CUMULONIMBUS INCUS at Mykonos, Greece - photo by Sfortis, via Wikipedia; a cumulonimbus cloud which has formed the characteristic flat, anvil-top shape. They can form into supercells resulting in severe storm phenomena such as tornadoes.
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Cumulonimbus cloud over western Africa near the Senegal-Mali border. Cumulonimbus clouds rise vertically until they hit a natural barrier, known as the tropopause, and then flatten out. Cumulonimbus clouds usually herald the onset of a severe storm.