(a) TECHNICAL SPECIFICATION
Cumulonimbus calvus, with or without Cumulus, Stratocumulus or Stratus.
(i) The principal characteristic of calvus is that the summits of the Cumulonimbus lack sharp outlines and none of the Cumulonimbus present have yet reached the stage of Cumulonimbus capillatus.
(ii) Cumulonimbus calvus evolve from Cumulus congestus and usually develop rapidly into Cumulonimbus capillatus. Cumulonimbus calvus generally constitutes a very short-term intermediate stage of development.
(iii) They are distinguished from Cumulus congestus by the at least partial disappearance of clear-cut outlines and cauliflower appearance, both of which are characteristic of the upper part of Cumulus congestus.
(iv) They are distinguished from Cumulonimbus capillatus by no portion of their upper part having a clearly fibrous or striated appearance, or any development in the form of an anvil, a plume or a mass of hair.
(v) The smooth part of a Cumulonimbus calvus may become hidden by new domes produced by other convective updrafts. Although the cloud mass may temporarily assume the appearance of Cumulus congestus, it is still to be identified as Cumulonimbus calvus and coded CL = 3.
(vi) Sometimes, a cloud identified as Cumulus congestus is accompanied by lightning, thunder or hail. The cloud is identified as Cumulonimbus calvus and coded CL = 3.