Physics Photo of the Week

April 1, 2011

Cloud Fall

Sometimes fog cascades over the mountains spilling into the valley beyond, similar to a waterfall going down a cascade.  Late March at Warren Wilson College found the weather to be downright miserable with cold, wet, foggy weather for a number of days.  Last Tuesday (March 29, 2011) there was a brief clearing during the night hours.  This formed lots of fog overnight due to the cooling effects of clear nights.  By noontime most of the fog had "burned off" due to the Sun, except some fog still on the other side of the Craggy mountain chain near Swannanoa, NC. 

There was a wind from the east (the camera is looking northeast).  The fog that was lingering on the far side of the mountain ridge was simply blowing over the ridge and descending on the near side into Bee Tree Cove.  The animation at right shows the fog descending in a time-lapse video clip.  The animation is sped-up by a factor of 200.  On the left the fog is evaporating before it descends out of view due to the adiabatic warming.   As the fog descends to higher pressure, the increase in temperature results in the fog droplets evaporating. 

Adiabatic processes occur when the thermodynamic system is thermally insulated from its surroundings.  Weather phenomena, on time scales of several minutes, are adiabatic - there is not enough time for the temperature changes to equilibrate with the surroundings.  Sometimes when strolling in the mountains on a cool night, we feel a weak draft of warmer air.  This must be air descending from a mountain warming due to the adiabatic heating.

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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