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Arctic Treats: Sun Dogs

    A common misperception about the North is that it's dark and dreary all winter. While the days do get short, and above the Arctic Circle the sun actually does stay below the horizon for a while, the extremely low humidity commonly produces bright, exhilarating days. The winter sun throws little warmth, but one of the fairly common bonuses to its appearance are parhelia, usually called sun dogs, mock suns or subsuns.

Sun dogs in Whitehorse, Yukon, brightening up a winter day
Sun dogs near Whitehorse, Yukon.
A Nikon 24mm super-wide-angle lens was used, with no filters.
Photographed 2 stops under-exposed so the sundogs would show up better.

    Sun dogs are formed by the refraction of the sun's rays through a very specific type of ice crystal in the air. These crystals, called plates, are hexagonal (they have six sides), are quite flat, and slowly settle to the ground in a similar manner to maple leaves, wobbling back and forth somewhat, with the flat sides horizontal. These crystals grow in temperatures between -9° and -20° C. (15-25° F.). Temperature inversions, though, often allow sun dogs to be visible when it's much colder than that at ground level.

    Sun dogs always appear 22° on either side of the sun, so it takes a very wide lens to capture them on film. They are multi-coloured, but the degree to which the rainbow colours are visible depends on the amount of wobbling the ice crystals are doing. The small amount of colour seen in the photo above is fairly typical. The amount of vertical "stretch" of the sun dogs is caused by the wobble (more wobble = more stretch), and to a lesser degree by the height of the sun above the horizon.

    Those of you who live in temperate climates occasionally get a sampling of what sun dogs look like, when ice crystals form in very high clouds. They are pretty faint, though, so give yourself a treat and come on up and see ours - maybe we'll even turn on the Northern Lights in honour of your visit!

Photo ©1999-2006 by Murray Lundberg

Related Links:

Further Reading:
  • Alaska Science Nuggets - Neil Davis, Editor (Fairbanks, AK: Geophysical Institute, University of Alaska, 1982)
  • Clouds of the World: A Complete Colour Encyclopedia - Richard Scorer (Melbourne, Australia: Lothian, 1972)
  • The New York Public Library Science Desk Reference - Patricia Barnes-Svarney, Editorial Director (New York, NY: Macmillan, 1995)