Physics Photo of the Week

March 16, 2007

Lenticular Cloud
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 mountains.  

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 from the condensation is not enough to rise in the warmer upper air.


Large animated file - please wait...
Cumulus clouds
Animated lenticular cloud
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.

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 dcollins@warren-wilson.edu.


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