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

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.

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