Physics Photo of the Week
Fog and Stratus Clouds
This bleak midwinter
morning in February 2006 seems very uninteresting. It
depicts lingering winter and the "blahs" of a cold, overcast day.
However, if you look closely, you can
notice a layer of fog in the valley infront of Buckeye Cove across the
valley from Warren Wilson College. In addition to the ground fog
the cloud layer is like a fog layer at higher elevation. What
motivated me to take this photograph was either the strong westerly
wind from the right hand side of the photo or the counter-moving clouds
that were seen moving in the opposite direction of the main wind.
By taking a series of still images over a 10 minute period, playing the
pictures in a rapid sequence, I was able to produce an time lapse
The results of the time lapse photo (right) are truly amazing.
As expected the main
layer of clouds is moving from right to left as well as the
level fog. However, in the middle of the animated sequence, you
can notice some mid-level clouds - or fog - moving in the opposite
direction as the main wind flow! The exact cause of the counter
flow is mostly a mystery to me. The major idea that I have is
that the reversed motion of the inermediate layer of clouds/fog is due
to an eddy effect of wind blowing over the mountains. On the
downwind side of an obstacle wind (or water in a river) often whips
around and forms eddys or vortices where the flow reverses. I
would welcome any other explanations from readers.
To produce the time lapse, I snapped a photo every 10 sec, selected
every other image and played them back at 10 images/sec. That is
a speed-up factor of 200 times. Some WWC students are currently
engaged in projects to produce similar time lapse photographs with this
technique. Look for those in the near future.
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 email@example.com.
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the Week Archive.
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