Physics Photo of the Week
Lenticular clouds are one of my
favorite subjects. The clouds are so smooth, isolated, and
"uncloud like". They are named "lenticular" for their resemblance
to lenses. They are formed by streamlined flow of air briefly to
higher elevations where the cooling condenses the cloud. They
also tend to form in the wake of mountains - the air oscillates up and
down similar to the waves in a stream as water flows over
obstacles. I have featured these fascinating cloud formations on
several previous PPOW's: (April
9, 2007, April
15, 2005, March
10, 2006, and on "Earth
Science Picture of the Day" for July 17, 2006.
The picture below (shown
in an earlier PPOW from April 9, 2007) illustrates schematically the physics of the
formation of lenticular clouds in the downwind side of
Unlike usual cloud formations that show billowing, towering cumulus
piles, lenticular clouds remain smooth. This poses a physical paradox,
especially when the latent heat of condensation is considered.
When clouds usually form, the condensation of water vapor into the
liquid cloud drops releases considerable heat. The "heated" cloud
expands and becomes less dense than the surrounding air and
rises. The rising condensed cloud forms the usual cumulus clouds
(below, left) as opposed to the smooth streamlines clouds featured
here. The streamlined lenticular clouds require a somewhat rare
condition of exceptional atmospheric stability. Such a stability
would be obtained if the upper layers of air were considerably warmer
than the lower layers. When lenticular clouds form, the buoyancy
the condensation is not enough to rise in the warmer upper air.
The animated picture on the right was made on February 8, 2007
just before sunset by taking a picture every 20 sec and played back at
10 frames/sec. This is a speed-up by a factor of 200 times normal
speed. The featured lenticular cloud all but disappeared at the
end of the loop, but it actually re-appeared later in the early
evening. Notice more lenticulr clouds off the right hand side of
the animated picture.
|Animated lenticular cloud
I am indebted to meteorologist Grant Goodge of Ashevile for providing
the answer to the stability question.
Due to Spring Break at Warren Wilson College, there will be no Physics
Photo of the Week for March 23, 2007. The next Physics Photo of
the Week will be posted on March 26, 2007.
All illustrations by Donald F. Collins
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 email@example.com.
here to see the Physics Photo of the Week Archive.
Observers are invited to submit
digital photos to: