Cumulonimbus clouds just before sunset over Tucson, Arizona. Cumulonimbus is the thunderstorm cloud. It is vertically developed, and appears as exceptionally dense and billowed mountains or towers. The base is at an altitude of 500 to 15,000 feet (150 to 4500 metres) and the top is at 10,000 to 50,000 feet (3000 to 15,000 metres). Cumulonimbus occurs as either isolated clouds or as a line or wall of clouds with seperated upper portions. The upper portion often spreads out in the form of a large anvil or vast plume, and is known as incus, anvil cloud or thunderhead.

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