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.
The same effect is seen in fine sand around beaches, deserts, and
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
backwash of Martian air rebounding from the obstacle.
Photo of the
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 firstname.lastname@example.org.
All photos and discussions are copyright by Donald
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