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._)]