From AMS Glossary
A strongly sprouting cumulus species with generally sharp outlines and, sometimes, with a great vertical development; it is characterized by its cauliflower or tower aspect, of large size.
Mainly in the Tropics, cumulus congestus may produce abundant precipitation. It may also occur in the form of very high towers, the tops of which are formed of kinds of cloudy puffs that, detaching themselves successively from the main portion of the cloud, are carried away by the wind, then disappear more or less rapidly, sometimes producing virga. Cumulus congestus is the result of the development of cumulus mediocris and, sometimes, of that of altocumulus castellanus or stratocumulus castellanus. Cumulus congestus often transforms into cumulonimbus; this transformation is revealed by the smooth, fibrous, or striated aspect assumed by its upper portion. The species congestus is unique to the genus cumulus.