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. 
Large animated file loading - please be patient

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.


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