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   Circumzenithal Arc  

The CZA is produced by oriented plate crystals, the same crystals that form sundogs.

Downcoming sunrays enter the uppermost horizontal face and leave through a vertical side face.

The refraction of rays nearly parallel rays through faces inclined at 90 produces very pure and well separated prismatic colours.   The colours of the circumzenithal arc are purer than those of the rainbow.

When the sun is higher than about 32.3, the rays cannot leave the side face but are instead totally internally reflected. They then eventually emerge through the lower horizontal face to contribute to the almost devoid of colour parhelic circle.

The arc is also formed by Parry oriented columns .

Smudge on the sky. Near the critical solar altitude of 32, rays just graze out of the vertical sides of plate oriented crystals. The result - a smudge of colour almost directly overhead.