Physics Photo of the Week

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.

Lenticular 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 going anywhere.

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 top photo. 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