Why do some clouds have flat bases?

Ask Tom Why
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Dear Tom,
I have observed that many clouds appear to have flat bases. Why is that?
Kevin Clammensheld, Chicago
Dear Kevin,
The kinds of clouds that you are referring to are known as cumulus clouds. On warm days, especially in the summer, cumulus clouds are the puffy little clouds that resemble dobs of cotton, and you’re correct to observe that they have flat bases.
Cumulus clouds form when sunshine warms air near the ground enough so that it rises in invisible columns. As the air rises, it cools (generally at a rate of about five degrees per thousand feet of elevation increase). When it has cooled to its saturation temperature, invisible water vapor gas begins to condense into visible water droplets, and a cumulus cloud forms. At any given time, this “saturation level” is a flat surface in the atmosphere, and so cumulus clouds also have flat bases.


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