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Sun Dogs

(January 13, 1997)
This report was sent out onto a science news group on USENET:

CHICAGO, Jan. 11 (UPI) -- The bitter cold gripping the Midwest is generating what are called "sun dogs," an atmospheric phenomenon usually seen only by inhabitants of the polar regions.

Also called mock suns or false suns, sun dogs form when incoming sunlight is refracted through suspended ice crystals in the atmosphere, creating the image of two brilliant spots on either side of the sun.

The phenomenon is common in Arctic regions, but rare in the Midwest.

The explanation is right, but the statement about the rarity is wrong. Sundogs are a very common event. I personally see sundogs a couple of times or more every winter month, and I live at 38 degrees north latitude in Washington, DC! Not exactly the Arctic Circle. I have also seen them in Georgia, Florida and other points south. All you need is high altitude cold air with ice crystals in it and the correct viewing geometry.

Actually, I am always amazed that many people have not seen sundogs (or the even more common halo around the Sun) considering how common they are. I think two things are the cause: one is that people tend not to look at the Sun too closely (a good plan!), and the other is that people simply tend not to notice the sky around them. I remember a few winters ago, there was a spectacular halo around the Sun, with two vivid sundogs and even some of the more esoteric formations like circumzenithal arcs and tangential arcs (arcs of light around the primary halo). A bunch of us grad students were gawking at it, and some of the people walking by would stop and stare as well. Many people walking by didn't look up at all and simply walked past us, but the ones that did got a view they'll never forget. All they had to do was look up.

Want to know more? Try the WW2010 University of Illinois web site, with fantastic diagrams and descriptions of all sorts of atmospheric phenomena.

©2008 Phil Plait. All Rights Reserved.

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