This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

Dear Tom,
Why does a cumulus cloud form as a discrete puff instead of the moisture being evenly distributed through the atmosphere?
—Chester Szmurlo
Dear Chester,
It has to do with the way a cumulus cloud forms. A cumulus cloud develops at the top of a column or bubble of rising warm air that has moved up high enough into cooler air aloft to reach its “saturation temperature,” the temperature at which it has cooled as it rises for its load of water vapor to begin condensing into a visible cloud. That is why cumulus clouds have flat bases. The base of the cloud is at the exact height at which the rising air has reached its saturation temperature. On any given day, the height of the saturation temperature is about at the same level above the ground and so the cumulus bases are at the same level.