C_M2 Thick altostratus or nimbostratus

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Gallery
© R.K. Pilsbury
© C.S. Broomfield
© R.K. Pilsbury

Altostratus C_M2 is denser and of a darker grey or bluish grey than altostratus C_M1 from which it may develop. The greater part is sufficiently dense to completely mask the sun or moon. Ragged shreds of pannus clouds C_L7 (C=7) may form at a considerable distance below the altostratus. Later, with a thickening of the altostratus and a lowering of the base, this distance is greatly reduced. The three photographs on this page show examples of nimbostratus, together with pannus.

With further thickening of the altostratus and a lowering of its base, the cloud may eventually become thick enough to mask the sun throughout. At this stage it becomes nimbostratus (C_M code remains 2 but the C code changes to C=5) and has a diffuse appearance when rain or snow is continuously falling through it (middle photograph). Pannus clouds, generally moving fast and changing shape rapidly, frequently occur beneath its base. These clouds appear dark or grey against the lighter background of the cloud above. During heavy precipitation the pannus may disappear.

If pannus clouds merge into a continuous layer obscuring the cloud above, the coding C_M=2 should be replaced by a / and the pannus coded as C_L=7. In the tropics, particularly during short lulls in the rainfall, nimbostratus may break into several different cloud layers, which rapidly merge again. The clouds then often show a very livid colour with variations in brightness.

Classification
Description: Altostratus, the greater part of which is sufficiently dense to hide the sun or moon. Nimbostratus, a dark grey layer whose base is made diffuse by continuously falling precipitation.	Designation: CM=2 (C4 altostratus, C5 nimbostratus)
	Latin name: Altostratus opacus (As op), or nimbostratus (Ns).

Related pages: Cloud classification | C_L guide | C_M guide | C_H guide | Cloud index