Physics Photo of the Week

April 11, 2008

Precipating Contrails
These jet contrails (condensation trails photographed early in the morning on Dec. 6, 2007) are rather unusual in two features. First, the contrails are rather "clumpy" on one side - resembling the teeth of an open zipper; second, the viewer can see shadows from the clumps cast on the other clouds at the top of the picture.

The clumpyness of the contrails is attributed to mini-clouds of condensed water vapor falling under the influence of gravity.  Usually airplane contrails disperse horizontally as the condensed water droplets diffuse in all directions.  The atmosphere that produced these contrails is unusual in that the cloudlets formed in the contrails became cooler than the surrounding air and descended due to the smaller bouyancy.  I do not have an explanation why the contrails should become cooler than the surrounding air.

In the above image the Sun is near the horizon - below the base of the picture left of the center.  The shadows cast by the contrail point away
from the Sun towards the upper right.  In the second, smaller, close-up picture the Sun is again near the horizon - below the picture, but the camera is looking further north.  As a result the Sun, still near the horizon, is located below the picture and off to the right, so the shadows stream off to the upper left in the second picture. 

Further thought on the geometry of the shadows indicates that the contrails are higher in altitude than the thin clouds that show the shadows.  Even though the Sun is located below the pictures, it is still above the horizon.  Because the Sun is above the horizon it is shining "down" onto the clouds and contrails, which are in layers parallel to the surface of the Earth.  The distance between the Earth and the Sun is so very large that the angle between Sun's rays and the cloud layers (about 1000 m above the Earth's surface) is the same angle between the Sun's rays and the surface of the Earth.  These pictures are taken very close to sunrise (the time stamp on digital photos is extremely helpful); the sunrise is slightly higher than the horizon due to the mountains; and the Sun's rays are thus ever-so-slightly pointing down.  The long path of the Sun's rays through the thin cirrus clouds makes the shadows more visible.

Physics Photo of the Week is 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 

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