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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.




CHAPTER II

CIRRUS


A CLOUD is sometimes defined as any visible mass composed of small
particles of ice or water suspended in the air, and formed by
condensation from the state of vapour. As a general rule this is exact
enough, but under certain circumstances it is possible to have the
particles so small, and so thinly scattered, that it is not fully
satisfied. The resulting mass may not be actually visible. The presence
of the condensed particles may be indicated by nothing more than a
slight whitening of the blue sky, or by the formation about the sun or
moon of bright circles of light known as halos. If such a halo appears,
it is generally a phenomenon of brief duration. Sometimes the circle
breaks and becomes incomplete by the passing away of the thin patch of
cloud, sometimes the cloud increases in density until the rings are
destroyed.

The thinnest variety of this halo-producing structure is quite
invisible to the eye. It is so thin as to have no distinctly noticeable
effect upon the colour of the sky, but the optical results of its
presence may be very remarkable. Highly complicated systems of rings
are sometimes produced, the rings, as a rule, falling into two groups.
The commonest form has the sun (or moon) in the centre, and a circle
of pale light at a distance of about 22 degrees. Larger rings are seen
less frequently, which have an angular radius of about 46 degrees, and
as a rule have the sun situated on the ring itself. In Plate 1 we have
a part of such a great halo. The camera was directed towards the east,
and tilted upwards at an angle of about 40 degrees. The sun was behind
the camera, in the south-west, and the ring could be traced right up to
it on either side.

At the same time the sun was surrounded partially by a halo of the
more ordinary type, which was brightly coloured, making an effective
contrast to the dull white of the greater ring. The phenomenon did
not last more than half an hour, and the changes in its appearance
coincided with a growing density of cloud. When first noticed the
great ring was alone, and the sky was of a full blue, but a silvery
film came gradually up from the south-west, and the smaller and
brighter halo flashed out as the delicate curtain came near the sun.
Slowly the cloud spread to the north-east, gathering density from the
opposite point of the compass; and by the time the ordinary halo was at
its best, the great white ring had completely vanished.

[Illustration: Plate 1.

PART OF A GREAT HALO.]

[Illustration: Plate 2.

PART OF A SOLAR HALO.]

These circles, and the bright spots called mock-suns or mock-moons
which often accompany them, can all be explained on the assumption
that their cause is the passage of light through a veil composed of
hexagonal crystals of ice. The simple halo of 22 degrees radius is
common in most parts of the world, being very generally formed by the
film of high cloud which marks the advancing edge of a cyclonic cloud
system. A portion of one is shown in Plate 2, in which the rudimentary
fibrous structure of the sheet of cloud is distinctly seen. Halos of
this sort are frequently coloured, often most brilliantly so; but the
tints are seldom noticed unless a black mirror is used. They are
sometimes quite as bright as those of an ordinary rainbow, but instead
of being projected upon a background of dark rain-clouds, they are
seen against a part of the sky which is near the sun, and therefore
exceptionally bright.

The red is always on the inside of the ring, the violet outside,
thereby distinguishing them at once from the so-called coronæ, which
are formed around the sun or moon when shining through a sheet of alto
or other lower cloud made up of liquid particles. In these the radius
of the rings is much less, and the red is on the outside, the violet
actually touching the central luminary.

The cloud which produces halos is called cirro-nebula. It is much
thinner, and on an average higher than cirro-stratus. Mr. Ley named
it cirro-velum (or cirro-veil), but cirro-nebula has now got to be
fairly well understood. It sometimes appears and disappears in a
curious manner, showing that it occurs in patches, which drift about or
which keep forming and melting away, only to repeat the process. If,
however, it forms part of an advancing cyclone fringe, then the sky
gets whiter and whiter, until it is covered with a sheet of undoubted
cirro-stratus. This process of growing density is shown in progress in
Plate 3.

[Illustration: Plate 3.

CIRRO-NEBULA CHANGING TO CIRRO-STRATUS.]

Cirro-nebula, as we shall call it, floats at very great altitudes in
temperate regions; but in polar latitudes, where the optical phenomena
peculiar to it are most brilliant and diversified, it seems probable
that the ice dust is much lower down, even in actual proximity to the
ground. In England its height varies greatly with the time of year, and
other circumstances, but mounts up in summer to such altitudes as nine
miles or more; the greatest height yet recorded being 9·6 miles, or
about 15,500 metres, at Exeter.

The change from cirro-nebula to cirro-stratus is generally accompanied
by the formation of a distinct fibrous structure, easily observable by
the black mirror. This is not really a new feature, but only a further
development of a structure already existing, but too minute to be
easily seen. True halo-producing cirro-nebula may usually be shown to
possess more or less of a fibrous texture in an indirect way, which is
worth a brief description.

In order to observe the spots on the sun and other features of the
solar surface, it is a common practice to hold a white screen, say,
about a foot from the eyepiece of a telescope, while the instrument
is pointed to the sun. An image, considerably magnified, is thus
projected on to the screen, and the solar details can be studied with
ease and safety. If thin clouds drift before the sun, their images are
similarly projected as they pass across its disc, and it is possible
thus to detect not only the fibrous texture but also the movement of
cirro-nebula.[2]

The change into cirro-stratus is also attended by a marked fall in
altitude, but whether this is due to an actual descent of the cloud
particles, or to a downward spread of the conditions which give rise
to them, cannot at present be definitely settled. The balance of
probability points very strongly towards the downward spread of the
conditions. It is likely that the clouds, particularly the cyclonic
specimens, are wedge-shaped, and that as they pass overhead we see
first the thin edge, and later on the thicker parts, which project
much lower down. This is just one of those many minor problems in
cloud mechanics which we are not able to solve from the scanty data on
record.

Occasionally cirro-nebula breaks up into little detached
semi-transparent cloudlets, all of them exceedingly thin, and showing
a complicated mottling, resembling, on a minute scale, the ripple
clouds of much lower altitudes. Such a sky is depicted in Plate 4, but
no reproduction can possibly do justice to the minute and delicate
features of the real thing. The arrangement of the faint markings was
in a state of continual flux, curiously similar to the ever-changing
aspect of the sun’s photosphere when seen under adequate power. Some
parts of the cloud stratum would at one moment break up into distinct
granules arranged in complicated patterns, other parts would assume a
fibrous texture, and yet other places would show a continuous smooth
sheet. In a minute or two all would be changed--the smooth part
granulated, the fibres vanished, and the granules fused together, and
so on, no two of a series of photographs representing the same details.

[Illustration: Plate 4.

CIRRO-NEBULA CHANGING TO CIRRO-CUMULUS.]

These changes of form continued until the whole was hidden from view
by a veil of much lower stratiform cloud, one advance portion of which
is shown. Plate 4 does not represent a type or a distinct variety of
cloud. It is an intermediate form, or a temporary condition, showing
cirro-nebula in the act of changing into cirro-cumulus, or possibly
cirro-stratus.

Cirro-nebula itself, in its simpler form, is, however, a distinct type.
It is true that it never persists over one locality for more than an
hour or two without passing into some denser form, but while it lasts
its features are so distinctive, and the optical phenomena to which it
gives rise are so striking and significant, that it is a matter for
surprise that it should in the International system have been relegated
to the position of a subordinate variety of cirrus. It is more nearly
related to cirro-stratus, but is sufficiently distinct from that to
deserve at least specific rank.

True typical cirrus must have a plainly shown fibrous structure. The
fibres may cross and interlace, they may radiate in fan-like manner,
or they may curl and twist like a well-trimmed ostrich feather. The
clouds so formed must not be arranged in a continuous level sheet, or
they at once become cirro-stratus, and it is impossible to invent a
definition which will mark the exact limits of either type. Typical
cirrus consists of detached clouds. They cast no shadows on the
landscape, for the simple reasons that they are semi-transparent and
their component parts too narrow. If the sun is shining down obliquely
through the naked boughs of a tall tree, it will be seen that the
lowest twigs cast fairly sharp shadows on the ground, but that even
these are bordered by a fading rim; the twigs further up cast no sharp
shadows, but broader faint bands of shade; while the topmost boughs
cast no shadows which can clearly be identified. In other words, the
more distant the narrow twig is from the ground the narrower the real
shadow or umbra, and the broader the penumbra becomes, until when the
distance is sufficient the shadow is all penumbra. Cirrus filaments
throw nothing but a faint penumbra. Indeed, it is only when they lie
in the earth’s shadow, and stand against the background of a faintly
lighted sky, that they show any sign of shadow even on themselves.

There is no doubt that they are composed of particles of ice. They are
formed at altitudes where the thermometer must be many degrees below
freezing-point, and not a few of the thinner examples show fragmentary
halos like those of cirro-nebula.

Their actual altitudes are very variable, being greater in summer than
in winter, and reaching a maximum for any given station after a long
spell of hot weather. Exact measurements have not yet been made in
tropical latitudes or in polar regions, but there is every reason to
expect that the upper limit of cirrus for equatorial districts will
be found to be much higher than in the temperate zones where actual
observations have been made. In places nearer to the Arctic Circle it
is also almost certain that the altitudes will be less.

In the New England states, as shown by the Blue Hill observations, the
maximum altitude for summer was found to be little under 15,000 metres.
At Upsala, in Sweden, it was 13,300 metres. The average altitudes at
the same observatories were, respectively, about 9900 and 8800 metres.
At Exeter the writer’s own measurements give an average for the summer
months of 10,200 metres, with a minimum rather lower than was the case
in America or Sweden, and with a maximum far above the foreign values.
In winter cirrus certainly comes much lower down, but the number of
observations is fewer.

[Illustration: Plate 5.

HIGH CIRRUS.

(_Cirrus Excelsus._)]

The loftiest variety of cirrus appears in the afternoon in very hot
weather, sometimes quite late in the evening; and in autumn it is by no
means a rare event for it to suddenly form just when the sunset colours
are fading, or even after they have paled into twilight. Under such
circumstances it stands out of a shining silvery grey colour against
the background of the darkening sky. A specimen of it is shown in Plate
5, which shows its extreme slightness of form and delicacy of texture.
Sometimes it remains visible so long after the stars have begun to
show as to give the idea that it is self luminous, and the illusion
is certainly very strong. The writer has noted several instances in
which it was plainly visible, like a silvery curtain, though the sky as
a whole was so dark that stars like the five brightest points of the
Great Bear could be seen through the cloud, and much smaller stars down
to the third and fourth magnitude were plainly visible in the clear
intervals. It has sometimes been called luminous cloud, and Mr. Ley
estimated its altitude at upwards of 90,000 metres; but if we think of
it as reflecting the light of the distant colourless twilight there is
no need to regard it as anything fundamentally different from other
clouds, or to assume a greater altitude than we know to have been the
case. The specimen figured occurred in the early afternoon on June
12, 1899, at Exeter, and careful measurements of its altitude were
made. This worked out as 17·02 miles, or more than 27,000 metres, a
value so much greater than all other measurements of the kind that it
was only after most careful verification and reference to duplicate
records that it could be accepted. It differs in several ways from the
lower varieties, being thinner, more glistening, and in every way more
delicate. A suitable distinctive name would be high cirrus, or cirrus
excelsus.

[Illustration: Plate 6.

WINDY CIRRUS.

(_Cirrus Ventosus._)]

Lower down by thousands of metres come the feathery masses of typical
windy cirrus, such as are shown in Plate 6. Indeed, in cold winter
weather they occur within three or four thousand metres of the ground.
In the instance figured the wind was blowing from left to right, and
the clouds were travelling swiftly. The upper filaments appeared to be
repeatedly torn away from the main masses, while the long faint streaks
which trail below and behind are evidently due to streams of fine
particles falling from the main centres of condensation into a less
rapidly moving stratum below. There is no room for doubt that these
clouds, like others of a similar order, are formed by a direct passage
from the vapour to the solid, or that the fibres are made of minute
snowflakes. The condensation is evidently attended by rapid movements,
which draw out the cloud, as fast as it is formed, into long curving
lines which mark lines of motion. The variety is always, therefore, an
indication of strong winds and rapid eddying movements in the region
in which it occurs. Such strong disturbances overhead almost always
accompany similar but less intense movements at the ground-level, and
when they do not accompany them they precede them. The cloud is well
named windy cirrus, which may be converted into a specific name, cirrus
ventosus.

The next variety we come to (Plate 7) is in some ways rather similar.
It is, however, thinner, more delicate, and is entirely composed
of fine threads, which are more systematically arranged. Generally
there is a bundle, or several bundles, of long parallel fibres, which
form, so to say, the quill of the feather, with numbers of shorter
threads branching out from them at various angles. Cirrus ventosus was
indicative of irregular movements in various directions; this variety
points also to complicated movements, but executed in accordance with
some sort of system, strangely complex and wonderfully ordered. The
specimen figured is the type of what Mr. Ley called cirro-filum, or
thread cirrus, and his name can hardly be bettered. It is a cloud of
summer, and occurs rather high up in the cirrus zone, but no actual
measurements can be quoted. It is fairly common, but not nearly so
frequent as the last.

[Illustration: Plate 7.

THREAD CIRRUS.

(_Cirro-filum._)]

A somewhat more familiar variety is shown in Plate 8. Little irregular
feathers of cirrus, from which long tapering streamers point downwards
in graceful curves, or else lag behind in the direction from which
the clouds have travelled. If clouds of this type are carefully
watched, it will soon be seen that each feathery head is a centre of
condensation, and that the tails or streamers are nothing else than
falling particles, which dwindle slowly away by evaporation, and which
gradually sink below the level of the heads. It is usual, in dealing
with cloud-forms like these, to speak of air-currents of different
velocities almost as if the winds at different levels were as clearly
separated as oil and water, or even air and water. This can hardly
be the case, for if such a thing should occur as an air-current of
one velocity flowing over another of less speed, or of a current in
one direction over another moving in a different course, the two must
inevitably mix at their junction, and in a very short time the passage
from the lower current to the upper one would be quite gradual. No
doubt we can often observe two, three, or more layers of cloud moving
in different directions; but if we were to send up a balloon, it would
be rare indeed to find its direction of horizontal movement changed in
a few metres of ascent. Different and distinct air-currents are often
invoked to explain cloud-forms quite unnecessarily. It is far more
likely that the differential movements involved in the explanation of
the features of these cirrus varieties are due to increased velocity
with greater altitude, to progressive change of direction, to irregular
eddies, or to the interaction of ascending and descending convection
currents. Indeed, it is probable that careful study of the growth and
decay of these clouds will, in time, lead to a clearer understanding
of atmospheric movements, and so enable us to say more precisely why
they are as we see them to be. The variety shown in Plate 8 is rare
except in combination with other forms. It might well be termed tailed
cirrus or cirrus caudatus.

[Illustration: Plate 8.

TAILED CIRRUS.

(_Cirrus Caudatus._)]

The form of cirrus shown in Plate 9 is far more frequently seen than
either of those which have been described. In this the fibrous texture
is very imperfect, and the cloudlets show a tendency to arrange
themselves in a kind of ribbed structure in two directions almost at
right angles to each other. But this last is an accidental feature
of the particular example, and not in any way a specific character
of the cloud. The reason for regarding it as a distinct variety is
the total absence of sharply defined lines, not only the heads of
condensation, but even the long streamers attached to them being
uniformly hazy and ill-defined. It is a form of cirrus which comes at
all seasons, but most frequently in summer; it moves always with great
slowness, indicating a quiet atmosphere free from disturbance of any
kind. The conditions necessary for its appearance are a nearly uniform
distribution of pressure over a considerable area, chequered by little
shallow depressions of some trifling fraction of an inch. In hot
weather these are the conditions under which thunder-storms develop,
and this hazy cirrus, or cirrus nebulosus, may be taken as a certain
sign of such an atmospheric state.

[Illustration: Plate 9.

HAZY CIRRUS.

(_Cirrus Nebulosus._)]

So far as permanency of form is considered, hazy cirrus is one of the
most persistent, and affords a marked contrast to the species shown in
Plate 10, which represents the most fugitive. Five minutes before the
photograph was taken the same part of the sky was a deep, clear blue,
without any trace of cloud. Suddenly a few short curling wisps made
their appearance. These rapidly increased in number, until a delicate
filmy network extended over the greater part of the field of view. But
while the camera was being adjusted for an exposure, part of the net
had broken up into the granular structure shown in the lower part of
the photograph. The granulation rapidly spread through the net, almost
as if the fibres had been curdled, and five minutes later the whole had
been converted into a patch of cirro-cumulus which soon fused into a
uniform sheet. Meanwhile the same series of phenomena were taking place
in other parts of the sky.

[Illustration: Plate 10.

CHANGE CIRRUS.

(_Cirrus Inconstans._)]

On other occasions exactly the same set of events have been seen to
follow each other in the inverse order. Beginning with a fairly even
sheet, this broke up into granules, and they in turn seemed to be
frayed out into short hazy and wavy fibres which slowly melted away.

Clearly we have here to do, not with a distinct type of cloud, but
rather with the first step towards the formation of one, or the last
stage in the life of one which is drying up. But sometimes the life of
the cloud is so short that it never passes beyond this first stage;
and it is by no means a universal rule for a growing sheet of cirrus
to pass through this stage at all. It therefore represents a peculiar
state of instability, and requires a name of its own. Sometimes patches
of it will come and go in an apparently capricious manner for an
hour or more before permanent condensation is effected or before the
sky finally clears. But this is a rare event, since the slow change
of conditions which has brought the stratum of air to the unstable
condition is generally progressive, and instead of stopping at the
critical point, goes beyond it, with the result that the condensation
grows or the cloud disappears entirely. Change cirrus, or cirrus
inconstans, would be an appropriate name for a kind of cloud which is
so plainly indicative of instability.

The critical condition referred to is, of course, that in which a
particular stratum of air is just saturated, or is just on the point
of forming visible cloud. If any cause is brought to bear on such a
stratum which brings about even slight cooling, cloud must be produced;
and, conversely, anything which results in the slightest heating must
cause it to disappear. The shortness and haziness of the fibres, and
the fact that they gather themselves into granules, shows that the
cloud is formed in a stratum of air which is either still, or is moving
as a whole, without any of those differential movements which seem to
be necessary for the longer fibrous details.

The causes which may bring about the local cooling and heating are easy
to understand when we remember how the air will be affected by the
uneven contours of the ground. As it passes over hill and valley the
up-and-down movements of the lower layers, or even the disturbances
caused by passing over a wood or clump of trees, all must be propagated
upwards. Each disturbance must slowly spread laterally and diminish
vertically, so that it will reach the cirrus zone as a broad and
gentle dome-like oscillation. Suppose now a series of such slight
upheavals to reach the critical level. The passage of the waves will
mean alternate expansion and compression. Expansion means cooling, and
therefore cloud-production; compression means heating, and therefore
the destruction of cloud.

From the most transient form of cirrus we pass, in Plate 11, to the
most persistent and probably the most frequent. It occurs in detached
masses which have very variable forms but are wholly fibrous, with the
details arranged in a very irregular manner. The example figured was
taken in the evening during a long spell of fine weather. If such a
cloud is watched, its permanence of detail is very striking, and must
be due to a persistence of slow eddying movements and to a continual
renewal and waste of the component particles of each wisp. This is
the kind of cirrus selected generally as the type of cirrus, and the
selection is a good one. Common cirrus, or cirrus communis, it should
be called. Settled conditions and fine weather are its usual attendants.

[Illustration: Plate 11.

COMMON CIRRUS.

(_Cirrus Communis._)]

We next come to a variety which is anything but a harbinger of good,
namely, the long stripes or bands of cirrus which stretch outwards
from the margin of the cloud canopy of a cyclonic storm. In some ways
these appendages to the great nimbus resemble the strips of cirriform
cloud which fringe the summit of a thunder-cloud. They look as if they
must have been formed by the blowing away, by a rapid wind, of the top
of an uprising column of vapour-charged air. Their main outline may
thus be easily accounted for, but we have only to study their detailed
structure for a few minutes to feel that they really present a problem
of a very high order. Plate 12 shows a fairly simple example, but Plate
13 represents a cloud of very great complexity. To take this last the
camera was tilted upwards at an angle of 45 degrees, so that the top of
the picture is not far from the zenith. The wonderful plume of cloud
rose from the southern horizon, and ended in a great sheaf of fibres
and films spread out like a partly opened fan whose edge was only about
50 degrees above the northern horizon. Its length as it passed overhead
lay between a point a little east of south to a little west of north;
and the broad band moved as a whole, without any marked internal
changes, from the south-west towards the north-east. The weather was
very unsettled. A long procession of cyclones had been sweeping along
our western shores, and the barometer was just beginning a fresh and
rapid fall. During the ensuing night a heavy gale burst over the south
of England.

[Illustration: Plate 12.

BAND CIRRUS.

(_Cirrus Vittatus._)]

[Illustration: Plate 13.

BAND CIRRUS.

(_Cirrus Vittatus._)]

The whole phenomenon was highly characteristic. These great bands with
the divergent striation might well be known as storm bands, from their
almost invariable connection with the violent atmospheric movements
to which they are most probably due. Plate 12 shows a much less
dangerous variety of the same species, which is distinguished from it
by the comparative absence of internal detail and by the curled ends.
Clouds of this character have sometimes been called cirro-filum, but
a comparison of the plates with the typical cirro-filum of Plate 7
will show that there is little resemblance; and the attendant weather
is also in marked contrast, both of which facts imply a fundamental
difference in the causes to which their features are due. Banded or
ribboned cirrus is the name which they immediately suggest, and this
may be rendered cirrus vittatus.

This ends our survey of cirrus clouds. Any one who compares the plates
so far given will see that they represent forms so diverse that it is
impossible to avoid the conclusion that the conditions under which they
are produced must differ not only in degree but also in kind. What
those conditions are we have attempted here and there to suggest, but
in no case can we feel that the explanation has been at all complete.
In some cases, notably the last, we are face to face with such
complicated details that it is hopeless to attempt to explain them in
the present state of our knowledge. Fact upon fact must be accumulated
until we can give their history from their earliest beginnings; and
far more accurate and detailed knowledge of the attendant atmospheric
conditions must be acquired before we can hope to rob such elaborate
structures of their present mystery.

This requires the co-operation of many eyes and many minds, and exact
specific names must be an essential preliminary. Those which have been
suggested in these pages are--

  1. Cirro-nebula, or Cirrus haze.
  2. Cirrus excelsus, or High cirrus.
  3. Cirrus ventosus, or Windy cirrus.
  4. Cirro-filum, or Thread cirrus.
  5. Cirrus nebulosus, or Hazy cirrus.
  6. Cirrus caudatus, or Tailed cirrus.
  7. Cirrus vittatus, or Band cirrus.
  8. Cirrus inconstans, or Change cirrus.
  9. Cirrus communis, or Common cirrus.

FOOTNOTE:

[2] The telescope with which these observations have been made is a
6·8-inch refractor equatorially mounted.




CHAPTER III

CIRRO-STRATUS AND CIRRO-CUMULUS


SEVERAL of the varieties of cirrus already discussed may gather so
abundantly at some given level in the atmosphere, that the most obvious
feature comes to be this arrangement in a sheet. The cloud then
becomes cirro-stratus, and should be so named. We have described how
cirro-nebula frequently grows in density until it fails to produce halo
phenomena, and may even reduce the sun to a hazy patch of light showing
no outline. This is the most typical of all forms of cirro-stratus.
It has always a distinctly fibrous or streaky appearance, whereby it
is at once distinguished from a similar but lower cloud which will be
described later on.

A similar sheet may be formed from the fusion together of the streaks
of cirrus-nebulosus, the bands of cirrus vittatus, or the development
of cirrus inconstans. But the general rule is that the cirro-stratus
retains more or less of the specific characters of the parent form.

Plate 14 shows a hazy form of cirro-stratus developed from the nebulous
cirrus. Its altitude was great, being about 10,000 metres. The
processes of growth and change could be studied easily. First would
appear some faint spots and streaks; these quickly fused together
into larger patches, which again joined to their neighbours. In a few
minutes the cloud so formed would return to the mottled or streaky
appearance, and either disappear entirely or become very thin, only
to recommence the process. This went on for more than an hour, the
cloudy patches getting larger and larger, until the critical condition
was passed, and the sky was covered with a general veil of typical
cirro-stratus.

[Illustration: Plate 14.

HAZY CIRRO-STRATUS.

(_Cirro-stratus Nebulosus._)]

In Plate 15 we have an example of a cloud which is clearly
cirro-stratus, but the sheet is broken up into long bands, and each of
these is made up of common cirrus. In the upper part of the picture
the sprays of cirrus are forming, and as they come into being they
are arranged in rows. We have here to do with a phenomenon of a very
different order from the one presented by the true banded cirrus. The
arrangement into belts must here be due to some kind of wave-movement
in the air, breaking up the critical plane into long ribs transverse
to the direction of the wave-movement. The specimen shown was moving
in a direction nearly at right angles to the bands, though the surface
wind was nearly parallel to their length. The question at once
presented itself as to whether the movement of the bands was really a
drift of the cloud, or whether it was not a case of the propagation
of cloud production with the advancing wave. This was easily answered
by watching the details of a band. The advancing side was always
feathery, and careful observation showed that the edge advanced by
throwing out new threads and curls. A given thread or curl, one moment
at the edge, would in a few minutes be well in the band. Quite opposite
events were taking place in the rear of each band. The cloud was there
obviously melting away. Indeed, to sum the matter up, the cloud bands
flowed past their details just as the waves on the sea flow past the
floating foam. Evidently we have in Plate 15 the result of a plane of
commencing cirrus formation broken into a series of troughs and waves
by an undulatory movement of the air. But, as we have already said
when speaking of cirrus inconstans, the condition in which trifling
up-and-down movements can determine whether condensation shall, or
shall not, take place seldom lasts long. It is usually only a stage
in a continuous change, and in this particular instance the banded
structure was soon replaced by a fairly continuous sheet of typical
cirro-stratus.

[Illustration: Plate 15.

CIRRO-STRATUS.]

The next plate, No. 16, shows a similar process. In the upper part we
have cirrus inconstans forming in patches out of a deep clear-blue sky.
Its hazy fibres grow closer and closer, betraying a slight tendency to
gather in narrow ripple-like bands, but the structure is soon lost in
the uniform white sheet of interlacing fibres, which differ from common
cirrus in little else than their number and closeness. Nevertheless,
the stratiform arrangement is quite obvious enough to warrant the use
of the term “cirro-stratus.”

[Illustration: Plate 16.

CIRRO-STRATUS.

(_Cirro-stratus Communis._)]

The change of cirro-nebula into cirro-stratus is shown in Plate 4, to
which reference has already been made. The structures are remarkably
delicate, showing in the middle a distinct irregular mottling; and
rather further towards the top right-hand corner a ripple structure
appears, and in the top left-hand corner the sheet is denser and
whiter. The altitude of this cloud was evidently great, and actual
measurement showed it to be 7·6 miles. It did not last long, and after
its change into broken patches of denser cirro-stratus, still higher
clouds were revealed through the gaps.

Cirro-stratus often forms almost simultaneously at more than one level,
and when that happens the full stratiform appearance is generally
reached first by the lower layer. In Plate 17 we have two layers. The
fluffy bits of cirrus nebulosus, in the lower part of the picture, are
really the higher clouds. Below them, probably by many thousands of
feet, floats the denser cloud shown in the upper part of the picture.
This is an interesting link between the fibrous and the granular forms
of cirrus, and is probably best described as spotted cirro-stratus,
or cirro-stratus maculosus. It is a form very frequently met with,
but seldom showing any persistence. It is indicative of condensation
in a calm atmosphere, and not unfrequently marks either the small
irregularities of pressure which form the conditions for thunderstorms,
or the beginning of the break up of an anticyclone.

[Illustration: Plate 17.

FLOCCULENT CIRRO-STRATUS.

(_Cirro-stratus Cumulosus._)]

A coarser texture and greater density are shown in Plate 18, where we
have cirro-stratus in the lower part of the picture, and cirro-cumulus
in the upper. The altitude of this cloud was only about 4000 metres,
one of the least values recorded at Exeter for cirro-stratus of any
kind. The intimate admixture of the fibrous and granular forms is very
clearly shown.

[Illustration: Plate 18.

CIRRO-STRATUS AND CIRRO-CUMULUS.]

This close relation is equally obvious in Plate 19, where the cloudlets
are arranged in loosely marshalled rows, dimly resembling the banded
structure of Plate 15. But in this case the direction of movement was
with the long lines, and the propagation of cloud production followed
the same course. Some of the little cloudlets have an opacity, and
therefore brilliancy, quite unusual for cirro-cumulus, but their
intimate association evidently in the same plane with undoubted cirrus
shows that they must fall under that general description. It is a cloud
indicative of unsettled weather, and the exceptional brilliancy is
doubtless due to an unusual quantity of vapour at the cloud plane,
which must mean that the change from the dry stratum above to the damp
one below must be much more sudden than is ordinarily the case. Clouds
of this kind might well be called cirro-stratus cumulosus.

[Illustration: Plate 19.

CIRRO-CUMULUS.]

We now come to two companion pictures, Plates 20 and 21, which were
taken within half a minute of each other. In the first the camera
was directed towards the west, and in the second it was facing the
north-west. The sun was nearing the horizon, and was only just outside
the field of view in each case, so that the two photographs form
a panorama of the western sky. A solar halo had disappeared about
half an hour previously, and the cirro-nebula had changed into the
remarkable forms of cloud depicted. Plate 21 shows cirrus ripples in
the upper part, and cirro-cumulus in soft, ill-defined balls in the
lower part; but they were at the same level, and are only different
parts of the same cloud plane. In Plate 22 we see similar ball-like
cloudlets ranged in long lines which run almost at right angles to
the ripples of the companion picture. Clouds like these are rare.
They are almost unknown during the early part of the day, and, so
far as the writer’s experience goes, they are only to be found in
the afternoon towards sunset. Some of our most gorgeous sunset skies
are due to them; for their altitude is considerable, and they do not
light up with the sunset colours until the lower clouds have become
dark shadows against the glowing background. The hottest months of the
year, the still air and great evaporation which are the contributing
causes of thunderstorms, are also the conditions under which such
skies may be seen. Indeed, while these photographs were being taken,
heavy thunderstorms were in progress within less than a hundred miles.
Cirro-cumulus nebulosus, or hazy cirro-cumulus, describes the form
correctly.

[Illustration: Plate 20.

HAZY CIRRO-CUMULUS.

(_Cirro-cumulus Nebulosus._)]

[Illustration: Plate 21.

HAZY CIRRO-CUMULUS.

(_Cirro-cumulus Nebulosus._)]

The next plate, No. 22, gives a view of an evening sky about half an
hour after sunset. The lower clouds, cirro-cumulus and cirro-stratus,
of a deep purple brown, standing out dark against a gold-coloured sheet
of higher cirro-stratus, which comes out white in the photograph, while
the purple-tinted sky comes dark. We have here three distinct layers,
all cirrus. First, the hazy cirro-cumulus, forming two bars across the
lower part of the picture; then long bands of cirrus or cirro-stratus,
best seen in the bottom right-hand corner; and, far above both, the
cirro-stratus which was reflecting the yellow sunlight. Such a sky
might be an indication of thunder conditions, or it might be due to an
unusual quantity of vapour in the atmosphere produced by some other
cause. The actual conditions were the gentle flow over England of
vapour-laden air from the western ocean, heralding the change from a
long spell of fine hot weather, due to a July anticyclone, to a month
of heavy rains and western gales, accompanying the passage of a long
procession of cyclones along our western shores.

[Illustration: Plate 22.

A SUNSET SKY.]

Again, a marked contrast is shown in Plate 23. Here we have the highest
and thinnest form of cirro-cumulus, the one named cirro-macula by
Mr. Ley. It is rarely, if ever, seen before eleven o’clock in the
morning, and is far commoner in the afternoon. The example shown was
photographed at sunset at the close of a day which had been almost
cloudless. Cirro-macula forms here and there in a clear sky. A hazy,
whitish patch appears, which at first shows no definite structure, but
looks almost like a little bit of cirro-nebula. This suddenly splits
up by clear blue lanes running through it, and cutting the patch up
into irregular segments, which quickly round themselves off into minute
bits usually whiter on their edges and semi-transparent in the centre.
The process can be strikingly imitated by scattering on water some fine
powder which will float. If left without disturbance, the particles
draw together into numerous small groups, leaving lanes of clear water
between them.

[Illustration: Plate 23.

SPECKLE CLOUD (Ley).

(_Cirro-macula._)]

Cirro-macula frequently gives rise to the fibrous form of cirrus we
have called cirrus caudatus. The granules of the cirro-macula grow
denser, and begin to drop their frozen particles as soon as they become
large enough. Indeed, a cloudlet of cirro-macula may sometimes be seen
to turn bodily into a fine line of falling crystals, which will be a
curving line of cirrus. On the other hand, it will sometimes remain
visible for an hour or more without any trace of descending streaks or
floating fibres. Pure cirro-macula such as Plate 23 is not often seen;
it is far more frequently mixed with more solid-looking cloudlets and
descending fibres, such as are shown in Plate 24, which gives the same
point of view as 23, but a quarter of an hour later, and photographed
with a longer focus lens. These two photographs, together with 20 and
21, give excellent examples of the use of the black mirror. In none
of the four could the naked eye detect all of the cloud structures.
The whole sky was a blaze of dazzling light, but by adjusting exposure
and development the details are fully brought out without the least
difficulty.

[Illustration: Plate 24.

CIRRUS CAUDATUS AND CIRRO-MACULA.]

Cirro-stratus, we see from the examples which have been considered,
hardly deserves to be treated as a distinct genus of cloud. Its
formation is identical with that of many species of cirrus, or in some
cases with that of the speckle cloud, cirro-macula, or even the coarser
kinds of cirro-cumulus. The different varieties which it shows are best
rendered by reference to the specific names of the detached forms which
have similar features.

Cirro-cumulus, on the other hand, does present clearly marked
varieties. Cirro-macula is so distinct that it might well be given
the name awarded to it by Mr. Ley, while the term “cirro-cumulus” is
reserved for the coarser and rounder forms. The hazy, ripple-like
structures of Plate 4 and Plate 20 should also have some distinctive
appellation, as will be suggested later on when dealing with wave
clouds as a whole.

It is difficult to find any short way of expressing the various ideas
which should be summed up in the name of a cloud. There seems no
alternative to the use of additional words, unless it be to follow
the example of chemists, and compound appalling names similar to
those which terrify the uninitiated who think they would like to read
something about, let us say, the coal-tar dyes.

If a cloud belongs to the order cirrus, is in a level sheet, and
that sheet is composed of interlacing or curling fibres, like those
of common cirrus, we can hardly express the facts more briefly than
by calling it cirro-stratus communis, or common cirro-stratus. If
it consists of cirrus bands fused together, but still showing the
banded structure, it is cirro-stratus vittatus. Again, if it is finely
speckled, like cirro-macula, it may be described as cirro-stratus
maculosus, and if the structure is coarser it may be called
cirro-stratus cumulosus.

As a general average, cirro-stratus lies somewhat lower in the
atmosphere than the detached forms, probably because the conditions
which give rise to the latter reach to greater altitudes in patches
than it is possible for them to reach in a continuous manner. Vapour
becomes rarer with increased height and with diminished temperature,
so that it must, on the whole, be less frequently present in
cloud-producing quantity as the height increases. At great altitudes
it will be seldom that the quantity is great enough to produce a
stratiform cloud, though it may well be enough for cirro-macula, or the
detached forms of cirrus, like cirrus excelsus.

The production of cirro-cumulus and cirro-stratus sometimes spreads
across the sky with astonishing speed, and this rapid advance of the
edge of the cloud may lead to quite mistaken ideas as to the velocity
of the wind at that altitude. In the case of cirro-cumulus, or
cirro-macula, it is easy to fix attention on a single cloudlet. If this
has the usual ball-like form, it can only be regarded as floating in
the air and moving with it. Meanwhile new cloudlets may be forming and
growing denser, so that the cloud patch as a whole may be apparently
advancing at a much greater rate. Careless observation would then lead
to the idea that the cloud was moving much faster than it really is,
but if the attention is rigidly fixed on a particular cloudlet the
mistake is impossible. If the cloud is a variety of cirro-stratus,
it is not always easy, or even possible, to distinguish between the
advance of condensation and the movement of the whole, but it can
nearly always be done if the cloud shows any definite features upon
which attention can be fastened. Sometimes none sufficiently marked can
be seen, and when that happens it is still possible in most cases, by
watching the edge of the cloud-mass, to see whether new cloud is being
added to that edge. The wave-like forms present a special case, which
will be dealt with in a later chapter, after the general principles
of cloud formation have been discussed in connection with the great
clouds of the lower air, whose causes and conditions are far better
understood.




CHAPTER IV

ALTO CLOUDS


FROM cirro-cumulus and cirro-stratus we pass through almost insensible
gradations to the denser forms classed together in the alto group.
These clouds are fundamentally different, in that they are always
composed of liquid particles, though there is no doubt, from their
great altitude, that their temperature must often be many degrees below
the ordinary freezing-point of water. When this is the case, they
are not unfrequently more or less mixed with streaks and filaments
exactly like those described under the name of cirrus, which have been
explained as due to slowly falling snowflakes. It is not immediately
obvious how such apparently contradictory statements can be reconciled.
The explanation is that minute droplets of water may be cooled many
degrees below freezing-point without changing into ice, and that such
super-cooled droplets congeal instantly if a few of them join together
to form a larger drop. Practically the same process may be watched any
day when there is a sharp frost and dense fog drifting slowly along.
The fog-particles are liquid, and produce optical effects in the
neighbourhood of any brilliant light, like an arc lamp, absolutely the
same as those which would be produced if the temperature were above
freezing-point, while there are none of the different phenomena which
might be expected if the particles were crystalline ice-dust. As these
liquid particles drift along they come in contact with branches of
trees and other obstacles, the surface stratum which surrounds them and
binds them into spheres is broken, and the drop instantly solidifies.
It is to be noted, moreover, that the drop does not freeze as such,
but merely adds some more particles to the branching crystals of hoar
frost, which grow outwards always towards the direction from which the
fog is drifting.

Most liquids, when freed from contact with solid bodies, or when
surrounded by a smooth envelope of uniform character, can be cooled
below their normal freezing-point without solidification taking place;
but the introduction of a particle of the solid, or sometimes of any
foreign body, instantly brings about a rapid freezing of the whole.
These phenomena of surfusion, as it is called, have long been known,
and many of them are very interesting and difficult to understand.
Indeed, it is probable that we shall have to add largely to our
knowledge of the forces which bind the molecules of a body together to
form a solid, and which direct the processes of crystallization before
we shall be able to interpret with any certainty a series of facts
depending on the attributes of those very forces.

Water is no exception. If finely divided, as by placing it in fine
capillary tubes, in the pores of wood, or in the narrow spaces of a
wick, it may be cooled several degrees below normal freezing-point.
In a cloud, or fog, all the conditions necessary for surfusion to
take place are undoubtedly present. The water is pure, the envelope
is uniform, the subdivision is exceedingly minute, and the drops are
free from most of the mechanical disturbances which bring about the
solidification of larger masses in the laboratory.

Thus we see there is nothing at all surprising in the fact that clouds
composed of liquid particles may exist at temperatures below the
ordinary freezing-point. On the contrary, we should expect that the
solidification of the cloud particles would not take place until the
temperature was many degrees below freezing, as is certainly the case
with clouds of the cirrus order. At temperatures between this unknown,
but low value, and the normal freezing-point, the clouds will be
composed of liquid; but when the particles join together, snowflakes
will result instead of raindrops; and this will be just as true of
alto clouds as it is of the great vaporous mountains of the lower
regions of the air which bring falls of snow. The streaks often mixed
with alto-cumulus are cirrus threads, and are, no doubt, of exactly
the same nature as the tails of cirrus caudatus, or even the fibres of
cirro-filum.

The simplest alto cloud is alto-stratus. When this is complete, so as
to cover the sky, it can be distinguished from cirro-stratus by the
absence of fibrous structure, and by the facts that it never produces
any halo or fragment of a halo, but instead surrounds the sun or
moon with a white blur, or, if it is thin enough, with a close ring
of coloured light much nearer than a halo, and with the colours in
the inverse order--that is, with the red furthest from the centre.
Some of these so-called coronæ are very beautiful when seen in the
black mirror, and some of those formed around a full moon show quite
brilliant tints to the unaided eye. Of course, these meteorological
coronæ have no relation whatever to the true solar corona; they are
simply formed by the passage of the rays of light through the veil of
small particles, and may be easily imitated. Take a piece of glass such
as a lantern-cover glass, breathe on it, and hold it close before the
eye while looking at some small source of light. If the dew deposit
is thin, bright colours are shown in a luminous ring surrounding the
light, and the thinner the deposit of dew the larger the ring will be.
Breathe heavily so as to give a thick deposit, and the light will be
seen to be the centre of a patch of white brightness without any colour.

The phenomena are due to what is known as diffraction, and if the
other conditions are unchanged the diameter of the ring is inversely
proportional to the size of the particles. Purity of colour in these
rings is an indication of uniformity in the size of the particles.
When the moon is shining through a sheet of alto-stratus, which thins
off to one edge, very beautiful effects may often be noticed, and the
change from the colourless blur, when a thicker part of the cloud
is interposed, to the brilliant colours of the corona formed by the
thinner edges is very striking. Similar phenomena are shown almost
equally well by any of the alto clouds, but cirrus thin enough to
produce a coloured corona will generally produce a halo.

Alto-cumulus of the kind most nearly allied to cirro-cumulus is shown
in Plate 25. The upper part of the picture shows ragged, irregular
patches, with slight indications of fibrous streaks. The lower portion
shows rounder, ball-like cloudlets, a few of the larger of which have
distinct shadows on the side away from the sun. This plate gives
alto-cumulus in a partly formed condition, but it is not a mere passage
form. Sometimes exactly such a cloud will float overhead for hours,
showing very little movement and only slow changes of detail. It is
therefore a distinct variety, and may be called alto-cumulus informis.

[Illustration: Plate 25.

ALTO-CUMULUS INFORMIS.]

A less definite form is shown in Plate 26. It may also be regarded as
only partly formed, but its construction is quite different, every part
being misty and ill defined. It is a common cloud, especially in sultry
summer weather with still air. Under those circumstances, after a hazy
morning, it may be seen slowly forming during the afternoon, growing
in density as the hours go by, until it reaches a maximum about five
or six o’clock, after which it melts away, or settles down into small
patches of high stratus. Most frequent in summer, it is by no means
rare in autumn and winter, but still air is essential. From its hazy
appearance it may be called alto-cumulus nebulosus.

[Illustration: Plate 26.

HAZY ALTO-CUMULUS.

(_Alto-cumulus Nebulosus._)]

Fixity of detail and slow movement characterize both the foregoing
forms, and in that respect our next picture (Plate 27) shows a cloud
which is a great contrast. Its detached cloudlets are rather flatter
and thinner, and though the cloud as a whole will often persist for
hours, it is undergoing continual change, and is formed when the air
is far from still. Cloudlets form and gather into stratiform patches,
which soon break up again and disappear; and the process goes on
here and there, sometimes accompanied by fairly rapid movement of
the patches as a whole. This cloud may be described as alto-cumulus
stratiformis.

[Illustration: Plate 27.

FLAT ALTO-CUMULUS.

(_Alto-cumulus Stratiformis._)]

We now come, in Plate 28, to a cloud of singular beauty. It forms
rapidly in a clear sky, its first traces bearing a striking
resemblance to cirro-macula, but the floccules, instead of remaining
semi-transparent or dropping cirrus threads, rapidly become opaque
balls of cloud which lengthen upwards. This upward tendency causes
the formed cloudlets to have their longer axes vertical, which is
very characteristic. It might be named alto-cumulus castellatus,
or high-turreted cloud. Mr. Ley named it stratus castellatus, or
turret-cloud, but it certainly belongs to the cumulus section of
the alto group. Thunder weather is the invariable condition for
its production. If it is seen, at least in England, thunderstorms
are certain to be recorded not very far away. When this particular
photograph was being taken in South Devon, very destructive storms were
recorded in Brittany and in the English Midlands, and the anvil-shaped
tops of unmistakable thunder-clouds were visible above the horizon
while the exposure was being made.

[Illustration: Plate 28.

HIGH TURRETED CLOUD.

(_Alto-cumulus Castellatus._)]

Another form of almost equal beauty is shown in Plate 29. The rounded
balls make their appearance as semi-transparent spots upon the sky, and
in their general characters might easily be mistaken for cirro-macula.
But a few minutes will be enough to decide the question. The little
spots rapidly grow denser, frequently becoming ragged at the edges;
they never drop down the slender filaments which usually descend from
cirro-macula, and their edges are never denser than their central
parts, which, it will be remembered, was a frequent feature of the true
speckle cloud. The cloudlets are obviously rounded balls arranged in
patches, which may turn gradually into alto-stratus by their fusion,
or, after an existence of minutes or hours, the whole may disappear
by a disintegration of each ball, by its breaking up into a ragged
mass and melting away. The altitude at which this cloud forms is
between 5000 and 9000 metres, according to measurements made by the
writer, the actual specimen figured being about 7000. It is almost as
characteristic of thunder weather as the last, but whereas Plate 28
shows a variety which is most often seen before 3 p.m., since it only
occurs while the cloud planes are rapidly rising, the one before us
may be formed at almost any time of day, but most frequently occurs in
the afternoon. An imperfect form of it is frequently met with about
sunset, in which the rounded balls are not usually so well defined as
when the sun is high above the horizon. Alto-cumulus glomeratus would
be a suitably descriptive name.

[Illustration: Plate 29.

HIGH BALL CUMULUS.

(_Alto-cumulus Glomeratus._)]

If it were possible to take a good typical example of the variety
just described and roll it flat, so that each cloudlet should be
reduced to a lenticular shape, we get a type which seems seldom to
appear during the heat of the day, and to be most frequent about
sundown. It consists, as shown in Plate 30, of distinct cloudlets,
with considerable spaces between them, and gives the impression of a
discontinuous level sheet. But the component cloudlets are much too
definite, and preserve their individuality far too well to suggest any
idea of a broken stratus; the spotted structure is the predominant
feature, while the stratiform arrangement is almost equally plain.
Alto-stratus maculosus would be a suitable term. It is not so high a
cloud as the glomeratus type, the one shown being at an altitude of
about 5600 metres. The plate shows the position of the setting sun,
which is partly hidden behind some dark patches of broken alto-stratus
(fracto-alto-stratus), the hazy form and boundaries of which form an
effective contrast to the shining cloudlets 2000 metres or so above
them. Many of our most beautiful sunsets are due to this form of cloud,
particularly in the late autumn. It is a cloud of calm weather, and
often floats apparently motionless, and undergoing little change,
like flakes of glowing fire against the background of a fading sky
long after the sun has disappeared. It is not indicative of thunder
conditions, and it may occur on the margins of an anticyclone.

[Illustration: Plate 30.

MACKEREL SKY.

(_Alto-stratus Maculosus._)]

A lower and coarser form of the spotted alto-stratus is shown in
Plate 31, where it is seen through the gaps in a thin sheet of broken
stratus. In this case also the sun was getting low in the sky, being
hidden by the denser bit of stratus in the bottom left-hand corner.

[Illustration: Plate 31.

MACKEREL SKY.

(_Alto-stratus Maculosus._)]

Alto-stratus does not often, if ever, grow from the fusion of the
cloudlets of the maculosus type. But it does come from alto-cumulus
glomeratus, and also from a form shown in Plate 32. Here we have
alto-stratus in process of growth. Small irregular lumps of cloud
forming on the right-hand side of the picture grow larger and more
irregular, begin to fuse together towards the centre, and on the
left-hand side the fusion is almost complete. Still, although the
sky is covered with cloud, the lumpy form is plainly visible. The
term “alto-strato-cumulus” is suitable, as it differs from the more
frequent and much lower cloud, which will be described further on as
strato-cumulus, in little else than altitude and general massiveness
of texture. This high strato-cumulus is common enough, too common,
indeed, in England, as it produces many a dull grey sky both in summer
and in winter. In the latter season it is not unfrequent with the cold
east winds of February and March. It is probably the lowest of the alto
clouds; the lowest measurement made by the writer being 1828 metres at
Exeter, but lower altitudes seem to have been recorded elsewhere.

[Illustration: Plate 32.

ALTO-STRATO-CUMULUS.]

Alto-cumulus castellatus, which is breaking up and disappearing, is
shown in Plate 33. It was photographed with a long-focus lens, so that
the scale of representation is about eight times as great as that of
Plate 27. This view was taken at Exeter while a thunderstorm was in
progress at Bristol.

[Illustration: Plate 33.

SUNSET.

(_Alto-cumulus Castellatus Fractus._)]




CHAPTER V

LOWER CLOUDS


THE clouds of the lower portions of the atmosphere are formed in
regions where water vapour is abundant, and frequently in easy reach
of the strong ascending and descending air currents produced by the
varying temperatures and irregular surface of the ground. It is
sufficient to recite these conditions to show that these lower clouds
will be denser, larger, coarser in texture, and characterized by
greater definiteness of form than those we have, so far, considered.

In the International system they are classified thus--

  Group C. Lower clouds.
    (_a_) Strato-cumulus.
    (_b_) Nimbus.
  Group D. Clouds of diurnal ascending currents.
    Cumulus and cumulo-nimbus.
  Group E. High fogs.
    Stratus.

This is certainly the least satisfactory part of the whole scheme,
and it is not at all easy to see upon what grounds it was adopted by
the International Committee. Group D--cumulus and cumulo-nimbus--do
show important differences from the other groups, though it is often
difficult to say whether the sky should be described as covered with
strato-cumulus or as covered with numerous small cumulus. It is the
separation of stratus--placing it in a group by itself, and making that
the lowest--which is the worst point. As a matter of fact, stratus
may exist at any altitude from sea-level up to such heights that we
should not hesitate to call it alto-stratus. Indeed, there is no
essential character of alto-stratus which distinguishes it from some
of the lower forms. Whatever its altitude, its thickness and the size
of its particles may vary in a precisely similar manner. We may have
the particles exceedingly small, when the fog will be dry, and such a
stratus may be so thin as hardly to dim the sun; or it may be so thick
as to completely hide it. On the other hand, the fog may consist of
particles easily visible to the naked eye, forming the so-called Scotch
mist, or the “dry” fog of Dartmoor, which will wet things as rapidly
and more thoroughly than a smart shower. When such a fog accumulates
to a sufficient depth, the particles in their fall pick up others, and
the result is a distinct fine rain. This may occur not only near the
ground, but at almost any level below that at which the cloud would
pass into the region of cirrus.

Plate 34 shows three layers of stratus, in each case much broken up.
The highest layer is a good example of alto-stratus maculosus. Lower
down, by half a mile or more, come parts of a grey sheet considerably
denser and thicker. It is a matter of taste whether this should be
called high stratus or low alto-stratus. There is no test by which
the one can be distinguished from the other. Lower again come the
detached darker clouds, which are fragments of a sheet of stratus
which is breaking up and disappearing. The photograph was taken in the
afternoon, after a wet morning, and all three layers were probably
relics of the great rain-cloud system, or nimbus, which produced the
rainfall.

[Illustration: Plate 34.

THREE LAYERS OF STRATIFORM CLOUD AFTER RAIN.]

We have referred to the production of fine rain from a thick fog. If
now such a thick layer of coarse-grained fog--if we may use such a
phrase--is suspended overhead at a moderate altitude, the result is
a drizzling rain underneath, and the cloud at once becomes a nimbus.
When Luke Howard adopted the term “nimbus,” he proposed to employ it,
apparently, for a vast mass of cloud such as that which forms the rainy
region of a cyclone; a huge pile of clouds containing representatives
of all his other types in some unknown but close relationship. It was,
in fact, a comprehensive term, and as such there was a good use for
it. At present it is applied to any cloud from which rain is falling,
except when the cloud can be identified as a variety of cumulus which
is called cumulo-nimbus. But we have already said that a stratus may be
a rain-cloud, and so may other varieties. Moreover, whenever a nimbus
breaks sufficiently for us to be able to see its upper surface, we
invariably find that, if it were viewed from above, we should, without
a moment’s hesitation, place it in one of the other groups. It is only
when we are underneath it we can see its rain-producing character, and
give it the orthodox name. The real fact is that nimbus should be an
adjective, meaning rain-producing, and not a substantive.

However, it has its allotted place in the International system, and
it is better to adhere as far as possible to a defective but widely
recognized system until it can be authoritatively amended, rather than
to make an individual attempt to ignore it. The facts are sufficiently
obvious, and the days of nimbus as a type are numbered. The two plates,
Nos. 35 and 36, are fair typical representations of the clouds usually
known as nimbus; but they are both of them only the under-surfaces of
other clouds, Plate 36 showing the under-surfaces of a group of heavy
cumulo-nimbus all joined together so as to cover the sky, while Plate
35 shows a mass of dense strato-cumulus. The rain-cloud is always
a form of either stratus or cumulus, or a combination of the two,
sometimes in further combination with clouds of the alto class, or even
extending upwards to cirro-stratus and cirro-nebula. Where it consists
of a single layer, that layer differs from its rainless representative
only in greater thickness from base to summit, or in greater density;
and when there are several distinct layers of cloud, so that the
lowest is shaded by the higher, rain may fall, even though they differ
in no visible way from clouds which would be rainless if alone. Plate
33 is an example.

[Illustration: Plate 35.

RAIN-CLOUD.

(_Nimbus._)]

[Illustration: Plate 36.

RAIN-CLOUD.

(_Nimbus._)]

Nimbus, indeed, is not a type-form, but is merely a typical condition,
and when used as a substantive is only a convenient way of expressing
our ignorance as to the real form of the cloud we so describe.

The altitude of the base of a rain-cloud may vary considerably. It
may be anything from sea-level up to heights which vary with the
geographical conditions and with the conditions of temperature and
pressure, but probably in this country never greater than 7000 or 8000
metres.

Rain, or snow, often falls from clouds at greater altitudes than these,
but unless in its descent it passes through other lower clouds, the
drops, as a rule, will dry up and disappear. The author has often seen
quite heavy rain descending from a cloud, and disappearing completely
within a thousand feet or so of the cloud-base. On rarer occasions a
still more remarkable thing may be seen--namely, a shower falling from
an upper cloud into a lower, and none between this lower cloud and the
ground. This curious phenomenon can only be explained by supposing
that the convection currents which make the lower cloud are strong
enough to support the small raindrops.

Pure stratus is a level sheet of cloud with little variation of
thickness, not ascending every here and there into rounded lumps. Its
most typical form covers the whole sky with a uniform grey pall, which
may or may not completely hide the sun. Such a cloud does not lend
itself to pictorial representation. A frequent form, in which the sheet
is more or less broken, is shown in Plate 37. This is a variety which
is frequent in the summer mornings, and generally breaks up and clears
away before eleven o’clock. If, however, it appears in autumn and
winter with layers of alto cloud above, it may grow denser, and turn
into a stratiform nimbus, or it may go on drifting overhead for several
days without sign of change.

[Illustration: Plate 37.

STRATUS COMMUNIS.]

Break up such a sheet of cloud by numerous meandering cracks, and round
off the detached pieces so as to give them a more or less rounded
or pyramidal section, and the cloud becomes strato-cumulus, typical
representations of which are shown in Plates 38 and 39, which depict
different parts of the same sky. In Plate 38 the camera was pointed due
west, and in Plate 39 it was turned round to the north-west, so that
the two views do not quite meet.

[Illustration: Plate 38.

STRATO-CUMULUS.]

[Illustration: Plate 39.

STRATO-CUMULUS.]

Plate 40 is a different variety. It is stratiform, each component
cloudlet being rather ragged at the edges. In some ways it resembles
cirro-macula and the speckled varieties of alto cloud, but it
is coarser in texture and obviously at no great altitude. The
International system would call it strato-cumulus, but Mr. Ley gives
a representation from another negative taken at the same time as the
type of what he calls stratus maculosus, a name which seems far more
suitable, since the cloud bears a much closer relation to stratus than
to cumulus. In the particular instance figured, the broken structure
did not last long; the spaces gradually closed in, and a complete
stratus was the result.

[Illustration: Plate 40.

STRATUS MACULOSUS.]

Strato-cumulus often lasts for hours, with little or no perceptible
change, but stratus maculosus rarely persists for more than half an
hour. The first is a cloud of fairly stable conditions, the latter
is dependent for its existence upon the near approach to critical
conditions at one particular level, and, as we have said in other
cases, such a critical state is almost always soon passed, with the
result that the cloud either masses into a denser form, or else breaks
up and disappears. If the up and down currents are strong enough to
persist, the result will be strato-cumulus and not stratus maculosus.

A kind of stratus which is frequently seen in the daytime is shown
in Plate 41. This is literally a lifted fog, having been formed
about midday, after ground fog in the early morning. It would be
called common stratus, or stratus communis. When it appears it is a
fairly persistent form, sometimes breaking up or swelling up into
strato-cumulus, but more often splitting into long rolls of cloud,
with margins like those of cumulus. This phenomenon is shown in Plate
42, which was taken in December at 11 a.m., on a day which opened
with a thick ground fog. A precisely similar cloud is frequent in the
early hours of a summer morning, as a stage in the dispersal of a
radiation ground fog. The fog first lifts from the ground, until it
reaches a height of a few hundred metres, when it splits into the long
rolls whose axes are at right angles to the direction of drift. The
consequence is very strange if you stand on a hilltop close under the
drifting mass, and look towards the horizon in the direction of drift.
The changing shadows give the impression that the clouds are actually
rolling along, though of course no such thing is really taking place.
As time goes on the rolls grow larger and the interspaces wider; then
transverse fissures appear, and gradually the rolls break up into small
detached cumulus. Cumulus radius, from the Latin for a rolling-pin,
might be a suitably descriptive name, but it should not be forgotten
that it is only an intermediate link between stratus and cumulus,
and, indeed, is more nearly related to the former, since it is never
produced except on the break up of stratus, while it may dry up and
disappear without reaching the cumulus stage at all. Stratus radius
would therefore be a better name.

[Illustration: Plate 41.

COMMON STRATUS.

(_Stratus Communis._)]

[Illustration: Plate 42.

ROLLER CLOUD.

(_Stratus Radius._)]

Cumulus is closely related to another form of stratus, which Mr. Ley
has named stratus lenticularis, but this appears to be so frequently
the last stage in a disappearing cumulus that its history will come
better later on. It is mentioned here as it is, after all, one of
the commonest of all forms of stratus, the form which appears at,
or after, sunset, and is one of the few clouds which have an English
popular name--Fall cloud. Plate 47 gives a representation of it,
standing out dark against an evening sky, with a sheet of alto-stratus
far above it in the upper part of the photograph.

To sum up, then, we have among the lower clouds of more or less
stratiform pattern--

Stratus communis, or Common stratus.

Stratus lenticularis, the Fall cloud.

Stratus radius, or Roll cloud.

Stratus maculosus, or Mottled stratus.

Strato-cumulus, or Sheet cumulus.

The last leads naturally to the consideration of cumulus and
cumulo-nimbus, while the term “nimbus” does not belong to any one
type-form, but sometimes to one, sometimes to another, and generally to
a mixture of two or more.

A good many years ago the writer made a series of measurements of
the thickness of detached clouds of the stratus and cumulus types,
such as those which may produce a shower. The conclusions reached in
consequence of those determinations have since been amply confirmed
by subsequent observations. In winter no rain will fall from a
cloud unless it reaches a minimum thickness of at least 100 metres,
while in summer it must have rather greater thickness. There is one
exception, and that is in winter, when the temperature is so low that
the drop starts on its downward journey as a flake of snow. When this
is the case, rain may fall from a layer of thin lifted fog, not quite
thick enough to hide the blue colour of the sky. But under ordinary
conditions of temperature, if the cloud has a thickness less than 2000
feet, or 616 metres, rain is unlikely, but if it does come, the drops
will be small and the fall of rain quite trifling.

Above this thickness the heaviness of the rain and size of the drops
increases, so that if the distance from base to summit be between
2000 and 3000 feet, or 600 to 1000 metres, the fall will be gentle. A
thickness of 4000 to 6000 feet, or 1200 to 1800 metres, gives large
drops and a fairly heavy shower, while, in summer time at least,
cold heavy rain and hail come from clouds measuring 6000 to 10,000
feet, or in round numbers 1800 to 3000 metres or more. In winter the
necessary dimensions seem to be less, but the rule still holds equally
good, that the rain-cloud does not necessarily differ in any way from
the rainless one, except in thickness, and that when the requisite
thickness is present rain is not always the result.




CHAPTER VI

CUMULUS


UNDER the general term cumulus there are grouped the most common, the
best known, and the grandest forms of cloud. Indeed, beautiful as the
cirrus and alto clouds may be, there is a solid grandeur about the
greater forms of cumulus which gives them a beauty of their own quite
comparable with the charm afforded by the delicate tracery of their
more lofty rivals.

Cumulus can be divided into several types, which are best considered
in the order of growth. They are all formed in the lower part of the
atmosphere, their under-surfaces varying in altitude from about 600
metres, or even less, up to 3000 metres, or slightly more. The writer’s
own measurements vary from a minimum of 584 metres to a maximum of 2286
metres, with an average of a little more than 1000 metres.

They are described in the International system as “clouds in a rising
current,” and there is no doubt the description is correct. Each
cumulus must be looked upon as simply the visible top of an ascending
pillar of damp air. The vapour which makes its appearance in the cloud
is present in the transparent air beneath, and the base of the cloud
is simply the level at which that vapour begins to condense into
visible liquid particles. Since cumulus clouds are caused by ascending
currents, these currents must be brought about either by the general
disturbance of the air due to a cyclonic movement, or by the local
irregularities of temperature on the ground produced by the sun’s heat.
As a matter of fact, we do get cumulus produced in great abundance in
the rear of every cyclone, and we get them also under the conditions
of still air and hot sun, which specially favour evaporation and the
development of differences of temperature. The cyclone cumulus may come
at any hour of the day or night, though comparatively rare between
midnight and the morning. Heat cumulus is generally formed during the
afternoon, and it is only under relatively uncommon conditions that it
persists during the night. If the cloud has not grown to very great
size it usually begins to break up and disappear about sunset, but
if it has grown to the enormous dimensions of a summer thunder-cloud
it may go on growing, piling mass on to mass, until it generates a
thunderstorm, even in the hours of early morning.

In the case of some of the higher kinds of cloud, we are not able to
give any certain account of the mechanics of their production from a
study of those clouds themselves. We have already referred incidentally
to some of the speculations as to their origin and some of the facts
definitely known, but considerable light can be thrown on the genesis
of all the varieties of cirro-cumulus and alto-cumulus by a careful
study of their larger and more accessible representatives of lower
regions.

The cyclone cumulus does not differ in any essential from the clouds
of calm weather. The only difference is that the uprising currents are
perhaps partly eddies, and the rate of fall of temperature with ascent
is often more rapid.

Given any mass of air at a particular temperature, it can take up and
hold in the form of invisible vapour a fixed quantity of water, and no
more. When it holds the maximum possible it is said to be saturated. If
it is nearly saturated it would be called damp; if far from saturated,
dry. Now, the warmer the air the larger the quantity of vapour
necessary to saturate it, so that if a quantity is saturated at a high
temperature, and is then cooled, it will no longer be able to retain
all its moisture in the invisible form, but the surplus quantity will
make its appearance as liquid particles, that is to say, as mist or
cloud.

Similarly, if a quantity of air is not fully saturated at its
particular temperature, and is then cooled, it will approach nearer and
nearer to saturation, and if the process is continued long enough the
result will be cloud formation.

All clouds, without exception, are produced by exactly such cooling
of air containing water vapour, first to the temperature at which the
quantity it contains is the maximum possible, and then beyond that
point. Now, if we start with very warm air, and cool it 1 degree, we
decrease its vapour-holding power, and the decrease per degree grows
less and less as the temperature falls. Suppose, for instance, we have
air saturated at 61 degrees and cool it to 60 degrees, the quantity
of vapour condensed will be equal to the difference of holding power.
Suppose, again, we have air saturated at 31 degrees and we cool it to
30 degrees, the quantity of vapour condensed will again be equal to the
difference of holding power; but this quantity will be very greatly
less than in the former case. Cooling air saturated at 61 degrees to 60
degrees might produce a dense cloud; but applying a similar reduction
of 1 degree to air saturated at 31 degrees, if we take the same volume
of air, will only produce a very much thinner result. Here we see one
good reason why the highest clouds are the thinnest and the alto clouds
of intermediate density.

The necessary cooling may be brought about in several ways. Firstly,
the air is capable of radiating its heat into space, and therefore
of cooling. But we know little of the laws which govern atmospheric
radiation, and presumably, if cloud could be produced by such means,
it ought to make its appearance most frequently in the small hours of
the morning before sunrise. We are, however, unaware of any variety
of cloud which answers those conditions, unless it be the ground
fogs which so often form during the night; and these, we know, are
certainly due to the chilling of the air by contact with the ground,
which has been cooled by radiating away its heat. On the contrary, it
is well known to astronomers that the sky is, on the whole, clearer
and freer from clouds after midnight than in the earlier hours of the
night--a circumstance which is particularly unfortunate for the amateur
star-gazer, who has to be up and about at the same time as the rest of
the working world. Cooling by radiation we may then dismiss as a cause
of cloud formation of no great efficacy, and certainly one which has
little to do with the production of cumulus.

Cooling by contact with a cold body is another and more potent cause.
We often see it in a mountain district, where a frost-bound peak
stands facing the wind with glittering snow-slopes on which the sun
is shining, while a long tongue of cloud hangs like a banner on its
leeward side. In such a case it is easy to understand how the air
sweeping by the icy mass is chilled below its saturation point; but as
it passes on, the chilled portions become mixed with the rest, and the
cloud evaporates again. It is not quite so easy to see how far this
cause is responsible for the clouds which are formed when the warm damp
air of the ocean drifts over a comparatively cold land. It is probable
that the contact chilling is in this case only part of the explanation,
and that other causes co-operate.

The mixing of warm damp air with cold has often been adduced as a cause
of clouds. No doubt it might be, and some of the stratiform types may
possibly be formed at the junction between a warm damp stratum of air
and a cold one, but no example is certainly known. It may also be a
contributing cause in producing the sharply defined upper surfaces of
some cumulus or strato-cumulus clouds, but these are in the main most
certainly due to the chief cause of cloud production--namely, what is
known as dynamic cooling.

If a quantity of air exists under a certain pressure and at a certain
temperature, on reducing the pressure it will expand, and in the act of
expanding it will become cooler. This may easily be illustrated with
an air-pump. Let a damp sponge or a piece of wet blotting-paper stand
under a glass receiver over an air-pump until the air has become damp.
If the apparatus is in a darkened room, and a powerful beam of light
from a lantern is sent through the receiver, the damp air will be seen
to be quite clear; but a stroke or two of the pump removes some of the
air, the remainder is chilled by its own expansion, and a dense cloud
is precipitated. If this cloud be viewed closely, it will be seen to
be composed of minute particles, which, on looking towards the light,
glow with the colours of a corona. In a few minutes the cloud will
disappear, but it can be recalled again and again by successive strokes
of the pump, getting thinner and thinner as the air gets more and more
rarefied; an illustration of a second reason why the high clouds are
thinner than the lower.

Some years ago Mr. John Aitken showed that if the damp air used in
this experiment were carefully filtered, so as to remove all foreign
particles, no cloud was produced, and the introduction of a puff of
unfiltered air was attended by immediate condensation. The deduction
was that vapour, even below its saturation temperature, cannot produce
cloud unless nuclei of some sort are already present, presumably dust
particles. Later on it was shown by Mr. Shelford Bidwell and others
that gaseous particles, such as those produced by the burning of
sulphur, would serve the purpose, and that the brush discharge from an
electrified point was in some mysterious way particularly effective.
It has recently been shown by Mr. C. T. R. Wilson that causes such as
the radiations of radium, or the impact of ultra-violet rays, acting on
the air itself, splits up some of its particles into the smaller bodies
known as ions, and that these are efficient nuclei. These experiments
open up many most interesting questions, but, unless it is to explain
the extreme density and darkness of a thunder-cloud, they do not seem
to play any important part in determining the forms to be assumed.
Nuclei in sufficient abundance are probably always present at any
height which can be reached by enough vapour to form a cloud.

Now, if we have a quantity of air, say at sea-level, damp but not
saturated, and it is caused to ascend, either because it is warmer and
therefore lighter than the surrounding air, or for some other reason,
as it moves upwards the pressure upon it will decrease, it will
expand, and in the act it will be steadily cooled. This cooling may
after a time bring it down to the same temperature as the rest of the
air at its particular level. If so, it will no longer be lighter, and
the ascent will come to an end. But before this state of affairs is
attained it may have reached its saturation point, and cloud production
will begin.

It is true that the rarefaction of the air tends to enable it to retain
more vapour than it could if it were cooled without change of density.
The temperature of the air being fixed, its holding power increases
with decrease of pressure. But this increase is much less than the
diminution due to cooling, and the result in nature must be similar to
what we can see happen under the receiver of the air-pump.

The condensation of water introduces another factor of great
importance. It has just been said that the ascending air may be cooled
so rapidly as to be reduced to the same temperature as the rest of
the air at that level, and if so the ascent will end. Clearly the
cessation or persistence of the upward motion depends upon whether the
diminution of temperature per 100 metres of ascent is most rapid in
the rising column or in the air outside it. As long as the ascending
air is warmer than that outside, but at its own level, so long will
ascent continue. Now, as long as no condensation was taking place, the
rate of cooling would follow a simple law which produces a cooling of
1 degree for about 100 metres of ascent; but as soon as water vapour
begins to pass into the liquid form, a large quantity of heat is set
free, and the rate of cooling is consequently greatly lessened. Cloud
production tends, therefore, to accelerate ascent, and the greater the
amount of condensation, the more important will this consideration
become; though, on the other hand, when once the cloud is formed, it
tends to stop the rising current by shading the air and ground beneath
it.

On an ordinary day the rate of decrease of temperature as we ascend is
rather less than the value given above, and uprising currents are soon
checked. If they do extend far enough to reach cloud production, the
clouds will be small, forming the smallest variety of cumulus. This is
shown in Plate 43. Small irregular uprising currents have just been
able to reach far enough up to have their summits tipped with cloud.

[Illustration: Plate 43.

SMALL CUMULUS.

(_Cumulus Minor._)]

After the foregoing explanation, it is easy to see why at a given time
the floating cloudlets should have a common base level. This is the
height to which the air must attain before reaching its saturation
temperature. Each cloudlet marks an uprising current, and the intervals
show the position of the counterbalancing descending streams.

A larger variety is shown in Plate 44. In this the level base and
generally pyramidal shape is shown, and also the hard, rounded upper
surface. The thickness of this cloud was about 500 metres. When clouds
like these are visible, they may be the beginning of larger ones, and
the only way to judge whether they are likely to develop into rain- or
shower-clouds is to watch them. If they are seen to be growing larger,
and particularly if detached fragments are developing into clouds,
further growth is almost certain, and rain is probable.

[Illustration: Plate 44.

CUMULUS.]

If great towering masses are making their appearance with little dark
fragments between them, as shown in Plate 45, then smart showers may
be confidently expected. The cloud figured was a shower-cloud, and
the distance is seen through the veil of falling rain. The height
and thickness of this particular cloud were measured just after its
photograph had been taken. Its base was 1200 metres above the ground,
and its summit was 1500 metres further. Its thickness from summit to
base was, therefore, not much short of a mile, and the total contents
of the cloud were probably between one and a half and two cubic miles.
The upper contour is hard and rounded, as in the smaller cloud of Plate
44, but the whole cloud is much larger.

[Illustration: Plate 45.

LARGE CUMULUS.

(_Cumulus Major._)]

We have already explained that there seems to be a definite connection
between the thickness of such clouds and the amount of precipitation
from them. Small cumulus, less than 120 metres thick, rarely produces
rain, and nothing like a heavy shower is likely unless the thickness
exceeds 400 metres. In winter, especially in hard frost, snow crystals
may fall from the smallest cloud, even from little fragments only a
few metres thick, but the quantity of water so precipitated will, of
course, be small.

As long as the top of the cumulus is rounded and clearly defined,
the conditions of aërial equilibrium are stable, and the growth of
the cloud has been brought to an end by a stoppage of the ascending
current. In Plate 45 the ascent has been hindered both by the
mechanical action of the falling raindrops and by the cooling of the
lower parts of the ascending column by the descent into it of the cool
drops from its colder upper part. This is probably one of the chief
reasons why a shower-cloud never maintains its activity as a rain
producer for more than a very limited period. As the cloud drifts over
the landscape, it seldom maintains its showery character for more than
ten or twenty miles, often for much less.

Cumulus, like any of these three, is a cloud of the daytime. It
generally begins about ten or eleven o’clock in the morning, grows
larger until about four o’clock, and then begins to break up and
disappear. After the ascending currents have ceased, the component
cloud particles slowly settle down into the warmer air beneath,
until the mass has lost its proper pyramidal form, and has become
an irregular cloud, such as is shown in Plate 46. This is known as
degraded or fracto-cumulus.

[Illustration: Plate 46.

FRACTO-CUMULUS.]

One consequence of the arrest of the uprising currents is the
formation of lenticular patches of stratus, called by Mr. Ley stratus
lenticularis. This is often formed about sunset, and has been named
fall cloud, from its appearance at the fall of night. The name is
appropriate in another way. The ascending currents having ceased,
the cloud particles slowly subside until they dry up in some warmer
stratum. The water vapour does not continue its descent, but slowly
diffuses in all directions, and if the fall of cloud particles is
sufficient, this stratum, which is approximately coincident with the
base of the original cumulus, soon becomes saturated, and further
particles which fall into it remain visible. This saturated zone will
slowly sink lower and lower with the descent of the particles, until it
reaches regions in which the temperature is high enough for the whole
to be evaporated without reaching saturation point. Evening stratus in
calm weather always goes through this sequence of changes. It usually
forms at, or soon after, sundown, and begins to break up and disappear
as the stars are becoming visible in the darkening sky. Plate 47 shows
a specimen of this evening stratus.

[Illustration: Plate 47.

FALL CLOUD.

(_Stratus Lenticularis._)]

A curious feature is sometimes shown on the underside of a thick cloud,
which is probably due to the upper part of the ascending column having
been carried beyond its position of equilibrium by its own inertia,
and then falling back again in the teeth of the still rising lower
part. The result is to give the base of the cloud an appearance of
a number of rounded masses hanging downwards below the cloud, very
suggestive of the idea that the cloud is upside down. Such an event
will not often occur, and when it does the conditions are quite wanting
in stability, and the consequent features will be very transient. When
the base of a cumulus or cumulo-nimbus is so affected, the cloud is
known as festooned cumulus, or cumulus mammatus. A precisely similar
structure may be seen under strato-cumulus, or even thick stratus. In
some countries it seems to be frequently observed, but in England it is
so uncommon that the writer has only noted it about a dozen times in
twenty years, and on no one of these did it last long enough to allow
of its portrait being taken. It is an indication of very disturbed
conditions, and is usually followed by heavy rain.

When cumulus clouds are formed in air which is steadily moving as
a whole, that is to say, when there is a steady breeze, they have
a very decided tendency to follow each other in long lines. It may
often be noticed that in a particular place with a certain direction
of wind these long processions follow definite tracks in relation to
the geographical features. The phenomenon does not seem to have been
recorded except in hilly country, but has frequently been observed by
the writer. It is not the same thing as the formation of stationary
belts of cloud transverse to the wind. These cumulus float along with
the movement of the air, and the question to be answered is, why should
they follow each other so persistently, and why should the intervening
belts of sky be so continuously free from cloud.

If we consider that the warm damp air which supplies them is drawn
from the ground, it seems that any cause which tends to direct this
warm stratum into definite channels, as it is carried on by the wind,
will be a competent cause of the whole phenomenon. This we find in
the presence of lofty hills which stand in the way of the warm
surface winds, causing them to follow more or less the general trend
of the valleys, and so delivering the rising convection currents of
cloud-producing air at the same spot.

It is easy to conceive that other causes, such as a difference in
temperature or dampness of neighbouring tracts, resulting from whether
they are bare or wooded, marshland or sandy plain, might equally
suffice; or might, at least, powerfully co-operate with, or counteract,
the effect of hill and vale. But in any case it is plain that the
geographical conditions to the windward of the place of observation not
only may affect the occurrence and distribution of cloud, but if the
wind is steady it is difficult to see how they could avoid affecting it.

Another puzzling phenomenon, sometimes presented by cloud and fog, is
that our instruments for detecting humidity show that the air within
them is not always fully saturated. It seems probable that this is due
to such cloud or fog having begun the process of drying up, or that
in some way not fully understood the presence of the cloud particles
after they have first come into existence may cause the withdrawal of
some of the moisture from the intervening damp air. The surface of
each minute droplet exerts a pressure on its interior similar to the
pressure exerted by the film of a soap-bubble on the air within it,
and it is conceivable that some of the uncondensed vapour from outside
may diffuse through this enclosing surface film, and be retained
there in consequence of the pressure. If this is so, and subsequent
investigation can alone decide the matter, it will follow that when
once cloud production has begun it will be continued until the air
between the cloud particles is reduced so far below its saturation
point that the tendency of the drops to evaporate, that is to say, for
the imprisoned water to escape through the confining film, balances the
retaining pressure.

This consideration, however, is quite incompetent to affect the general
explanation of cloud formation which has been given. Its result would
be to carry condensation a little further than the exact saturation
point, and to retard equally slightly the subsequent evaporation of the
cloud particles.

We have spoken of the typical cumulus as having a roughly pyramidal
shape, and if the horizontal movement of the air is small, the
loftiest point of the cloud will be situated approximately above
the centre of its base. But if the horizontal movement increases in
velocity, so that the top is in a more rapidly moving stratum than the
base, it will lean forward in the direction of movement. This is a very
common phenomenon, being generally shown by cumulus on a windy day.

On much rarer occasions the converse occurs, and the top of the cloud
lags behind the base, the explanation being a lessening of the velocity
of the wind as the height above ground increases. But such conditions
rarely occur, and when they do they are due to local eddies and affect
only a limited area. Hence such clouds are isolated, and indicate a
disturbed state of the air and uncertainty of weather. The clouds which
lean forward are formed under conditions which are spread over wide
districts, such as the rear of a large cyclone, and cumulus of that
kind may follow one another across the sky for hours or even days as
long as the wind persists.

So far we have considered only the round-topped types of cumulus--those
which mark the tops of ascending currents whose ascent has been
stopped at a comparatively early stage, or those whose ascent is still
in that early stage, though the upward movement has not yet come to an
end. The full story of the growth of a cumulus is identical with that
of the youth of a cumulo-nimbus, the later stages of which we will
consider in another chapter.




CHAPTER VII

CUMULO-NIMBUS


GRANDEST of all clouds are the huge mountains of vapour which are the
parents of summer thunder-storms. They are at once distinguished from
ordinary cumulus by their upper parts, which sometimes reach beyond the
region of the alto clouds high into the realm of cirrus, and extend
outwards as a broad disc, which is occasionally indistinguishable from
the cirro-nebula and cirro-stratus which form the van of a cyclone
cloud canopy. Indeed, there seems to be no essential dividing line
between a large cumulo-nimbus and the cloud pile of a small cyclone,
and no real difference between them except their size.

As a matter of fact, the term cumulo-nimbus would only be given to the
cloud when a large fraction of the whole can be seen at once.

In dealing with common cumulus, it has been pointed out that the
cessation of the uprising convection currents which determines the
maximum height to which the cloud will grow is due to the rate of
cooling within the ascending column being greater than the rate of
cooling outside it. It follows that when the ascending current has
reached a certain height it will, as a whole, be just as heavy as an
equal column outside. Ascent must then cease. The equilibrium of the
air in such a case is said to be stable, and the condition of such
stability is simply that the general rate of fall of temperature per
100 metres of ascent is less than the rate of cooling dynamically
produced in an ascending current.

If, however, the general rate of fall of temperature is greater than
that produced dynamically, the consequence will be that the upward
tendency of the rising air will increase as it moves upward, and the
taller the column becomes the greater will be the difference of weight
between the inside and outside columns. In such a case the equilibrium
is said to be unstable, and the result will be the production of
cumulo-nimbus.

Just as cumulus may be divided into heat cumulus and the clouds of
the rear of a cyclone, so cumulo-nimbus may be divided into the same
two groups. In the case of the heat thunder-clouds the instability
of the air is effected by the rapid heating of its lower layers in
contact with the ground, those lower layers being so quickly warmed
that there is not time for them to become mixed with the overlying
air in which the rate of decrease is normal. If there is much wind we
rarely get cumulo-nimbus, because the heated air is mixed mechanically
with the overlying parts, and the rate of decrease is approximately
normal throughout. Calm air and hot sun are then one set of necessary
conditions for the production of instability.

But it is well known that thunder-showers and lesser examples of
cumulo-nimbus are by no means infrequent in the rear of a cyclone, and
such storm clouds are usually attended by considerable wind. They are,
as a rule, much smaller than those produced by heat, but they have the
same form, and are evidently due to instability in the lower part of
the air, and the question is how can that condition be produced. In
order to find the answer it is necessary to refer to the temperature
phenomena of a cyclonic area. If a cyclone be divided into four
quadrants by two lines drawn through its centre, one in the direction
in which the system is travelling and the other at right angles to
it, then the front right-hand quadrant is the warmest and the rear
right-hand quadrant much colder. The cumulo-nimbus clouds of a cyclone
are limited to the first part of this cold quadrant, that is to say, to
the portion of the storm in which a great volume of cold air is flowing
over a district which has just been warmed and wetted by the preceding
part. The result is that, the air being warmed by contact with damp
ground at a temperature many degrees above that of the air itself, we
have produced exactly the same unstable state at a low temperature as
we have at a high temperature in the case of heat storms. The lower
temperature of the whole is enough to account for the smaller volume
of the cloud, and that in turn explains why cyclone thunderstorms are,
generally speaking, on a much smaller scale than heat storms.

The life history of a cumulo-nimbus is easily studied on a suitable
day. The rapid heating of the lower layers of air causes them
to expand bodily, and as they do so they lift the overlying air,
frequently in broad domes or waves. The first result is the expansion
of these upper zones, which are lightened by the flowing away of
still higher layers. Expansion means chilling, and sooner or later
its effects become visible in the formation of cirro-stratus,
cirro-cumulus, or alto-cumulus. Simultaneously the heated air near the
ground begins to rise up in tall columns, while the cooler air from a
little higher descends to take its place. Soon patches of lower cloud
appear, at first hazy and indistinct, but gradually shaping themselves
into cumulus with hazy base and rounded summits. These rapidly assume
the typical pyramidal shape, with level base and sharply contoured
top, and so far there is little to distinguish them from an ordinary
cumulus (see Plate 48). But watch them carefully. Here and there some
will be growing taller than their fellows, and as they grow their rate
of growth increases until the top begins to show signs of spreading
outwards. Rapidly the bulging summit throws out long fingers of
cloud, radiating from the central column almost as if propelled by
some repulsive force. At first, these fingers are merely projecting
lumps of cloud with rounded ends, but in a few minutes they undergo a
sudden and striking change. The whole summit becomes frayed out, drawn
out into long radiating lines, which thin off against the blue sky
exactly like the edges of a sheet of cirro-stratus. False cirrus is
the name commonly given to this, but there seems no valid reason why
it should be regarded as “false.” The top of the cloud rapidly spreads
horizontally, forming a disc of cirriform cloud, which sometimes
spreads several miles ahead of the rest of the storm. Meanwhile, the
original cumulus column loses all its deep folds and convolutions, and
other round-topped cumulus arise around it until the completed system
consists of a more or less disc-shaped mass of cumulus, with a common
base, rising higher and higher towards some central point, where these
are connected, by an uprushing column of vapour, to an upper disc with
cirriform margins.

[Illustration: Plate 48.

THUNDER-CLOUDS FORMING.]

In Plate 49 we have on the left hand a specimen in which the
outspreading is just beginning, and the same cloud is shown half an
hour later on the left of plate 50. A complete cumulo-nimbus in full
work is shown on the right of Plate 49, and the same appears on the
right in Plate 50.

[Illustration: Plate 49.

THUNDER-CLOUDS.

(_Cumulo-nimbus._)]

[Illustration: Plate 50.

THUNDER-CLOUDS.

(_Cumulo-nimbus._)]

These clouds were thunder-clouds, the larger one being a smart
thunderstorm with heavy hail. They were photographed in the evening,
and in the second picture the sun was just below the horizon.

But, to continue the story of a thunder-cloud, we always find that
after a time the cirriform top flattens out and gradually subsides,
and this is usually accompanied by a descent of the cloud base to a
lower level. Meanwhile, it frequently happens that the whole series of
phenomena is repeated in one of the attendant cumulus. Plates 51 and 52
are also two views of the same cloud at different times. In Plate 51 we
have the main part of the storm on the right, while on the extreme left
a lower part of the cloud is rising rapidly into a tall dome. In Plate
52 the central top has lost its cirriform margin and has distinctly
flattened, while the left-hand dome has risen much higher and is
beginning to throw out the projecting bits.

[Illustration: Plate 51.

THUNDER-CLOUD.

(_Cumulo-nimbus._)]

[Illustration: Plate 52.

THUNDER-CLOUD.

(_Cumulo-nimbus._)]

The hard-topped cumulus which fringe the lower disc, and the vast pile
of cirriform and hazy cloud which forms the centre of a cumulo-nimbus,
are shown in Plate 53, which represents part of the side of a great
thunder-cloud. In this case the diameter of the lower disc was about
15 miles, and the upper disc was rather larger. The uprising column
in the middle was about 7 miles across, and the height from base to
summit about 3 miles. The whole system contained between 100 and 150
cubic miles of cloud. When photographed it was over the northern part
of Salisbury Plain. Lightning played repeatedly between the back of the
white cumulus and the hazy mass behind it, and the rumble of thunder
was all but continuous for nearly half an hour as the great cloud
passed by.

[Illustration: Plate 53.

THE FLANK OF A GREAT STORM.]

This was an unusually large cloud for this country, but specimens of
10, 20, or 30 cubic miles are quite common.

Now for the explanation of the series of events. To begin with, we have
the production of an ordinary cumulus, but the equilibrium is unstable,
the growth of the cloud, therefore, becomes more and more rapid, and
the rapid condensation adds to the instability until the rising
column is so much lighter than an equal column outside that a powerful
updraught is created, strong enough for a time to hold up the raindrops
or even hailstones. At length the condensation is complete, the upper
part of the cloud consisting of snow crystals exactly like those of any
other cirrus. In the mean time, the rapid ascending current necessarily
involves an indraught from around, and consequent descending currents
to supply it. The result is to set up a circulating system, moving
inwards along the ground, upwards in the central column, and outwards
in the upper disc. The downward currents are sometimes shown by a
curling over of the edges of the upper disc, but the phenomenon is not
often seen, as the descending movement is generally enough to dry up
the cloud particles.

The rapid rising of damp air drawn from the ground brings about rapid
condensation and heavy rain. The large size of thunder-drops is almost
certainly due to the fact that it is only the larger drops which can
fall in the teeth of the strong updraught. But when these drops begin
to fall, and still more when cold hailstones begin to fall through
this ascending air, it becomes chilled from top to bottom, and the
column is broken or even stopped altogether. The frozen particles
which make up the top subside gradually, and the chilling of the air
immediately below the cloud brings the saturation level nearer the
ground, and we say the cloud base descends.

The arrangement of a thunder-cloud into the upper and lower discs with
a connecting uprush gives to the typical cloud a shape something like
that of an anvil when seen sideways, but in the larger clouds the
disc-like form is more obvious. In any ordinary thunderstorm the great
majority of the discharges of lightning play between the two discs, and
the larger the cloud the more frequent these are. Such discharges as
pass between the cloud and the earth come exclusively from the base of
the lower disc if the cloud is large, and generally follow immediately
after or simultaneously with one between the two discs. The phenomena
of lightning are intensely interesting, but the purpose of these pages
is confined to the study of clouds and cloud forms, and it would be
going beyond our scope to discuss either lightning or hail. Both are,
however, so closely related to cumulo-nimbus that they can hardly be
passed over in silence. One thing is certain, and that is that neither
the electrical developments nor the hail has anything to do with the
growth of the cloud. On the contrary, both are consequences of the
cloud, the hail being due to the great altitude, and consequent low
temperature of the upper part of the cloud, and also to the violent
uprising currents within it; while the electrical phenomena are due
to either the enormous amount of condensation, or to friction due
to the rapid uprush, or more probably to the fact that considerable
differences of electrical condition exist in the distant parts of the
air connected by the cloud, and between which its circulating currents
move. These differences are known to exist at all times, and we cannot
here discuss their origin.

The formation of cumulo-nimbus and cumulus is dependent upon the
presence of a large amount of water vapour. It is worth while to
consider whether the atmospheric movements which bring about the
condensation could exist without moisture. Wherever we find differences
of temperature between neighbouring places we must get currents of
hot air rising from the warmer spots, and compensating descending
currents around them. But we have pointed out that if the rate of
cooling as we ascend in the still air is less than the rate at which
an ascending current will be dynamically cooled, such a rising current
will come to rest. If, on the other hand, the rate of cooling in the
ascending current be less than in the still air, the equilibrium will
be unstable, and a violent uprush will result.

Now, in a climate such as our own, where the lower regions of the air
contain large quantities of water vapour, any considerable rise brings
about more or less condensation, and that condensation is attended by
a liberation of very large quantities of heat, which retard cooling in
the ascending current, and so facilitate the production of instability.
But if this cause is put aside it is still possible to have a similar
circulation. When discussing the causes of instability, it has been
pointed out that the prime condition was an unusually rapid rate of
fall of temperature in still air, such as may be produced by hot
sunshine. Now, these conditions are exactly those which will give
rise to the phenomenon of the mirage, and which reach their fullest
development in great desert districts when the air is still.

Again, it has been pointed out that the causes which bring showers and
thunderstorms to an end include the chilling of the lower parts of the
ascending column by the descent of cold rain or hail from above. We may
also add the shading of the underlying ground by the cloud itself, and
the absorption of heat in the partial evaporation of some of the rain
during the lower part of its fall. In a desert district the arising
currents are so dry that even a very great ascent does not often result
in visible cloud; and when it does, the cloud is produced at so great
a height that the air is too rarefied to produce anything much denser
than thin alto-stratus, from which no falling droplets could reach
the earth. It seems, then, that there will be no such automatic check
on the growth of the circulating system, and it will go on growing in
volume and intensity indefinitely. As a matter of fact, this is not the
case. A different check does come into operation, but not until the
indraught and updraught have become so powerful as to draw up the dust
and sand and generate a sandstorm, the weight and shade of which, in
time, destroys the circulating currents which uplifted them.

Since, however, condensation is a considerable factor in producing
instability, we should expect that such sandstorms would be rarer than
thunderstorms are in an equally hot but well-watered district, which
is the fact. Again, since rain and cloud are checks upon such systems,
we should expect the sandstorm systems to be larger and far loftier
than thunderstorms, and to consist of far more violent atmospheric
movements. This also is the case, and when we know that some of these
disturbances have the dimensions of a cyclonic storm, it is easy to
understand how the finest dust may be raised to vast altitudes, into
the great upper currents of the air, by which it may be borne hundreds
of miles before returning to the ground. It is thus that the dust of
the African deserts is carried across the Mediterranean to Europe, and
the yellow loess from Mongolia even to the eastward of Japan.




CHAPTER VIII

WAVE CLOUDS


REFERENCE has already been made on more than one occasion to the
remarkable rippled or wavy structure sometimes assumed by clouds. The
waves may be of almost any dimensions, from the broad bands into which
a sheet of cirro-stratus or of alto-stratus is sometimes divided, down
to the most minute ripples. Sometimes they are ranged in long straight
lines, sometimes they are bent into sharp angles, and sometimes curved
in very elaborate patterns; but whether they be large or small,
straight or curved, no one can see them and fail to conclude that they
must be due to an action more or less analogous to the causes which
produce waves on the sea or ripple marks upon the sand.

Wave clouds occur at all heights where clouds are formed. The break-up
of a lifting fog into roller clouds is probably the lowest example,
but it may more frequently be seen in higher clouds of the alto or
cirrus kinds.

A low example is given in Plate 40, which represents stratus maculosus,
and which has already been described. A higher type is shown in Plate
54, which is a wave-like arrangement of alto-cumulus. Rather higher
come the long zig-zag bands of Plate 55, in which the stratiform
arrangement is more obvious, and which would be best described as a
wave-form of alto-stratus. These two plates form striking contrasts.
The clouds shown in the first are distinctly of the cumulus order, and
a prominent feature is the way in which the right-hand side of each
wave has a clear-cut rounded contour like that of the upper edge of
a small cumulus, while the left-hand edge of each band is frayed out
into a ragged fringe. The whole cloud was moving slowly in a direction
nearly, but not quite, at right angles to the waves, and the fringed
edge formed the rear. It is evident that this peculiar structure must
be due to a series of narrow waves intersecting a plane in which the
air is just on the point of producing alto-cumulus. If there were no
such waves, the little uprising currents, with their intervening
down currents, would be irregularly distributed, and all the wave
disturbances have had to do is to arrange them. The consequence is
that as the waves pass along the stratum the air is alternately raised
and lowered. Where it is rising condensation takes place, where it is
falling evaporation results.

[Illustration: Plate 54.

CRESTED ALTO WAVES.

(_Alto-cumulus Undatus._)]

[Illustration: Plate 55.

ALTO WAVES.

(_Alto-stratus Undatus._)]

The cloud, like most other wave clouds, did not retain its features for
any length of time, but the gaps closed slowly in as the cloud-bands
increased in size, until a sheet of alto-stratus was produced. Since
the time of day was the morning, it is almost certain that the plane
of saturation was rising in accordance with the general law, which is
that the planes of condensation rise steadily, until about two or three
o’clock in the afternoon, and then slowly descend.

In Plate 55 each band is much flatter and less dense. They are just
as evidently formed by wave movements intersecting the plane of
condensation; but this was formed in the evening when the sun was
nearing the horizon, and at a time when the cloud planes are as a rule
rapidly descending.

Among the alto clouds wave-forms sometimes persist for a fairly long
time, and in this case the bands moved steadily onward in a direction
equally inclined to their length and breadth, that is to say, from the
bottom left-hand corner of the photograph to the top right-hand corner.
As they passed across the sky new bands kept on making their appearance
at about the same spot, each band persisting with little change until
it had passed out of sight.

Going much higher up into the region of cirrus, we meet with the
most minute and delicate ripple clouds. Some of these have already
been referred to. They are connected with either cirro-macula,
cirro-cumulus, or cirro-stratus, just as the coarser textured waves we
have been considering are connected with alto-cumulus or alto-stratus.
In Plate 56 we have an example in which we can see the stages in
the process. Nearest to the zenith we have cirro-cumulus, which is
here and there irregularly distributed, but is generally arranged in
delicate ripples, which are variously curved. Nearer the horizon the
troughs of the waves are filled in, and sheets of cirro-stratus are the
result. Here, again, the wave-form is evidently not typical. It is an
arrangement of either cirro-cumulus or cirro-stratus, produced by the
intersection of the plane of condensation by a series of wave movements.

[Illustration: Plate 56.

CIRRO RIPPLES.

(_Cirro-cumulus Undatus._)]

The arrangement is, however, so striking a feature when it is well
shown that any description of the cloud which contains no reference to
the waves is manifestly incomplete, and this would be best effected
by adding the word undatus or waved to the name of the cloud. Plate
54 will then be alto-cumulus undatus, Plate 55 alto-stratus undatus,
and Plate 56 would be described as cirro-cumulus undatus, passing into
cirro-stratus undatus and cirro-stratus. In popular language Plate 55
might be called alto waves, Plate 54 crested alto waves, and Plate 56
cirro ripples.

If we are satisfied that the wave clouds are due to a wave movement
intersecting a plane of incipient cloud formation, the whole question
of their mode of production resolves itself into two parts--how is that
plane of incipient condensation produced? and how can we account for
the intersecting waves?

The first question has by far the greater importance, since it amounts
to asking for a general explanation of the production of high clouds,
especially the forms of cirro-cumulus, cirro-stratus, cirro-macula,
and the corresponding alto varieties. There are, again, two divisions
also to this question. How does the water vapour reach the stratum
in sufficient quantity to saturate it? and when condensation takes
place, why does it so frequently assume the characteristic mottled
and granular forms like crowds of little cumulus clouds arranged in
one level? This last sentence gives the clue. They are, in truth,
little cumulus clouds, and must be formed in exactly the same way as
their vastly larger prototypes of lower regions. It has been explained
that low cumulus is the result of large upward moving air columns or
convection currents, each one being initially caused by the heating of
the vapour-laden air near the ground, and each uprising column being
supplied by cooler descending air which flows down in the intervening
spaces. It has also been explained that these movements result in
changes of temperature, which tend to check those movements and restore
the original equilibrium. Suppose this to occur, as it constantly does,
without any column reaching sufficiently high to produce a cloud. There
will be no visible effect, but, nevertheless, an important change has
taken place. Every ascending current has lifted some water vapour with
it to a higher level, and the descending drier air has come down in
contact with the ground or damper air to become equally charged with
moisture in its turn. The process will be repeated again and again,
and at one level after another, so that the water vapour travels ever
higher and higher.

This process of interchange between ascending and descending air
has been called by Mr. Ley inversion, but the term does not seem
very suitable, and interconvection would be better. The two opposite
currents pass through each other, as if the ascending air gathered
itself into definite channels, and passed through holes in the
descending mass like the passage of water upwards through a descending
plate of perforated metal. Moreover, just as the holes in such a
descending plate might have any size, so that the ascending streams
might vary in breadth from the finest hair to a column of huge
diameter, in exactly the same way the ascending columns of air may vary
from the smallest imaginable size to the great cumulo-nimbus currents.
It is the little currents which account for the constant quiver of the
margins of any object which is viewed through a large telescope by day,
and for the haze, so characteristic of a hot day, which makes distant
objects seem ill-defined and in a state of continual tremble. The rays
of light in passing through the intersecting streams are bent a little,
now this way, now that, as the air currents sway to and fro.

The near neighbourhood of the ground is not essential. As long as the
temperature of the air at any level is rising, so long interconvection
must occur. The process will be independent of the presence or absence
of wind. All that wind can do is to mix up the air at different levels,
breaking the system of currents and reducing it to, so to say, a finer
texture, or producing eddies, if strong enough, which direct the
currents and gather them into definite channels. The final result in
any case is that, with rising temperature, water vapour is steadily
borne upwards from the ground.

As it ascends the air becomes cooler, and yet retains its water vapour.
When the rising currents are large they mix little with the descending
dry air, and on reaching a certain level condensation takes place,
and we have the beginning of a cumulus. If they are of a more moderate
size they will ascend less rapidly, the admixture with descending air
will bear a larger proportion to the whole, and the plane at which
condensation will begin will be higher, and then each small column
will be tipped with a ball of alto-cumulus. Make the interconvection
currents smaller still, and the cloud plane will be lifted yet higher,
and we shall have cirro-cumulus or cirro-macula.

Now, the more even the distribution of temperature on the ground the
less the probability of coarse interconvection, and the same is true
of any higher stratum of air, provided it is free from disturbing
influences from outside. If, therefore, we have large currents near
the ground, ending, as they must, in cumulus, it has already been
explained that these clouds stop the action, and the general system of
large currents will be restricted to the region in which they occur. At
some distance above the lower clouds the only difference will be that
water vapour has been brought up to their level in great abundance.
Smaller systems of interconvection can then exist, and so we may have
the spectacle of several layers of cloud--cumulus capping the great
currents of lower regions, alto-cumulus forming the summits of the
smaller currents of intermediate regions, and cirro-cumulus floating
far above both.

Frequently it happens that before the ascent of vapour has gone quite
far enough to produce a cloud, other causes co-operate, and the cloud
makes its appearance suddenly over considerable patches of sky. The
most potent of these is a fall of the barometric pressure, which is
brought about by some of the air far above the region of even the
highest clouds flowing away to some other district. The air at all
lower levels being thus relieved of the superincumbent pressure,
immediately expands, and is thereby cooled throughout. Consequently, if
at any level it was near its point of saturation, it will be carried
beyond that point, and cloud will rapidly make its appearance over a
large part of the sky, possibly at more than one level. Stratiform
arrangements will be the rule; but if interconvection is going on at
the time, its presence will be betrayed by a granular or cumuloid
structure. Interconvection clouds should then be most frequent, and
best formed when the air as a whole is still or moving slowly (so as
not to create great eddies), when the temperature is rising rapidly,
and when the barometer is making a sudden fall. All these conditions
are met in thunder weather, and at the time when a summer anticyclone
is giving way. It will be remembered that many of the most beautiful
forms have been described as forming under one or the other of these
very conditions.

A second contributing cause, and one which tends to make the
condensation in patches or long broad bands ranged roughly at right
angles to the direction in which the air is moving, has been referred
to earlier. It is the passage of the air over an undulating country;
the up-and-down movements of the lower air being transmitted upwards
to great altitudes, as ever broadening and flattening waves. If the
upper air is flowing more rapidly than the lower, these broad waves
may be far ahead of their real cause, which will, therefore, quite
escape recognition, but the phenomenon is constantly to be detected
in the arrangement of the lower clouds. Two instances in the writer’s
experience will suffice. It was desired one morning to measure the
altitude of some small clouds which were passing from the north-west
at a height of probably between 2000 and 4000 metres, over a hill only
about 150 metres higher than the valley in which the apparatus was
fixed. In order to make the measurement, it was necessary for the cloud
to cross the valley and appear in the same field of view as the sun,
according to the method that will be described further on. But in order
to cross the valley the air had to descend, and so, of course, had the
cloud stratum, though to a less extent. But small as the descent was,
it was enough to dry up the clouds entirely, and for more than a couple
of hours the clouds came sailing over the hill, disappearing entirely,
and then reforming so far beyond that no measurement was possible,
since not one single fragment came near enough to the position of the
sun, which remained shining brightly through a broad clear gap between
two patches of cloud-strewn sky.

On another occasion considerable preparations had been made for some
photographic observations during an eclipse of the sun. The observatory
stands on the eastern side of the valley of the Exe, which is flanked
on its western side by a long ridge of hills going up to 800 feet above
the sea. Beyond these hills lies the deep, narrow valley of the Teign,
and beyond that the granite ramparts of Dartmoor, 1000 feet above the
sea. The wind was blowing gently across the two valleys, and shortly
before the eclipse began a broad strip of thin cloud formed above and
rather towards the eastern side of the Exe valley, just where the sun
was, while at the same time the sky was practically clear half a mile
further east, and bright sunlight was streaming down on the ridge
between the two rivers a few miles towards the west. The cloud was
never thick enough to quite hide the sun, so that the eclipse was easy
to watch with the naked eye; but in spite of fairly rapid movement of
the cloud masses as they drifted before the sun, they kept on forming
in just the same place, and completely prevented the carrying out of
the programme planned. It is almost certain that the phenomenon was
brought about by an upward moving wave marking the place where the
level of approaching saturation was upheaved by the disturbance caused
by crossing the two valleys and intervening ridge.

These two instances are not quoted as examples of a rare occurrence,
but as definite simple instances of a phenomenon which may be
constantly observed, and as proof that the conformation of the ground
does exercise an influence upon the distribution of cloud.

But no irregularities of the ground will suffice to explain the minute
waves and ripples which have been described at the beginning of this
chapter. These must be due to wave disturbances in the air itself.
They have been explained as due to two different currents of air,
either a warm damp current flowing over a cold one, or _vice versâ_.
Now, such an occurrence as a warm damp current flowing over a cold
one must be very rare, though it is impossible to deny that it might
occur. The immediate contact of a cold current above a warm damp one is
equally unlikely, unless the general atmospheric condition were greatly
disturbed, which is the same thing as saying that wave clouds would not
occur. They are most frequent at just those times when interconvection
has freest play, and this is amply sufficient to account for a plane of
saturation without any necessity for a hypothesis of two layers of air
at different temperatures all but producing cloud at their junction.
No convincing evidence of cloud production by such means has yet been
adduced, and it is better to rely upon causes which we know do operate
than to call in theories as to what might possibly happen. This is
one of those points in the study of clouds which need investigation,
and until proof is forthcoming it is better to say that the admixture
of two strata of air might conceivably produce cloud, but most forms
can be accounted for by other causes of which we have more positive
evidence.

Still, the wave clouds are due to waves, and there seems no other way
of accounting for them than the supposition of gentle differential
currents. But if such currents occur the ripples and waves will not
be limited to a definite surface, so to say, of contact, but will be
propagated upwards and downwards for considerable distances from the
level of greatest disturbance. Whether, therefore, the level at which
the natural operation of interconvection has produced saturation is
high or low in this region, the result will be the marshalling of the
ascending and descending elements of the convection system in the
characteristic waves.

The differential currents, then, which cause the waves must not be
conceived as producing those waves at a surface of contact, nor must
the currents be thought of as separated by any definite surface, but
rather by a region of variable but usually considerable depth, in which
the air is disturbed by a series of small slow eddies and oscillatory
movements. When the waves are parallel straight lines the air currents
may be really portions of a whole, having the upper part more rapid
than the lower. In such a case the direction of movement should be
at right angles to the cloud lines. If the upper current differs in
direction as well as velocity, the direction of movement of the clouds
will be intermediate, and will resemble that of the upper or lower
current, according to their relative distances from the plane at which
the clouds are formed.

The behaviour of the clouds will depend upon the relative shares in
their production borne by interconvection pure and simple and by the
wave oscillations. If the stratum is one in which cloud would actually
be formed independently of the up-and-down movements, all this will
be able to do will be to arrange the cloudlets at their birth, and
these will then continue to exist, drifting with the general horizontal
movement of the air like any other cloud of the same order.

On the other hand, if the production of cloud is dependent upon the
vertical oscillations, the cloudlets or lines of cloud will move with
the air waves, and their rate of motion and direction of motion will
be determined by the rate and direction of the waves, which may be
quite different from that of the air at that stratum as a whole. The
ascending waves will be marked by lines of cloud generally rounder and
better defined on their advancing sides, while the descending troughs
will be marked by clear intervals.

Wave movements of the necessary kind are frequently very complicated,
and it is not by any means a rare occurrence to see the wave lines in
one part of the sky at all sorts of angles with similar lines in other
parts, or even to see two or more sets of waves at different altitudes
crossing one another. Either phenomenon is always accompanied by rapid
changes in the cloud, and the rippled structure is short-lived. This
was the case with the clouds shown in Plate 54. Plate 53, on the
contrary, shows great uniformity in the wave lines, and although the
vertical oscillation is probably the main cause of condensation, the
form was unusually persistent.

Irregular patches of wave disturbance, affecting a plane occupied by
cirro-stratus vittatus, are shown in Plate 57. In this case the wave
systems only touch the cloud plane here and there, and the places of
contact varied rapidly. It is pretty clear from this photograph that
the idea of the waves being formed at a surface of contact between two
diverse currents will not suffice. The bands of the cirro-stratus are
for the most part unbroken and unaffected; it is only here and there
that the wave region touches them.

[Illustration: Plate 57.

WAVED CIRRO-STRATUS.

(_Cirro-stratus Undatus._)]

The conclusions at which we have arrived are simple, and there is
little room for doubt as to their main correctness, but there are
numerous minute features presented by these beautiful cloud patterns
which await interpretation, and they reveal complicated oscillatory
movements in the air which are difficult to account for, whether we
seek their originating causes or the mechanics of their motions.




CHAPTER IX

CLOUD ALTITUDES


DURING an extended experience of cloud photography, it was found that
it was quite possible to get pictures which showed the cloud detail
even when the sun was in the field of view. Sometimes the solar image
was reversed, but if the exposure was very short this was not the
case. In such photographs the structure of the cloud was exceedingly
clear and sharply defined quite close to the sun. Indeed, the intense
illumination seemed to reveal minute details of internal arrangement
which could not be detected in similar clouds some distance away.

The methods which had been employed for the measurement of cloud
altitudes elsewhere have already been briefly referred to. Some of
them required two observers, who were equally responsible, each of
them having to direct his apparatus or camera to the same point of the
cloud, and to record the exact direction in which the instrument was
pointed. The instruments, if accurate, were costly, and there were many
opportunities for error in reading the graduated circles which gave the
directions. Moreover, in most of these methods the two observers were
connected by telephone, and had to agree on the exact point towards
which their instruments should be directed; either the exact point
of the cloud, or the precise direction as shown by the mounting of
the camera or other instrument. At Kew some of these sources of error
were avoided by fixing the two cameras with the axes of the lenses
and centres of the plates in a vertical position and exposing the two
plates simultaneously. The Kew observations were not long continued,
and for some years the only measurements in progress were those carried
out abroad, particularly at the Blue Hill Observatory and at Upsala.

The experience gained in photographing clouds in order to record their
forms suggested a way in which many of the sources of error in previous
measurements of altitude could be avoided, especially by simplifying
and reducing the operations at the moment of making the observation.

If two cameras are placed at the opposite ends of a measured base line,
whose direction is known, and if they are both pointed towards the sun,
on making the exposures by electrical means at the same moment, the
position of the image of the sun upon the plate gives the direction in
which the cameras are pointed. It will be in the same direction as seen
from both ends of the line.

Now, if we note the time at which the exposure is made, this with the
date gives all that is required for ascertaining the sun’s position
in the sky, and is, therefore, the only exact observation which need
be made at the time of taking the photographs. Mistakes are almost
impossible, as each plate contains its own record of the sun’s
position, and even if some of the plates should get mixed the images of
the clouds will generally suffice to pair them properly. For general
measurements there is one grave defect in the method, and that is that
it can only be used when the sun and cloud can be got into the same
field of view. But with the higher varieties of cloud this is generally
possible, and it was just these higher sorts about which knowledge was
least certain, and which it was proposed to study.

An initial difficulty was the finding of a level site, flat land being
very uncommon in Devonshire, but fortunately a suitable place was
found in some artificially levelled ground close to Exeter, belonging
to the London and South Western Railway Company. It was a stretch of
ground intended to be covered with sidings, but had not been finished,
and had become overgrown with grass, stunted sallows, and other wild
plants. Being railway ground, it was, comparatively, though by no means
entirely, free from mischievous and inquisitive people. The next point
was a suitable camera. It must have fairly long focus in order to give
a large image, and therefore large displacement; it must be capable
of being pointed in any direction and clamped there; and it must be
capable of standing considerable extremes of temperature and variations
of dampness, as it was intended that they should be kept on the spot in
wooden structures, which served for stands as well as to contain the
apparatus.

The pattern finally decided upon is represented in Plate 58, which
shows one of the cameras pointed up to the sky and standing on one of
the stands. These cameras were to take plates of whole plate size, two
double dark slides of the ordinary pattern being attached to each.

[Illustration: Plate 58.

CAMERA FOR MEASURING ALTITUDES.]

The camera looks rather complicated, but it is really simple. Its
body consists of front and back, each attached to a central part by a
short bellows and sliding on a base board, to which it can be clamped
by screws of the usual pattern. The central part carries trunnions,
such as are used for looking-glasses, which swing in sockets carried
by two upright supports, so as to give the whole free motion in a
vertical plane. In order to be able to fix it firmly at any angle, the
base board of the camera body carries on its underside a thin board
projecting beneath it and forming a segment of a circle whose centre
would be the horizontal axis through the trunnions. The board passes
between the jaws of a small wooden clamping vice in front, which is
carried by the square base to which the uprights are fixed. The whole
is firmly made of well-seasoned pine, and has stood well the hard usage
of half a dozen years.

There is no focusing screen. Focusing was done with great care once for
all, and then a coat of hard varnish was put over all the adjusting
screws. A small view-finder is attached to one side, and it was by
this that the camera was pointed in the desired direction.

In order to lessen risk of mistake, it was so arranged that the two
slides belonging to one camera would not fit the other. The lenses,
of 18 inches focus, and giving sharp detail all over the plate, were
carefully matched, and the focus adjusted until the images given by
them when placed side by side appeared to coincide exactly. They were
provided with iris diaphragms, which were shut down to an aperture of
a quarter of an inch, and with shutters which could be released at the
same moment by an electric current, acting through the electro-magnet
shown under the lens on the front of the camera.

The shutters were of the kind known as the “Chronolux,” which will give
any exposure from the sixty-fourth of a second up to three seconds. But
it was found in practice that the highest speed was sufficient and gave
satisfactory results. Of course, there was no idea of adjusting matters
on each occasion so as to get the best possible negatives capable of
yielding good prints. Measurement was the object, and if the negative
showed the sun and sufficient cloud detail for the identification of
cloud points, that was all that was wanted. The shutters gave a good
deal of trouble at first. Their sliding parts were made of ebonite, and
when the cameras were left in their stands with an August sun shining
down upon them, everything inside got very hot and the ebonite warped;
but the difficulty was got over by substituting aluminium.

The two camera stands were placed 200 yards apart, and were connected
by a line of telegraph wire carried on short poles. At each end of
the wire an insulated connecting piece was brought down to the camera
stand, and to the batteries and other apparatus. The current which was
sent through this wire by pressing a contact at one end of the line did
not directly make the exposures; but two similar relays were brought
into action, and each of these sent the current from a local battery of
Leclanché cells through the electro-magnet on the camera and made the
exposure.

After development the two negatives showed the image of the sun, not
far from the centre of the field of view, and the cloud whose altitude
was required. Since this was taken from two different points of view,
the negatives were not alike, but the distances between the centre
of the sun’s disc and any special point of the cloud were different.
For instance, if the cloud were east of the sun, with its edge just
apparently touching the solar image as photographed from the eastern
station, then the negative taken from the western end of the base would
show an interval of clear sky between the two, which would be greater
as the cloud was lower.

It often happened that after developing the plates the image of the
sun was lost in a black blur, but it was easy to reduce this part of
the image by local application of a reducing agent[3] by means of
a paint-brush, until the disc became clear enough. Two lines were
then drawn on the negative, one vertical and the other horizontal,
intersecting each other at the centre of the sun’s image. These lines
served as the starting-points for exactly measuring the distance from
their point of intersection to any selected point of the cloud.

The distances could generally be determined to a fiftieth or a
hundredth part of an inch, and their difference was, of course,
dependent upon the direction of the sun relative to the base line and
the altitude of the cloud, but for low level clouds the difference
was sometimes so great that no pair of corresponding points could be
detected, while it was often as much as an inch. With higher clouds
the differences were smaller, but unless the sun was very low in the
sky, either east or west, the displacements of the cloud image were
great enough to give reliable measures. Specimen prints from pairs of
negatives are shown in Plates 59 and 60.

[Illustration: Plate 59.

PRINT FROM A NEGATIVE USED FOR MEASURING ALTITUDE.]

[Illustration: Plate 60.

PAIR OF PRINTS SHOWING THE DISPLACEMENT OF THE CLOUD.]

The processes by which the measurements are worked out are
laborious,[4] and consist of two parts, the first being the
determination of the exact position of the sun from the date, hour, and
latitude and longitude of the place, and the second, the determination
of the position of the cloud. Two points which represent the same part
of the cloud are selected, and their respective distances from the two
lines drawn through the sun are measured as accurately as possible.
Now, a certain distance on the negative corresponds with a definite
angular displacement, and a scale can be constructed showing how much
should be added to or subtracted from the sun’s position to get the
exact position of the cloud. This being done, it is then a simple piece
of trigonometry to deduce the actual height of the cloud above the
place of observation. The work of computation, however, was greatly
lightened by the fact that many of the pairs of negatives showed more
than one layer of cloud; thus Plate 59, which is a fair specimen,
shows three layers, and, consequently, one determination of the sun’s
position sufficed for three distinct results.

For the highest clouds the displacements were, of course, small, and
could only be made with certainty of a correct result within about
three hours of noon. Earlier than 9 a.m., or later than 3 p.m., the
sun was too nearly in a line with the two stations, or too low in the
sky, to give a sufficient displacement of image. A base line of 400
yards instead of 200 would have been better for the high clouds. But,
on the other hand, when low level clouds are viewed from two different
spots their outlines may seem so changed that it may be impossible to
identify a pair of corresponding points, and the same difficulty may
also arise when high clouds are seen through a gap in a lower stratum.
The longer the base line the more frequent and more obtrusive would
this perspective difficulty become, so the distance of 200 yards
between the stations was adopted as a convenient mean.

The method of making the observations was simple. Each observer was
provided with some signal flags, by which the necessary communications
were made in accordance with a simple code. Call the two observers A
and B, and suppose A directed the operations. He watched the sky until
a favourable opportunity seemed to be approaching. He then signalled
to B, and both cameras were turned to the sun, the dark slides were
inserted, the shutters set, and everything made ready. Signals were
then interchanged, to signify that preparations were complete, and when
A saw that the edge of the cloud had reached a suitable position to be
in the same field of view with the sun, the contact key was pressed and
the plates simultaneously exposed. At the moment when this was done the
time was noted. Several observations were thus made in a short time.

Measurements were carried out as opportunity allowed over four
consecutive seasons, from the beginning of April until the end of
October. During the last of the four years, the site had become less
convenient owing to an extension of the railway work, and early in
November the series was brought to an abrupt conclusion by a heavy
gale, which snapped off all the poles carrying the connecting wire. But
by that time 423 measurements had been obtained, the great majority of
which referred to clouds of the cirrus and alto groups.

The general results may be tabulated thus, giving heights in metres:--

  -------------------+-------------+---------+---------+---------
                     |  Number of  | Maximum | Minimum |  Mean
                     |observations.|altitude.|altitude.|altitude.
                     +-------------+---------+---------+---------
  Cirrus             |       58    |  27,413 |   4,114 |  10,230
                     |             |         |         |
  Cirro-stratus      |       64    |  15,503 |   3,840 |   9,540
                     |             |         |         |
    „   cumulus      |       63    |  11,679 |   3,657 |   8,624
                     |             |         |         |
  Alto-cumulus       |       83    |   9,390 |   1,828 |   5,348
                     |             |         |         |
  Cumulus top        |       42    |   4,582 |     --  |   3,006
                     |             |         |         |
    „     base       |       48    |   1,959 |     584 |   1,290
                     |             |         |         |
  Strato-cumulus     |       27    |   6,926 |     823 |   2,248
                     |             |         |         |
  Cumulo-nimbus top  |       15    |   6,409 |   2,004 |   8,002
                     |             |         |         |
    „      „    base |       15    |   2,286 |     766 |   1,045
  -------------------+-------------+---------+---------+---------

These values are not very different, on the whole, from those which
have been arrived at elsewhere, and in making a comparison it must
be borne in mind that there is always a little want of precision in
cloud nomenclature. As a whole, the Exeter maxima are greater than the
foreign ones, and this is very markedly so in the case of cirrus, for
which the American highest record is 14,930 metres, the Swedish record
is 13,376, while the Exeter value is 27,413 metres, or about 17 miles.
But this extreme measurement, and several others unusually large,
were made in one morning, a day of very hot damp weather, when cloud
formed at seven different levels: cumulus at a height of 1·9 miles,
alto-cumulus at 3·9 miles, cirro-cumulus at 4·7 miles, cirro-stratus
(No. 1) at 8 miles, cirro-stratus (No. 2) at 9·6 miles, cirrus at 11·5
miles, and cirrus excelsus at 17 miles. By about half-past one in the
afternoon the sky was completely overcast with dull grey clouds, which
cleared off at half-past four, and at half-past five in the evening the
cirrus had fallen to 7·9 miles, and the cirro-cumulus to 4·3 miles.
If this one day’s observations had been omitted, the Exeter maximum
would only have been little more than 1000 metres above the record from
across the Atlantic, but 1000 metres is a height worth noting.

While the Exeter maxima are all rather greater, we find the minima
for cirrus, cirro-stratus, and cirro-cumulus are rather less than at
the foreign stations; that is to say, that clouds are formed over
Devonshire both at lower and at higher levels than seems to be the case
in Massachusetts or Sweden. It seems probable that this is due to a
greater humidity on our western coasts, such as we should suppose would
be the case from their position and the prevailing winds and ocean
currents. If so, we should expect the great convection clouds to be
larger. Thus, at Exeter, out of only fifteen examples of cumulo-nimbus,
the top varied from 2004 metres to 6409, with an average base level
of 1045. At Upsala the maximum was 5970 and the minimum 1400, with an
average base level of 1400. The mean thickness of the Swedish clouds
was only 1400 metres, while that of the Devonshire specimens was more
than 2000 metres.

Again and again, during the progress of these measurements, it was
found that the greatest altitudes and the richest development of the
higher varieties occurred towards the end of a spell of fine calm
weather, when convection had had free play day after day. A slight
fall of the barometer, only the hundredth part of an inch, would
usually, under those circumstances, bring about abundant formation
of high clouds, frequently of the undatus kind. All the cumulus
clouds, by which we mean to include alto-cumulus and cirro-cumulus,
are most frequent when the levels of condensation are rising, while
the stratiform clouds are an indication of no vertical movement or
of active descent. Pure cirrus is indicative rather of movement in a
horizontal direction, and may occur when the condensation levels are
stationary, or when they are rapidly changing either way.

In broken weather the natural movements of the atmosphere and of its
vapour are masked and disturbed by the strong eddies brought by the
cyclonic systems. It not unfrequently happens that the region of
disturbance does not reach up to the level of the highest cirrus, or,
what is more probable, the cyclonic system leans so far forward that we
may have in its rear the upper clouds floating quietly far above the
comparatively shallow region of disturbance, while in front the upper
part of the storm system projects above undisturbed air.

The frequent appearance of cloud almost at the same time at more than
one level is at first rather difficult to understand, but it will be
noticed that when this occurs the barometer almost invariably falls.
Now, if we suppose that the air is nearly saturated at more than one
level, and that the whole is then bodily relieved of some of the
superincumbent mass, so that the barometer falls, the mass of air will
at once swell up, being cooled from top to bottom simultaneously, and
wherever it is damp enough cloud will be formed.

The converse is equally true. If we have cloud at several levels, and
the whole is compressed by the addition of more air above, which is the
case when the barometer rises, that compression will be accompanied
by the generation of heat and the consequent disintegration and
disappearance of the clouds.

FOOTNOTES:

[3] Ferricyanide of potassium and hyposulphite of soda.

[4] From the declination of the sun corrected for variation and from
the known latitude, the meridian zenith distance is calculated.

From the Greenwich time, the longitude, and the equation of time, the
hour angle is obtained.

Now, if H be the hour angle, D the reduced declination, and M the
meridian zenith distance, the sun’s altitude may be calculated by the
formula--

 log versin H + L cos lat. + L cos D-20 = log _n_,

where _n_ is a natural number, and

 _n_ + vers M = covers alt.

Again, to find the azimuth--

 vers sup. (lat. + alt.)-vers polar dist. = _m_,

where _m_ is another natural number, and

 log _m_ + L sec. lat. + L sec. alt.-20 = log vers azim.,

reckoned from the south.

Hence the position of the sun is ascertained for both negatives.

By actual measurements on the plates and reference to a previously
constructed scale the position of the cloud as seen from each camera
is next determined, and the angle subtended by the base line at a
point X vertically beneath the cloud is calculated. If A and B are
the stations, and _a_ and _b_ the angles from them respectively, the
distance AX is given thus--

 log AX = L sin _b_-L sin AXB + log AB,

and the height _h_ of the cloud above X is given by--

 log _h_ = log AX + L tan alt.-10.




CHAPTER X

CLOUD NOMENCLATURE


SINCE a considerable number of new terms have been suggested in the
foregoing pages, it may be convenient to collect them and tabulate
them, so as to show their relation to those already recognized by the
International system.

In the atlas put forward by the committee, sixteen varieties are
recognized by distinct names, and these are drawn up in tabular form
with appropriate abbreviations for use in making records.

The names are--

  Cirrus. Ci.
  Cirro-stratus. Ci. S.
  Cirro-cumulus. Ci. Cu.
  Alto-cumulus. A. Cu.
  Alto-stratus. A. S.
  Strato-cumulus. S. Cu.
  Nimbus. N.
  Cumulus. Cu.
  Cumulo-nimbus. Cu. N.
  Stratus. S.
  Fracto-cumulus. Fr. Cu.
  Fracto-nimbus. Fr. N.
  Fracto-stratus. Fr. S.
  Stratus-cumuliformis. S. Cf.
  Nimbus-cumuliformis. N. Cf.
  Mammato-cumulus. M. Cu.

During our survey of these groups we have found that some of them
include clouds of many shapes, which must be due to very diverse
conditions. It follows that if observations are to be made on the
occurrence of these special kinds, with a view to arriving at a
thorough understanding of the circumstances to which they owe their
forms, it becomes necessary to devise a code of names and symbols
whereby an interchange of ideas and records may be rendered possible.
Specific names have been proposed as each form was considered, and
it only remains to sum them up concisely. Subsequent observation,
particularly in other climates, may show that further additions should
be made; but if the principle of specific names be once admitted, it
will be easy to fill any omission.


GROUP CIRRUS.

Under the general head of cirrus we have found nine distinct forms--

1. _Cirro-nebula_ (Ley) (Plates 2 and 3). Cirrus veil.

Characterized by comparative absence of structure and by the formation
of halo. Ci. Na.

2. _Cirro-filum_ (Ley) (Plate 7). Thread cirrus.

Built up of fine long threads, straight, curved, or crossing, but free
from hazy curling or flocculent structures. Ci. F.

3. _Cirrus excelsus_ (Plate 5). High cirrus.

Characterized by great altitude, thinness, irregular branching
structure. Ci. Ex.

4. _Cirrus ventosus_ (Plate 6). Windy cirrus.

Characterized by curving branches leaning forward in the direction of
movement, and other long curving streamers lagging behind and below.
Fluffy parts are usually present, and mark the origins of the long
curling fibres. Ci. V.

5. _Cirrus nebulosus_ (Plate 9). Hazy cirrus.

Characterized by the absence of sharply defined lines, fibres, or
streamers; all parts of the cloud being hazy, and suggestive of other
varieties of cirrus out of focus. Ci. Neb.

6. _Cirrus caudatus_ (Plate 8). Tailed cirrus.

Characterized by small hazy or fluffy heads behind or below which hang
long streamers, which taper away more or less to a point. The tails are
sharply defined, and so are the edges of the heads. Ci. Ca.

7. _Cirrus vittatus_ (Plates 12 and 13). Ribbon cirrus.

Characterized by formation in long bands of cloud, sometimes made of
parallel long fibres with cirrus haze linking them together, sometimes
consisting of a long bundle of fibres, from which others diverge at an
angle as shown in the plate. Ci. Vt.

8. _Cirrus inconstans_ (Plate 10). Change cirrus.

Characterized by a peculiar ragged, wavy appearance. It is
generally only the beginning or the end of a mass of cirro-stratus
or cirro-cumulus, but occasionally it vanishes shortly after its
appearance, without reaching the further stage. Ci. In.

9. _Cirrus communis_ (Plate 11). Type cirrus or common cirrus.

Characterized by short irregularly curling fibres collected together
in considerable patches. No definite arrangement into any of the forms
already described. Ci. Com.


GROUP CIRRO-STRATUS.

Under this group the cloud usually shows some structure, being
apparently built up from a massing together of detached forms at a
common level. When this is so it should be described by adding the
specific name of the detached form most nearly related.

1. _Cirro-stratus nebulosus_ (Plates 3, 4, and 14). Hazy cirro-stratus.

Characterized by absence of visible structure. Ci. S. Neb.

2. _Cirro-stratus communis_ (Plate 16). Common cirro-stratus.

Characterized by the presence of short curling fibres matted together.
Ci. S. Com.

3. _Cirro-stratus vittatus_ (Plate 57). Ribboned cirro-stratus.

Characterized by being made up of long stripes or bands of cloud. Ci.
S. Vt.

4. _Cirro-stratus cumulosus_ (Plate 17). Flocculent cirro-stratus.

Characterized by an obscurely granular structure. Ci. S. Cu.

Many forms of cirro-stratus are arranged in waves or ripples. This is
indicated by attaching the word undatus, or waved, after the ordinary
specific name, or the letter U after the abbreviation.


GROUP CIRRO-CUMULUS. Divisible into three species.

1. _Cirro-macula_ (Ley) (Plate 23). Speckle cloud.

Characterized by semi-transparency, by the fact that the particles
are frequently whiter and more opaque on their edges. A patch of
cirro-macula always looks like a thin sheet which has curdled. Ci. Ma.

2. _Cirro-cumulus nebulosus_ (Plates 20 and 21). Hazy cirro-cumulus.

Characterized as rounded balls of semi-transparent cloud, but
ill-defined and hazy. No shadows. Ci. Cu. Neb.

3. _Cirro-cumulus_ (Plates 18 and 19).

Characterized as opaque rounded balls clearly defined, but showing no
shadows on their under sides. Ci. Cu. Com.

Wave forms again are indicated by the addition of the word undatus.


GROUP ALTO CLOUDS. Divisible into nine species.

1. _Alto-stratus._ High stratus.

A uniform veil of cloud showing no details of structure except local
variation in density in patches. Rarely dense enough to completely hide
the sun, or even the full moon. A. S.

2. _Alto-stratus maculosus_ (Plate 30). Mackerel sky.

Characterized as numerous nearly equal and small lenticular patches
ranged on a level and about equi-distant from each other. A. S. Mac.

3. _Alto-stratus fractus_ (Plate 34).

Patches and bits of cloud of irregular shape, but resembling broken
bits of a level sheet. A. S. Fr.

4. _Alto-strato-cumulus_ (Plate 32).

Intermediate between alto-stratus and alto-cumulus. A. S. Cu.

5. _Alto-cumulus informis_ (Plate 25).

Characterized as more or less rounded cloudlets interspersed with
ragged bits of cloud and occasionally with streaks of cirrus, the
cloudlets showing no clear-cut outlines, but having distinct shadows.
A. Cu. In.

6. _Alto-cumulus nebulosus_ (Plate 26).

Hazy alto-cumulus. A. Cu. Neb.

7. _Alto-cumulus castellatus_ (Plate 28). Turret cloud.

A high cloud resembling a number of tall narrow cumulus clouds on a
very diminutive scale. The cloudlets show distinct shadows, are very
opaque, and their upper margins are sharply defined. Vertical axes
longer than the horizontal ones. A. Cu. Ca.

8. _Alto-cumulus glomeratus_ (Plate 29).

Characterized by the roundness and regularity of the cloudlets, which
have sharp margins, cast distinct shadows, and have their axes about
equal in all directions. A. Cu. Gl.

9. _Alto-cumulus communis._

Small high cumulus of the ordinary pyramidal pattern. A. Cu. Com.

10. _Alto-cumulus stratiformis_ (Plate 27).

Flattened cloudlets gathering into small detached sheets. A. Cu. S.


Lower clouds. GROUP STRATUS.

1. _Stratus communis_ (Plates 37 and 41).

In its most typical state, stratus consists of a sheet of cloud of
approximately uniform thickness. The most common form, however, does
vary considerably, though usually dense enough to hide the sun.
Portions of such a sheet would take the same specific name, unless the
portions are very small and ragged, which would be expressed by adding
the word fractus. S. Com.

2. _Stratus maculosus_ (Plate 40).

Formed either by the appearance of cloud in lumps, which are always
lenticular in shape, and ultimately join together to form a stratus, or
by the break up of the typical stratus. S. Mac.

3. _Stratus radius_ (Plate 42). Roll cloud.

Formed during the break up of a low stratus, which separates up into a
number of parallel lines of cloud. S. R.

4. _Stratus lenticularis_ (Plate 47). Fall cloud.

Formed by the collapse of cumulus or strato-cumulus. A cloud of
evening, easily recognized as lenticular patches. S. L.

5. _Strato-cumulus_ (Plates 38 and 39).

A term applied to either a stratus which has thickened every here and
there into cumulus, or a number of cumulus which have joined together
so as to show a nearly continuous common base. S. Cu.


GROUP CUMULUS.

1. _Cumulus minor_ (Plate 43). Small cumulus.

Cumulus clouds so small as to present the appearance of rounded lumps,
no definite pyramidal form or flattened base. Cu. Mi.

2. _Cumulus major_ (Plates 44 and 45). Large cumulus.

Characterized by a flattened base and rounded clear-cut upper surfaces.
Cu. Ma.

3. _Cumulo-nimbus_ (Plates 49 to 52). Storm cloud.

Characterized by the expanded, anvil-shaped, or disc-shaped top,
cirrifying at its edges.


GENERAL TERMS

_Nimbus_, a term applied to a cloud from which rain is falling. When
the form of the cloud is visible, the term should be attached to that
belonging to the cloud. It may, however, be used as a substantive alone
when there is nothing to show from what sort of cloud, or combination
of clouds, the rain is falling (Plates 35 and 36).

Nimbus is either heavy stratus, massive strato-cumulus, or a
combination of these with stratiform clouds above, and possibly ragged
masses of fracto-cumulus below. N. either alone or after the sign of
the cloud.

_Fracto-_ is a term placed as a prefix before the name of a cloud to
indicate that the cloud has ragged irregular margins, as if it had been
more or less torn to pieces. It is sometimes less awkward to append the
word fractus after the name of the cloud.

A convenient abbreviation would be to write F. after the name of the
cloud.

_Undatus_, or waved, should always be added to the name of any cloud
which shows the arrangement so described.




CHAPTER XI

CLOUD PHOTOGRAPHY


REFERENCE has been made in the first chapter to the fact that those
who wish to make a photographic study of clouds must follow a special
course of procedure. For every photographic purpose there is some
particular process or some special kind of apparatus which is better
fitted for the end in view than any other, and half the difficulty in
attaining success is to find out the best tools and the best methods.

There is no difficulty whatever in securing excellent photographs of
heavy grey clouds, or of clouds which stand out dark against a twilight
sky. Any camera and any plate can be used, and in an experienced hand
will ensure success after a few trials, but except under these special
conditions, cirrus, in all its varieties, the alto clouds, and even
many of the lower ones, present a real difficulty due to two causes.
In the first place, they and their surroundings are so brilliant that
a very short exposure is sufficient, far shorter than would be needed
for a sunlit landscape; and in the second place, the actinic value of
the light they reflect is very little greater than that received from
the background of blue sky. When so minute a difference comes to be
represented in the monochrome of the ordinary photograph, the eye fails
to appreciate it, and all the finer details are lost.

Now, if proper care is taken in the development of a negative,
satisfactory results may be attained even if the exposure is twice as
great, or only half as great, as it should have been to get the best
result. But if the exposure is four or more times the best duration,
the negative will generally yield but poor contrasts, if any result at
all can be coaxed out. Again, if the exposure is only a quarter or less
of the ideal time, little or no image will come out. Suppose, now, we
have a brilliant object, and the correct exposure for the plate and
aperture of lens employed should be one-fiftieth of a second; if we
make an error either in judging or in effecting the exposure, which
amounts to one twenty-fifth of a second too much, we get the negative
exposed three times as much as it should be. Suppose, again, the object
is less brilliant, and the correct exposure should be one-fifth of a
second, an equal error of one twenty-fifth will make little difference.
But in photographing cirrus and such clouds, if we used the same plates
and the same lens apertures as we employ for ordinary landscape work,
we should want exposures of the order of those given by a focal plane
shutter, and a mistake either in judging or in making the exposure, of
even the hundredth part of a second, would be fatal to good results,
and would probably completely spoil the plate. Evidently one of our
first steps must be to lengthen the correct exposure.

There are four ways in which this can be done--by using a slow-acting
plate, by lessening the aperture of the lens, by putting some
transparent screen in front of the lens to shut off some of the light,
and, finally, by pointing the camera, not at the cloud itself, but at
its image in a black mirror.

Of these, of course the slow plate and small aperture are the simplest
to adopt, and all the cloud studies shown in the illustrations to
these pages have been taken on plates prepared for photo-mechanical
purposes or for transparencies. There seems to be nothing to choose
between these two brands. Orthochromatic, isochromatic, double-coated,
and many other special types of plate had previously been tried, both
with coloured filters in front of the lens and without them, without
showing any marked superiority over an ordinary plate of low rapidity.
At last the photo-mechanical plates were tried, and the efforts made
to get satisfactory cloud portraits, which had previously been marked
only now and then with satisfactory results, became uniformly and
continuously successful.

If the slow plates are exposed in the camera without either a screen or
the black mirror, the diaphragm should be reduced to a small size and
the exposure suitably adjusted. The length of exposure may generally
be judged by looking at the image on the focusing screen, and reducing
the aperture until the picture shows its detail easily. Then, regarding
the picture as that of a sunlit sea or distant landscape, judge the
necessary exposure by the brightness of the image.

No definite rule can be given. The light varies enormously from day to
day, and hour to hour, and especially with the position occupied by
the cloud relative to the sun. Thus, working with a lens of six inches
focus and an aperture of a quarter of an inch, the exposure may vary
from the quickest snap of a Thornton-Pickard roller blind to as much as
a quarter of a second, or even more. Again, using a lens of eighteen
inches focus and an exposure of a fiftieth of a second, the necessary
aperture might vary from an eighth of an inch up to an inch and a
half. But if we suppose that we are dealing with an ordinary bright
summer sky between 9 a.m. and 5 p.m., and that the clouds are cirrus
or cirro-cumulus, an aperture of about one thirty-second of the focal
length will probably give some sort of image with a snap-shot exposure.
At first the failures will be many, but a little practice will soon
enable very respectable pictures to be taken by varying either the
diaphragm or the speed of shutter. Heavier clouds of the alto types
will need rather longer exposure or larger aperture.

The lens may be of any kind, as long as it gives a well-defined image,
but there are many advantages in using one of the rectilinear type
provided with an iris diaphragm. A rapid lens is not needed; indeed, it
has been pointed out that slowness is a very great desideratum, and if
the camera is provided with a rapid lens it must be ruthlessly stopped
down. For general cloud purposes the best kind of lens is a wide-angle
rectilinear, but many occasions will present themselves on which a
lens of longer focus will be wanted in order to give more insight into
the details of some specially delicate clouds. If the lenses are good,
and the focusing is accurate, enlargements will go a long way towards
revealing the minuter structures, but the results can never be quite so
well defined as a direct photograph in a long camera.

A shutter will be essential, and it should be one which opens in the
middle, or which travels across the lens. The shutters which are
ingeniously contrived to give more exposure to the lower part of the
picture than to its upper part are useless for the purpose in view. It
should have some latitude of exposure, from about one-sixtieth of a
second up to a full second or more.

Then as to the camera. Any light-tight camera will do, and, as
the objects will all be at a great distance, it may very well be a
fixed-focus one, or may be kept set up and fixed in focus for a distant
object. If not, on setting it up it should be focused on the horizon or
most distant object possible, and not on the cloud itself. As, however,
the clouds present themselves at all heights above the horizon, even
in the zenith, it becomes necessary to have some means of pointing the
camera in such directions. To a certain extent the ordinary stand does
allow of tilting, but a special support which will allow the camera to
be fixed firmly in any position is of the greatest convenience.

If the study is meant to be at all prolonged, the best plan is to make
a suitable camera, once for all, which can be left in fixed focus, so
as to be always ready, and which can be directed with equal ease to any
part of the sky, from the horizon to the zenith. If it is intended to
use a black mirror, then a special mount becomes almost essential.

Many of the most delicate of the photographs reproduced here have been
taken with a camera of peculiar pattern, the structure of which is
shown in Plate 61. The lens is an ordinary rapid rectilinear, and the
stop used was generally one-sixteenth of the focal length. The shutter
is a light slip of aluminium, which can be drawn across from side to
side at any desired pace. The body of the camera is mahogany, with a
bellows part for getting correct focus, but when once this was obtained
the back was clamped to the tail-board and a little varnish brushed
over the clamping screws.

[Illustration: Plate 61.

CLOUD CAMERA FOR STUDIES.]

The camera swings on a couple of screws, which act as trunnions. These
pass through two upright arms, which spring on either side from the
base board, which is attached to the ordinary camera stand. This base
board can be rotated into any horizontal position desired, and the
camera can be tilted through any vertical angle by swinging it between
the uprights, and can be clamped by tightening the two trunnion screws.
These screws are so placed on the front of the camera that the lens and
its attachments on the one side nearly balance the back part of the
camera on the other side, and so lessen the danger of slipping.

Supported in front of the lens by light brass-work is the black mirror,
made of a very dark glass optically worked on the front face. It is
a curious fact that, although bits of plate-glass blackened on the
back seem to the naked eye to give a single image of sufficient truth,
if such a mirror is placed in front of the camera the second faint
image formed by reflection from the blackened surface is almost always
to be detected. Moreover, the lens with its large aperture at once
detects irregularities in the surface of the glass, which are quite
imperceptible through the narrow limits of the pupil. Black glass,
with a truly worked surface, is essential then, but the surface need
not be of the high order of excellence required for mirrors used for
telescopic work, since the first image is not, as a rule, intended to
be highly magnified.

The mirror is held so that its surface makes an angle of about 33
degrees with the axis of the lens, and the block carrying shutter and
mirror can be turned round into any position by slipping it round the
lens mount as an axis. The mirror thus always retains the correct angle.

The action of the mirror is to a large extent due to mere diminution
of brightness, but it also partly extinguishes the blue light of the
sky without exerting any such influence on the white light from a
cloud. This is due to the fact that the blue light of the sky is partly
polarized, while that reflected from the cloud is not. Now, polarized
light which falls upon a black mirror held in a particular position is
not reflected by it. This position depends upon various circumstances,
but one condition is that the reflected ray must make an angle of about
33 degrees with the surface of the glass. The amount of the polarized
component of the blue light varies greatly, but is at a maximum at all
points 90 degrees away from the sun. This, then, is the best possible
position for photographing a cloud, as the whole of this polarized
component may be suppressed by adjusting the mirror to the proper
position, and then the most delicate cirrus fibres stand out brilliant
on an almost black background.

The black mirror could with some advantage be replaced by a Nicol’s
prism mounted between the components of the lens, so that it could be
turned in any position; but Nicol’s prisms are expensive, and such an
arrangement would cost many times the sum sufficient for an excellent
mirror, and then would narrow down the field of view in a very
inconvenient way.

With this apparatus exposures of a tenth to a fifth of a second were
usually required for high clouds in bright daylight, while longer
times, up to a second, might be required under less actively actinic
conditions.

The exposure having been made, the next step is development.

Now, every practical photographer has his own pet formula, his own
particular favourite among the numerous developing compounds now on
the market. It is, therefore, rather a thankless task to offer advice
as to which should be selected. In all probability as good results may
be got by other methods and other formulæ, and the description which
follows must be understood rather as an account of the process actually
adopted, than advice as to that which should be chosen.

The developer used has been always pyro and ammonia, made up in
accordance with the formula--

  Pyro                           30 grains
  Potassium metabisulphite       30   „
  Ammonium bromide               30   „
  Water                          10 ozs.

But if much work was anticipated the solution was made up in a more
concentrated form, and diluted to this strength of 3 grains of pyro per
ounce for actual use.

The ammonia solution is prepared by mixing 3 drams ammonia fortiss.
with 20 ozs. of water.

In developing it is necessary to remember that our object is to make
the most of a very small difference in effect. The plate is first
flowed over with a mixture of sufficient developer, with not more than
a quarter of its bulk of the ammonia. If the cloud should flash out in
a few seconds add more of the pyro solution, but unless the exposure
has been much overdone this will not happen. If the image begins to
appear after from thirty to forty seconds it is probable that the best
result will be reached by leaving it alone, but if there is any hanging
back of the detail another quarter bulk of ammonia should be put into
the glass, the developer mixed with it, and the whole returned to the
developing dish.

If no image appears after about forty seconds, add more ammonia as
above described, and leave for another forty seconds, and so on,
until by this method of trial the right quantity of alkali for the
particular exposure has been ascertained. The development must never be
hurried, or the background of sky will blacken too soon, and in some
cases it may take a quarter of an hour or more to get enough density on
the cloud. But as a general rule the image is fully out in about two
minutes, and the plate is then washed and fixed in the usual way.

If a black mirror is used there will seldom be any necessity for
intensification, but if not, it may frequently be required, especially
for the more delicate kinds of cirrus. Indeed, the image may sometimes
be so thin that the common process of intensification by mercury and
ammonia does not give density enough. If that seems at all likely to
be the case, it is wiser to use the formula known as Monckhoven’s,
since that simply adds silver to silver instead of replacing the silver
image by some other body, and the process can consequently be repeated
more than once, if sufficient density is not secured by the first
application. The formula does not seem to be very often used, so it may
be best to quote it.

  A. Potassium bromide                    100 grains
     Mercuric chloride                    100   „
     Water                                 10 ozs.

  B. Potassium cyanide (pure)             100 grains
     Silver nitrate                       100   „
     Water                                 10 ozs.

Place the washed negative in A until it has gone white, then rinse it
well and transfer to B, in which the image turns to a velvety black.
After washing, the process can be repeated.

Intensification is, however, only a way of saving photographs which
cannot be secured again. If the first photograph of a particular
variety of cloud is not satisfactory, it ought at least to tell the
operator where he had gone wrong, and a second attempt should produce
a better result than any image built up by chemical action on an
imperfect base.

There is nothing novel in any of these methods, and there is no doubt
that other formulæ would be as good; but the one thing essential is to
have a developer whose action can be held under control, and to apply
that developer in such a way that very considerable over-exposure
will not result in the ruin of the plate. If a number of photographs
have been taken in about the same part of the sky, and within a short
time of each other, then the correct proportions of developer and
alkali will be nearly the same for all, but the first of such a batch
will always have to be attacked in the cautious step-by-step method.
Patience and perseverance, backed by a steady refusal to be discouraged
by the failures which are at first inevitable, are as certain to be
crowned by success as they are in other studies.

The workers are few, and there is much to be done; for it is mainly to
those who will photograph the higher clouds, and so trace the stages
of their growth and decay, that we must look for the data which will
enable us to solve the problems they present, and so enlarge the narrow
boundaries of our knowledge of some of the most beautiful things in
Nature.




REFERENCES


 1. “International Atlas of Clouds” (Atlas International des Nuages).
     Hildebrandsson, Riggenbach, and Teisserenc de Bort. Paris. 1896.


This is the atlas referred to in the text. The letter-press is short,
and is repeated in English, French, and German.

 2. “Annals of the Astronomical Observatory of Harvard College.”
     Vol. XXX. Observations made at the Blue Hill Meteorological
     Observatory. Part III. Measurement of Cloud Heights and
     Velocities. By H. H. Clayton and S. P. Fergusson. Part IV.
     Discussion of the Cloud Observations. By H. H. Clayton.

This last gives a very concise account of all the different proposals
which have been made for the systematic naming of clouds.

 3. “Études International des Nuages.” 1896-1897. Observations et
     Mesures de la Suède. I., II. Publication de l’Observatoire
     Météorologique de l’Université Roy. d’Upsala. H. H. Hildebrandsson.

An account of the Upsala observations referred to in the text.

 4. _Quarterly Journal of the Royal Meteorological Society_.

  Helm Wind. Marriott. 1886 and 1889.

  The Thickness of Shower Clouds. Clayden. 1886.

  Methods of Cloud Measurement. Ekholm. 1888.

  Cirrus Formation. Clayton. 1890.

  Nomenclature of Clouds. Hildebrandsson. 1887.

      „              „    Abercromby. 1887.

      „              „    Wilson-Barker. 1890.

      „              „    Gaster. 1893.

      „              „    Scott. 1895.

  A New Instrument for Cloud Measurement. Ekholm. 1893.

  Calculation of Photographic Cloud Measurements. Olsson. 1894.

  The Motion of Clouds. Shaw. 1895.

 5. Reports of the British Association. Reports of the Committee on
     Meteorological Photography. Clayden. 1891 to 1900.

The reports for 1896 and 1900 refer mainly to the measurements
described in the text.

 6. “Cloudland.” Clement Ley.

The work in which Mr. Ley set forth his proposed scheme.

 7. “A Popular Treatise on the Winds.” Ferrel.

Not a “popular” work in the usual sense, but contains lucid
descriptions of the mechanics of the atmosphere.

 8. There are many excellent text-books on meteorology, all of which
     deal more or less with the movements of the atmosphere and the
     formation of clouds.




INDEX


  Abercromby, 10
  Aitken, 3, 91
  Altitude of clouds, 19, 149
  ---- of rain-clouds, 76
  Alto clouds, 59, 159
  Alto-cumulus communis, 161
  ---- castellatus, 66, 160
  ---- glomeratus, 67, 161
  ---- informis, 64, 160
  ---- nebulosus, 65, 160
  ---- stratiformis, 65, 161
  Alto-strato-cumulus, 70, 160
  Alto-stratus maculosus, 68, 160
  Ascent of vapour, 124
  Atlas, International, 11


  Band cirrus, 41
  Bidwell, Shelford, 92
  Black mirror, 14, 173
  Blue Hill, 20, 138


  Cameras, 140, 171
  Change cirrus, 37
  ---- of velocity, 35
  Cirriform top of thunder-cloud, 110
  Cirro-cumulus, 45, 159
  ---- nebulosus, 52, 159
  Cirro-filum, 33, 156
  Cirro-macula, 53, 159
  Cirro-nebula, 26, 27, 155
  Cirro-stratus, 45, 157
  ---- communis, 47, 158
  Cirro-stratus cumulosus, 48, 158
  ---- nebulosus, 45, 158
  ---- vittatus, 158
  Cirro-velum, 24
  Cirrus, 21, 155
  ---- altitudes, 30, 149
  ---- communis, 40, 157
  ---- caudatus, 34, 156
  ---- excelsus, 31, 156
  ---- inconstans, 37, 157
  ---- nebulosus, 36, 156
  ---- ripples, 51
  ---- ventosus, 32, 156
  ---- vittatus, 41, 156
  Cloud altitudes, 149
  ---- nuclei, 3
  ---- photography, 17, 165
  Condensation, 93
  Cooling by contact, 89
  ---- by expansion, 92
  ---- by mixture, 90
  ---- by radiation, 88


  Development, 175
  Differential currents, 133
  Diffraction, 63
  Dimensions of clouds, 112
  Dry fog, 73


  Equilibrium, stable and unstable, 106
  Exeter measurements, 137
  Exposure, 167


  Fall cloud, 81, 88
  False cirrus, 110
  Fog particles, 60
  Fracto, 164


  Gravitation, 4
  Great waves, 129


  Halos, 21, 22, 23
  Heat cumulus, 85
  ---- thunderstorms, 107
  Hildebrandsson, 10
  Hoar frost, 60
  Howard, Luke, 9, 10


  Intensification, 177
  Interconvection, 125
  International Code, 11
  ---- Committee, 10
  ---- System, 12, 71
  Ions as nuclei, 92
  Irregularities of ground, 39


  Kepler, 4
  Kew, 138


  Ley, Clement, 24, 34, 55
  Lightning, 114
  Lower clouds, 71


  Mammato-cumulus, 99
  Meteorological Conference, 9
  Methods of computing altitudes, 143
  Munich, 9


  Newton, 4
  Nimbus, 74, 163
  Nuclei, 92


  Photographic methods, 17, 165


  Rain-clouds, altitude, 76
  ----, thickness, 81
  Rate of fall of temperature, 94


  Sandstorms, 117
  Saturation, 87
  Scotch mist, 73
  Slow plates, 167
  Spread of condensation, 57
  Stable equilibrium, 106
  Strato-cumulus, 77
  Stratus, 72, 161
  ---- communis, 77, 161
  ---- lenticularis, 80, 98, 162
  ---- maculosus, 78, 162
  ---- radius, 79, 162
  Subsidence of cloud top, 111
  Surfusion, 61
  Swing stand, 171


  Turreted cloud, 66
  Tycho Brahe, 4
  Types, 8


  Umbra and penumbra, 29
  Undatus, 164
  Unsaturated cloud, 101
  Upsala, 10, 138


  Wave clouds, 47, 119, 133
  Wilson, C. T. R., 92


THE END


PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES.




       *       *       *       *       *


Transcriber’s Note

Variations in hyphenation (i.e. thunderstorm and thunder-storm) have
been retained. The following apparent typographical errors were
corrected:

Page 171, “focussed” changed to “focused.” (it should be focused on the
horizon)

Page 173, “aperature” changed to “aperture.” (the lens with its large
aperture)