Cloud of the Day – Sun Dogs
Many of the optical effects in meteorology, such as halos around the Sun and Moon, are the result of the way light interacts with ice crystals. Rainbows are a different matter. They’re the result of light reflecting back to you from inside water droplets. But sun dogs, parahelia in the argot, are caused by the refraction of sunlight (or moonlight) in tiny ice crystals high in the atmosphere.
Because light refracts about 22 degrees passing through ice, the sun dogs appear about 22 degrees on either side of the Sun (or Moon.) Because red light refracts the least and blue the most, sun dogs are redder closer to the source and bluer farther away. The Wikipedia article contains some interesting discussion of the etymology of the term.
The best time to see sun dogs is when a weather system is approaching. The first clouds to appear are the highest, cirrus. The form cirrostratus gives the best show, its thin, even layer of ice crystals creating a perfect canvas for the light.
All of the images are linked to their larger originals.