why does the top of a strom cloud flatten out into an anvil shape?

Answer 1

The scientific name for these clouds are called Cumulonimbus Incus meaning king of clouds. For the cloud to develop to it's "anvil" shape the convective process must combine with instability to produce a powerful updraft.
As long as the air in the vicinity of the updraft remains unstable the cloud will continue to tower. Eventuall the cloud reaches the top of the troposphere(lowest level of the atmosphere-about 6 miles high)where the air temps level off and and begin to rise with altitude. This change in temperature has the effect of placing a lid(aka:cap) on the updraft, and the cloud can rise no further. However, the momentum of the air below continues to push upward, and spreads the cloud out in a lateral fashion forming the anvil shape.
Now if that "lid"(cap) were to "break" and more of that bottled up air should push upward and punch through the troposphere and you were to see a large bulge atop that nice flat anvil, it would be a good indicator of severe weather which may produce large hail damaging winds and maybe even tornadoes.

Answer 2

The anvil form on top of the fully developed cumulonimbus cloud is considered to indicate the prsence if ice crystals or snowflakes.It quickly assumes a fibrous appearance and is often stretched out from the main cloud columns by changes of wind with height.The updrafts(air vertically moving up)present in such storm clouds spread in all directions at the top of the cloud when it becomes matured (after which the cloud does not grow vertically)and make the cloud also spread like an anvil.

Answer 3

You are talking about a thunderhead.
Those clouds are caused by updrafts of warm moist air.
As the air 'runs out of steam` the winds aloft tend to flatten the top of the rising air column causing the classic 'anvil` shape.

Answer 4

Strong lateral winds.

Answer 5

cold air causes a barrier and the heated storm clouds spread out

Answer 6

Good answer, Irv. That's my understanding.