C_L9 Cumulonimbus with anvil

The characteristic shape of these clouds can only be seen as a whole when viewed from a distance (top photographs). The tops of these massive clouds show a fibrous or striated structure that frequently resembles an anvil (2nd row left), plume or huge mass of hair (2nd row right). They may occur as an isolated cloud or an extensive wall (3rd row left) and squalls, hail and/or thunder often accompany them.

Underneath the base, which is often very dark, pannus clouds C_L7 frequently form and, in storms, these may be only a few hundred feet above the earth's surface and they can merge to form a continuous layer. There may be rugged cumulus (3rd row right) or a dense horizontal roll at the shower's edge. Mamma may form, especially on the underside of the projecting anvil (bottom left) and may appear particularly prominent when the sun is low in the sky. Virga may often be seen (bottom right). A whole variety of other clouds such as dense cirrus, altocumulus, altostratus, stratocumulus, cumulus and stratus may also be present.

If the cumulonimbus passes nearly or directly overhead, the characteristic top is lost to view. Consequently an observer, seeing only the under surface, may confuse it with nimbostratus, if a watch has not been kept on the sky. By convention, the cloud is reported as cumulonimbus if accompanied by lightning, thunder, hail or other precipitation of a showery nature. C_L9 is used when it is impossible to differentiate between C_L3 and C_L9. Cumulonimbus most frequently develops from large cumulus C_L2. They sometimes develop from altocumulus castellanus C_M8, when the base is unusually high and can be embedded in altostratus or nimbostratus. Cumulonimbus can disintegrate into dense cirrus C_H3.