Physics Photo of the Week

April 20, 2007

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 photograph. 

The results of the time lapse photo (right) are truly amazing.   Loading large time lapse image.  Please have patience....As expected the main thick layer of clouds is moving from right to left as well as the ground 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.

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

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