the type of dense cloud that develops vertically through all cloud levels, consisting of water droplets, ice crystals, and sometimes hail, and associated with thunder, lightning, and heavy showers
See cumulonimbus in American Heritage Dictionary 4
cu·mu·lo·nim·bus
nounpl.cu·mu·lo·nim·bus·es or cu·mu·lo·nim·bi(-bī)
An extremely dense, vertically developed cumulus with a relatively hazy outline and a glaciated top extending to great heights, usually producing heavy rains, thunderstorms, or hailstorms.