Physics Photo of the Week

Clouds and Sunshine on Groundhog Day

Photos by Donald Collins

 On Groundhog Day (Feb. 2, 2024) we had nice, sunny weather with a few clouds.  The groundhog must have seen his shadow because we have had continuing cold temperatures - but no snow - for the beginning of the second half of the season.  Groundhog Day historically was chosen to occur approximately halfway between the winter solstice and the spring equinox.

I had been examining the weather satellite photos (from https://weather.ndc.nasa.gov/GOES/ ) and noticed the wave-like clouds in the area northeast of my location near Warren Wilson College - the 'X' in the image (right) in western North Carolina.  These wave clouds are quite fascinating, so I grabbed my camera and looked from my deck in the direction of the clouds northeast of our house to photograph the Groundhog Day clouds in the top photo.

Since clouds are dynamic - always moving - cloud photos are much more interesting if we take time-lapse photos (one photo every 10 seconds with an inexpensive interval timer that plugs into the digital camera mounted on a rigid tripod).  We play these photos at a rate of a video (20 frames /sec in this case) - see the image below.  The first and the last frame of the short video are displayed for about 1 second each; the middle 18 frames are displayed for 1/20 sec each.  Thus the playback for the time-lapse video is a speedup of 200 times.

Notice that the clouds are in a stationary location while the wind is blowing through them.  We see the clouds form on the left (northwest); the clouds move through a wave that is stationary relative to the Earth; and the clouds disappear as the cloud particles move off to the right (southeast).  The wind at the clouds' elevation actually moves in an up-and-down wave, similar to the surface ripples that appear in a stream or river downstream of a submerged rock.  As the air-stream rises upon blowing over a mountain ridge, the air expands due to the altitude and cools below the dewpoint to form a cloud.  After rising and having a chance to cool by expanding - thus cooling the air - the air then contracts and sinks.  The increased pressure due to descending altitude rapidly reheats the air and the cloud evaporates.  These wave oscillations continue for 10's of kilometers downstream of a mountain ridge.

The image below was taken from the website (https://weather.ndc.nasa.gov/GOES/, which allows easy viewing of animated clips that are assembled from images taken about 5 minutes apart.  The viewer can also control the playback speed.  The images are the top halves of each frame from the satellite.

 

The satellite videos taken one frame in 5 minutes and played back here at 1/20 sec per frame works to a speed-up factor of 6000 times.  The area of the image (about 300 km) is much larger than the area of the sky from the ground-level video (covers about 15 km).  We see the same stationary cloud waves on a much larger scale.  Sometimes these cloud waves appear over topography that is flat with no mountain range to induce the up-and-down motion.  In those cases I believe that the waves are caused by wind shear between varying layers of air which over top of each other with different speeds.  That involves very complicated physics - similar to explaining the generation of water surface waves from the layer of air blowing over the stationary water.  Another mysterious question is "why are these stationary waves in the atmosphere 'stationary'?"  With water waves from flowing over a submerged rock, sometimes the waves propagate upstream and downstream.  Profound mysteries to me!

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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. 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.

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