Snow Stripes

April 16, 2007

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Provided and copyright by: Michael Blanpied, U.S. Geological Survey
Summary authors & editors: Michael Blanpied

Every boy scout knows that one can find north by looking for moss or snow on the shadowed side of trees. In this Reston, Virginia back yard, the same principle is vividly revealed to work with split-rail fences too. On two or three occasions each winter, the melting of a light snowfall reveals stripes of snow on the leaf-covered ground to the north of this east-west-running fence.

This is obviously a shadow effect, but since the Sun's height in the sky changes throughout the day, why are the stripes not wider to reflect the integration of the shadow over several hours of daylight? It's difficult to make precise measurements on rolling, leafy ground, but the stripes in this case were about as far from the fence as the fence is high. Simple trigonometry suggests a sun angle of about 45 degrees, which exceeds the maximum inclination of the sun on February 11 at Reston's latitude of 38.9 degrees north, as pointed out by colleague Steve Hickman. This may be because the surface of accumulated snow was inches higher than the ground when differential melting began.

Elsewhere in the yard, shadows behind trees and fence posts point 10-20 degrees east of north, rather than exactly north. This suggests that the key time of day for making shadows is early afternoon, when the Sun is still high and air temperature has risen higher than it was at noon. Hopefully this small difference will never lead a boy scout astray!

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