Explanatory remarks

(Section 2.3.6.8)

Nimbostratus usually develops from thickening Altostratus, the base of which gradually lowers. When the cloud becomes thick enough to mask the Sun, it is classified as Nimbostratus.

Nimbostratus usually appears as if illuminated from inside. This is a result of the absence of small cloud droplets in its lower parts, whereby more light penetrates from above than in the case of non-precipitating clouds of the same depth. The small cloud droplets in the lower parts of the cloud are swept out by precipitation or they evaporate owing to the presence of colder raindrops or snowflakes in the cloud.

Although Nimbostratus generally has no clear undersurface, 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 undersurface 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 undersurface 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 re-form. 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 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 gaps.