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
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 firstname.lastname@example.org.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.