What forces cause the flattened tops of thunderstorm clouds?

Dear Tom,
You often refer to thunderstorm clouds as “anvil topped.” What forces cause the flattened tops of those clouds?

Doug Barnum,

Dear Doug,
Cumulonimbus clouds (the clouds that produce thunderstorms) build upward into the atmosphere to 30 to 60 thousand feet, sometimes higher. The primary force that drives those clouds to such great heights is an updraft, a powerful current of warm air (warmer, at least, than air surrounding the updraft).
The updraft rises higher and higher so long as its temperature is greater than that of the surrounding air. Eventually, though, the updraft encounters air aloft that is warmer than the updraft and it can no longer rise. The updraft air then spreads horizontally into a so-called “anvil top.” Upper-level winds sometimes carry portions of the anvil several tens of miles away from the main updraft part of the cloud.