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.