Physics Photo of the Week

May 16, 2008

Surf in the Clouds
On April 22, 2008 at about 7 pm I noticed a strange linear feature in the clouds east of Warren Wilson College, near  Asheville, North Carolina.  The wind was blowing from the east towards the camera at about 10-15 mi/hr, but the cloud bank with the rolled edge - a long linear feature - was not advancing.  To investigate the phenomenon further, I took a series of photos to play back as a time-lapse sequence.

The resulting time-lapse series shown at right (one frame taken every 10 seconds and played back at about 150 times the normal speed) shows several extremely interesting weather phenomena.

1)  The roll front resembles a water wave breaking and spilling out onto a beach - except there is no beach!  The bank of clouds in the distance is approaching.  When the bank of clouds reaches warmer air, the cooler cloud spills over and breaks into the "beach" of a barrier of warmer air.

2)  The
overhead clouds in the forground are moving from the northwest to the southeast indicating that the two groups of clouds are converging on each other at an angle.  The cloud shadows on the Four Brothers mountain confirms the motion of the overhead clouds.  The Sun was behind the camera.

3)  A small cumulus cloud behind the Four Brothers is trying to form, perhaps due to air rising over more distant mountains, and is blown toward the southeast along with the nearby stratus clouds - opposite the distant stratus clouds that are approaching.

4)  Finally, the surface winds at the camera were coming right towards the camera - almost opposite the overhead clouds and the distant cumulus cloud.  However, the distant clouds are approaching the camera along with the surface winds.  All these wind directions is highly confusing!  The surface wind direction from the southeast is confirmed by the Warren Wilson Farm Weather station.

The whole process resembles the convergence of two air masses.  Clouds moving from the northwest and clouds coming from the southeast.

The converging air masses is supported by satellite animation loops obtained from NASA Earth Science Office ( and reproduced in the animation on the left..  The time of the satellite images matches the time for the breaking waves image (7 pm EDT).  The clouds in the satellite image are clearly converging in the area.  In the final satellite image a north-south clear strip can be seen just east of Asheville, NC where clouds were photographed - marked by the cross in the final frame of the sequence.

This animation clearly represents the convergence of two air masses.  The air may be warmer where the sky is clear.  As the distant colder cloud bank meets the warmer air, the larger density of the cold cloud layer causes the clouds to sink into the less dense warmer air and form the breaking wave appearance.  As the front of the cool cloud descends into the warmer air the cloud droplets soon evaporate - due both to the presence of warmer air and to adiabatic warming as the cold air descends into higher pressure due to the sudden lower altitude.

This is the last Physics Photo of the Week until classes resume in late August, 2008 at Warren Wilson College.  The next Physics Photo of the Week will be published on August 29, 2008.  Have a nice summer!

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.

Click here to see the Physics Photo of the Week Archive.

Observers are invited to submit digital photos to: