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Energy Release as Air Cools, Rises
Name: Dale
Status: educator
Grade: 9-12
Location: NC
Country: USA
Date: N/A
Question:
What happens to the heat energy release to surrounding air
when air cools by rising?
Replies:
Dale,
As an air parcel rises, it cools, assuming that it's rising through
air that is cooler (and/or has lower atmospheric pressure) above
it. The air parcel will also expand as it rises, in the decreasing
pressure with height. The energy released from cooling of the parcel
is radiated to the air surrounding the parcel, thereby warming that
air. This tends to cause the parcel and the air surrounding it to
approach a temperature equilibrium. However, since the parcel has
momentum in rising, that equilibrium is not reached and the parcel
continues to rise, unless the decrease in temperature of the
surrounding air with height is so small as to allow sufficient time
for the equilibrium to occur or the decrease in temperature with
height of the surrounding air ends.
David R. Cook
There actually is not very much heat transfer between a rising air
parcel and the air surrounding it. Air parcels are so large and the
rate of heat transfer so slow that the processes are very close to adiabatic.
When air cools by rising, it does not do so by heat transfer. The
rising air does work by expanding, and in so doing it lowers its own
internal energy. Basically, since the rising and expanding air
parcel moves the surrounding air outward, the air molecules in that
parcel don't rebound as fast when they collide with a retreating target.
Now, the descending air that replaces the rising parcel has exactly
the opposite process occurring--it is compressed by the surrounding
air, which makes its molecules move faster. So the rising becomes
cooler and the sinking air becomes warmer, but that is not due to
heat transfer between them.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
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Update: June 2012
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