Dear Tom,
I have always wondered why the tops of cumulonimbus clouds are flat.
James McConner, Dubuque, Iowa

Dear James,
Cumulonimbus clouds (the clouds that produce thunderstorms) build upward into the atmosphere 30 to 65 thousand feet, sometimes higher. The primary force that drives those clouds to such a great height is an updraft, a powerful current of rising warm air (warmer, at least, than air surrounding the updraft). The updraft rises higher and higher as long as its temperature is higher than that of the surrounding air. But when the updraft encounters air that is warmer than the updraft, it can no longer rise. It spreads horizontally into the so-called flat “anvil top.” Upper-level winds sometimes carry portions of the anvil several miles away from the main updraft portion of the cloud.