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.

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.

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.