
Cumulonimbus clouds usually form from cumulus clouds at a much lower height, thus making them, like cumulus clouds, grow vertically instead of horizontally, thus giving the cumulonimbus its mushroom shape. The base of a cumulonimbus can be several miles across, and it can be tall enough to occupy middle as well as low altitudes; though formed at an altitude of about 3000 to 4000 meters (10000 to 13000 feet), its peak can reach up to 23000 meters (75000 feet)[citation needed] in extreme cases. Typically, it peaks at a much lower height (usually up to 5000 meters / 16500 feet).[verification needed]. 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, as well as by an inversion over the thunderstorm caused by rising temperatures above the tropopause. This anvil shape can precede the main cloud structure for many miles, causing anvil lightning. This is the tallest of the clouds.
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