Question

How are Sun dogs formed ?

Answer 1

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 facesalmost 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 theminimum deviationof 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 avariety of halos.