FOR OFFICIAL USE.
M.O. 233.
METEOROLOGICAL OFFICE.
CLOUD FORMS
ACCORDING TO THE
INTERNATIONAL CLASSIFICATION
The Definitions and Descriptions
APPROVED BY THE
INTERNATIONAL METEOROLOGICAL COMMITTEE
IN 1910.
WITH AN
ATLAS OF PHOTOGRAPHS OF CLOUDS
SELECTED FROM THE COLLECTION OF MR. G. A. CLARKE
OF THE OBSERVATORY, ABERDEEN.
Issued By the Authority of the Meteorological Committee.
LONDON :
PUBLISHED BY HIS MAJESTY'S STATIONERY OFFICE.
To be purchased through any Bookseller or directly from
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1918.
Price 6d. Net.
LIST OF SOME OF THE PUBLICATIONS ISSUED
BY THE AUTHORITY OF THE METEORO-
LOGICAL COMMITTEE.
1. Hand-books, Text-books, Tables. (8vo.)
(M) Calendar, with Notes and Diary of Operations for the use
of Observers. (No. 213.) Issued annually. Is.
The Observer's Handbook. Approved, for the use of Meteoro-
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Meteorological Society, the Scottish Meteorological Society,
and the British Rainfall Organization. (No. 191.) Issued
annually. 3s.
The Marine Observer's Handbook. (No. 218. 1915.) 3s.
[Second Edition in preparation^}
Barometer Manual for the use of Seamen. A Text-book of
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Seaman's Handbook of Meteorology. A companion to the
Barometer Manual for the use of Seamen. (No. 215.) Third
Edition, 1918. 3s. 6d.
M) The Computer's Handbook (No. 223)*. The following
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Introduction. C.G.S. Units of Measurements in Meteor-
olog} T , with their Abbreviations and their Equivalents.
1916. % ls.
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of Atmospheric Air : Humidity and Density. 1916. 6d.
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1915-16. 5d.
Subsections II. -IV. Computation of Height and
Temperature by means of Registering Balloons.
The Dynamics of the Upper Air. Tables for the
Estimation of Geostrophic Winds. 1917. Is. 3d.
Section V. Computations related to the Theory of Proba-
bilities, 1915. 6d.
FORECASTING :
(M) The Weather Map. An introduction to Modern Metero-
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(M) Meteorological Glossary in continuation of the Weather
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(II) Forecasting Weather. By W/N. Shaw, Sc.D., F.R.S. Con-
stable & Co., Ltd. 12s. 6d. (Demy 8vo.)
M.O. 233.
/"
^METEOROLOGICAL OFFICE.
CLOUD FORMS
ACCORDING TO THE
INTERNATIONAL CLASSIFICATION
The Definitions and Descriptions
APPROVED BY THE
INTERNATIONAL METEOROLOGICAL COMMITTEE
IN -1910.
WITH AN
ATLAS OF PHOTOGRAPHS OF CLOUDS
SELECTED FROM THE COLLECTION OF MR. G. A. CLARKE
OF THE OBSERVATORY, ABERDEEN.
Issued by the Authority of the Meteorological Committee.
LONDON :
PUBLISHED BY HIS MAJESTY'S STATIONERY OFFICE.
To be purchased through any Bookseller or directly from
H.M. STATIONERY OFFICE at the following addresses :
IMPERIAL HOUSE, KINGSWAY. LONDON, W.C. 2. and
28, ABINGDON STREET, LONDON, S.W.I;
37^ PETER STREET, MANCHESTER ;
1, ST. ANDREW'S CRESCENT, CARDIFF :
23. FORTH STREET, EDINBURGH ;
or from E. PONSONBY, LTD., 116, GRAFTON STREET, DUBLIN.
Price 6d, Net. \, :
ATLAS OF CLOUD FORMS.
PLATE I. Type 1. Stratus (St.). Level sheet of low cloud below 3,000 feet.
PLATE II. Type 01. Fracto-stratus (Fr. St.). Ragged Stratus. Drifting
masses of low cloud.
PLATE III. Type 2. Nimbus (Xb.). Shapeless cloud with ragged lower edge
and rain falling (base be!ow 7,000 feet).
PLATE IV. Type 3. Cumulus (Cu.). Detached cloud with flat base (mean
height 4,500 feet) and rounded top (mean height 6,000 feet).
PLATE V. Type 03. Fracto-cumulus (Fr. Cu.). Ragged cumulus in drifting
masses, 4,000 feet 0,000 feet.
PLATE VI. Type 32. Cumulo-nimbus (Cu. Xb.). Mountainous mass (reach-
ing sometimes from 5,000 feet to 25,000 feet).
PLATE VII. Type 32 with 84. Cumulo-nimbus (Cu. Xb.). Thunder cloud
with large u anvil " of false cirrus.
PLATE VIII. Type 32 with 84. Cumulo-nimbus (Cu. Xb.). Shower cloud ;
top fringed with false cirrus.
PLATE IX. Type 82. Mammato-cumulus (M. Cu.). The last stage of
cumulo-nimbus.
PLATE X. Type 13 Strato cumulus (St. Cu.). Layer of lumpy clouds below
7,000 feet.
PLATE XI. Type 51. Alto-stratus (A. St.). Level sheet at middle height
10,000 to 23,000 feet.
PLATE XII. Type 53. Alto-cumulus (A. Cu.). Layer of large cloudlets in
rows or waves at middle height.
PLATE XIII. Type 93. Cirro-cumulus (Ci. Cu.). Higher than alto-cumulus.
Mackerel sky : layer of small cloudlets in waves.
PLATE XIV Type 73. Alto-cumulus-lenticularis (A. Cu. lent.). Almond-
shaped banks of cloudlets at alto-cumulus level.
PLATE XV. Type 73. Cirro-cumulus-lenticularis (Ci. Cu. lent.;. Almond-
shaped banks of cloudlets at cirro-runntlits level.
PLATE XVI. Type U4. Cirrus (Ci.). Mare* tails with tufted ends (Cirrus
uncinus) (often at about 30,000 feet).
PLATE XVII. Type 91. Cirro-stratus (Ci. St.). Uniform sheet of very high
cloud (30,000 feet), line of ragged cumulus about 25,000 feet below.
METEOROLOGICAL OFFICE.
CLOUD FORMS
ACCORDING TO THE INTERNATIONAL
CLASSIFICATION.
The Definitions and Descriptions approved by the International
Meteorological Committee in 1910, with an Atlas of Photo-
graphs of Clouds selected from the Collection of Mr. Gr. A.
Clarke, of the Observatory, Aberdeen.
The international classifi cation 01 cloud-forms is based upon the
four fundamental types of the classification proposed by Luke
Howard at the beginning of the 19th Century, namely, cirrus, the
thread-cloud ; cumulus, the heap-cloud ; stratus, the flat cloud or
level sheet ; and nimbus, the rain-cloud. The details of a more
precise classification occupied the attention of meteorologists in
many countries during the latter part of the century, among whom
were specially prominent our own countrymen, the Rev. Clement
Ley and the Hon. Ralph Abercromby. A book by Mr. Clement
Ley entitled " Cloudland " is well known to meteorologists. Mr.
Abercromby contributed a number -of papers on the subject, laying-
stress upon the most important fact that cloud-forms are not
peculiar to special localities or latitudes, but are the same for all
parts of the world. Both these gentlemen unfortunately died before
the classification was settled. The other meteorologists who were
specially active in this work were Professor H. H. Hildebrandsson,
of Upsala, Sweden ; M. Leon Teisserenc de Bort, of Paris ; and
M. A. Riggenbach, of Zurich, Switzerland.
In 1891 the International Conference at Munich recommended
the following classification of clouds elaborated by Messrs. Aber-
cromby and Hildebrandsson :
(a) Detached clouds with rounded upper outlines (most
frequent in dry weather).
(b) Clouds of great horizontal extent suggesting a layer or
sheet (suggestion of wet weather)
A. Upper Clouds: average altitude, 9,000 metres (30,000 feet).
(a) 1. Cirrus.
(b) 2. Cirro-stratus.
B. Intermediate Clouds : between 3,000 metres and 7,000 metres
(10,000 feet and 23,000 feet).
/ \ ( 3. Cirro-cumulus.
^ ) \ 4. Alto-cumulus.
(b) 5. Alto-stratus.
C. Lower Clouds : below 2,000 metres (7,000 feet).
(a )6. Strato-cumulus.
(b) 7. Nimbus.
(1698712). Wt. 1864 864. 3,500. 5/18. 1) X S. Gr 3.
420225
D. Clouds of diurnal ascending currents.
(a) 8. Cumulus: top, 1800 'metres (6,000 feet); base, 1,400
metres (4,500 feet).
(b) 9. Cumulo-nimbus : top, 3,000 metres to 8,000 metres
(10,000 feet to 26,000 feet) ; base, 1,400 metres (4,500
feet).
K High Fogs under 1,000 metres (3,300 feet).
10. Stratus.
N. B. The equivalents in feet of the heights given in metres are only
roughly approximate.
After Abercromby's death, at the request of the International
Meteorological Committee in 1894 a Cloud Atlas was prepared by
Messrs. Hildebrandsson, Riggenbach and Teisserenc de Bort and
published in 1895, with excellent illustrations in photochromotype
executed at Zurich. It was naturally very acceptable to meteoro-
logists in all countries, and the number of copies was soon exhausted.
In 1905 the International Meteorological Conference at Innsbruck
requested Messrs. Hildebrandsson and Teisserenc de Bort to
prepare a new edition, making at the same time some suggestions
for the improvement of the classification and definitions. Among
these suggestions the following should be noted because it indicates
a special type of cloud. " Certain cloud forms which are particu-
larly frequent on days of sirocco, mistral or fohn, have an oval
shape and occasionally show irisation. These cloud forms are to be
described by the term lenticularis, r.y., cumulus lenticularis (cu.
lent.), stratus lenticularis (st. lent.). A plate illustrating this cloud
form should be included in the Cloud Atlas."
These lenticular, lentil-shaped or almond-shaped clouds have
attracted some attention in recent times. They have a peculiar
outline. In many cases they are very suggestive of an airship, and
are perhaps the clouds in"" Hamlet" which are "very like a
whale " (Plate XIV). In others the inner part of the cloud
becomes very thin, or disappears, so that the shape looks like a large
horse-shoe as seen from beneath at a great distance (Plate XV).
Photographs of typical specimens have been secured by Mr. Gr. A.
Clarke at Aberdeen, and by Captain Cave at Ditcham Park. At
the Meteorological Office there is a very fine specimen of a stereo-
photograph, with a wide base, of one of horse-shoe-shape, which
shows admirably the detail of the structure of curled wisps which
form the cloud. The photographs and eye-observations show that
the bank .of clouds which keeps its position with little apparent
change is really composed of a mass of cloudlets, forming and
drifting into the cloud-bank with the wind at one side drifting away
from it and dissolving at the other. Thus the stationary appear-
ance of the cloud-bank is illusory as regards the wind. The wind
blows throuyli the cloud-bank, which is formed by the massing of
the drifting cloudlets. The cloudlets belong apparently to the
type of alto-cumulus or cirro-cumulus, not to the massed clouds,
stratus or cumulus, as suggested by the Conference. The example
inserted in the new edition of the Cloud Atlas is rirro-cumulus-
lenticularis. The relation of this cloud to sirocco, which is a
southerly wind in front of an advancing depression of the Mediter-
ranean Sea, or to the fnlin, which is the continuation of the same
wind on the northern side of the Alps, or to the mistral, which is
a dry wind from the North- West on the French Mediterranean
coast, has not been investigated.
Upon two examples of this type of cloud Mr. Clarke remarks
as follows : " Very often the intermediate clouds of the cirro-
cumulus, alto-cumulus and strato-cumulus types may be seen
massed together in long oval or torpedo-shaped sheets. These
are termed Lenticular clouds, from the resemblance of their
form to that of the cross-section of a lens. These lenticular
masses are found sometimes detached but at other times cover
the sky in dense sheets at several different levels, and are
generally seen when the wind is blowing from some point in the
south-west quadrant. The following conditions are found to
accompany their appearance : (1) The sky, when visible, is usually
of a very intense blue colour ; (2) the barometer is exceedingly
unsteady, rising and falling jerkily at very short intervals of time ;
(3) the wind is usually strong or high and of a very gusty charac-
ter, and in addition there is a periodic rise and fall in its average
velocity. At times the lower clouds, such as Cumulus and Stratus
are seen to assume a somewhat similar form in quiet weather, but in
such cases the conditions above-mentioned will be absent."
That clouds similar to those associated with mistral or fohn
are to be found in south-westerly winds at Aberdeen, where
there are mountains to the south-westward, and over the South
Downs at Ditcham Park, points perhaps to the orographical influence
of the land surface upon the currents of the upper air, and it seems
desirable to have a definite class to which such clouds can be
referred.
The new edition of the Cloud Atlas was issued in 1910, and the
definitions and descriptions of cloud forms are as follows :
[The translation into English has been altered in certain respects from that
which appears in the English version of the introduction to the International
Cloud Atlas, in order to represent more closely the original French.]
INTERNATIONAL DEFINITIONS AND DESCRIPTIONS
OF CLOUD-FORMS.
1. * Cirrus (Ci.). Detached clouds of delicate appearance, fibrous
(threadlike) structure and feather-like form, generally white in colour.
Cirrus clouds take the most varied shapes, such as isolated tufts of
hair, i.e., thin filaments on a blue sky, branched filaments in
feathery form, straight or curved filaments ending in tufts (called
cirrus uncinus), and others. Occasionally cirrus clouds are arranged
in bands, which traverse part of the sky as arcs of great circles,
and as an effect of perspective appear to converge at a point on the
horizon, and at the opposite point also if they are sufficiently
extended. Cirro-stratus and cirro-cumulus also are sometimes
similarly arranged in long bands. Plate XVI.
* It may be noted that the outline of the sun is visible, and his rays cast a
shadow in spite of the presence of clouds of this type, unless the clouds and
the sun are both low down on the horizon.
(16987) A 2
2. * Cirro-Stratus (Ci.-St.). A thin sheet of whitish-cloud ; some-
times covering- the sky completely and merely giving- it a milky
appearance ; it is then called cirro-nebula or cirrus haze ; at other
times presenting more or less distinctly a fibrous structure like
a tang-led web. This sheet often produces halos around the sun
or moon. Plate XVII.
3. *Cirro-Cumulus (Ci.-Cu.) (Mackerel Sky). Small rounded masses
or white flakes without shadows, or showing very slight shadow :
arranged in groups and often in lines. French, Moutons German,
Sckafchen-wolken. Plate XIII.
4. Alto-Stratus (A.-St.). A dense sheet of a grey or bluish colour,
sometimes forming a compact mass of dull grey colour and fibrous
structure. At other times the sheet is thin like the denser forms
cirro-stratus, and through it the sun and the moon may be seen
dimly gleaming as through ground glass. This form exhibits all
stages of transition between alto-stratus and cirro-stratus, but
according to the measurements its normal altitude is about one-half
of that of cirro-stratus. Plate XI.
5. Alto-Cumulus (A.-Cu.). Larger rounded masses, white or
greyish, partially shaded, arranged in groups or lines, and often so
crowded together in the middle region that the cloudlets join. The
separate masses are generally larger and more compact (resembling
strato-cumulus) in the middle region of the group, but the denseness
of the layer varies and sometimes is so attenuated that the indivi-
dual masses assume the appearance of sheets or thin flakes of
considerable extent with hardly any shading. At the margin of
the group they form smaller cloudlets resembling those of cirro-
cumulus. The cloudlets often group themselves in parallel .lines,
arranged in one or more directions. Plate XII.
6. Strato-Cumulus (St.-Cu.). Large lumpy masses or rolls of dull
grey cloud, frequently covering the whole sky, especially in winter.
Generally strato-cumulus presents the appearance of a grey layer
broken up into irregular masses and having on the margin smaller
masses grouped in flocks like alto- cumulus. Sometimes this cloud-
form has the characteristic appearance of great rolls of cloud
arranged in parallel lines close together. (Roll-cumulus in England,
Wulst-cumulus in Germany.) The rolls themselves are dense and
dark, but in the intervening spaces the cloud is much lighter and
blue sky may sometimes be seen through them. Strato-cumulus
may be distinguished from Nimbus by its lumpy or rolling
appearance, and by the fact that it does not generally tend to bring
rain. Plate X.
7. Nimbus (Nb.). A dense layer of dark, shapeless cloud with
ragged edges from which steady rain or snow usually falls. If there
are openings in the cloud an upper layer of cirro-stratus or alto-stratus
may almost invariably be seen through them. If a layer of nimbus
separates in strong wind into ragged cloud, or if small detached
clouds are seen drifting underneath a large nimbus (the " Scud "
of sailors), either may be specified as fracto-nimbus (Fr.-Nb.).
Plate III.
* It may be noted that the outline of the sun is visible, and his rays cast ;t
shadow in spite of the presence of clouds of these types, unless the clouds and
the sun are both low down on the horizon.
S. Cumulus (Cu.) (Woolpack or Cauliflower Cloud). Thick cloud of
which the upper surface is dome-shaped and exhibits protuberances
while the base is generally horizontal. These clouds appear to be
formed by ascensional movement of air in the daytime which is
almost always observable. When the cloud and the sun are on
opposite sides of the observer, the surfaces facing the observer are
more brilliant than the margins of the protuberances. When 011
the contrary, it is on the same side of the observer as the sun it
appears dark with bright edges. When the light falls sideways, as
is usually the case, cumulus clouds show deep shadows. Plate IV.
True cumulus has well-defined upper and lower margins ; but one
may sometimes see ragged clouds like cumulus torn by strong
wind of which the detached portions are continually changing ; to
this form of cloud the name Fracto-Cumulus may be given. Plates
V and XVIJ.
9. Cumulo-Nimbus (Cu.-Nb.) the Thunder Cloud ; Shower Cloud.
Great masses of cloud rising in the form of mountains or towers or
anvils, generally having a veil or screen of fibrous texture (false
cirrus) at the. top and at its base a cloud-mass similar to nimbus,
From the base local showers of rain or of snow, occasionally of hail
or soft hail, usually fall. Sometimes the upper margins have the
compact shape of cumulus or form massive heaps round which
floats delicate false cirrus. At other times the margins themselves
are fringed with filaments similar to cirrus clouds. This last form
is particularly common with spring showers. The front of a thunder-
storm of wide extent is frequently in the form of a large low arch
above a region of uniformly lighter sky. Plates VI, VII and VIII.
10. Stratus (St.) A uniform layer of cloud like fog but not lying on
the ground. Plate I. The cloud layer of stratus is always very low.
It' it is divided into ragged masses in a wind or by mountain tops, it
may be called Fracto-Stratus. Plate II. The complete absence of
detail of structure differentiates stratus from other aggregated forms
of cloud.
The following remarks are added in the international atlas as
instructions to observers.
(a) In the daytime in summer all the lower clouds assume, as a
rule, special forms more or less resembling cumulus. In such cases
the observer may enter in his notes " Stratus- or nimbus-cumuli-
formis."
(b) Sometimes a cloud will show a mammillated surface and the
appearance should be noted under the name mammato-cumulus.
Plate IX.
(c) The form taken by certain clouds particularly on days of
sirocco, mistral, fohn, etc., which show a^i ovoid form with clean out-
lines and sometimes irisation, will be indicated by the name lenticular,
for example : cumulus lenticularis, stratus lenticuiaris (Cu.-lent., St.-
lent.).
(d) Notice should always be taken when the clouds seem motion-
less or if they move with very great velocity.
The illustrations in colour in the Cloud AfcJas, twenty-nine in all,
comprised
Four examples of cirrus. Two of cumulus,
One of cirro-stratus, One of fracto-cuniultis,
One of cirro-cumulus, Four of cumulo-nimbus,
Two of alto-stratus, One of mammato-cumuliis.
Two of alto-cumulus, One of stratus,
Two of strato-cumulus, One of fracto-stratus,
Three of nimbus, One of cirro-cumulus-lenticularis.
One of fracto-nimbus, One additional alto-cumulus,
One of tufted cirrus.
The Atlas is now unfortunately again out of print, and, in
consequence of the war, it is not likely that a reprint can be
obtained. In order to meet immediate requirements, a number of
photographs have been selected from Mr. G. A. Clarke's collection
and are reproduced here as a provisional atlas of cloud-forms.
It includes two specimens of lenticular cloud banks, one at the
alto-cumulus-level and the other at the cirro-cumulus-level. Plates
XIV and XV. A definition of this peculiar form of cloud is
necessary. It may be put into the following words.
11. Lenticular Cloud Banks. Banks of cloud of an almond or air-
ship shape, with sharp general outlines, but showing, on close
examination, fretted edges, formed of an ordered structure of cloudlets
similar to alto-cumulus or cirro-cumulus which is also seen in the
bank itself when the illumination is favourable. Sometimes the body
of the cloud bank is dense, and the almond shape is complete, fore
and aft, but sometimes the bank thins away from the forward edge
to clear sky within, so that the bank presents the appearance of a
horse-shoe seen in perspective from below at a great distance. The
bank appears nearly or quite stationary, while the cloudlets move
rapidly into it at one side and away from it at the other.
Although we have at present no illustrative plate we may add a
description of a form of cloud to which Clement Ley gave a separate
name.
12. Alto-Cumulus-Castellatus. ' : Little miniature cumulus rising
in many heads from a more or less compact layer of alto-cumulus."
" Not a very common cloud in these latitudes but sometimes seen in
Summer, and when, coming from a westerly or south-westerly point
is almost always a sign of the approach of shallow depressions which
bring thunderstorms."
Captain C. J. P. Cave, R.E. The Form of Clouds. Q. J. Roy : M<>t : No,-.
Vol. XLIII, p. 68. 1917.
CLOUD TYPES.
In making a cloud Atlas it has always been customary to begin
with the highest clouds, viz., cirrus and cirro-stratus, and gradually
to let our view descend to the surface, where we find the only form
of cloud actually known to many of us by practical experience of
its interior, namely, fog or mountain-stratus. The learner or
inexperienced reader may find it easier to proceed the other way,
and starting from the fog, with which he is familial 1 , pass to the
consideration of stratu 5 , and nimbus, with which lie lias also a
working acquaintance, to cumulus, which is the first cloud to prompt
an answer to the question of its origin and development, and so on
to the cloud forms some ordered, some irregular of the upper
regions of the air, about the formation and structure of which we
know little.
It should be noticed in this connexion, that the problem presented
to those who classify clouds is of a dual character. There are first
the forms of individual clouds, stratus, nimbus., cumulus, and cirrus,
while the other forms are really aggregates, or groups of clouds or
cloudlets, arranged sometimes in a continuous mass, sometimes in
rows or waves, not infrequently in double or even triple sets of
waves. There are all sorts of gradations, from the dappled
mackerel sky of cirro-cumulus to the alto- cumulus, with a dense
central portion and separate clouds on the margins, the irregular
masses of strato-cumulus, and finally the continuous stratus which is
to be found at various different levels low, intermediate, and high.
We can hardly exclude the continuous stratus itself from con-
sideration as a group or aggregate, because when it thins it breaks
up into detached clouds.
I have arranged the photographs according to these ideas, and in
numbering the types I have adopted the following scheme of
enumeration :
TYPES OF SiNGLE-Ci,QUD-FoRMS.
The figure 1 means stratus
,, ,, *2 nimbus
,, ,, 3 ,, cumulus
,,4 cirrus
TYPES OF CLOUD GROUPS.
The figure 5 means clouds in ordered groups of middle height
between 7,000 feet and 25,000 feet.
,. 6 ragged clouds drifting in detached masses
or irregular groups.
7 lenticular groups ; almond-shaped or
horseshoe- shaped banks of cloudlets,
generally between 10,000 feet and
25,000 feet.
OTHER TYPES.
The figure 8 means portions of a cloud-mass having a peculiar
structure
9 the highest forms of clouds.
The combination of two figures gives the appropriate number for
the types which appear in the international classification, and are
illustrated in the Atlas.
A word must be added about cirrus. It is generally understood
to be not only a cloud of thread-like structure, as its name implies,
but at the same time a very high-cloud, its normal height being
about 9 kilometres, or nearly 30,000 feet. No doubt the best and
most durable examples are to be found at those great heights, but
10
from personal observation and from the cQnsideration of many
observations by Mr. Clarke, I think it must be allowed that thread-
like clouds, indistinguishable iii appearance from wisps of true
cirrus, mav be found at much lower levels just as the so-called false
cirrus is formed at various heights. Captain C. K M. Douglas,
from close observation in an aeroplane, expresses the opinion that
false cirrus, or, in other words, thread-like structure, is always
attributable to clouds formed of ice crystals, and if that be the
properly distinctive characteristic of the thread-like structure, it
only hampers our conception of the atmospheric processes if we
assume all clouds which show that structure to be at a very high
level. The form is really suggestive of the formation of cloud by
some special physical process, such as the reduction of the pressure
of a mass of air which contains exceptional quantity of water
vapour in streaks or wreaths. It is better, therefore, to regard
cirrus as being a special form of cloud which may be developed in
suitable circumstances at any level where ice crystals can form, and
where a reduction of pressure, in consequence of external changes,
may occur ; this may be any region beyond the four- kilometre
level in our latitude. ' Consequently, J give a separate number 4 to
cirrus as a cloud of thread-like structure, and another number 9
to the highest clouds, so that the clouds that we are accustomed to
call " true cirrus " that is, thread-like clouds in the region of
9 kilometres height are designated by the composite number 94.
The-information given here is concerned almost entirely with the
identification of cloud forms which is only the first step in the
meteorological study of clouds. It should be noted that in the year
1896-97. known among meteorologists as the international cloud-
year, observations were made on an international basis of the height
and the velocity of motion of all the typical forms of cloud at the
following observatories : Bossekop (Norway), Upsala (Sweden),
Pavlovsk near Petrograd (Russia), Potsdam (Germany), Trappes
near Paris (France), Toronto (Canada), Blue Hill near Boston and
Washington B.C. (U.S.A.), Allahabad (India), and Manilla
(Philippine Islands). A very instructive summary of these and
other observations of clouds is given in Chapters IV and VII of
Let bases de la mWowfaf/ic (li/naini</u<< par H. H. Hildebrandss&n ^
LI-OH Triwrt-m' (fr Bort. ' According to the discussion of the
heights of clouds at Potsdam, clouds tend to favour the following
levels (the heights are in metres) 500, 2,000, 4,300, 6,500, 8,300,
9,900. These heights are, however, only general means. They
vary from one station to another and are also subject to diurnal and
seasonal variations.
NAPIFK SHAW.
Meteorological Office, London,
May, 1918.
2. Reports of Investigations in Meteorology
and Geophysics.
(M) Geophysical Memoirs (4to.) :
VOL. I., PART L, FOR 1911 :
No. 1. The Effect of the Labrador Current upon the Surface
Temperature of the North Atlantic ; and of the latter upon
Air Temperature and Pressure over the British Isles. By
M. W. Campbell Hepworth, C.B., R.D., Commander R.N.R.,
Marine Superintendent. (No. 210a. 1912.) 9d.
No. 2. The Free Atmosphere in the Region of the British Isles.
Second Report by W. H. Dines, F.R.S., with a Preface by
W. N. Shaw, Sc.D., F.R.S., Director. (No. 2106. 1912.) Is.
No. 3. Graphical Construction for the Epicentre of an Earth-
quake. By G. W. Walker, M.A., Superintendent of Eskdale
Observatory. (No. 210c. 1912.) 3d.
VOL. L, PART II., FOB 1912 :
No. 4. On the Radiation Records obtained in 1911 at South
Kensington. By R. Corless, M.A., Secretary to the Director.
(No. 210d. 1912.) 3d.
No. 5. The International Kite and Balloon Ascents, by Ernest
Gold, M.A., Superintendent of Statistics. (No. 2100. 1913.)
Is. 6d.
No. 6. The Free Atmosphere in the Region of the British Isles.
Third Report by W. H. Dines, F.R.S. (210/. 1914.) 3d.
No. 7. A comparison of the Electrical Conditions of the
Atmosphere at Kew and Eskdalemuir, by Gordon Dobson,
B.A., Graduate Assistant. (No. 210^. 1914.) 8d.
No.* 8. Lag in Marine Barometers on Land .and Sea. By
Charles Chree, Sc.D., LL.D., F.R.S., Superintendent of Kew
Observatory. (No. 2107?. 1914.) 4d.
VOL. L, PART III., FOR 1913 :
No. 9. On the Relation between the Velocity of the Gradient
Wind and that of the Observed Wind at certain M.O.
Stations. By J. Fairgrieve. (No. 2101. 1914.) Is.
No. 10. The Effect of the Labrador Current upon the
Surface Temperature of the North Atlantic, and of the
latter upon^Air Temperature and Pressure over the British
Isles. Part II. (see ftfbmoir, No. 1). By M. W. Campbell
Hepworth, C.B., R.D., Commander R.N.R., Marine Superin-
tendent. (No. 210j. 1914). 8d."
[Geophysical Memoirs, Volume I., is on sale at the Meteoro-
logical Office, complete, with title page, table of contents,
and index, unstitched, price 7s. (No. 210.)]'
. VOL. II., PART L, FOR 1914 :
No. 11. The South Wales Tornado of October 27, 1913.
(No. 220a. 1915.) 6d.
VOL. II., PART II., FOR 1917 :
No. 12. The Travel of Circular Depressions. By Sir Napier
Shaw, F.R.S., Director. (No. 220b. 1917.) 9d.
(M) Professional Notes (8vo.) :
No. 1. On the Inter-relation of Wind Direction and Cloud
Amount at Richmond (Kew Observatory). By Lt. David
Brunt, R.E. (No. 232a. 1918.) 3d.
(16987.)
3. Tl
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