Home   OPOD   What's New    Rays & Shadows   Water Droplets   Rainbows   Ice Halos     Contents     Crystals     Frequent Halos       22° circular       Sundogs         Formation         Sun Altitude         Colours         Moon Dogs         More Images       Tangent Arcs       Circumscribed       Pillars       Circumzenithal       Circumhorizon       Parhelic Circle     Infrequent Halos     Multiple Displays     Other Worlds     Observing Halos     HaloSim   High Atmosphere   Links & Resources   Search - Index   123456789012345678		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.

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