a principal cloud type (cloud genus) of vertical development, exceptionally dense and vertically developed clouds, occurring either as isolated clouds or as a line or wall of clouds with separated upper portions; these clouds appear as mountains or huge towers, at least a part of the upper portions of which are usually smooth, fibrous, or striated, and almost flattened; this part often spreads out in the form of an anvil (incus) or vast plume; under the base of cumulonimbus, which is very dark, there frequently exist virga, precipitation, and low, ragged clouds, either merged with it or not; its precipitation is often heavy and always of a showery nature.