CUMULONIMBUS (CB)

A cumulus cloud of significant vertical extent which is producing heavy precipitation. The cloud often has an anvil-shaped appearance on top, and it is bubbly (cauliflower) shaped on the sides. The top of the cloud is sufficiently dense to filter out sunlight so that the cloud base looks dark. Cumulonimbus clouds are one of four genera that are hybrids of cumulus clouds. [See also anvil cloud.]