Why are clouds flat on the bottom?
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
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.
Argonne National Laboratory
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Update: June 2012