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Concept
Sun dog
Summary
A sun dog (or sundog) or mock sun, also called a parhelion (plural parhelia) in meteorology, is an atmospheric optical phenomenon that consists of a bright spot to one or both sides of the Sun. Two sun dogs often flank the Sun within a 22° halo.
The sun dog is a member of the family of halos caused by the refraction of sunlight by ice crystals in the atmosphere. Sun dogs typically appear as a pair of subtly colored patches of light, around 22° to the left and right of the Sun, and at the same altitude above the horizon as the Sun. They can be seen anywhere in the world during any season, but are not always obvious or bright. Sun dogs are best seen and most conspicuous when the Sun is near the horizon.
Sun dogs are commonly caused by the refraction and scattering of light from horizontally oriented plate-shaped hexagonal ice crystals either suspended in high and cold cirrus or cirrostratus clouds, or drifting in freezing moist air at low levels as diamond dust. The crystals act as prisms, bending the light rays passing through them with a minimum deflection of 22°. As the crystals gently float downwards with their large hexagonal faces almost horizontal, sunlight is refracted horizontally, and sun dogs are seen to the left and right of the Sun. Larger plates wobble more, and thus produce taller sun dogs.
Sun dogs are red-colored at the side nearest the Sun; farther out the colors grade through oranges to blue. The colors overlap considerably and are muted, never pure or saturated. The colors of the sun dog finally merge into the white of the parhelic circle (if the latter is visible).
The same plate-shaped ice crystals that cause sun dogs are also responsible for the colorful circumzenithal arc, meaning that these two types of halo tend to co-occur. The latter is often missed by viewers, since it is located more or less directly overhead. Another halo variety often seen together with sun dogs is the 22° halo, which forms a ring at roughly the same angular distance from the sun as the sun dogs, thus appearing to interconnect them.
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