Physics Photo of the Week

October 3, 2008

Valley Fog

Click on the image or here for a larger view.

Fog is very common in the valleys during late summer and early fall.  Cool nights and clear skys allow the ground to cool to considerably lower temperatures then when the sky is cloudy.  This cooling is called radiative cooling.  As a result the air at ground level drops below the dew point and fog forms.  Air on the slopes of the surrounding mountains also cools, but because the mountain air is originally cooler than the valley air, the mountain air sinks to the valley floor enhancing the fog concentration in the valleys.  As a result, the air temperature in the valleys during the nights often drops more than on the mountain peaks and slopes forming an inversion and a sharp demarcation between the fog and the clear air above.

Such an inversion and valley fog form only on relatively windless nights.  When there is considerable wind and moisture, mountaintop fogs form as was shown in the Physics Photo of the Week for September 19, 2008

This photo was taken on soon before sunrise on September 22, 2008 only a couple of days after the mountain top fog photo.

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.

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