Re: why are cumulus clouds flat on the bottom and fluffy on the top?

Clouds are made of ice crystals that form from water vapour carried in the 
air. Warm air can carry a lot of water vapour, but when the air is cold the 
water vapour cannot remain as a gas, and turns to ice. You can see this 
happening when you breathe out warm air with a lot of water vapour on a 
winter day.

Cumulus clouds form when warm moist air rises from the ground. As you go 
higher the air gets colder. When you come to a height where it gets cold 
enough that the water vapour cannot remain as a gas, it starts to form ice 
crystals. The flat bottom of a cumulus cloud marks the height where that 
happens. It marks a particular temperature level in the atmosphere.

Once it passes that height, the stream of rising air might continue to rise 
for a while, and then it might start to break up into little eddies, or to 
move sideways. As it moves it carries the ice crystals with it, and the 
fluffy tops of the cumulus clouds show the direction the airstream has been 
moving, and the way the air has been welling and eddying.