CLOUD STUDIES
[Illustration: A SUNSET SKY.
_Frontispiece._]
CLOUD STUDIES
By ARTHUR W. CLAYDEN, M.A.
PRINCIPAL OF THE
ROYAL ALBERT MEMORIAL COLLEGE, EXETER
LONDON
JOHN MURRAY, ALBEMARLE STREET
1905
PRINTED BY
WILLIAM CLOWES AND SONS, LIMITED,
LONDON AND BECCLES.
PREFACE
To the meteorologist I hope the following pages may prove not only of
some interest, but of practical value as a small step towards that
greater exactness of language which is essential before we can attempt
to explain all the details of cloud structure, or even interchange
our ideas and observations with adequate precision. The varieties
depicted and described have been selected from many hundreds, as those
which seem to me to show such differences of form as to imply distinct
differences in the conditions to which they are due. I have not
attempted to deal with the physical causes of condensation except in a
general way, being unwilling to introduce diagrams of isothermals and
adiabatics and such purely scientific methods into a work also intended
for a wider public. For those who wish to pursue this part of the
subject I have appended a list of papers from the _Quarterly Journal
of the Royal Meteorological Society_ and other sources, which may
serve as references. I also hope that some more votaries of the science
may be induced to realize that meteorology does not consist solely of
the tabulation of long columns of records, but includes subjects for
investigation as much more beautiful as they are more difficult.
To the artist I trust they may also be of some use, by calling
attention to the variety and exquisite beauty of the sky. Nothing is
more extraordinary in art than the general negligence of cloud-forms.
Many of them are quite as worthy of careful drawing as the leaves of a
tree, the flowers of a field, the ripples on a stream, or the texture
of a carpet, or a marble pavement. Yet it is the common rule to find
pictures, which are otherwise marvellous examples of skill and care,
disfigured by impossible skies with vague, shapeless clouds, as untrue
to nature as it would be possible to make them. Grace of outline,
delicacy of detail and texture, richness of contrast, beauty of form
and light and colour, all are present in the skies, and combine to make
a whole well worthy of the best that art can give. The illustrations I
offer are not selected for pictorial effect; they are chosen from a
purely scientific point of view; but they are enough to indicate what
could be done if the facts of nature were treated with high artistic
skill.
In addition to the meteorologist and the artist, there are a much
larger number who follow neither profession, but who love Nature in
all her moods; and to them also I hope these pages may be of interest.
Indeed, if only a few of them should be stimulated to take up a branch
of nature study which has given me many an hour of quiet enjoyment, the
labour of bringing these notes together will not have been in vain.
ARTHUR W. CLAYDEN.
St. John’s,
Exeter.
CONTENTS
CHAPTER PAGE
I. Introductory 1
II. Cirrus 21
III. Cirro-stratus and Cirro-cumulus 45
IV. Alto Clouds 59
V. Lower Clouds 71
VI. Cumulus 84
VII. Cumulo-nimbus 105
VIII. Wave Clouds 119
IX. Cloud Altitudes 137
X. Cloud Nomenclature 154
XI. Cloud Photography 165
References 181
Index 183
LIST OF ILLUSTRATIONS
PLATE PAGE
A Sunset Sky _Frontispiece_
1. Part of a Great Halo 22
2. Part of a Solar Halo 23
3. Cirro-nebula changing to Cirro-stratus 24
4. Cirro-nebula changing to Cirro-cumulus 27
5. High Cirrus. (_Cirrus Excelsus_) 31
6. Windy Cirrus. (_Cirrus Ventosus_) 32
7. Thread Cirrus. (_Cirro-filum_) 34
8. Tailed Cirrus. (_Cirrus Caudatus_) 35
9. Hazy Cirrus. (_Cirrus Nebulosus_) 36
10. Change Cirrus. (_Cirrus Inconstans_) 37
11. Common Cirrus. (_Cirrus Communis_) 40
12. Band Cirrus. (_Cirrus Vittatus_) 41
13. Band Cirrus. (_Cirrus Vittatus_) 42
14. Hazy Cirro-stratus. (_Cirro-stratus Nebulosus_) 46
15. Cirro-stratus 47
16. Cirro-stratus. (_Cirro-stratus Communis_) 48
17. Flocculent Cirro-stratus. (_Cirro-stratus Cumulosus_) 49
18. Cirro-stratus and Cirro-cumulus 50
19. Cirro-cumulus 50
20. Hazy Cirro-cumulus. (_Cirro-cumulus Nebulosus_) 51
21. Hazy Cirro-cumulus. (_Cirro-cumulus Nebulosus_) 51
22. A Sunset Sky 52
23. Speckle Cloud (Ley). (_Cirro-macula_) 53
24. Cirrus Caudatus and Cirro-macula 55
25. Alto-cumulus Informis 64
26. Hazy Alto-cumulus. (_Alto-cumulus Nebulosus_) 65
27. Flat Alto-cumulus. (_Alto-cumulus Stratiformis_) 65
28. High Turreted Cloud. (_Alto-cumulus Castellatus_) 66
29. High Ball Cumulus. (_Alto-cumulus Glomeratus_) 67
30. Mackerel Sky. (_Alto-stratus Maculosus_) 68
31. Mackerel Sky. (_Alto-stratus Maculosus_) 69
32. Alto-strato-cumulus 70
33. Sunset. (_Alto-cumulus Castellatus Fractus_) 70
34. Three Layers of Stratiform Cloud after Rain 73
35. Rain-Cloud. (_Nimbus_) 75
36. Rain-Cloud. (_Nimbus_) 75
37. Stratus Communis 77
38. Strato-cumulus 77
39. Strato-cumulus 78
40. Stratus Maculosus 78
41. Common Stratus. (_Stratus Communis_) 79
42. Roller Cloud. (_Stratus Radius_) 80
43. Small Cumulus. (_Cumulus Minor_) 94
44. Cumulus 95
45. Large Cumulus. (_Cumulus Major_) 96
46. Fracto-cumulus 97
47. Fall Cloud. (_Stratus Lenticularis_) 98
48. Thunder-clouds forming 109
49. Thunder-clouds. (_Cumulo-nimbus_) 110
50. Thunder-clouds. (_Cumulo-nimbus_) 111
51. Thunder-cloud. (_Cumulo-nimbus_) 111
52. Thunder-cloud. (_Cumulo-nimbus_) 111
53. The Flank of a Great Storm 112
54. Crested Alto Waves. (_Alto-cumulus Undatus_) 120
55. Alto Waves. (_Alto-stratus Undatus_) 121
56. Cirro Ripples. (_Cirro-cumulus Undatus_) 122
57. Waved Cirro-stratus. (_Cirro-stratus Undatus_) 136
58. Camera for measuring Altitudes 141
59. Print from a Negative used for measuring Altitude 144
60. Pair of Prints showing the Displacement of the Cloud 145
61. Cloud Camera for Studies 171
CLOUD STUDIES
CHAPTER I
INTRODUCTORY
ALL who have the faculties proper to man must have been to some extent
students of cloud form. Go where we will, do what we will, we cannot
easily escape from the sky, or avoid noticing some of its features
and coupling them with the varying conditions of weather. We all
sometimes want to know if it is likely to rain, or whether some other
change is probable; and experience soon shows us that the clouds give
the simplest and most obvious indication of what we may expect. It is
almost impossible to avoid noticing that certain types of cloud, or the
simultaneous appearance of certain types, is the usual accompaniment
of definite kinds of weather or of particular changes. Thus it is that
most people acquire some small measure of weather wisdom before their
schooldays are over.
Generation after generation, through all human history, the same causes
must have led to the same conclusions; and the study of clouds must,
therefore, be one of the oldest of all branches of scientific inquiry.
Yet, old as it is, it is still in its infancy, having made very little
advance indeed towards the precision of an exact science.
There are many reasons for this want of growth, and so far as
the theoretical aspects of the subject are concerned it is easy
enough to understand. Clouds are among the most inaccessible of
terrestrial objects. Except by balloon ascents, by sending up kites
bearing recording instruments, or by making observations among the
mountain-tops, we have no means of getting at them to study the
conditions under which they exist. Temperature, pressure, humidity,
have generally to be guessed at, those guesses being based on the
scanty data which have been laboriously obtained by one or another of
these cumbrous methods. Moreover, many clouds have such vast dimensions
that it is very difficult to grasp all that goes on in such a space.
Besides the difficulty of attacking the problems presented by cloud
formation, it is probable that even if we could have got among the
clouds at will, we should have understood little more than we do, from
a want of sufficient certainty on many of the purely physical questions
involved. It is not many years since Mr. J. Aitken discovered the
necessity for material nuclei as a first step in the formation of cloud
particles, and not many months have elapsed since Mr. C. T. R. Wilson
showed that those particles can be formed by the action of radiation
on the air itself. There is nothing surprising, therefore, in the fact
that our theoretical knowledge of the why and wherefore of the facts
revealed by a study of clouds is limited to general principles, and
quite fails to say exactly why each special form should be assumed. The
matter for surprise is quite different.
Theoretical explanations are not the first step in the working out
of a branch of science. It begins with the acquisition, by diligent
and painstaking observation, of a great mass of facts. This may go on
for centuries, the accumulation growing greater and greater, until
at last some one comes who examines the records, classifies them
carefully, and finally makes a summary in the form of a number of
generalizations, which are announced under the name of Laws.
Two examples of such “Laws” will suffice. Astronomers for centuries had
observed the movements of the planets, always with increasing accuracy,
until Tycho Brahe made his famous series of observations on the planet
Mars. These materials fell into the hands of Kepler, and the result of
his work was the announcement of Kepler’s Laws, which state the rules
which govern the movements of the planets in their orbits. He found
that the records could not be accounted for unless the planets moved
in a certain way, but he knew nothing of the reasons for a method and
order which clearly existed.
Kepler’s Laws, in fact, rest upon another set, namely, Newton’s Laws of
Gravitation, and these are themselves a second example. They are the
summary of accumulated experience, and even at the present day we know
nothing certain as to why two bodies should attract each other, and
nothing as to why that mutual attraction should act as it was found to
act by Newton.
The observational part of cloud study, however, is still in its
infancy, in spite of the fact that it has been going on for such
countless ages. We are still in the condition of the humble observers
engaged in the comparatively humdrum task of gathering facts for
future arrangement and interpretation. Cloud observers, in all ages,
have suffered from a peculiar difficulty. They have had no common
language, no code of signs by which they could benefit from the work
of those who had gone before them, no means of transmitting their own
experience to each other, or to those who would come after them. No
progress would be possible in any study under such conditions, for
each person would begin where the previous generation began, instead
of taking up the task where others had left it. In all languages there
is an extraordinary scarcity of cloud names, and such as do exist are
frequently applied to quite different forms by different people. So
pronounced is this lack of terms, that any one who tries to describe
a sky without using any of the modern scientific names, finds himself
obliged to rely on long detailed descriptions, backed with references
to well-known objects, whose outlines or structures resemble the
clouds more or less vaguely; and even then he has to be a word-painter
of singular skill if his description calls up in the mind of the reader
a picture much like the original.
It was to meet this want of a common tongue that Luke Howard, in 1803,
proposed his scheme of cloud names. He recognized three main types of
cloud architecture, which he named Cirrus, Stratus, and Cumulus. Cirrus
included all forms which are built up of delicate threads, like the
fibres in a fragment of wool; Stratus was applied to all clouds which
lie in level sheets; and Cumulus was the lumpy form.
By combinations of these terms other clouds were described. Thus, a
quantity of cirrus arranged in a sheet was called cirro-stratus, while
high, thin clouds like cirrus, but made up of detached rounded balls,
was cirro-cumulus. Many cumulus clouds, arranged in a sheet with little
space between them, became cumulo-stratus, while the great clouds from
which our heavy rains descend partake, to some extent, of all three
types, and were therefore distinguished by a special name--Nimbus.
This system had much to recommend it. The three fundamental types
were obvious to all. Their names were descriptive, and were derived
from a dead language, so that no living international jealousies were
raised. It was sufficiently detailed to serve the purposes of the time,
when accurate observation was in its infancy. Hence it was universally
adopted, and will pretty certainly hold its own as the broad basis upon
which any more detailed system must necessarily rest.
It has done excellent service; but although observation of clouds in
a general way is far from complete, attention is now being given to
much smaller details and much more minute differences of form, and our
vocabulary must be amplified. Precision of description is the first
essential of a satisfactory system, and the question is, what sort of
edifice can we build on Luke Howard’s foundation.
The great difficulty is the infinite variety of clouds. Certain forms
may be arbitrarily selected as types, and names may be given to them;
but however well they are chosen, a very short period of observation
will show that there are all manner of intermediate forms, which make
a perfect gradation from one type to another. This fact should never
be forgotten. There is always a danger that the use of any system
of names based on types shall lead to the neglect of everything not
typical. A curious illustration is afforded by the well-known fact,
that in arranging collections of fossil shells, it is frequently found
that some specimens do not exactly match the type examples to which
names have been assigned. In former days it was the custom to throw
aside such “bad specimens,” as they did not show plainly the specific
characters. It is now realized that they have a value of their own,
in that they are the links in the evolutionary chain, once supposed
to be missing. Indeed, it is not unfrequent nowadays to see carefully
selected series, showing the gradual change whereby one species
passed into another, displayed in the place of honour, while the type
specimens are relegated to humbler places in the general collection.
Types there must be, no doubt, and where the series is continuous,
some one must make the selection. With clouds the series is absolutely
continuous. The task is like choosing typical links from a long chain
in which each link is almost exactly like its neighbours, yet no
two are alike, and the greater the distance between them the less
their likeness. Clearly any system put forward must be accompanied
by illustrations, so that all may know exactly which links have been
chosen.
Many attempts have been made to meet the want; some of the systems
proposed being based on the forms assumed by the clouds, some on
their supposed mode of origin, and some on their altitudes. Those
which were not founded on Luke Howard’s types had no chance of being
accepted, while knowledge was not yet sufficiently far advanced to make
classifications based on origin of form at all possible. But the great
reason why none of the proposed schemes could come into general use was
that they were put forward without adequate illustration, so that none
but their authors knew exactly what they meant.[1]
Matters came to a head in 1891, when an International Meteorological
Conference met at Munich. One object of this gathering was to promote
inquiries into the forms and motions of clouds, by means of concerted
observations at the various institutes and observatories of the globe.
Luke Howard’s system was not enough for the purpose in view, and the
addition of more detailed terms had to be settled before work could be
begun.
Professor Hildebrandsson, of Upsala, and the Hon. Ralph Abercromby
jointly submitted a revised scheme, the main feature of which was the
introduction of a new class of clouds, to be distinguished by the
prefix alto-before the other name. Such alto clouds were less lofty and
denser than cirrus. This scheme was the best before the Conference, and
without waiting to discuss, and possibly improve it, it was formally
adopted, and a committee appointed to arrange and publish an atlas
showing pictures of the type-forms. This atlas did not appear until
1896, and in the mean time the Rev. W. Clement Ley had published
proposals of his own, some of which had much to recommend them. But
he was too late. The International Committee had come to a decision,
and, although it may be far from ideal, the system backed by such an
authority must be regarded as the standard until some similar gathering
has given worldwide sanction to a change, and even then it would be
better to modify by addition rather than by substitution.
The subjects of the following pages are named in general accordance
with this International Code, but they are by no means restricted to
types. Their object is not to attempt any repetition of the work which
has already been well done by the Atlas Committee, but rather to show
the chief varieties within a type. It will, however, become abundantly
evident that the standard system is far from complete, and that any
minute and detailed study of cloud-form must take note of the precise
variety.
This at once raises the question whether many of these varieties
are not sufficiently distinct to be given definite names. If a
meteorologist is told that cirrus clouds were seen on a particular
occasion, he instinctively asks--What sort of cirrus? and is utterly
unable to form any mental picture of the clouds until the question
has been answered by a detailed description. A glance at a few of the
plates further on will show the difficulty plainly, and it occurs with
other forms of cloud as well as cirrus.
Is it not time that the International names were regarded as those of
the cloud genera, and to add specific names for those varieties which
seem to imply some difference in kind in the conditions which have led
to their formation? This has been here attempted by translating into
Latin the ordinary English term by which the variety would naturally
be described. More extended observation will probably show that other
species should be introduced, and possibly some of those suggested
in these pages may have to be subdivided. Whatever the names may be,
specific distinction of some sort is an essential preliminary to
detailed study of the why and wherefore of the particular forms.
The International system is as follows:--
A. Upper clouds.
(_a_) Cirrus.
(_b_) Cirro-stratus.
B. Intermediate clouds.
(_a_) Cirro-cumulus and alto-cumulus.
(_b_) Alto-stratus.
C. Lower clouds.
(_a_) Strato-cumulus.
(_b_) Nimbus.
D. Clouds of diurnal ascending currents.
(_a_) Cumulus and cumulo-nimbus.
E. High fogs.
(_b_) Stratus.
In this tabulation the forms marked (_a_) are detached and occur in dry
weather, while those marked (_b_) are widely extended. The original
scheme also gives the mean heights of the various types, but these
values have been omitted here because they are extremely variable,
and impossible to ascertain with any approach to accuracy by mere eye
estimates. They vary also with the season, and probably also with the
locality. Moreover, the altitude is no guide to the name, except that
on the whole the types occur in the order given, taking group A as the
highest and group E as the lowest. In the chapter on cloud altitudes
this subject will be further considered, and under the descriptions
of cloud-forms their average height or actual measurements for the
particular specimen figured will be given whenever possible.
Before coming to the description of individual forms, it may not be out
of place to give brief consideration to the best means of observing
them in nature. For eye observation, of course, no directions are
needed when we are dealing with the lower and denser varieties; but
when we come to the highest groups it sometimes becomes necessary to
protect the eye from the brilliant glare which may make it impossible
to detect the real structure. Smoked glass, neutral-tinted spectacles,
or yellow glass all have something to recommend them; but by far the
most convenient means is to look, not at the clouds themselves, but
at their images formed in a black mirror. A lantern cover glass, or a
thin piece of plate-glass, blacked on the back with some black paint,
serves admirably. But all black paints are not equally good. The best
are oil paints which dry with a glossy surface, the so-called enamels.
They have the advantage that the varnish with which they are mixed has
an index of refraction not very different from that of the glass. The
consequence is that so little light is reflected from the blackened
back, compared with that which is reflected from the front surface of
the glass, that the second image can only be detected with difficulty.
If the mirror is a piece of black or deeply coloured glass all trace of
the second image is lost.
With this simple appliance it is easy to study the details of the
thinnest clouds right up to the sun, and even the image of the sun
itself may be glanced at without serious discomfort. Nor is the general
diminution of brightness the only gain. If the glass is so held that
the light from the cloud makes an angle of about 33 degrees with the
surface, some of the blue light from the sky is suppressed altogether,
while that from the cloud is practically unaffected. The exact fraction
suppressed depends upon the part of the sky relative to the sun, and
also on the position of the mirror, but a few minutes’ trial will show
when the maximum effect has been reached.
It is astonishing to see for the first time how the delicate filaments
of cirrus or the beautiful structures of cirro-cumulus stand out
shining white on the deep blue background; and the use of the black
mirror is a revelation to most. It also has one indirect advantage,
which is really more important than it seems. By gazing down into a
mirror long-continued observations can be made, and one form of cloud
may be watched changing into another, and possibly back again into
its original shape, without any danger of incurring that unpleasant
result of much looking upwards which is sometimes known as exhibition
headache. Such a mirror may be quite small, so that it can be carried
in a pocket-book, a point of some moment, as many of the forms of
cirrus are exceedingly transient, coming and going in a few minutes,
while others are in a state of continuous change. This is particularly
often the case with the exquisite ripple clouds, and the delicate
lacework of the higher kinds of cirrus.
Still another advantage possessed by the mirror is that it makes it
easy to see the solar halos formed on the verge of a cyclone, and
to detect their iridescent colouring in a way which is quite beyond
the reach of the naked eye or any protective spectacles. Every one
is familiar with the faint halos formed round the moon, but the
corresponding solar phenomenon is comparatively little known, though it
is far commoner, much more brilliant, and often glows with colour. Its
very brightness, and that of the background on which it is projected,
hides it from the eye, except on those rare occasions when the sun is
conveniently hidden by some thicker cloud.
If some permanent record is desired, much can be done with a few light
strokes of a pencil, but more ambitious pictures are best secured
by the use of soft pastels, aided by a liberal use of the finger or
leather stump. Ordinary paints, whether oil or water-colour, are of
little use for actual study of cloud detail, except in the hands of a
highly skilled artist who knows how to get the effect he wants in the
minimum of time.
But no sketching or drawing can make records of cirrus or alto clouds
with the speed and accuracy necessary for careful study. Photography
is really the only way in which the amazing wealth of detail can be
truthfully portrayed. Yet even the camera has its limitations. It
does not record colour, and completely fails to delineate the forms
of alto-stratus, stratus, or nimbus, if they are present in the most
typical condition, that is to say, when they cover the whole sky with
a uniform tint. It is only when these forms are more or less broken up
that a photograph, or anything other than a carefully coloured picture,
will represent them at all.
Cloud photography, even of the most delicate and brilliant varieties,
is easy enough when the right methods are followed; but these are not
the same as those which are right for portraiture or landscape work
of the usual kind. The background of blue sky produces almost the same
effect on the plate as the image of the cloud itself, and the whole art
consists in an adequate exaggeration of the minute difference so as to
reveal the details of form and structure.
A slow plate--the accompanying illustrations have all been taken
on Mawson and Swan’s photo-mechanical plates--extremely cautious
development, and sometimes intensification of the image, are all that
is necessary; but the process becomes easier if, instead of pointing
the camera to the cloud, it is directed to the image formed in a
properly constructed black mirror. Many of the following studies have
been taken by this method, and details of the camera and processes
employed will be found in a later chapter, for the convenience of any
one who may be inspired to take up a fascinating branch of photography.
It has been said that reference will be made to the average altitudes
of the different types of cloud, and to the actual altitude of some of
the varieties shown. The question will, no doubt, have occurred to
some as to how those altitudes have been measured. The methods are all
more or less complicated, involving rather laborious calculation. They
generally depend upon simultaneous observations made from two stations
at opposite ends of a measured base line. Sometimes the observations
are made directly by pointing an instrument at each station to some
agreed point of the cloud. It is obvious that the two directions must
converge to this point. If the convergence is measured, the exact
distance from either station can be calculated, and if the angle
between the cloud-point and the horizon beneath it is noted, it is a
simple matter to deduce the actual altitude of the cloud. At other
places the observers have relied upon the comparison of photographs
simultaneously taken from the two stations. In this method it is
necessary to know the exact direction in which the camera is pointed,
and the position of the image upon the plate then gives the direction
of the cloud as seen from that particular station, and the subsequent
calculations are the same.
Measurements by one or the other of the above methods have been made at
several places, the most extensive series being those which have been
compiled at Upsala, and at the Blue Hill Observatory in Massachusetts.
The method employed by the writer at Exeter has been rather different,
and a description will be found later on in the chapter on Cloud
Altitudes, the fuller consideration of which comes naturally after the
different forms have been described and compared.
FOOTNOTE:
[1] See reference No. 2 on p. 181.