Downwind Fog

March 12, 2006


Provided by: Donald Collins, Warren Wilson College
Summary authors & editors: Donald Collins

This non-descript image of on an overcast day in the Smoky Mountains (Great Caaggy Mountains) of North Carolina shows an interesting phenomenon of fog accumulating on the downwind side of the mountain. The wind is blowing towards the right in the image, from the northwest. By the time the air rounds the downwind side of the mountain, it begins to expand since the wall of the mountain is no longer squeezing (compressing) it. The sudden expansion is adiabatic (no heat input), which causes the air temperature to suddenly drop. If conditions are right, the decrease in temperature falls below the dew point and clouds form. In terms of thermodynamics, the expanding air does work on the surrounding atmosphere -- there's no heat input so the internal energy and temperature must drop. An animated image at the link below shows the dynamics of this more clearly. Photo taken on September 26, 2005 from near Swannanoa, North Carolina.

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