EDITOR’S NOTE: SEPTEMBER 22, 2012: Greetings from the future, people of January 2008! I think that this is the first post that I’ve ever re-done. The photos here were really cool but they were removed from Flickr. I probably would have waited for winter but as today’s post about rainbows refers here, I figured I’d do it now! Also, this post is in the new science category that I created today. If you have suggestions for other posts from Michigan in Pictures to be included, post a comment on them!
Sundogs, parhelia, are formed by plate crystals high in the cirrus clouds that occur world-wide. In cold climates the plates can also be in ground level as diamond dust.
The plates drift and float gently downwards with their large hexagonal faces almost horizontal. Rays that eventually contribute their glint to a sundog enter a side face and leave through another inclined 60° to the first. The two refractions deviate the ray by 22° or more depending on the ray’s initial angle of incidence when it enters the crystal. The condition where the internal ray crossing the crystal is parallel to an adjacent face gives the minimum deviation of about 22°.
Red light is refracted less strongly than blue and the inner, sunward, edges of sundogs are therefore red hued.
Rays passing through plates crystals in other ways form a variety of halos.
More science on Michigan in Pictures!