Cumulonimbus (Cb) 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 anvil (incus) or vast plume. Under the base of cumulonimbus, which is very dark, there frequently exists virga, precipitation, and low, ragged clouds, either merged with it or not. Its precipitation is often heavy and always of a showery nature. Cumulonimbus