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Name: H.
Status: educator
Grade: 6-8
Location: IL
Country: N/A
Date: 7/8/2005

How do clouds float? Since they are made up of condensed (liquid or solid) water which is more dense than air, how do they stay up? What are the forces involved?


Upward air motion, whether updrafts in a cumuliform cloud (fair weather cumulus or thunderstorms) or general layer lifting (of stratus clouds) counter the force of gravity on the water droplets or ice crystals. When the droplets or crystals become too heavy or the lifting weakens, they fall out of the cloud. Sometimes this is seen as wisps coming out of the bottom of the cloud, which meteorologists call virga (this evaporates before reaching the ground) or as precipitation (which does reach the ground).

David R. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory

Dear H.

Clouds are bubbles of moisture in the atmosphere that have condensed into water droplets that form the cloud. These bubbles of air rise due to several reasons, such as differential heating at the surface, convergence of airstreams due to slight differences in wind direction, and airflow across a frontal boundary which forces the air to ascend.

Once the bubbles of air begin to rise, they will continue to ascend as long as the bubble temperature is warmer than the air outside the bubble. The air inside the bubble cools as it rises, due to a drop in air pressure. When the bubble air cools to its saturation point, liquid water droplets form, and a cloud is visible. These bubbles drift with the winds at the height of the cloud. Eventually enough air from outside the bubble is mixed with the bubble air, and the cloud "evaporates." When the surface air is no longer heated enough to make the bubbles ascend, that cloud formation process stops.

Here is a link that also explains cumulus cloud formation...

Other types of clouds are formed by different physical processes, but they all involve the ascent of air parcels, such that the moisture in the air is cooled and condensed.

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

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