[SolanaBogon]

Hello,
the cumulus have a flat-bottom (in France in any case) because this clouds because they progress vertically. At a certain altitude, they spread because the armosphere doesn't contains water at this altitude. At this time, the cumulus it called cumulo-nimbus, the only cloud who can creat tornadoes and light-storm, because there is some wind in it. But this transformation is (in the south of France in summer) very fast, and not realizable in IL-2. And cumulus are big clouds, and there aren't very big clouds in IL-2. Otherwise, here some photos:



cumulus





cumulo-nimbus

I hope I've helped you a little on the cumulus.