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Sun Dog Days - Page 2

© Keith C. Heidorn

Sun dogs emerge when sunlight passing through the ice plate's thin sidefaces is refracted. The more perfectly aligned the falling crystals are to the horizontal, the more compact the resulting sun dog. Crystal misalignment from true horizontal will spread the sun dog vertically -- its angular height being approximately four times the maximum crystal angular tilt.

Sun dogs frequently display a reddish tint on the side facing the sun and may sport bluish-white tails which stretch horizontally away from it. The degree to which colours are visible depends on the amount of wobble in the ice crystal's fall: the more wobble, the more colour. The sun dog's tail is formed by light passing through the crystal at angles other than the optimal deviation angle.

Sun dogs typically appear when the sun is low to the horizon, usually just prior to sunset or after sunrise, or during winter months at mid-latitudes. If the sun is low (horizon to about 15° above it), each sun dog is separated from the sun by 22^o (or about two handsbreadth on extended arms), and both will lie on the circle of the 22^o halo if one is present.

Sun dogs form tightest to the sun at lowest solar altitudes, but they are never less than 22^o from it. As the Sun climbs in the sky, the sun dogs slowly move away from the 22^o separation, although they remain on the line through the sun parallel with the horizon. When the sun has climbed to more than 45^o altitude, sun dogs are fainter and noticeably off the 22^o circle, and they vanish altogether above 61^o solar altitude.

Over two millennia ago, the Greeks recognized that haloes and sun dogs foretold rain. Today we known this is often a valid prediction, because haloes and sun dogs are produced by ice crystals that form the cirroform clouds which make up the typical cloud sequence preceding a precipitating warm front.

One last word. There are also moon dogs that appear alongside the moon and are formed by lunar light passing through ice crystals. Moon dogs (or paraselenae) are less commonly seen because the moon can only produce them when bright and because they appear during the night when most of us are asleep.

Copyright 2001, Keith C. Heidorn, All Rights Reserved. (Illustrations courtesy of Spectrum Educational Enteprises, ©2001. Photos courtesy of NOAA/US Dept. of Commerce)

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