C_M = 2

(a) TECHNICAL SPECIFICATION

Altostratus opacus or Nimbostratus.

(b) EXPLANATION

(i) Altostratus corresponding to the code figure C_M = 2 is of a darker grey or a darker bluish grey than Altostratus translucidus and is sufficiently dense over the greater part of its extent to completely mask the Sun or Moon. It may occur in several layers. 

(ii) Altostratus opacus may result from the thickening of a layer of Altostratus translucidus, from the merging of the elements of a sheet or layer of Altocumulus, from the spreading out of the middle or upper part of a Cumulonimbus, from thinning of Nimbostratus or from the horizontal extension of Cirrus spissatus.

(iii) Nimbostratus, which is also C_M = 2, has a denser and darker appearance than Altostratus opacus; its base is at a comparatively low level and generally has a diffuse and “wet” appearance. 

(iv) Nimbostratus results either from the evolution of a thick layer of Altostratus opacus or from the merging of the elements of a thick sheet or layer of Altocumulus opacus or, rarely, Stratocumulus opacus. It may also evolve from Cumulonimbus clouds.

(v) When pannus clouds accompanying the layer of Altostratus opacus or Nimbostratus become merged into a continuous layer such that the Altostratus or Nimbostratus can no longer be seen, the coding C_M = 2 should be replaced by C_M = /; the pannus clouds are coded as C_L = 7.