Dear Tom,
I have noticed that many clouds have flat bottoms and puffy tops. What are such clouds called and what causes them?
—Ronaldo Verpersa, Chicago
Dear Ronaldo,
You have described cumulus clouds. They come in all sizes and shapes. They occur when bubbles of rising warm air push far enough upward so that the rising air (and its load of water vapor) cools to its saturation temperature. Then, any additional rise causes condensation and the development of a visible cloud — at the “saturation height.” The rising bubbles cool at a rate of about five degrees per thousand feet of elevation increase. The saturation height is a horizontal surface in any given weather situation, usually two thousand to six thousand feet above the ground. All rising bubbles produce visible clouds at exactly that height..