Physics Photo of the Week

September 19, 2008

Midnight Mountaintop Fog
At about midnight between Sept. 13 and 14, 2008, I noticed this lingering fog on top of the mountains near Warren Wilson College.  There was enough light from a nearly full moon that with a sufficient shutter time, I could obtain a "satisfactory" exposure of the mountains and fog illuminated by the bright Moon. 

Nightime fog usually forms in valleys, not the tops of mountains - especially in late summer and early fall.  The valleys cool by radiadiative cooling of the ground beneath the night sky.  Cool air from the surrounding hills sinks into the cooler valleys.  How do we explain the fog on top of the mountains rather than in the valleys?

The answer is explained by examining a time-lapse motion picture.  The picture at right was produced by taking a series of 15 second exposures, Large file loading, please be immediately after the other, then playing them back rapidly (1/20 sec between playback frames).  This speeds up the apparent motion by a factor of 300 times.  As can be seen by the time-lapse animation, the fog is formed by wind coming from beyond the mountains.  As the warm air was forced to rise over the mountain, the drop in atmospheric pressure due to  the higher elevations causes the air to expand.  The expansion of the rising air volume is adiabatic - i.e. it is insulated from its environment.  The tremendous volume of air that is rising is essentially insulated from its surroundings.  According to thermodynamics, as the air expands adiabatically, it cools.  In this case the temperature drops below the dew point so the water vapor condenses into clouds.  Further study of the animated sequence shows the reverse process occuring as the air descends on the downwind side of the mountain.  As the cloud-laden air descends, the pressure rises, and temperature rises above the dew point.  The water droplets in the cloud then evaporate when the temperature rises above the dew point. 

The apparent flashes of light in a couple of frames of the animation are automobile headlights.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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.

Click here to see the Physics Photo of the Week Archive.

Observers are invited to submit digital photos to: