Fog on downwind
side of mountain
Donald F. Collins
Professor of Physics - Warren Wilson
This looks like a
dreary day: Completely overcast, fog lingering on the mountain side,
smoke from a brush fire - not scenic at all.
However, when one sees with all
senses, one sees that the wind is blowing from the left (north) fairly
stiffly. The clouds in the sky are moving quite rapidly to the
right, but the fog on the side of the mountain lingers. I set-up
my digital camera on a tripod and took a frame every 10 seconds and
made an animated loop (see below). The frames in the animation
are played back at 5 frames per second. With the images snapped
every 10 sec, the animated image represents real time sped up by a
factor of 50. The 11 frame sequence represents a real time of
just uner 2 minutes.
In the animation, notice that the
sky clouds move to the right, the smoke plume in the foreground also moves to the right
with the prevailing wind. The cloud on the mountain side seems
trapped in wind eddies on the downwind side of the mountain. They
even appear to move against the wind and climb up the mountain.
This phenomenon also represents a
sudden cooling of the air mass as it moves around the mountain.
On the upwind side of the mountain - the side facing the viewer, the
air is partially compressed. The air compression in air movements
tends to be adiabatic - i. e. thermally insulated and allowing no heat
to be exchanged with the surroundings. This is because air masses
are quite large and the rate of heat transfer takes much longer than
the time for the air to be compressed. By the time the air rounds
the downwind side of the mountain, the air expands without the wall of
the mountain squeezing it. The sudden expansion is also
adiabatic, which causes the air temperature to suddenly drop. If
conditions fit, the drop in temperature falls below the dew point and a
cloud forms. In thermodynamics, we say that the expanding air
does work on the surrounding atmosphere. There is no heat input
(adiabatic) so the internal energy and temperature must drop.
The photos were made September 26, 2005 at Warren Wilson College,
Swannanoa, NC. The mountain supporting the fog is called White
Oak Flats, part of the Great Craggy Mountains of N. Carolina.