Why are clouds flat on the bottom?

Dear Matthew:

Why are clouds flat on the bottom?

-- A. Miracle, San Diego

Some clouds are flat on the bottom. I'm pleased to apparently be the first to tell you that there is an X-rated formation (well, maybe PG-13) called mammatus clouds (Latin, "having breasts") that is associated with tornadoes. Pockets of cold, damp air sink from the bottom of a thunderhead and form what looks very much like the effect achieved by a hundred topless dancers bending over to tie their shoes. But flat bottoms on some other types of clouds mark the lowest altitude at which the temperature is low enough to cause the air's water vapor to condense and become visible in the form of a cloud.