Grey cloud layer, often dark, the appearance of which appears 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. It is often formed by a thickening and generally lowering Altostratus layer.
Although a middle level cloud, a Nimbostratus base is frequently observed in the low level.
Nimbostratus does not present any species or varieties, however low, ragged clouds frequently occur below the layer and can merge with the Nimbostratus base.
Distinguishing Ns from other genera
Nimbostratus differs from thick Altostratus (opacus variety) by the absence of thinner parts through which the sun is vaguely revealed. It is also darker grey in colour. An observer will have no indication of the position of the sun or moon with Nimbostratus cloud. If on dark nights it is difficult to distinguish between Nimbostratus and Altostratus, the cloud is by convention called Nimbostratus if rain or snow is reaching the ground.
Nimbostratus is distinguished from thick Stratus in that its base is more diffuse than that of Stratus, and that it produces rain, whereas the precipitation associated with Stratus is drizzle.
Nimbostratus differs from Cumulonimbus in that Nimbostratus is never associated with lightning and thunder or showery precipitation, including hail.
The precipitation associated with Nimbostratus is (non-showery) rain or snow or ice pellets, often of a continuous nature and greater in intensity to that from Altostratus.