Physics Photo of the Week
January
29, 2010
Orogenic Cloud Front
Cloud fronts are often formed over mountains. As the surface
winds encounter mountain ranges, the air is diverted to cooler, higher
elevations where the water vapor condenses to form clouds. The
Great Craggy Mountains that overlook Warren Wilson College are an
excellent place to observe these orogenic clouds - clouds that are
formed by mountains. January 9, 2010 produced a rather
unusual orogenic cloud front. This front was present before dawn
on January 9 and persisted most of the day. The weather at Warren
Wilson College (where this photo was taken) remained fairly clear most
of the day, but the cloud front gradually became more dense over the
valley. The weather had been quite windy for several days before
this event with a persistent cold north wind bringing cold weather to
the Asheville, NC area.
The photo below shows a time-lapse animation of the cloud front.
The main thing to notice is that the clouds are approaching the camera
from the north along with the ground-level wind, but the clouds are
evaporating as soon as they traverse the mountains. The camera
was set to take an image every 10 seconds. The pictures are
played
back at about 20 frames/sec, enhancing the motion by a
factor of 200. 
The weather had changed during the previous night. Snow had
been predicted for the area, but a cold front had moved from the
northwest during the night causing the clearing. My theory as to
why the clouds are suddenly evaporating after passing the mountains is
one of two: 1) the air is free to descend to lower elevations where the
higher pressure causes it to warm up and evaporate the clouds; or 2)
the overnight front drove out much of the humidity and the air coming
across the mountains suddenly meets drier air that forces the clouds to
evaporate.
I was
able to obtain a series of weather satellite images from the NASA site:
http://wwwghcc.msfc.
nasa.gov/GOES/ that cover the same time - early morning. The
satellite mage clearly shows the receding sharp front southeast of
western North
Carolina. The dark band next to the front is the shadow of
the receding cloud front. The other cloud front, featured in this
week's photograph near the Asheville area in the center of the image is
stationary in the
satellite animation. The massive cloud area can be seen
approaching the area from the north in Kentucky, but disappears as soon
as it transverses the Blue Ridge along western North
Carolina. For this reason I believe that lack of humidity is the
main reason for the orogenic front evaporating.
Physics
Photo of the
Week is
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature 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 dcollins@warren-wilson.edu.
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.
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