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Physics Photo of the Week

October 14, 2005

Cloud Cascade

This cloud looks rather ordinary hovering over the mountain, but when we study the animated image it is stunning.  What is most surprising is that the wind and the cloud is moving to the left in the photo.  The camera is looking east on October 10, 2005, a gentle wind is blowing from the southeast.  The cloud at the top of the Four Brothers Knobs near Warren Wilson College is blowing to the left of the photo, then it cascades down into the cove.  To see the animated photo, click here, but have patience, the file is over 440 kB and will take awhile on a slow connection.

Clouds present very interesting thermodynamics.  The wind is blowing from the right hand side of the photograph.  The moist air is forced to rise to get over the mountain.  However, as the air rises, it expands due to lower barometric pressure.  Because the rising air is unable to exchange heat with the surroundings (not enough time, large volume), it is insulated from the surroundings.  This is called an adiabatic process - a thermally insulated process.  Whenever a mass of air expands adiabatically, or in insulated surroundings, its temperature drops.  In this case, the temperature drops below the dew point and the water vapor in the air condenses to form a cloud.  When the risen air arrives at the other side of the mountain away from the wind, it flows down the mountain due to gravity, and brings the cloud with it.  The cloud soon evaporates as it descends and warms up, but the evaporation is hidden from view in this photo beyond the ridge of Davidson Cove.  See the schematic airflow diagram below.  A similar "cloudfall" is seen in Physics Photo of the Week for Aug. 27, 2004.

There will be no Physics Photo of the Week on Oct.  21 due to Fall Break at Warren Wilson College.  The next PPOW will be published on Friday, Oct. 28, 2005.

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

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.