Physics Photo of the Week
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.
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,
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
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.
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
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the world around us. Students, faculty, and others are invited to
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