On January 7, 2006 these
clouds stayed in the same position all afternoon. In fact they
are a series of lenticular clouds caused by the wind oscillating up and
down downwind of the Great Graggy Mountains near Swannanoa, NC.
This photo looks east with the Four Brothers Knobs in the immediate
left of center. The Great
Craggy Mountains are just off
the left edge of the photo. White Oak Flats is visible on the
skyline at the extreme left. In the photo above three distinct
small lenticular clouds are seen in a row from left to right.
Hints of a fourth cloud can barely be seen at the horizon near the
right of the photo. The whiter clouds behind the Four Brothers
are also a lenticular cloud system.
clouds are named after their lens-like appearance. The drawing at
right shows the formation of these unusual clouds in the down-wind side
of mountains as a series of waves - similar to ripples in a stream
downstream of an obstacle in the stream. Downwind of the
mountain, the air flows up and down for several cycles (only two cycles
in the drawing). At higher altitudes, the air is chilled below
the dew point and the cloud forms. Unlike ordinary cumulous
clouds that move along with the wind, the lenticular clouds are
stationary as the wind blows through them. In the case of the
January 7, 2006 lenticular clouds near Swannanoa, these clouds
persisted practically all afternoon. I did not get the camera out
until late in the afternoon after I realized that these clouds were not
|The animated photo at left represents a
140 sec time span of frames taken 10 sec apart. The speed of the
display is approximately 50 times the normal speed. In this
animated photo you can see that the clouds are "stationary" while the
wind moves through them. Notice that there are two rows of
lenticular clouds, and each row shows two cycles of the waves.
See if you can identify each cloud of the animation with a cloud in the
By studying the flow rate of the cloud features through the wave crests
and "guessing" that the wind speed was a moderate 20 mi/hr (9 m/sec),
the cloud in the center of the animation is about 1200 meters
across. This size is consistent with the span of the Four
Brothers Knobs of 2000 m according to topographical maps. With
the scale thus roughly determined, the succeeding cycle of the wind
wave is about 4 x 1200 = 4800 meters downwind. From the guessed
wind speed of 9 m/sec it takes the wind about 530 sec to travel from 1
crest to the next. Thus the air mass oscillates up and down with
a period of about 530 sec. I hope to relate this period to the
thermodynamic and elastic properties of the atmosphere. Any
suggestions would be welcome!
There will be no Physics Photo of the Week next week (March 17, 2006)
due to spring break at Warren Wilson College.
Photos and drawing by Donald Collins
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 email@example.com.
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