Circumzenithal Arc, Perth, Australia
imaged by Graham Watts. ęGraham Watts, shown with permission.
Circumzenithal arcs change fast. These two images are 2 minutes apart.

The arc is the collective glints of millions of plate crystals in high cirrus cloud. The glints light the cirrus wisps and the resulting arc changes quickly as the cloud fragments transit the the zenith sky. Typically a CZA will alter its appearance over less than a minute and be visible over a few.
Atmospheric
Optics

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