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Cumulonimbus (Latin, "Accumulated Cloud") is a type of cloud that is tall, dense, and involved in thunderstorms and other intense weather. It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold front in a squall line. They create lightning through the heart of the cloud. Cumulonimbus clouds form from cumulus clouds and can further develop into a supercell, a severe thunderstorm with special features.

Well-developed cumulonimbus clouds are also characterized by a flat, anvil-like top (anvil dome), caused by straight line winds at the higher altitudes which shear off the top of the cloud.