From the Latin cumulus meaning ‘heap’ and nimbus meaning ‘cloud’, a genus of clouds of bulging, dense form, often towering to great height in unstable air. Young clouds have distinctive fibrous or lined features; older, glaciated types, with abundant ice crystals, are lustrous. Typically, the upper parts are spread into incus or plume features. The cloud base is dark and usually gives rise to precipitation, often with virga. See also cloud classification.