cumulonimbus calvus cloud

cumulonimbus calvus cloud

[¦kyü·myə·lō′nim·bəs ′kal·vəs ‚klau̇d]
(meteorology)
A species of cumulonimbus cloud evolving from cumulus congestus: the protuberances of the upper portion have begun to lose the cumuliform outline; they loom and usually flatten, then transform into a whitish mass with a more or less diffuse outline and vertical striation; cirriform cloud is not present, but the transformation into ice crystals often proceeds with great rapidity.