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.