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
The arc is also formed by Parry oriented