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  Sundog formation
 
  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.

When the sun is relatively high, rays cannot pass through the crystal unless they are channeled by being internally reflected from the upper and lower basal (large hexagonal) faces. The skewed angle of incidence also causes the ray deviation to increase and high sun sundogs are farther from the sun.

Plate crystals rarely float exactly horizontal, they wobble and the wobble increases with crystal size. Wobbly plates produce tall sundogs and in the more extreme cases the distinction between a tall sundog and fragments of a 22° halo becomes somewhat arbitrary.