Dear Tom,
Why do some clouds have flat bottoms and all their development is upward? Tom Mucha, Hudson, Wis.
Dear Tom,
You are referring to cumulus clouds. Cumuli come in all sizes and shapes, but they have two common characteristics: They are flat-bottomed and they grow vertically rather than horizontally. Cumulus clouds appear when bubbles of rising warm air push far enough upward into the atmosphere so that the rising air (and its load of water vapor) cools to its saturation temperature–after which any additional rise causes condensation and the beginning of a visible cloud. Three factors are important. First, although bubbles start to ascend at different times, each ascent begins when the bubble has warmed to a specific temperature–a temperature that depends on the given weather situation. Secondly, once rising, all bubbles cool at the same rate (about five degrees per thousand feet). Lastly, air temperatures are horizontally stratified within an air mass. Because of those factors, the “saturation height” is a flat horizontal surface, usually 2,000 to 6,000 feet above ground, and all rising bubbles produce visible clouds exactly when they achieve that height.
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Tom Skilling is chief meteorologist at WGN-TV. His weather forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
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