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
photos to play back as a time-lapse sequence.
The resulting time-lapse series shown at right (one frame taken every
10 seconds and
at about 150 times the normal speed) shows several extremely
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
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
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
confirmed by the
Wilson Farm Weather station.
The whole process resembles the convergence of two air
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
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
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!
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.
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
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