The clouds of the lower portions of the atmosphere are formed in regions where water vapour is abundant, and frequently in easy reach of the strong ascending and descending air currents produced by the varying temperatures and irregular surface of the ground. It is sufficient to recite these conditions to show that these lower clouds will be denser, larger, coarser in texture, and characterized by greater definiteness of form than those we have, so far, considered.

In the International system they are classified thus—

Group C. Lower clouds.

(a) Strato-cumulus.
(b) Nimbus.

Group D. Clouds of diurnal ascending currents.

Cumulus and cumulo-nimbus.

Group E. High fogs.

Stratus.

This is certainly the least satisfactory part of the whole scheme, and it is not at all easy to see upon what grounds it was adopted by the International Committee. Group D—cumulus and cumulo-nimbus—do show important differences from the other groups, though it is often difficult to say whether the sky should be described as covered with strato-cumulus or as covered with numerous small cumulus. It is the separation of stratus—placing it in a group by itself, and making that the lowest—which is the worst point. As a matter of fact, stratus may exist at any altitude from sea-level up to such heights that we should not hesitate to call it alto-stratus. Indeed, there is no essential character of alto-stratus which distinguishes it from some of the lower forms. Whatever its altitude, its thickness and the size of its particles may vary in a precisely similar manner. We may have the particles exceedingly small, when the fog will be dry, and such a stratus may be so thin as hardly to dim the sun; or it may be so thick as to completely hide it. On the other hand, the fog may consist of particles easily visible to the naked eye, forming the so-called Scotch mist, or the “dry” fog of Dartmoor, which will wet things as rapidly and more thoroughly than a smart shower. When such a fog accumulates to a sufficient depth, the particles in their fall pick up others, and the result is a distinct fine rain. This may occur not only near the ground, but at almost any level below that at which the cloud would pass into the region of cirrus.

Plate 34 shows three layers of stratus, in each case much broken up. The highest layer is a good example of alto-stratus maculosus. Lower down, by half a mile or more, come parts of a grey sheet considerably denser and thicker. It is a matter of taste whether this should be called high stratus or low alto-stratus. There is no test by which the one can be distinguished from the other. Lower again come the detached darker clouds, which are fragments of a sheet of stratus which is breaking up and disappearing. The photograph was taken in the afternoon, after a wet morning, and all three layers were probably relics of the great rain-cloud system, or nimbus, which produced the rainfall.

We have referred to the production of fine rain from a thick fog. If now such a thick layer of coarse-grained fog—if we may use such a phrase—is suspended overhead at a moderate altitude, the result is a drizzling rain underneath, and the cloud at once becomes a nimbus. When Luke Howard adopted the term “nimbus,” he proposed to employ it, apparently, for a vast mass of cloud such as that which forms the rainy region of a cyclone; a huge pile of clouds containing representatives of all his other types in some unknown but close relationship. It was, in fact, a comprehensive term, and as such there was a good use for it. At present it is applied to any cloud from which rain is falling, except when the cloud can be identified as a variety of cumulus which is called cumulo-nimbus. But we have already said that a stratus may be a rain-cloud, and so may other varieties. Moreover, whenever a nimbus breaks sufficiently for us to be able to see its upper surface, we invariably find that, if it were viewed from above, we should, without a moment’s hesitation, place it in one of the other groups. It is only when we are underneath it we can see its rain-producing character, and give it the orthodox name. The real fact is that nimbus should be an adjective, meaning rain-producing, and not a substantive.

However, it has its allotted place in the International system, and it is better to adhere as far as possible to a defective but widely recognized system until it can be authoritatively amended, rather than to make an individual attempt to ignore it. The facts are sufficiently obvious, and the days of nimbus as a type are numbered. The two plates, Nos. 35 and 36, are fair typical representations of the clouds usually known as nimbus; but they are both of them only the under-surfaces of other clouds, Plate 36 showing the under-surfaces of a group of heavy cumulo-nimbus all joined together so as to cover the sky, while Plate 35 shows a mass of dense strato-cumulus. The rain-cloud is always a form of either stratus or cumulus, or a combination of the two, sometimes in further combination with clouds of the alto class, or even extending upwards to cirro-stratus and cirro-nebula. Where it consists of a single layer, that layer differs from its rainless representative only in greater thickness from base to summit, or in greater density; and when there are several distinct layers of cloud, so that the lowest is shaded by the higher, rain may fall, even though they differ in no visible way from clouds which would be rainless if alone. Plate 33 is an example.

Rain, or snow, often falls from clouds at greater altitudes than these, but unless in its descent it passes through other lower clouds, the drops, as a rule, will dry up and disappear. The author has often seen quite heavy rain descending from a cloud, and disappearing completely within a thousand feet or so of the cloud-base. On rarer occasions a still more remarkable thing may be seen—namely, a shower falling from an upper cloud into a lower, and none between this lower cloud and the ground. This curious phenomenon can only be explained by supposing that the convection currents which make the lower cloud are strong enough to support the small raindrops.

Pure stratus is a level sheet of cloud with little variation of thickness, not ascending every here and there into rounded lumps. Its most typical form covers the whole sky with a uniform grey pall, which may or may not completely hide the sun. Such a cloud does not lend itself to pictorial representation. A frequent form, in which the sheet is more or less broken, is shown in Plate 37. This is a variety which is frequent in the summer mornings, and generally breaks up and clears away before eleven o’clock. If, however, it appears in autumn and winter with layers of alto cloud above, it may grow denser, and turn into a stratiform nimbus, or it may go on drifting overhead for several days without sign of change.

Break up such a sheet of cloud by numerous meandering cracks, and round off the detached pieces so as to give them a more or less rounded or pyramidal section, and the cloud becomes strato-cumulus, typical representations of which are shown in Plates 38 and 39, which depict different parts of the same sky. In Plate 38 the camera was pointed due west, and in Plate 39 it was turned round to the north-west, so that the two views do not quite meet.

Plate 40 is a different variety. It is stratiform, each component cloudlet being rather ragged at the edges. In some ways it resembles cirro-macula and the speckled varieties of alto cloud, but it is coarser in texture and obviously at no great altitude. The International system would call it strato-cumulus, but Mr. Ley gives a representation from another negative taken at the same time as the type of what he calls stratus maculosus, a name which seems far more suitable, since the cloud bears a much closer relation to stratus than to cumulus. In the particular instance figured, the broken structure did not last long; the spaces gradually closed in, and a complete stratus was the result.

Strato-cumulus often lasts for hours, with little or no perceptible change, but stratus maculosus rarely persists for more than half an hour. The first is a cloud of fairly stable conditions, the latter is dependent for its existence upon the near approach to critical conditions at one particular level, and, as we have said in other cases, such a critical state is almost always soon passed, with the result that the cloud either masses into a denser form, or else breaks up and disappears. If the up and down currents are strong enough to persist, the result will be strato-cumulus and not stratus maculosus.

A kind of stratus which is frequently seen in the daytime is shown in Plate 41. This is literally a lifted fog, having been formed about midday, after ground fog in the early morning. It would be called common stratus, or stratus communis. When it appears it is a fairly persistent form, sometimes breaking up or swelling up into strato-cumulus, but more often splitting into long rolls of cloud, with margins like those of cumulus. This phenomenon is shown in Plate 42, which was taken in December at 11 a.m., on a day which opened with a thick ground fog. A precisely similar cloud is frequent in the early hours of a summer morning, as a stage in the dispersal of a radiation ground fog. The fog first lifts from the ground, until it reaches a height of a few hundred metres, when it splits into the long rolls whose axes are at right angles to the direction of drift. The consequence is very strange if you stand on a hilltop close under the drifting mass, and look towards the horizon in the direction of drift. The changing shadows give the impression that the clouds are actually rolling along, though of course no such thing is really taking place. As time goes on the rolls grow larger and the interspaces wider; then transverse fissures appear, and gradually the rolls break up into small detached cumulus. Cumulus radius, from the Latin for a rolling-pin, might be a suitably descriptive name, but it should not be forgotten that it is only an intermediate link between stratus and cumulus, and, indeed, is more nearly related to the former, since it is never produced except on the break up of stratus, while it may dry up and disappear without reaching the cumulus stage at all. Stratus radius would therefore be a better name.

Cumulus is closely related to another form of stratus, which Mr. Ley has named stratus lenticularis, but this appears to be so frequently the last stage in a disappearing cumulus that its history will come better later on. It is mentioned here as it is, after all, one of the commonest of all forms of stratus, the form which appears at, or after, sunset, and is one of the few clouds which have an English popular name—Fall cloud. Plate 47 gives a representation of it, standing out dark against an evening sky, with a sheet of alto-stratus far above it in the upper part of the photograph.

To sum up, then, we have among the lower clouds of more or less stratiform pattern—
Stratus communis, or Common stratus.
Stratus lenticularis, the Fall cloud.
Stratus radius, or Roll cloud.
Stratus maculosus, or Mottled stratus.
Strato-cumulus, or Sheet cumulus.
The last leads naturally to the consideration of cumulus and cumulo-nimbus, while the term “nimbus” does not belong to any one type-form, but sometimes to one, sometimes to another, and generally to a mixture of two or more.

A good many years ago the writer made a series of measurements of the thickness of detached clouds of the stratus and cumulus types, such as those which may produce a shower. The conclusions reached in consequence of those determinations have since been amply confirmed by subsequent observations. In winter no rain will fall from a cloud unless it reaches a minimum thickness of at least 100 metres, while in summer it must have rather greater thickness. There is one exception, and that is in winter, when the temperature is so low that the drop starts on its downward journey as a flake of snow. When this is the case, rain may fall from a layer of thin lifted fog, not quite thick enough to hide the blue colour of the sky. But under ordinary conditions of temperature, if the cloud has a thickness less than 2000 feet, or 616 metres, rain is unlikely, but if it does come, the drops will be small and the fall of rain quite trifling.

Above this thickness the heaviness of the rain and size of the drops increases, so that if the distance from base to summit be between 2000 and 3000 feet, or 600 to 1000 metres, the fall will be gentle. A thickness of 4000 to 6000 feet, or 1200 to 1800 metres, gives large drops and a fairly heavy shower, while, in summer time at least, cold heavy rain and hail come from clouds measuring 6000 to 10,000 feet, or in round numbers 1800 to 3000 metres or more. In winter the necessary dimensions seem to be less, but the rule still holds equally good, that the rain-cloud does not necessarily differ in any way from the rainless one, except in thickness, and that when the requisite thickness is present rain is not always the result.