Grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less continuously falling rain or snow, which in most cases reaches the ground. It is thick enough throughout to blot out the sun. Low, ragged clouds frequently occur below the layer, with which they may or may not merge.
MAIN DIFFERENCES BETWEEN NIMBOSTRATUS AND SIMILAR CLOUDS OF OTHER GENERA
Thin Nimbostratus may be confused with thick Altostratus. Nimbostratus generally has a darker grey colour than Altostratus. By definition, Nimbostratus is sufficiently opaque though-out to hide the sun or moon, whereas Altostratus hides the luminary only when the latter is behind the thickest parts. If on dark nights, doubt exists regarding the choice of the Nimbostratus or Altostratus, the cloud is by convention called Nimbostratus when rain or snow reaches the ground.
Nimbostratus is distinguished from a thick layer of Altocumulus or Stratocumulus by the lack of clearly defined elements or its lack of a distinct lower surface.
Nimbostratus is distinguished from thick Stratus by the fact that it is a dense cloud producing rain, snow or ice pellets; the precipitation which may fall from Stratus is in the form of drizzle, ice prisms or snow grains.
When the observer is beneath a cloud having the appearance of a Nimbostratus, but accompanied by lightning, thunder or hail, the cloud should by convention be called Cumulonimbus.
CLOUDS FROM WHICH NIMBOSTRATUS MAY FORM
Nimbostratus usually develops from thickening Altostratus (Ns altostratomutatus); it may also, though rarely, result from the thickening of a layer of Stratocumulus (Ns stratocumulomutatus) or Altocumulus (Ns altocumulomutatus).
Nimbostratus also sometimes forms by the spreading out of Cumulonimbus (Ns cumulonimbogenitus) or, very rarely, when these clouds produce rain, by the spreading out of Cumulus congestus (Ns cumulogenitus).
Nimbostratus generally covers a wide area and is of great vertical extent. It is composed of water droplets (sometimes supercooled) and raindrops, of snow crystals and snowflakes, or of a mixture of these liquid and solid particles. The high concentration of particles and the great vertical extent of the cloud prevent direct sunlight from being observed through it. The cloud produces rain, snow or ice pellets which, however, do not necessarily reach the ground.
An observer at the earth’s surface usually sees Nimbostratus develop from thickening Altostratus, the base of which gradually lowers. When the cloud becomes thick enough throughout to mask the sun, it is called Nimbostratus.
Nimbostratus usually appears as if illuminated from inside. This is a result of the absence of small cloud droplets in its lower parts,l whereby more light penetrates from above than in the case of non-precipitating clouds of the same depth.
Although Nimbostratus generally has no clear under surface, an apparent base is sometimes discernible. This “base” is situated at the level where the snow melts into rain and is due to the poorer visibility in snow than in rain. The melting level can be seen only when it is sufficiently low and when the precipitation is not too heavy.
The under surface of Nimbostratus is often partially or totally hidden by pannus clouds resulting from turbulence in the layers under its base, which are moistened by partial evaporation of precipitation. At first, these pannus clouds consist of separate units; they may later merge into a continuous layer extending up to the Nimbostratus. When the pannus covers a large expanse of the sky, care should be exercised in order not to confuse it with the under surface of Nimbostratus. Although pannus clouds have a tendency to dissipate, chiefly by the coalescence of their small particles with raindrops or snowflakes falling through them, they continue to reform. In heavy precipitation, however, the pannus particles are swept out faster than they can be replaced and the pannus clouds disappear.
In the tropics, particularly during short lulls in the rainfall, Nimbostratus can be seen breaking
up into several different cloud layers, which rapidly merge again. The clouds then often show
a very characteristic livid colour with variations of luminance, probably due to internal lacunae.
No species are distinguished in Nimbostratus.
Nimbostratus has no varieties.
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
The main supplementary features of Nimbostratus are praecipitatio (rain, snow or ice pellets) and virga.
Pannus clouds may frequently be observed under Nimbostratus.