[ kyoo-myuh-loh-nim-buhs ]
/ ˌkyu myə loʊˈnɪm bəs /
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noun, plural cu·mu·lo·nim·bus.
a cloud of a class indicative of thunderstorm conditions, characterized by large, dense towers that often reach altitudes of 30,000 feet (9,000 meters) or more, cumuliform except for their tops, which appear fibrous because of the presence of ice crystals: occurs as a single cloud or as a group with merged bases and separate tops.
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Also called thundercloud, thunderclouds, thunderhead.

Origin of cumulonimbus

First recorded in 1885–90; cumulo- + nimbus
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

British Dictionary definitions for cumulonimbus

/ (ˌkjuːmjʊləʊˈnɪmbəs) /

noun plural -bi (-baɪ) or -buses
meteorol a cumulus cloud of great vertical extent, the top often forming an anvil shape and the bottom being dark coloured, indicating rain or hail: associated with thunderstorms
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Scientific definitions for cumulonimbus

[ kyōōm′yə-lō-nĭmbəs ]

Plural cumulonimbi (kyōōm′yə-lō-nĭm)
An extremely dense, vertically developed cloud with a low, dark base and fluffy masses that tower to great heights. Cumulonimbus clouds usually produce heavy rains, thunderstorms, or hailstorms. Also called thundercloud See illustration at cloud.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.