Circumzenithal arc formation Atmospheric Halos, description and simulation. The CZA is produced by oriented plate crystals although it can also be formed by Parry oriented columns .
Rays enter the uppermost horizontal face and leave through a vertical side face.
The refraction of rays all in nearly the same direction through faces inclined at 90 produces very pure and well separated prismatic colours. They then eventually emerge to contribute to the parhelic circle . Smudge on the sky. The result - a smudge of colour almost directly overhead.