Transparent, whitish cloud veil of fibrous or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena.
MAIN DIFFERENCES BETWEEN CIRROSTRATUS AND SIMILAR CLOUDS OF OTHER GENERA
Cirrostratus is distinguished from Cirrus by the fact that it occurs in the form of a veil which is usually of great horizontal extent. Cirrostratus differs from Cirrocumulus and Altocumulus by the lack of a more or,less regular macroscopic structure (grains, ripples, laminae, rounded masses, rolls, etc.) and by its diffuse general appearance. Cirrostratus differs from Altostraus by its thinness and by the fact that it may show halo phenomena. Cirrostratus near the horizon may be mistaken for Altostratus. The slowness of movement and the slowness of the variations in optical thickness and in appearance, both characteristic of Cirrostratus, give useful guidance in distinguishing this cloud from Altostratus and also from Stratus. Cirrostratus may be confused with very thin Stratus which, at angular distances of less than 45 degrees from the sun, may appear very white. Cirrostratus differs, however, from Stratus by being whitish throughout, and by the fact that it may have a fibrous appearance. Moreover Cirrostratus often displays halo phenomena, whereas Stratus does not, except occasionally at very low temperatures. Cirrostratus differs from a veil of haze by the fact that the latter is opalescent or has a dirty yellowish to brownish colour. It is sometimes difficult to discern Cirrostratus through haze.
CLOUDS FROM WHICH CIRROSTRATUS MAY FORM
Cirrostratus may be produced by the merging of elements of Cirrus or Cirrocumulus (Cs cirromutatus, Cs cirrocumulomutatus), by ice crystals falling from Cirrocumulus (Cs cirrocumulogenitus), by the thinning of Altostratus (Cs altostratomutatus) or by the spreading out of the anvil of a Cumulonimbus (Cs cumulonimbogenitus).
Cirrostratus is composed mainly of ice crystals. The smallness of these crystals, their sparseness and the fact that Cirrostratus has at most only a moderate depth, account for the transparency of this cloud through which the outline of the sun is visible, at least when the latter is not too close to the horizon.
In certain types of Cirrostratus, some of the ice crystals are large enough to acquire an appreciable terminal velocity, so that trailing filaments are formed, which give these Cirrostratus clouds a fibrous appearance.
Halo phenomena are often observed in thin Cirrostratus; sometimes the veil of Cirrostratus is so thin that a halo provides the only indication of its presence.
Cirrostratus, not completely covering the sky, may be straight-edged and clear-cut; more often, however, it shows an irregular border fringed with Cirrus.
Cirrostratus is never thick enough to prevent objects on the ground from casting shadows, at least when the sun is high above the horizon. When the sun is low (less than about 30 degrees) the relatively longer light path through a Cirrostratus veil may reduce the light intensity so much that shadows do not exist.
The remarks about the colours of Cirrus are, to a great extent, also valid for Cirrostratus.
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
None worth mentioning.