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.