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   Circumhorizon arc formation 
The arc is usually* produced by plate oriented crystals. Sunlight enters a near vertical side face and leaves via the lower near horizontal face. The refraction of the almost parallel solar rays through faces inclined at 90° produces the pure, bright and well separated prismatic colours. The colours are purer than those of the rainbow.

When the sun is ~58° high the outgoing ray grazes the crystal lower face (see left) . When the sun is lower still the ray is internally reflected by the lower face and no CHA is formed.

The CHA is closely related to the equally pure coloured circumzenithal arc. The latter is formed when sunlight enters the crystal top face and leaves a side face.




** 
Parry oriented crystals also form CHAs

Background image by Steve Bardwell