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Clouds and Suspension
Name: H.
Status: educator
Grade: 6-8
Location: IL
Country: N/A
Date: 7/8/2005
Question:
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?
Replies:
H.
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...
http://www.ace.mmu.ac.uk/eae/Weather/Older/Cumulus_Clouds.html
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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Update: June 2012
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