Physics Photo of the Week

February 26, 2010

Wave Clouds - ground level and satellite photos

Photo by Donald Collins
Applied Physics Laboratory, Johns Hopkins Univ.
On August 9, 2009, while traveling on Interstate 81 in northern Virginia, I noticed the parallel clouds running NE to SW, parallel to the mountain ridges and took ground-level photographs from my cell phone in the car.  Intrigued, I later found the archive collection of weather satellite images of the area.  The satellite images not only show the same cloud formations, but they also show the mountain/ridge system of the central Appalachian Mountains of the area.  The mountain ridges are parallel to the cloud crests.  The clouds thus appear to have been formed by winds blowing over the mountain ridges.  Traversing the mountain ridges, the air rises until it cools, then descends again, repeating many times as the winds continue to blow toward the southeast toward Baltimore and Washington. 

From the size of the features on the state lines of the satellite photo, and scaling from Google Earth images, the cloud ridges are about 6 miles apart.  From the lateral offset of the cloud shadows, the time of day (11:00 am), the heights of the clouds are calculated to be about 4000 feet (7.5 miles), well above the elevation of the Appalacian mountain ridges.  See PPOW for March 16, 2007 for more of an explanation.

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 

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