Physics Photo of the Week
Midnight Mountaintop Fog
midnight between Sept. 13 and 14, 2008, I noticed this lingering fog on
top of the mountains near Warren Wilson College. There was enough
light from a nearly full moon that with a sufficient shutter time, I
could obtain a "satisfactory" exposure of the mountains and fog
illuminated by the bright Moon.
Nightime fog usually forms in valleys, not the tops of mountains -
especially in late summer and early fall. The valleys cool by
radiadiative cooling of the ground beneath the night sky. Cool
air from the surrounding hills sinks into the cooler valleys. How
do we explain the
fog on top of the mountains rather than in the valleys?
The answer is explained by examining a time-lapse motion picture.
The picture at right was produced by taking a series of 15 second
immediately after the other, then playing them back rapidly (1/20 sec
between playback frames). This speeds up the apparent motion by a
factor of 300 times. As can be seen by the time-lapse animation,
the fog is formed by wind coming from beyond the mountains. As
the warm air was forced to rise over the mountain, the drop in
atmospheric pressure due to the higher elevations causes the air
to expand. The expansion of the rising air volume is adiabatic -
i.e. it is insulated from its environment. The tremendous volume
of air that is rising is essentially insulated from its
surroundings. According to thermodynamics, as the air expands
adiabatically, it cools. In this case the temperature drops below
the dew point so the water vapor condenses into clouds. Further
study of the animated sequence shows the reverse process occuring as
the air descends on the downwind side of the mountain. As the
cloud-laden air descends, the pressure rises, and temperature rises
above the dew point. The water droplets in the cloud then
evaporate when the temperature rises above the dew point.
The apparent flashes of light in a couple of frames of the animation
are automobile headlights.
Photo of the
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.
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