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Circumzenithal arc
Imaged by Sergey Eliseev at Sholohovo near Moscow.
©Sergey Eliseev, shown with permission.

The circumzenithal arc is the most beautiful of the ice halos. Its colour purity surpasses that of the rainbow.
As the sun rises its radius shrinks and the arc broadens. Here the 26° high sun lights crystals in high cirrus to form an arc fragment. At 32° the CZA becomes a mere smudge in the sky. It cannot form when the sun is any higher.

Circumzenithal arc
Effect of solar altitude
Colour purity
Overlapping rainbow colours