Plate 34.
THREE LAYERS OF STRATIFORM CLOUD AFTER RAIN.
Plate 34.
THREE LAYERS OF STRATIFORM CLOUD AFTER RAIN.
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.
