Description Stratus and differences with similar clouds

Generally grey cloud layer with a fairly uniform base, which may give Drizzle, Ice prisms or Snow grains. When the sun is visible through the cloud, its outline is clearly discernible. Stratus does not produce Halo phenomena except, possibly, at very low temperatures. Sometimes Stratus appears in the form of ragged patches.

Occasionally, owing to the wind, Stratus locally assumes the form of coarse fibres which differ from those constituting Cirrus in that they are much less white (except towards the sun), not so diffuse, and usually change their shape rapidly.

A thin Stratus layer may he confused with Cirrostratus. Stratus, however, is not so completely white except towards the sun; furthermore, coronae may be observed on Stratus.

Stratus is distinguished from Altostratus by the fact that it does not blur the outline of the sun (no ground glass effect).

A thick Stratus layer may be confused with Nimbostratus. The following criteria serve to distinguish between these two cloud genera:

(1) In general, Stratus has a more clearly defined and more uniform base than Nimbostratus. Moreover, Stratus has a "dry" appearance, which contrasts fairly strongly with the "wet" appearance of Nimbostratus.

(2) A relatively thin layer of Stratus allows the outline of the sun or moon to he clearly visible at least through its thinnest parts; Nimbostratus masks the luminary throughout.

(3) When the cloud under observation is accompanied by precipitation, it is fairly easy to distinguish Stratus from Nimbostratus

if it is borne in mind that Stratus can produce only weak falls of Drizzle, Ice prisms or Snow grains, whereas Nimbostratus nearly always produces rain, snow or ice pellets. A difficulty arises when precipitation falling from a higher cloud passes through the layer of Stratus. In this case, a dark and uniform layer of Stratus closely resembles a Nimbostratus and may very easily be confused with it.

(4) Stratus is more likely to occur during a calm or with a light wind than with a strong wind, whereas Nimbostratus is usually associated with moderate or strong winds. However, this criterion alone should not be used as a basis for distinction.

(5) The occurrence of a thick Stratus layer is not usually preceded by the existence of other clouds in the low and middle etages. Nimbostratus, on the other hand, nearly always succeeds other clouds, usually of the middle etage, or develops from a pre- existing cloud.

Stratus is distinguished from Stratocumulus by the fact that it shows no evidence of the presence of elements, merged or separate.

Stratus in ragged shreds is distinguished from ragged Cumulus in that it is less white and less dense; Stratus also has a smaller vertical extent.