To describe clouds, one of the supplementary features is mamma or mammatus. They appear as hanging protuberances, like udders, under the surface of a cloud. It occurs mostly with Cirrus, Cirrostratus, Altocumulus, Altostratus and Cumulonimbus. They form when cool air sinks in warm humid air and thus condensates into clouds. Especially on a Cb this feature may result in fascinating proportions. 

The picture shows sunlit  mammae behind a weak thunderstorm clearing the area northeastwards. Precipitation produced even a piece of a double rainbow.