From cirro-cumulus and cirro-stratus we pass through almost insensible gradations to the denser forms classed together in the alto group. These clouds are fundamentally different, in that they are always composed of liquid particles, though there is no doubt, from their great altitude, that their temperature must often be many degrees below the ordinary freezing-point of water. When this is the case, they are not unfrequently more or less mixed with streaks and filaments exactly like those described under the name of cirrus, which have been explained as due to slowly falling snowflakes. It is not immediately obvious how such apparently contradictory statements can be reconciled. The explanation is that minute droplets of water may be cooled many degrees below freezing-point without changing into ice, and that such super-cooled droplets congeal instantly if a few of them join together to form a larger drop. Practically the same process may be watched any day when there is a sharp frost and dense fog drifting slowly along. The fog-particles are liquid, and produce optical effects in the neighbourhood of any brilliant light, like an arc lamp, absolutely the same as those which would be produced if the temperature were above freezing-point, while there are none of the different phenomena which might be expected if the particles were crystalline ice-dust. As these liquid particles drift along they come in contact with branches of trees and other obstacles, the surface stratum which surrounds them and binds them into spheres is broken, and the drop instantly solidifies. It is to be noted, moreover, that the drop does not freeze as such, but merely adds some more particles to the branching crystals of hoar frost, which grow outwards always towards the direction from which the fog is drifting.

Most liquids, when freed from contact with solid bodies, or when surrounded by a smooth envelope of uniform character, can be cooled below their normal freezing-point without solidification taking place; but the introduction of a particle of the solid, or sometimes of any foreign body, instantly brings about a rapid freezing of the whole. These phenomena of surfusion, as it is called, have long been known, and many of them are very interesting and difficult to understand. Indeed, it is probable that we shall have to add largely to our knowledge of the forces which bind the molecules of a body together to form a solid, and which direct the processes of crystallization before we shall be able to interpret with any certainty a series of facts depending on the attributes of those very forces.

Water is no exception. If finely divided, as by placing it in fine capillary tubes, in the pores of wood, or in the narrow spaces of a wick, it may be cooled several degrees below normal freezing-point. In a cloud, or fog, all the conditions necessary for surfusion to take place are undoubtedly present. The water is pure, the envelope is uniform, the subdivision is exceedingly minute, and the drops are free from most of the mechanical disturbances which bring about the solidification of larger masses in the laboratory.

Thus we see there is nothing at all surprising in the fact that clouds composed of liquid particles may exist at temperatures below the ordinary freezing-point. On the contrary, we should expect that the solidification of the cloud particles would not take place until the temperature was many degrees below freezing, as is certainly the case with clouds of the cirrus order. At temperatures between this unknown, but low value, and the normal freezing-point, the clouds will be composed of liquid; but when the particles join together, snowflakes will result instead of raindrops; and this will be just as true of alto clouds as it is of the great vaporous mountains of the lower regions of the air which bring falls of snow. The streaks often mixed with alto-cumulus are cirrus threads, and are, no doubt, of exactly the same nature as the tails of cirrus caudatus, or even the fibres of cirro-filum.

The simplest alto cloud is alto-stratus. When this is complete, so as to cover the sky, it can be distinguished from cirro-stratus by the absence of fibrous structure, and by the facts that it never produces any halo or fragment of a halo, but instead surrounds the sun or moon with a white blur, or, if it is thin enough, with a close ring of coloured light much nearer than a halo, and with the colours in the inverse order—that is, with the red furthest from the centre. Some of these so-called coronÆ are very beautiful when seen in the black mirror, and some of those formed around a full moon show quite brilliant tints to the unaided eye. Of course, these meteorological coronÆ have no relation whatever to the true solar corona; they are simply formed by the passage of the rays of light through the veil of small particles, and may be easily imitated. Take a piece of glass such as a lantern-cover glass, breathe on it, and hold it close before the eye while looking at some small source of light. If the dew deposit is thin, bright colours are shown in a luminous ring surrounding the light, and the thinner the deposit of dew the larger the ring will be. Breathe heavily so as to give a thick deposit, and the light will be seen to be the centre of a patch of white brightness without any colour.

The phenomena are due to what is known as diffraction, and if the other conditions are unchanged the diameter of the ring is inversely proportional to the size of the particles. Purity of colour in these rings is an indication of uniformity in the size of the particles. When the moon is shining through a sheet of alto-stratus, which thins off to one edge, very beautiful effects may often be noticed, and the change from the colourless blur, when a thicker part of the cloud is interposed, to the brilliant colours of the corona formed by the thinner edges is very striking. Similar phenomena are shown almost equally well by any of the alto clouds, but cirrus thin enough to produce a coloured corona will generally produce a halo.

Alto-cumulus of the kind most nearly allied to cirro-cumulus is shown in Plate 25. The upper part of the picture shows ragged, irregular patches, with slight indications of fibrous streaks. The lower portion shows rounder, ball-like cloudlets, a few of the larger of which have distinct shadows on the side away from the sun. This plate gives alto-cumulus in a partly formed condition, but it is not a mere passage form. Sometimes exactly such a cloud will float overhead for hours, showing very little movement and only slow changes of detail. It is therefore a distinct variety, and may be called alto-cumulus informis.

A less definite form is shown in Plate 26. It may also be regarded as only partly formed, but its construction is quite different, every part being misty and ill defined. It is a common cloud, especially in sultry summer weather with still air. Under those circumstances, after a hazy morning, it may be seen slowly forming during the afternoon, growing in density as the hours go by, until it reaches a maximum about five or six o’clock, after which it melts away, or settles down into small patches of high stratus. Most frequent in summer, it is by no means rare in autumn and winter, but still air is essential. From its hazy appearance it may be called alto-cumulus nebulosus.

Fixity of detail and slow movement characterize both the foregoing forms, and in that respect our next picture (Plate 27) shows a cloud which is a great contrast. Its detached cloudlets are rather flatter and thinner, and though the cloud as a whole will often persist for hours, it is undergoing continual change, and is formed when the air is far from still. Cloudlets form and gather into stratiform patches, which soon break up again and disappear; and the process goes on here and there, sometimes accompanied by fairly rapid movement of the patches as a whole. This cloud may be described as alto-cumulus stratiformis.

We now come, in Plate 28, to a cloud of singular beauty. It forms rapidly in a clear sky, its first traces bearing a striking resemblance to cirro-macula, but the floccules, instead of remaining semi-transparent or dropping cirrus threads, rapidly become opaque balls of cloud which lengthen upwards. This upward tendency causes the formed cloudlets to have their longer axes vertical, which is very characteristic. It might be named alto-cumulus castellatus, or high-turreted cloud. Mr. Ley named it stratus castellatus, or turret-cloud, but it certainly belongs to the cumulus section of the alto group. Thunder weather is the invariable condition for its production. If it is seen, at least in England, thunderstorms are certain to be recorded not very far away. When this particular photograph was being taken in South Devon, very destructive storms were recorded in Brittany and in the English Midlands, and the anvil-shaped tops of unmistakable thunder-clouds were visible above the horizon while the exposure was being made.

Another form of almost equal beauty is shown in Plate 29. The rounded balls make their appearance as semi-transparent spots upon the sky, and in their general characters might easily be mistaken for cirro-macula. But a few minutes will be enough to decide the question. The little spots rapidly grow denser, frequently becoming ragged at the edges; they never drop down the slender filaments which usually descend from cirro-macula, and their edges are never denser than their central parts, which, it will be remembered, was a frequent feature of the true speckle cloud. The cloudlets are obviously rounded balls arranged in patches, which may turn gradually into alto-stratus by their fusion, or, after an existence of minutes or hours, the whole may disappear by a disintegration of each ball, by its breaking up into a ragged mass and melting away. The altitude at which this cloud forms is between 5000 and 9000 metres, according to measurements made by the writer, the actual specimen figured being about 7000. It is almost as characteristic of thunder weather as the last, but whereas Plate 28 shows a variety which is most often seen before 3 p.m., since it only occurs while the cloud planes are rapidly rising, the one before us may be formed at almost any time of day, but most frequently occurs in the afternoon. An imperfect form of it is frequently met with about sunset, in which the rounded balls are not usually so well defined as when the sun is high above the horizon. Alto-cumulus glomeratus would be a suitably descriptive name.

If it were possible to take a good typical example of the variety just described and roll it flat, so that each cloudlet should be reduced to a lenticular shape, we get a type which seems seldom to appear during the heat of the day, and to be most frequent about sundown. It consists, as shown in Plate 30, of distinct cloudlets, with considerable spaces between them, and gives the impression of a discontinuous level sheet. But the component cloudlets are much too definite, and preserve their individuality far too well to suggest any idea of a broken stratus; the spotted structure is the predominant feature, while the stratiform arrangement is almost equally plain. Alto-stratus maculosus would be a suitable term. It is not so high a cloud as the glomeratus type, the one shown being at an altitude of about 5600 metres. The plate shows the position of the setting sun, which is partly hidden behind some dark patches of broken alto-stratus (fracto-alto-stratus), the hazy form and boundaries of which form an effective contrast to the shining cloudlets 2000 metres or so above them. Many of our most beautiful sunsets are due to this form of cloud, particularly in the late autumn. It is a cloud of calm weather, and often floats apparently motionless, and undergoing little change, like flakes of glowing fire against the background of a fading sky long after the sun has disappeared. It is not indicative of thunder conditions, and it may occur on the margins of an anticyclone.

A lower and coarser form of the spotted alto-stratus is shown in Plate 31, where it is seen through the gaps in a thin sheet of broken stratus. In this case also the sun was getting low in the sky, being hidden by the denser bit of stratus in the bottom left-hand corner.

Alto-stratus does not often, if ever, grow from the fusion of the cloudlets of the maculosus type. But it does come from alto-cumulus glomeratus, and also from a form shown in Plate 32. Here we have alto-stratus in process of growth. Small irregular lumps of cloud forming on the right-hand side of the picture grow larger and more irregular, begin to fuse together towards the centre, and on the left-hand side the fusion is almost complete. Still, although the sky is covered with cloud, the lumpy form is plainly visible. The term “alto-strato-cumulus” is suitable, as it differs from the more frequent and much lower cloud, which will be described further on as strato-cumulus, in little else than altitude and general massiveness of texture. This high strato-cumulus is common enough, too common, indeed, in England, as it produces many a dull grey sky both in summer and in winter. In the latter season it is not unfrequent with the cold east winds of February and March. It is probably the lowest of the alto clouds; the lowest measurement made by the writer being 1828 metres at Exeter, but lower altitudes seem to have been recorded elsewhere.

Alto-cumulus castellatus, which is breaking up and disappearing, is shown in Plate 33. It was photographed with a long-focus lens, so that the scale of representation is about eight times as great as that of Plate 27. This view was taken at Exeter while a thunderstorm was in progress at Bristol.