Physics Photo of the Week
December 7, 2012
Adiabatic Expansion
Often
times in cloudy weather, a cloud forms on the downwind
side of the side of a mountain. This is a photo of
Mount Pisgah in northern Vermont. It was a very
moist day, completely overcast, and the wind was blowing
from the left. The face of the mountain recedes
where the cloud has formed. The wind is blowing up
and around the mountain from the left. As the moist
air blows up and around the mountain's "shoulder" from the
left, the contour forces a temporary compression of the
wind. As soon as the wind reaches the cliffs
on the front wall of the mountain, the pressure is
suddenly
lowered due to
the wind's expansion into the high valley.
Atmospheric expansions are nearly always adiabatic -
thermally insulated from the rest of the environment -
predominantly due to the large volumes of air involved and
the speed at which the expansion takes place.
Whenever air expands adiabatically, the temperature is
immediately lowered because expansion of air requires
energy, and that energy is obtained from the internal
thermal energy of the air. If the humidity is
relatively high, the rapid fall in temperature results in
a cloud formation.
In the time-lapse video at right the direction of the
wind and the formation of the cloud - along with
turbulence - is highly noticeable. Still images were
taken once every 10 seconds and replayed at 150 times the
actual speed.
A similar downwind fog near Warren Wilson College is
displayed in Earth
Science Picture of the Day on March 12, 2006, with
an animation
at this site.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
Click here to see the Physics Photo of the Week Archive.
