Formation and characteristics
Sun dogs are commonly caused by the
scattering of light from plate-shaped
ice crystals either
suspended in high and cold
cirrostratus clouds, or drifting in freezing moist air at low levels as
 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 sundogs.
Sun dogs are red-colored at the side nearest the Sun; farther out the colors grade through oranges to blue. However, the colors overlap considerably and so 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, however, 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. As the Sun rises higher, however, the rays passing through the plate crystals are increasingly skewed from the horizontal plane, causing their angle of deviation to increase and the sun dogs to move farther from the 22° halo, while staying at the same elevation.
It is possible to predict the forms of sun dogs as would be seen on other planets and moons.
Mars might have sun dogs formed by both water-ice and CO2-ice. On the giant gas planets—
Neptune—other crystals form the clouds of
methane, and other substances that can produce halos with four or more sun dogs.