Physics Photo of the Week
October 14, 2005
cloud looks rather ordinary hovering over the mountain, but when we
study the animated image it is stunning. What is most surprising
is that the wind and the cloud is moving to the left in the
photo. The camera is looking
east on October 10, 2005, a gentle wind is blowing from the
southeast. The cloud at the top of the Four Brothers Knobs near
Warren Wilson College is blowing to the left of the photo, then it
cascades down into the cove. To see the animated photo, click here, but have patience, the file
over 440 kB and will take awhile on a slow connection.
Clouds present very interesting thermodynamics. The wind is
blowing from the right hand side of the photograph. The moist air
is forced to rise to get over the mountain. However, as the air
rises, it expands due to lower barometric pressure. Because the
rising air is unable to exchange heat with the surroundings (not enough
time, large volume), it is insulated from the surroundings. This
is called an adiabatic process
- a thermally insulated process. Whenever a mass of air expands
adiabatically, or in insulated surroundings, its temperature
drops. In this case, the temperature drops below the dew point
and the water vapor in the air condenses to form a cloud. When
the risen air arrives at the other side of the mountain away from the
wind, it flows down the mountain due to gravity, and brings the cloud
with it. The cloud soon evaporates as it descends and warms up,
but the evaporation is hidden from view in this photo beyond the ridge
of Davidson Cove. See the schematic airflow diagram below.
A similar "cloudfall" is seen in Physics Photo of the Week for Aug.
There will be no Physics Photo
of the Week on Oct. 21 due to Fall Break at Warren Wilson
College. The next PPOW will be published on Friday, Oct. 28, 2005.
Physics 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.
Please send any photos to firstname.lastname@example.org.
here to see all Physics Photo of the Week for 2005
to see all Physics Photo of the Week for 2004.