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.