Physics Photo of the Week

January 29, 2010

Orogenic Cloud Front

Cloud fronts are often formed over mountains.  As the surface winds encounter mountain ranges, the air is diverted to cooler, higher elevations where the water vapor condenses to form clouds.  The Great Craggy Mountains that overlook Warren Wilson College are an excellent place to observe these orogenic clouds - clouds that are formed by mountains.  January 9, 2010 produced a rather unusual orogenic cloud front.  This front was present before dawn on January 9 and persisted most of the day.  The weather at Warren Wilson College (where this photo was taken) remained fairly clear most of the day, but the cloud front gradually became more dense over the valley.  The weather had been quite windy for several days before this event with a persistent cold north wind bringing cold weather to the Asheville, NC area. 

The photo below shows a time-lapse animation of the cloud front.  The main thing to notice is that the clouds are approaching the camera from the north along with the ground-level wind, but the clouds are evaporating as soon as they traverse the mountains.  The camera was set to take an image every 10 seconds.   The pictures are played back at about 20 frames/sec, enhancing the motion by a factor of 200.  Large animation file, please be patient

The weather had changed during the previous night.  Snow had been predicted for the area, but a cold front had moved from the northwest during the night causing the clearing.  My theory as to why the clouds are suddenly evaporating after passing the mountains is one of two: 1) the air is free to descend to lower elevations where the higher pressure causes it to warm up and evaporate the clouds; or 2) the overnight front drove out much of the humidity and the air coming across the mountains suddenly meets drier air that forces the clouds to evaporate.

I was able to obtain a series of weather satellite images from the NASA site: http://wwwghcc.msfc. that cover the same time - early morning.  The satellite mage clearly shows the receding sharp front southeast of western North Carolina.  The dark band next to the front is the shadow of the receding cloud front.  The other cloud front, featured in this week's photograph near the Asheville area in the center of the image is stationary in the satellite animation.  The massive cloud area can be seen approaching the area from the north in Kentucky, but disappears as soon as it transverses the Blue Ridge along western North Carolina.  For this reason I believe that lack of humidity is the main reason for the orogenic front evaporating.

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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