Physics Photo of the Week

April 13, 2012

Glaciating Cirrus
Occasionally cirrus clouds show long tails as this photo taken near sunset (5:47 pm) on November 11, 2011 so vividly illustrates.

Cirrus clouds consist of ice crystals and often give rise to halos around the Sun or Moon shown in last weeks Physics Photo.  In this case the ice crystals are forming in supersaturated water vapor near the tops of the clouds.  The condensing of water vapor into ice is referred to as "glaciating" in meteorology.  Because the air is often supersaturated with water vapor way below the freezing point of water, the resulting ice crystals grow rapidly and begin to fall due to gravity.  In the case of these clouds, the wind at the lower levels blows in a different direction than the wind at the tops of the clouds.  Thus  the falling ice crystals are carried laterally for considerable distance before they eventually evaporate.  Hence the long tails streaming towards the southeast.

The same phenomenon occurred about 3 weeks later (Dec. 3, 2011) shown in the mid-afternoon photo at right, except the precipitating ice crystals are being blown to the north and eventually disappearing as they evaporate before falling to the ground. 

The mid-afternoon event provided a rare opportunity to capture the motion of the cirrus tails to prove the hypothesis of the wind distortion of the clouds.  With the camera set to take a photo every 10 seconds, an animation was created.  However, the clouds are traveling away from the observer due to the wind velocities at the higher elevations.  The animation had to "catch-up" to the traveling clouds.  By anchoring the alignment of successive images on some relatively permanent features of the cloud "roots" at their tops, we were able to compose the animation shown at the left.  In the animation we can clearly see the tails blowing towards the north (left) due to the lower altitude winds blowing toward the north - a different direction from the wind at the tops of the clouds.  Thus the "comma-like" formations are the result of wind shear at high elevations.  The animation concentrates on the clouds in the left-center of the full-frame photo.  The animation is played back at about 20 frames/sec - a speed-up factor of about 200 from the original 10 sec between images.

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

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