Explanatory remarks

(Section 2.3.5.8)

The base of Altostratus occasionally shows a mamma-like or ragged appearance due to rain or snow virga. Isolated virga are clearly seen when rain, before evaporating, falls farther in some places than in others.

Precipitation sometimes makes it difficult to distinguish a cloud base, particularly when uniformly falling snow completely evaporates before reaching the ground. If, however, snow melts rapidly into rain, an apparent base may be observed at the melting level, as the visibility through rain is greater than through snow. This “base” is very clearly visible when the rain layer is thin, which is the case for instance if the raindrops quickly evaporate; it may be completely obscured when the rain layer is thick.

Pannus clouds may be present. They:

• Occur under the Altostratus in the lower turbulent layers when these are moistened by evaporation from precipitation
• Show a tendency to form near the level where the temperature is 0° C (the freezing level), where the cooling of the air by melting snow increases the instability of the layer underneath

In the initial stage of formation, pannus clouds are small, sparse and well separated. They usually occur at a considerable distance below the undersurface of the Altostratus.

Later, with a thickening Altostratus and a lowering of its base, this distance is greatly reduced. At the same time, the pannus clouds increase in size and number, and may merge into a quasi-continuous layer.