Why are clouds flat on the bottom?
The fundamental reason is that clouds are created when moist air rises and cools, condensing into water droplets...which make up clouds. There are two basic methods that occur. When stratus (flat, sheet-like, layered clouds) form, they result from warmer, moist air gliding up and over a layer of colder air. Although both layers are tilted, the 'tilt' is so slight that it's not noticeable...and the layer looks flat. When cumulus clouds form (the tall, puffy clouds)...they do so when warmer, moist air rises (often as the sun heats the ground). That moist air condenses at a set altitude, called the lifting condensation level. That's why cumulus clouds have flat bottoms.