Generally grey cloud layer with a fairly uniform base, which may give drizzle, ice prisms or snow grains. When the sun is visible though 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.
MAIN DIFFERENCES BETWEEN STRATUS AND SIMILAR CLOUDS OF OTHER GENERA
Occasionally, owing to the wind, Stratus locally assumes the form of coarse fibres (Stratus fractus) which differ from those constituting Cirrus in that they are much less white (except towards the sun), not so diffuse, and usually change their appearance rapidly.
A thin Stratus layer may be confused with Cirrostratus. Stratus, however, is not so completely white except towards the sun; furthermore, coronae may be observed in 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:
(a) 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.
(b) A relatively thin layer of Stratus allows the outline of the sun or moon to be clearly visible at least through its thinnest parts; Nimbostratus masks the luminary throughout.
(c) 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, however, 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.
(d) 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.
(e) 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 fractus is distinguished from Cumulus fractus in that it is less white and less dense. Furthermore, it shows a smaller vertical development, since it owes its formation mainly to turbulence without thermal convection.
CLOUDS FROM WHICH STRATUS MAY FORM
Stratus may develop from Stratocumulus. This occurs when the under surface of the latter lowers, or loses its relief or its apparent subdivisions, for any reason other than the release of precipitation (St stratocumulomutatus). A common mode of Stratus formation is the slow lifting of a fog layer, due to warming of the earth’s surface or an increase in wind speed. Stratus fractus of bad weather is often produced by Altostratus, Nimbostratus or Cumulonimbus (St fra altostratogenitus, St fra nimbostratogenitus or St fra cumulonimbogenitus); it may also result from precipitating.
Stratus is usually composed of small water droplets; this cloud may, when very thin, produce a corona round the sun or moon. At low temperatures, Stratus may consist of small ice particles. The ice cloud is usually thin and may, on rare occasions, produce halo phenomena. Stratus, when dense or thick, often contains drizzle droplets and sometimes ice prisms or snow grains; it may then have a dark or even a threatening appearance. Stratus with a low optical thickness, when observed at more than 90 degrees from the sun, often shows a more or less smoky, greyish tint like that of fog.
Stratus forms under the combined effect of cooling in the lower layers of the atmosphere, on one hand, and turbulence due to the wind, on the other. Over land, the cooling may be a result of nocturnal radiation, which is particularly marked when clouds are absent and the wind is weak, or by advection of relatively warm air over colder ground. Over sea, the cooling is mainly due to advection. Stratus is sometimes observed as more or less joined cloud fragments with varying luminance. These Stratus fractus clouds constitute a transitory stage during the formation or the dissipation of the more common extensive uniform Stratus layer. The transitory stage is usually very short. Stratus fractus clouds may also form as accessory clouds (pannus) under Altostratus, Nimbostratus, Cumulonimbus and precipitating Cumulus; they develop as a result of turbulence in the moistened layers under these clouds.
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
The only supplementary feature of Stratus is praecipitatio (drizzle, ice prisms and snow grains).