Physics Photo of the Week

September 14, 2012

September Fog
Fog is very common this time of year - especially late at night soon before dawn on clear, cold, still nights when the ground cools by radiation.

This scene was photographed at about 6:15 am on Monday, Sept 10, 2012, when most of the available light was due to the waxing crescent Moon (not shown) as well as outdoor lighting underneath the fog.  The fog had settled well into the Swannanoa River valley leaving very clear air above.  The sky was still quite dark permitting stars to be visible.  Venus is highly visible in the upper right of the photo.

Weather satellite images show many fog-filled valleys throughout Southern Appalachian mountains on this morning.  In the satellite sequence below (courtesy of NASA/GOES) shows fog in many valleys throughout Appalachia.  The fog is stationary because it is trapped in the valleys.  Clouds, however, move from frame to frame.  The Swannanoa Valley fog is indicated by the arrow in the first frame.  The satellite frames run from 8:15 am EDT until about 10:00 am when the Swannanoa Valley fog has mostly dissipated.

It is interesting to identify various river valleys in the region with the help of the state lines.  It is also interesting to compare the various geographic features revealed by the valley fog in the satellite images.  Note the linear valleys in the ridge and valley terrains of western Virginia; the highly branched valleys of the plateaus of Kentucky and West Virginia; and the isolated valleys flowing northwest through the mountains of western North Carolina.

The photo at right shows the same view of the Swannanoa Valley later in the morning (about the same time as the final frames in the satellite images) after most of the fog had evaporated.

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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