Sundog

Image Source     Bright sundog in Fargo, North Dakota

In meteorology, a sundog or mock sun is officially called a parhelion. It is an atmospheric optical phenomenon that is caused by ice crystals and is made up of a bright spot to the left or right of the Sun. They can be seen during any season and anywhere in the world, but they are not always bright or clear. They are best seen and are most clear when the Sun is near the horizon.

Sundogs are formed when the light is refracted and scattering from plate-shaped hexagonal ice crystals suspended in high and cold cirrus or cirrostratus clouds. They also form near the horizon by ice crystals drifting in the freezing moist air. The ice crystals act as prisms which bends the light rays passing through them with a minimum deflection of 22°. As the ice crystals gently float horizontally downwards with their large hexagonal faces, sunlight is refracted horizontally, and sundogs are seen on both sides of the Sun. Larger plates of ice crystals wobble more and thus create taller sundogs.

Sundogs are red in color at the side nearest to the Sun and farther out the colors grade from oranges to blue. The colors overlap considerably and are not very clear. The colors of the sundog than merge into the white of the parhelic circle, if it is visible.

Another halo (an atmospheric optical phenomenon caused by sunlight and ice crystals) variety often seen along with sundogs is the 22° halo. A ring is formed at roughly the same angular distance from the sun as the sundogs. 

Sundogs may also happen on other planets and moons. Sundogs on Mars might be formed by both water-ice and CO2-ice. On the giant gas planets like Jupiter, Saturn, Uranus and Neptune crystals of methane, ammonia, and other substances form clouds that can create halos with sundogs.

Source: Wikipedia

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