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.

CHAPTER VI
CUMULUS

Under the general term cumulus there are grouped the most common, the best known, and the grandest forms of cloud. Indeed, beautiful as the cirrus and alto clouds may be, there is a solid grandeur about the greater forms of cumulus which gives them a beauty of their own quite comparable with the charm afforded by the delicate tracery of their more lofty rivals.

Cumulus can be divided into several types, which are best considered in the order of growth. They are all formed in the lower part of the atmosphere, their under-surfaces varying in altitude from about 600 metres, or even less, up to 3000 metres, or slightly more. The writer’s own measurements vary from a minimum of 584 metres to a maximum of 2286 metres, with an average of a little more than 1000 metres.

They are described in the International system as “clouds in a rising current,” and there is no doubt the description is correct. Each cumulus must be looked upon as simply the visible top of an ascending pillar of damp air. The vapour which makes its appearance in the cloud is present in the transparent air beneath, and the base of the cloud is simply the level at which that vapour begins to condense into visible liquid particles. Since cumulus clouds are caused by ascending currents, these currents must be brought about either by the general disturbance of the air due to a cyclonic movement, or by the local irregularities of temperature on the ground produced by the sun’s heat. As a matter of fact, we do get cumulus produced in great abundance in the rear of every cyclone, and we get them also under the conditions of still air and hot sun, which specially favour evaporation and the development of differences of temperature. The cyclone cumulus may come at any hour of the day or night, though comparatively rare between midnight and the morning. Heat cumulus is generally formed during the afternoon, and it is only under relatively uncommon conditions that it persists during the night. If the cloud has not grown to very great size it usually begins to break up and disappear about sunset, but if it has grown to the enormous dimensions of a summer thunder-cloud it may go on growing, piling mass on to mass, until it generates a thunderstorm, even in the hours of early morning.