Physics Photo of the Week
April
1,
2011
Cloud Fall
Sometimes fog cascades over the mountains spilling into the valley
beyond, similar to a waterfall going down a cascade. Late March
at Warren Wilson College found the weather to be downright miserable
with cold, wet, foggy weather for a number of days. Last Tuesday
(March 29, 2011) there was a brief clearing during the night
hours. This formed lots of fog
overnight due to the cooling effects of clear nights. By noontime
most of the fog had "burned off" due to the Sun, except some fog still
on the other side of the Craggy mountain chain near Swannanoa,
NC.
There
was
a wind from the east (the camera is looking northeast). The
fog that was lingering on the far side of the mountain ridge was simply
blowing over the ridge and descending on the near side into Bee Tree
Cove. The animation at right shows the fog descending in a
time-lapse video clip. The animation is sped-up by a factor of
200. On the left the fog is evaporating before it descends out
of view due to the adiabatic warming. As the fog descends to
higher pressure, the increase in temperature results in the fog
droplets evaporating.
Adiabatic processes occur when the thermodynamic system is thermally
insulated from its surroundings. Weather phenomena, on time
scales of several minutes, are adiabatic - there is not enough time for
the temperature changes to equilibrate with the surroundings.
Sometimes when strolling in the mountains on a cool night, we feel a
weak draft of warmer air. This must be air descending from a
mountain warming due to the adiabatic heating.
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 dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
Click
here
to
see the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
