Physics Photo of the Week

February 6, 2009

Sculpted Snow Dune
The region's first significant snowstorm of the 2008-2009 winter occurred during the on February 3-4, 2009.  It was cold - well below freezing - and windy.  The wind had been blowing across the flat roof on the right and blowing against the brick wall at the left.  The well-below freezing snow is very powdery and blows easily. 

The shape of the snow dune tells us how the wind blows in the presence of an obstacle such as a vertical brick wall.  Most of the wind blows over the wall, but some wind "rebounds" and curls down the wall and flows backwards to meet the oncoming wind.  As a result there is a zone of still air where the ridge of the snow dune piles up since the snow grains fall out of still air.  See the drawing below on the left.

Drawing showing snow dune in front of obstacle
Sculpted sand dune in front of large boulder on Mars.  NASA photo.  APOD October 10, 1997.
The same effect is seen in fine sand around beaches, deserts, and even Mars whenever the wind meets obstacles as seen on the photo above right.  Looking carefully to the right of the large boulder in the center of the Mars photo (see arrow), the sand dune shows a ridge due to the backwash of Martian air rebounding from the obstacle.

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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