Physics Photo of the Week

April 23, 2010

Appalachian Fog
Fog is common both in April and October in valleys - especially on clear, cold nights of relatively high humidity.  When the sky is clear, the ground air cools because the heat radiates out into space.  The resulting cold air sinks into the local valleys and precipitates the formation of fog as the temperature descends below the dew point. 

This satellite image from October 25, 2009 is remarkable in that we can clearly see the outlines of the valleys where the fog accumulated.  The Satellite photo was taken at 9:45 Eastern Time, before the night's accumulation of fog has had a chance to evaporate.  The Piedmont regions of the Southeast USA was cloudy when this photo was taken, but the Appalachian region in the upper left half of the photo had cleared overnight allowing the fog to accumulate in the valleys.

The foggy areas clearly show the morphology of the
different regions of the Appalachians.  In western Virginia and eastern Tennessee, the fog delineates the long valleys that are separated by long ridges that are so noticeable when visiting these areas.  In eastern Kentucky and West Virginia, the valleys marked by the fog show many dentritic or tree-like branches typical of eroded plateaus.  See if you can identify the Tennessee River valley in northern Alabama, the Cumberland River valley in southern Kentucky and north-central Tennessee, and the Fontana Lake area in western North Carolina.  You can also see the narrow Sequatchie Valley in Tennessee - a narrow rift valley in the Cumberland Plateau just northeast of the Tennessee-Georgia-Alabama junction.

The photo at right shows a typical valley fog formation of the WWC Farm taken in late September, 2006.

The NASA satellite photo was obtained from Interactive Weather Satellite Images the Earth Science Office.

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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