Physics Photo of the Week
September
10,
2010
Alien Invasion?
It looks like
an invasion of flying saucers invading the Swannanoa Valley.
Actually these are lenticular clouds - clouds that are shaped like
lenses. This is an unusual display that occurred yesterday
afternoon and lasted from before 5:30 pm until past sunset 3 hours
later.
These clouds are even more spectacular when viewed with a time-lapse
animation shown in the picture below. The time lapse
animations are often published as UFO's. In the animation 30
frames are played back at 20 frames per second. The images were
taken 10 seconds apart. This represents 5 minutes
of clouds condensed to 1.5 seconds - a speed-up factor of 200.
Notice how the clouds are all stationary as the winds blow through
them. These stationary patterns kept their place in the sky at
least an hour. Notice also that high level cirrus clouds can be
seen in the background that move much faster than the air in the
relatively close lenticulars.
These amazing formations only form when the
air is very stable - when the air at higher altitudes is warmer than
normal. On normal days, clouds form from rising air
expanding due to lower pressure. As the air expands, the
expansion is adiabatic or
insulated from the surroundings, and the resulting falling temperature
causes the water vapor in the air to condense into a cloud releasing a
lot of heat called latent heat.
The
release of the latent heat prevents the cloud temperature from dropping
any further. As a result the formation of a cloud usually raises
its temperature higher than the surrounding air; the cloud becomes even
more bouyant - an unstabl situation. The normal instability leads
to the familiar puffy cumulus clouds.
However, if the surrounding temperature in the higher layers of the
atmosphere is warmer than normal, the rising air from cloud formation
loses its buoyancy
and doesn't rise any further and eventually sinks back down. The
thermal inversion creates a stable situation and we don't have the
puffy clouds typical of
convection.
These lenticular clouds are formed by the air rising over the
mountains, and developing lee waves
downwind of the mountains. The waves are stationary, the air
risess up and down as it passes through the stationary wave downwind
from the mountains. See the PPOW
for
March 10, 2006 for a diagram.
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.
Click
here
to
see the Physics Photo
of
the Week Archive.
Observers are invited to submit
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