Q: Why are clouds relatively flat on the bottom?

A: Often low clouds, like stratus and cumulus, appear to have flat bases. These clouds form as air near the ground is rising. As the air rises it expands and cools. This cooling causes the relative humidity to increase as the rising air temperature approaches the dew point temperature. When it reaches the height where those two temperatures are equal, the relative humidity is 100% and a cloud forms. Meteorologists call this altitude the lifting condensation level (or LCL) because it is the height at which water vapor in rising air starts condensing to form cloud drops.

The temperature and dew point temperature near the ground are quite uniform on the size scale of a cloud or a collection of clouds. So, as a layer of air near the ground rises, the height at which condensation occurs does not vary much and the cloud base height appears uniform. From the ground these bottoms appear very flat, but if you are in a plane and fly through these bases during take-off or landing, you'll be close enough to notice that the cloud bases are not a sharp boundary.

The closer the temperature and dew point, the lower the cloud base. So, cumulus clouds in the dry southwestern United States generally have much higher bases than those in the southeast.

-- Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller's WHA-AM radio show the last Monday of each month at 11:45 a.m.

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