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 dcollins@warren-wilson.edu.
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:
