Physics Photo of the Week
March
7, 2008
Precipitating Lenticular Clouds
Last Monday,
March 3, 2008, we woke up to these interesting clouds before
sunrise. Notice how the tops are all smooth, while the bottoms
seem to have clouds or rain falling beneath. These clouds are
basically "lenticular clouds" - named after their lens-like
appearance. They are most often caused by moist air rising due to
mountain
ridges. When the air rises, it cools due to adiabatic (or
insulated) expansion from the lower pressure. As the moist air
cools, the moisture condenses as fog (or cloud) if it cools below the
dew point. Usually the condensation of water vapor gives buoyancy
to the cloud on account of the latent heat of vaporization.
However, on rare occasions, the air is extremely stable. The
upper layers of the air (above the cloud forming levels) are warmer
than normal - a temperature "inversion" - and the cloud-forming heat
does not provide any additional buoyancy. This is the formation
of lenticular clouds.
The close-up image in this second photo shows the color from the low,
red sun illuminating the base, and the falling cloud condensate
underneath. 
An animated loop is displayed at right. The animation clearly
shows the falling clouds underneath the lens-like caps further
illustrating the stability.
The images were recorded with my digital camera taking photos about 1
in every 2 seconds and played back at one frame in 0.06 seconds.
This represents a speed-up by a factor of 33 times. Towards the
end of the loop you can see an airplane leaving a short contrail
"zipping" across the picture.
Usually lenticular clouds are stationary while the wind blows through
the cloud. See PPOW's for March
16, 2007 and March
10, 2006. The clouds last Monday, however, were not
stationary, but traveled with the wind as the animation shows. A
hypothesis for the formation of these clouds last Monday is a bit
different: Monday was a nice, balmy, sunny day with a gentle wind out
of the south as these clouds indicate. The south wind is warm,
bringing warm, moist air into the Swannanoa Valley. The warm wind
from the South flows on top of the colder air in the valley, forming
clouds at the interface between the warm moist upper level winds and
the colder air below. As the bottom of the clouds cooled, it lost
its bouyancy and proceeded to fall. Soon after sunrise, all these
clouds dissipated as the sunlight heated the air in the valley removing
the warm/cold interface.
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 dcollins@warren-wilson.edu.
Click
here to see the Physics Photo
of
the Week Archive.
Observers are invited to submit
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