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.

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.

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.

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.

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.

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.

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.

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.

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.

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.