Why do clouds have flat bottoms?

These clouds have a blend of conditions, where there is an atmospheric structure that makes the cloudy air locally buoyant, with flat bottoms and (mostly) flat tops, while the cloudy air has cumuliform elements. read more

Many clouds are flat on the bottom because as the air rises from the surface it cools, and at the saturation temperature it starts to condense, which is where the clouds start. Since the whole column is starting at the same conditions and going through the same process, it all condenses at the same height. read more

The flat bottoms of cumulus clouds define the exact height at which a critical combination of temperature and air pressure causes water vapor within the rising current to condense into a visible cloud. read more

Of course the bottoms are not exactly flat, but they may appear so from thousands of feet away, so deviations have to be large to be seen. again, clouds are large scale objects Cumulus are the lowest clouds - typically < 5000 feet and you can often see this effect across scores of miles of cloudscape. read more