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Name: June
Status: other
Age: 60s
Location: N/A
Country: N/A
Date: 2000-2001

Why are clouds flat on the bottom?

Dear June

Before we look at why clouds are flat on the bottom, a little review on how clouds form, or exactly what a cloud is, might be helpful.

Clouds are condensed water vapor in the atmosphere, either in a liquid, or solid form. (Yes, some clouds are composed of tiny ice crystals, and these are called "cirrus" clouds. They are very high in the atmosphere (above 20,000 feet), and they are the wispy, feathery-looking clouds in the sky. Their bases are not flat, but may appear that way, because they are so high, or far away from us.

The other kind of clouds, the ones composed from liquid water droplets, form in a variety of ways, but the end result is air containing water vapor is cooled to its saturation point, and then the water vapor condenses into visible water droplets, called clouds. When air rises, it cools, and at a particular level, it reaches its saturation point, and clouds form. That level constitutes the bottom, or base of the cloud. (This level depends on a variety of conditions, including temperature, air pressure, amount of moisture in the air, etc.) But the height of the cloud bases over a given area is fairly uniform, and the bases appear to be flat.
Here is a website that discusses cloud formation:

If you do an internet search on "clouds" you'll find lots of pages with pictures, descriptions, and explanations. Good Luck..!

Wendell Bechtold, Meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO

June, Sometimes clouds aren't flat on the bottom, although they usually are. As air rises, it cools until it gets down to the temperature at which the relative humidity is 100%. At that point water vapor forms droplets on particles; sometimes the particles are soil, or they could be particulate air pollutants. Since large parcels of air (cumulous clouds) or the entire layer of air (stratus clouds) is rising at the same time and at the same rate, the water droplets form at the same height, giving a fairly flat bottom to most clouds. There are exceptions to the flat bottom. Sometimes the droplets become too heavy to be suspended by the upward rising air; then they may slowly fall out of the cloud in bands (called virga) or bulbous shapes (such as the mammata cloud forms under the anvil of a thunderstorm) which appear under the cloud. Technically, these droplet fallouts could be called precipitation, but meteorologists still think of them as part of the cloud.

David Cook
Argonne National Laboratory

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