 462
WALLEN, C. CH., 317, 451
WALTER, A., 451
WARNECKE, G., 456
WEBER, R., 451
WEGENER, A., 9, 11, 37, 77, 121, 187,

293, 296, 416, 420, 429, 447
WEGENER, K., 132, 391, 418, 445, 459
WEGER, N., 136, 283, 292, 430, 445,

447, 462

WEHRHEIM, H., 441, 444
WEICKMANN, L., 387, 388, 459
WEINLANDER, A., 461
WELLENSTEIN, G., 371, 456
WENGER, R., 299, 447
WERTHEIMER, E., 424
WEST, E. L., 461
WIELE, H., 370, 456
WIEN, K., 295

WlESNER, J., 318, 452

WILD, H., 419, 432, 435, 447

WlTTERSTEIN, F., 439
WlTTMANN, A., 251, 442

WLISSIDIS, T., 451

WOEIKOF, A., 248, 284, 310, 419, 425,

442, 451

472

INDEX OF AUTHORS

WOELFLE, M., 208, 248, 284, 362, 393,

394> 396, 44*> 45i> 454» 459
WOLLNY, E., 134, 149, 224, 225, 431,
432, 440

WOLTERECK, H., 460

WREDE, C. VON, 348, 350, 454
WUST, G., 159, 160, 434
WYOTZKY, 460

YAKOTANI, S., 422

YAKUWA, R., 419, 432
YOUNG, F. D., 112, 206, 207, 209, 210,
277, 428, 439, 445, 462

ZATTLER, F., 448
ZEDERBAUER, E., 452
ZEDLER, P., 422, 460
ZIOBROWSKI, S., 462
ZOLYOMI, B., 265, 442
ZWOLFER, W., 374, 456

INDEX OF SUBJECTS

Absolute humidity, daily course of — ,

Absorption of insolation by air, 2
Absorption of insolation by water,

Absorption of outgoing terrestrial

radiation, 13
Absorption spectrum of water vapor

and carbon dioxide, 46
Absorptivity of a body, 47, 129
Absorptivity of leaves, 275, 321
Absorptivity of snow, 164 ff.
Acoustic phenomena, 117, 122 ff.
Adiabatic gradient, 8
Aerobium, 369
Aftereffect of night, 31
Agricultural microclimatology, 285
"Air avalanche," 116, 212
Air circulation in valleys by day, 255
Air film, 243, 244
Air layer above sod cover, 175 ff.
Air layer over water, 153 ff.
Air layer adjacent to the ground, xv
Air mass and temperature variation

with altitude, 251
Air plankton, 2
Air skin, 241

Albedo, 2, 129 ff., 154 ff., 164, 272
Albrecht platinum wire thermometer,

53
Anemometer, hot wire, 107

Anemometer, pressure plate, 305
Animals vs. microclimate, 191 ff.
Animate creatures vs. microclimate,

367 ff.

Annual course of ground temperature,

32
Annual course of temperature near the

ground, 68
Anthobium, 369
Asphalt pavement, temperatures above,

132 ff.

Aspiration thermometer, Assman, 102
Austausch coefficient, 39, 41
Austausch, magnitude of — in woods,

312

Atmospheric turbulence, 37
Avalanche, air, 116, 212

Back radiation, 14, 15, 17, 21

"Band" absorption, 13

"Band" radiator, 47

Bark cracks on the south side of a
beech, 234

Bark temperature, 230 ff.

Bats, hibernation places of — , 373

Bed climate, 389

Beehive, temperature in a — , 374

Beetles, 368, 369

"Bioclimate," 192

Bioclimatic index forms, 369

"Black body," 47, 164

Black-bulb thermometer, 134

Blooming process of a pine vs. micro-
climate, 236

"Blue shade," 275

Border climate, forest, 357 ff.

Boundary layer near the ground, 51 ff.

Braking effect (wind) in different
crops, 40, 102, 303

Brightness, decrease from top of
crowns to the floor, 318 ff.

Brightness on the ground, daily course
of — , 286

Bryobium, 369

Calm hours, frequency of — , 109

Calm hours, number of — at differ-
ent heights above forest floor, 338

Carbon dioxide, 46, 117, 125 ff.

Cave frost, 267

Cave with single opening, temperature
and humidity in a — , 266

Caves, dynamic, 266

Caves with several openings (wind
tunnels), 267

Caves, microclimate of — , 265

Caves, static, 265

Circulation system of a great city, 384

Circulation in valleys, 211, 255

City climate, 379 ff.

Climate, artificial, 390

474

INDEX OF SUBJECTS

Climate of bed, 389 fi.

Climate of city, 379 ff.

Climate of the house, 391

Climate, the influence of make up of

the forest stand on — , 342 ff.
Climate in the least space, xvii
Climate of man, xvi
Climate, manufactured, 390
Climate of plants, xvi, 269
Climate of room, 389 ff.
Climate in a stable, 392
Climate inside the stand, 314
Climate of the stand border, 357
Cloudiness, effect on daily course of

temperature, 78
Cloudiness, effect on insolation

amounts, 219, 226
Clouds, back radiation from, 17 ff.
Clouds, reflection from, of insolation,

Cold air dams, 195, 204
Cold air dome, 74
Cold air flood, 195, 196, 204
Cold air flow, 20, 203
Cold air, inrush of — in different

stands (forest), 345
"Cold air puddles," 195
"Cold air wind," 205, 211
"Cold lakes," 195
Coldness, convection, 63
Compass plant, 237 ff.
Conduction (See Heat Conduction)
Conimeter (Zeiss), 380
Content of dust, 365
Convection in air, 4, 26, 66, 94, no
Convection in water, 151, 157
Cooling of the atmosphere, 23
Cooling by evaporation, 27
Cooling of ground, 138
Cooling process, 62
Corrosion of sandstone, 76
Counterradiation of the atmosphere,

14, 15, 17, 21

Counterradiation of a wall, 136
Counterradiation from a stand

border, 362

Counterradiation, zenith angle de-
pendence of, 19 ff.

Daily course of relative humidity, 93,
95 ff ., 250, 334

Daily course of temperature, 72, 73, 78
Daily course of temperature, depend-
ence of — on cloudiness, 78
Daily course of temperature gradient,

8.3
Daily course of temperature in a

forest, 330 ff.
Daily course of temperature in a low

plant cover,, 291 ff.
Daily course of temperature in a

snow layer, 168 ff.
Daily course of water vapor gradient,

90 ff .

Daily course of wind velocity, 108 ff.
Daily march of soil temperature, 30,

140
Daily range of air temperature near

the ground, 75, 77
Daily range of water surfaced tem-
peratures, 156
Daily temperature march on slopes,

248 ff.

Damp moor, 140
Deep-tillage, 145
Density of frost changes, 76, 77
"Dependent" climate, 90
Depth of penetration of heat and cold

cycles, 33

Desert surface air temperatures, 53 ff .
Dew, 23, 27, 91 ff., 301
Dew (in the forest), 332
Dew plate, 301

Dew, utilization of — by plants, 293
Diffuse sky radiation, 218
Discontinuity of surface temperature,

7> 24, 52

Diurnal . . . (See Daily . . .)

Diurnal forest wind, 363

Double wave of vapor pressure, 93 ff.

Down-valley wind, 211

"Dry" type of water vapor distribu-
tion, 92 ff .

Duplicate forest stations, 312

Dust content, 124

Dust content at the stand border,

365 ff.

Dust distribution in a city, 381

Dustfree layer, 52

Dust layers above a city, 380

Dust particles, nocturnal cooling of,

INDEX OF SUBJECTS

475

Dust whirl, 9-12, 57, no
Dust wind, 10
Dynamic caves, 266
Dynamic eddy diffusion, 41

Eddies of hot air, 52, 56

Eddy diffusion, 26, 36, 37, 39, 90, 108

Eder-Hecht optical (grey) wedge
photometer, 218

Effective nocturnal outgoing radia-
tion, 20

Effective outgoing radiation, 15, 17,
19, 22

Effect of topography, 250

Emanation, 117, 126

Emissivity of a body, 47, 129

Evaporation 3, 7, 26, 90 ff., 178, 183,
186, 188, 225

Evaporation, cooling by, 27

Evaporation in different forest stands,

347
Evaporation from different soils and

water, 178

Evening wind type, 65
Exchange, fundamental equation

of -7 > 37
Experimental bodies for temperature

measurement, 70
Exponential wind profile, 103 ff.
Exposure, climatic effects of, 215 ff.

"Firn-wind," 213

Flooding as means against frost, 407

Flow of cold air, 20, 203

Flow of heat, 6, 26, 29 ff., 51

Fog, 87, 88, in, 173, 1 80

Fog, back radiation from, 18

Fog in great cities, 384

Fog precipitation at the stand border,

364 ff.
Fog, temperature stratification in

meadow type, 66
"Foot-ring disease," 176
"Fore-planting" against late frosts,

399
Forest ant dwellings, microclimate of,

37i

Forest circulation, 345
Forest climatology, 309 ff.
Forest entomology, 374

Forest influences on precipitation,

310 ff.

Forest meteorology, 309 ff .
Forest shade, 275
Frigorigraph, 70
Frigorimeter, 70
Frost, advective, 397
Frost area, 196
Frost, the battle against destructive,

403 ff.
Frost, brown etc. coal-heaters against

•—,408

Frost change density, 76, 77
Frost change number, 77
Frost changes, 75, 76
Frost control by artificial convection,

408, 410

Frost control by heating, 408
Frost controlled by small briquet

piles, 410

Frost in cranberry cultures, 407
Frost danger, increase with altitude,

397

Frost danger, increase of — , in clear-
ings of increasing size, 353

Frost danger in relation to soil type
and condition, 139

Frost danger in relation to topo-
graphy, 196 ff.

Frost danger decreased by snow
cover, 170 ff.

Frost danger decreased by wind, 112

Frost, destructive — as a microcli-
matic phenomenon, 396 ff.

Frost effects in soil formation, 77

Frost, factors conducive to, 397

Frost, flooding against — , 407

Frost forecasts, 401

Frost frequency, 353, 397

Frost heating, 407

Frost heaving, 37

"Frost holes," 195

Frost, oil heaters against — , 408

Frost penetration rate vs. soil type, 146

Frost prevention, 401, 403, 408

Frost protection, 86, 291

Frost protection by sprinkling, 407

Frost, radiation — , 397

Frost screens and caps, protection
means, 405

Frost smoking, 406

INDEX OF SUBJECTS

Frost, source region of — , 401
Frosts, early — , 396
Frosts, late, 396
Frosts, night — , 396
Full radiation, 215
Furrow temperatures, 20

Geobium, 369

Glacier wind, 211, 213, 214

"Glass covered" cavities, 174

Glaze, 152

"Gnomon plant," 239

Granite, temperature variations in,

139

Grass frost, 398 ff .
Grass-minimum thermometer, 175
Green shade, 275
Ground air layer, xvi
Ground-fog formation, 66
Ground temperature, 28 ff, 74, 128 ff.,

139, 146, 149, 224 ff.

Halo, 122

Haze hood over a city, 382

Health resort climatology, 386

Heat capacity of the dry soil, 149

Heat conduction, 6, 26 ff., 51

Heat cycle, 29, 33

Heat economy of the ground surface,

182 ff.

Heat economy of plants, 271
Heat economy of water, 26, 153 ff.
Heat exchange of the ground surface,

i86ff.

Heat exchange at night, 13 ff., 22
Heat exchange at noon, 2 ff .
Heat exchange over snow, 184
"Heaving" (frost), 137
Herpetobium, 369
Hoar frost, 398

Horizon, constrictions of the — , 262
Horizontal radiation, 215
Hot-house effect, 14
Hot wire anemometer, 107
House climate, 391
Humidity in caves, 266 ff.
Humidity gradient, 90 ff ., 288, 297
Humidity vs. low plant cover, 297 ff.

Humidity measurement near the

ground, 100

Humidity over great city, 383
Humidity relationships, 90 ff., 326 ff.
Humidity, relative (see Relative

Humidity)
Humidity stratification in a low plant

cover, 288
Hygrometer, hair, 100 '

Illumination conditions at the stand
border, 361 ff.

Illumination in forest cuttings, 354

Illumination and ground flora in for-
ests, 322

Illumination maps (forest), 323

Incoming radiation, 6

Incoming radiation pattern, 8

Incoming radiation type, 7, 58, 72,
82, 89, 146, 294, 312

Incoming radiation type in a flower
bed, 288 ff .

Incoming solar energy, 2

Incoming solar radiation, 5

"Independent" climates, 90

Index size of the clearing, 350

Inferior mirage, 119

Infrared, 13, 46, 129 ff.

"Infrared shade," 275

Insects and the microclimate, 367 ff.

Insolation absorbed in water, 153

Insolation for N, E, and S slopes of
all inclinations, 221

Interferometric temperature measure-
ments, 51

"Intermediate layer near the ground,"

Intermediate type, 65
Inversion, 23, 25, 48, 64, 8r, 103, 114,

123

Inverted mirage, 119, 120
Irradiation of a slope, influence of

cloudiness on — , 218 ff.
Irradiation of a standing tree trunk,

231

Isothermal condition (Isotherrny), 85,
86, 87

Killing frost, 112, 397
Kirchoff's law, 47, 129
Kleinklima, xvii

INDEX OF SUBJECTS

477

Kleinstklima, xvii

Lag in soil temperatures, 29 ff.
Laminar airflow, 36, 40, 161, 203
Large scale climate, xv
Leaf litter, low thermal conductivity

of, 144

Leaf orientation near the ground, 239
Leaf surface temperature, 277 ff.
Leaf temperature vs. sudden sunning,

278

Leaves, permeability of — , 273

Leaves, reflection from — , 271 ff .

Local climate, xvii

Locusts, 370

Long wave radiation, 13, 46, 129 ff.

Loss of heat by evaporation, 22, 27

(See also Evaporation)
Low plant cover, 284

Macroclimate, xv

Magnus effect, 12

Man vs. microclimate, 190, 367 ff.

Man modifying microclimate, 386 ff.

Mass exchange, 6, 37, 62, 90

Meadow-fog type, 66

Mesoclimate, 192

Mesoclimatology, xvii, 192

Melt craters, 174

Meteorological elements near the
ground, 90 ff,

Meteorological shelter, xv

Microclimate in an air conditioned
greenhouse, 392

Microclimate of caves, 265

Microclimate at high altitude, 4

Microclimate of hole cuttings, 350 ff .

Microclimatic zone on a fallen tree
trunk, 235

Microclimate, modification of the —
by man, 386 ff.

Microclimate, unintentional effect of
man on — , 375 ff.

Microclimatology, vii ff.

Midday heat exchange, 3

Miniature climate, xvii

Miniature tornadoes, 9

Mirage, 117 ff.

Moisture content of the ground sur-
face, 97

Molecular conduction and diffusion,

39» 5i» 90

Moor, temperatures in, 141
"Mountain atmosphere," 254
Mountain frost conditions, 77, 397
Mountain micrometeorology, 21, 35,

219

Mountain slope air temperatures,
241 ff.

Natural horizon, 262
Net outward radiation, 62
Nocturnal air circulation, 206
Nocturnal cooling, 23, 46, 62, 138
Nocturnal forest wind, 353
Nocturnal heat exchange, 13 ff.
Nocturnal temperature distribution,

24, 49, 6^ (See also Daily Course of

Temperature, also Temperature.)
Nocturnal temperature vs. plant

growth on a slope, 209
Nocturnal temperature stratification,

65 #•» 73, 82 ff., 102, 106, 114, 200,

329

Normal course of ground tempera-
ture, 26

Normal outgoing radiation type, 64

Normal refraction, 120

Number of frost changes, 76

Observation scaffold (forest station),

315
Oil heaters, frost combatting means,

403, 408, 409
Oimekon, cold pole, 201
Optical phenomena, 117
"Orienting plant," 237
Outer active surface (crown surface

of a forest), 284, 287, 289, 317,

330
Outgoing radiation, 6, 16, 21, 24, 65,

324, 352, 362, 405
Outgoing radiational type, 13, 24, 34,

62, 64, 72, 79,^81, 85, 146, 1 80, 186
Outgoing radiation type in a flower

bed, 290 ff .
Overhead light, 324
"Overlayer," 56
Ozone, 2

Parhelia, 122

INDEX OF SUBJECTS

Peaty soil, 141

Penetration of rain, 147

Penetrability of snow, 165

Phyllobium, 369

Physical heat conduction, 26

Plankton, air, 2

Plant climate, xvi, 269

Plant cover, influence on climate,

144, 176, 269 ff.
Plant, heat economy, 271
Plants vs. microclimate, 190
Plants, reflection from the surface

of — , 272

Plant temperature, 271
Plant temperature vs. air temperature,

2?6

Plants, temperature in the interior
of — , 277

Plants on trellis-work, 222

Platinum wire resistance thermom-
eter, 53

Pollen dispersal, 44

Pollution of city air, 380 ff .

Polygonal nets, 77

Power law wind profile, 163 ff.

Precipitation, distribution of — around
a hill, 246

Precipitation and forest, 338

Precipitation over great cities, 384

Precipitation and heat conductivity of
the ground, 148

Precipitation, horizontal, 364

Precipitation, influence of forests
on — , 311 ff.

Precipitation, obscure — , 364

Precipitation in an old stand (forest),

336

Pressure-plate anemometer, 305
Protecting effect of a hedge, 393
Pseudo-conduction, 26, 39
Psychrometer, 100

Rabbit burrows, 372

Radiation, back, 14 ff., 21

Radiation balance, 6 ff .

Radiation calculations, tabulation of,

216 ff.
Radiation, daily totals with regard to

cloudiness, 223
Radiation economy, 188

Radiation errors in temperature meas-
urement, 68 ff., 277, 326

Radiation exchange, 183

Radiation, gradual absorption of —
in a meadow, 285

Radiation vs. low plant cover, 284

Radiation in an old (forest) stand,

37 ff'
Radiation on various slopes, 220 ff .

Radiation on vertical walls, 222 ff.
Radiation permeability of leaves, 274
Radiation shield, 68
Radiation type, incoming — in a

flower bed, 288 ff .
Radiation type, outgoing — in a

flower bed, 290 ff .
Radiation, utilized in different kinds

of ground, 143
Radiation on water, 153
Radiative pseudo-conduction, 7, 50, 62
Radiator, band, 47
Radioactive material, 126 ff.
Radium emanation content, daily

course of — , 127
Rainbow, 121
Rain, effect on soil conductivity of,

148
Rain, effect on soil temperature of,

147
Range of validity of the observation

at a place, 259 ff .
Reflections from clouds, 2
Reflection number, 129
Reflectivity, 129
Reflectivity of plants, 271 ff.
Reflectivity of snow, 164 ff.
Reflectivity of soil, artificial control

of—, 134

Reflectivity of water surfaces, 154 ff.
Relative humidity, 93
Relative humidity vs. altitude, 91
Relative humidity and crop density,

298
Relative humidity, daily course of — ,

66, 93, 95 ff., 250, 334
Relative humidity, fluctuations of — ,

98.
Relative humidity in an old stand,

331 ff.

Relative humidity, vertical distribu-
tion in the forest, 332

INDEX OF SUBJECTS

479

Research auto, 379
Representativeness of a meterological

station, 259 ff.
Returning convection, 63
Rime banner, 103
Rime formation, 102
Room climate, 389 ff.
Roughness height, 302, 307

Sand, albedo of — , 129 fT.

Sand devil, 9

Sand soil, 140

Sand-sweep, 108

Sandy soil, heat economy of — , 31 ff.

Scintillation, 118

Screening angle (h), 351

Screening, horizon, 261

"Secondary temperature maximum,"

Seed dispersal by wind, 44
Seed distribution at the stand border,

363

Selective absorption, 13

Selective absorption by tree leaves,, 321

"Seven-mountain wind," 212

Shade, "blue," 275

Shade, "green," 275

Shade in forests, 275

Shade, "infrared," 275

Shadow in the margin of forests,

359 ff-

Shielding effect of moisture, 226
Short wave radiation, 3
Sink hole, 199

Situation for a vineyard, 154
Skin of air on mountain slopes, 241 ff.
Skin layer, 243
Skin temperatures and humidities,

387 ff.
Slashings (or cuttings) in forests,

351 ff.

Smoke experiments, 42, 249
Snow, albedo of — , 164, 165
Snow as a "black body," 164
Snow cover, isotherms in a — , 168
Snow cover, temperatures in the air

just above — , 169 ff .
Snow cover temperature, tautochrones

of — , 168
Snow, distribution of — in an old

stand, 340

Snow, influence upon the adjacent
air layer, 164

Snow interception by trees, 340

Snow on grounds of different ther-
mal conductivities, 151

Snowmelt, 150

Snow reflectivity, 129

Snow, smoking, 173

Snowstorm, 109

Snows weep, 108

Snow, wintering of plants under — ,
167

Soil color vs. temperature, 134

Soil conductivity vs. soil moisture,
148

Sod cover, air layer above — , 175, 181

Soil moisture vs. soil temperature, 149

Soil structures, 77

Soil surface temperatures (See Sur-
face Temperature, Ground Tem-
perature)

Soil temperature (See Ground Tem-
perature)

Soil thermal properties, 28

Soil, type and condition, 138 ff.

Solar constant, 3

Solar radiation, 3

Solar radiation at midday, tabulated
values of — , 5

Sound propagation vs. temperature
gradient, 122

Specific gravity (soil), 28

Specific heat (soil), 28

Spouts, 9-12, no

Stable equilibrium, 8

Stable stratification, 62

Stand border, climate of — , 357

Stand climate, 309 ff.

Stefan-Boltzmann law, 13, 15, 22

Stratification of air temperature,
nocturnal, 65 ff., 73, 82 ff., 102,
1 06, 114, 200, 329

"Streaking," 118

Street mirage, 121

Sunniness of different slopes, 215 ff.

Sun radiation, 2

Sunshine on the stand border, monthly
duration of — , 358

Super-adiabatic, 8, 81

Superheated layer, 121

Surface air temperature, 68 ff.

INDEX OF SUBJECTS

Surface temperature, 6, 13, 32, 74,

128 ff., 144, 149, 226
Surface temperature, meaning of — ,

131
Swamp land, temperature variations

in-—, 139

"Tablecloth" (Capetown, Africa), 364
Tautochrones, soil temperature, 29,

34
Temperature of air, directly over

water, 160

Temperatures in an ant nest, 371
Temperatures above an asphalt street,

132 ff.
Temperature at boundary between

earth and air, 13
Temperature in caves, 266 ff.
Temperature, course of — in an alder-
twig, 281
Temperature, daily course of air, 72,

73, 78, 248 if.
Temperature, daily — march on

slopes, 248 ff.

Temperature, daily — range in mil-
let crop and sugarcane, 295
Temperature, dependence of daily

course of — on cloudiness, 78
Temperature at different heights in

a forest, 327 ff.
Temperature discontinuity above a

water surface, 159 ff.
Temperature discontinuity at ground

surface, 7, 24, 52
Temperature discontinuity in a snow

layer, 167 ff.
Temperature distribution during a

frosty night, 113
Temperature distribution above an

oil heated experimental field, 411
Temperature distribution in a sink

hole, 200
Temperature distribution in an urban

area, 383

Temperature distribution in a vine-
yard by day and night, 293 ff.
Temperature, diurnal course of —

within a forest, 343
Temperature, diurnal course of — in

the ground, 29 ff.

Temperature, diurnal course in a

small lake, 157
Temperature, diurnal course of — in

a pine grove, 330 ff .
Temperature extremes in different

kinds of ground, 140
Temperature gradient near the

ground, 8, 52 ff., 80 rf .
Temperature gradient near the

ground, 80 ff.
Temperature gradient and weather,

88
Temperature of the ground, 30, 32,

128 ff., 149, 226
Temperature gustiness, 58
Temperature layers, both sides of the

ground surface, 74
Temperature, layer structure over sod,

181

Temperature vs. low plant cover, 284
Temperature measuring journeys, 379
Temperature, nocturnal in valleys, 204
Temperature range in different soils,

142 ff.
Temperature range in surface air, 75,

Temperature relationship in an old

stand, 326 ff.
Temperature, scattering at night — ,

at different altitudes, 253
Temperatures stratification over an

asphalt street, 60
Temperature stratification near the

ground, 58, 59, 73
Temperature stratification within the

ground, 74

Temperature of a tree trunk, 230 ff.
Temperature unrest, 43, 53, 57, 58,

288
Temperature variation with altitude

vs. air mass, 251

Temperature variations in water un-
der different conditions, 156, 162
Temperature of water surfaces, 156
Termites, compass nest of — , 373
Termites, tropical, nest of — , 372
Terrestrial scintillation, 118
Thermal belt, 206
Thermal conductivity, 27, 28, 140,

144, 148, 151
Thermal diffusion, 29

INDEX OF SUBJECTS

Thermal diffusivity, 28, 29, 35, 36
Thermal exchange, 41
Thermometer, Assman aspiration, 102
Thermometer, black-bulb, 134
Therjnometer, grass-minimum, 175
Thermometer, interferometric, 51
Thermometer, platinum wire, 53, 69,

Thermometer Six's, 71
Thermoneedles, 277
Tillage, deep, 145
Topography, influence of — , 194
Topography vs. microclimate, 190
Total reflection, 120
Transmissivity of leaves, 274
Transmissivity of snow, 166
Transpiration of leaves, 299, 399
Tree trunk temperatures, 230 ff.
True air temperature, 68, 69, 70
True heat conduction, 26
Trunk space climate, 284, 312
Turbidity factor, 220
Turbidity in ground air, 117
Turbulence, 37
Twilight, duration of — in a forest,

320

Ultraviolet, 129 ff., 154

"Under lighting" of vineyards, 154

Unstable equilibrium, 8

Upper tangential arc, 122

Upslope wind, 254

Upvalley wind, 254

Upward eddies of hot air, 56

Valley circulation, 204 ff., 211, 255
Vapor in the air directly over water,

160
Vapor pressure, daily course of — ,

94,95
Vapor pressure, measurement of — ,

100
Vapor pressure variation with height,

90 ff., 300

Vegetation climate, 269
Ventilation, 70

Ventilation of thermal elements, 69
Vespa vulgaris, temperature in the

nest of — - , 374
Vineyard situation, 154
Visible portion of the spectrum, 47

Visible spectrum, 129 ff.

Walls, conditions near — , 137, 222

Warm-air dome, 74

Warmest zone, nocturnal upward mi-
gration on a slope, 207

Warming process, 51

Warm slope zone, 206, 208, 255

Water, air layer over, 153 ff.

Water, albedo of, 154 ff.

Water balance of the atmosphere,
90 ff.

Water content of the soil, 147

Water, daily temperature ranges in
— , 156 ff.

Water vapor, 90 ff.

Water vapor absorption, 46

Water vapor conditions in crops,
300 ff .

"Wave-length transformation," 48

Wegener expedition, 188

Wet climate type, 98

"Wet type" (humidity distribution
with height), 91, 92, 95 ff., 300

Wien displacement law, 13, 46

Wind, artificial protection against — ,

393 ff-
Wind, braking action on — , 40, 302,

303

Windbreaks, 393 ff.
Wind, city effect on — , 384
Wind, cold air, 20, 195, 196, 203, 204,

205, 211, 345
Wind conditions in a forest cutting,

355
Wind conditions for a sanitarium,

39i
Wind, effect of plant cover on — ,

302 ff .
Wind, effect on thermal stratification

at night, 65 ff ., 87
Wind, glacier, 211, 213, 214
Wind, near the ground, 102 ff., 304
Wind movement in an opening and

in a thinned strip (screen), 349
Wind, nocturnal forest — , 353, 363
Wind pipes, 266

Wind vs. low plant cover, 106, 297
Wind relationship, 102
Wind over a snow cover, 106

482

INDEX OF SUBJECTS

Wind speed, average — in two dif-
ferent forest stands, 346

Wind speed, daily course at various
heights, 1 08

Wind speed vs. height, 103 ft.

Wind speed, variation of — with
height in the forest, 336 ft.

Wind in an old forest stand, 336

Wind at the stand border, 362 ft.

Wind stratification above the ground,
107

Wind, structure over crops, 304
Wind vs. temperature gradient, 117
Wind vs. temperature of ground air,

no ft.

Wind, warming effect, 114
"Wisper wind," 212

Zeiss conimeter, 380

Zone, warm — on a slope, 206, 208,

255
Zoology and microclimatology, 367 ff.

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