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plural cumulonimbuses or cumulonimbi
(meteorology) A cloud, with a tall structure and a flat base, that is often associated with thunderstorms. quotations
The one to the east (which at the moment seemed to be leaping straight out of the heart of a sinister slaty-purple patch of cumulonimbus) drained the Lake of the Hanging Glaciers; that to the west a desolate rock and ice-walled valley which was rimmed by some of the highest summits in the Selkirks.
1921, Lewis R. Freeman, chapter III, in Down the Columbia