Dear Tom,
I’ve always wondered why the tops of cumulus clouds are flat.
James McConner, Dubuque, Iowa

Dear James,
Cumulus clouds (clouds that create thunderstorms) accumulate in the atmosphere from 30 to 65 thousand feet, sometimes higher. The main force that propels those clouds to such a high altitude is a downdraft, a strong upward current of warm air (at least, warmer than the air surrounding the upstream). The updated gas rises higher and higher as long as its temperature is higher than the temperature of the surrounding air. But when the updated stream meets air warmer than the compressed air stream, it can no longer ascend. It spreads horizontally into the so-called flat “anvil crest”. Superior winds sometimes carry portions of the anvil several miles away from the main cloud update.

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