Circumzenithal Arc

The finest of all the ice halos formed here in fast moving tendrils of cirrus. Imaged in Finland by Jari Luomanen (atmospheric phenomena).

 All images ©Jari Luomanen, shown with permission
CZA rays

From inside the ice crystal

Rays enter the roof and leave through the left wall. The wall reflects some rays and these leave mostly through the floor to help light the parhelic circle.

When the sun climbs higher than 32° the rays from the roof strike the wall at too shallow an angle to be transmitted. To them, the wall closes and becomes a perfect mirror (total internal reflection). There is no longer any CZA.

Look overhead for a CZA whenever the sun is low and there is cirrus - especially if sundogs are seen.
Atmospheric
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parhelic circle
rays
         

The CZA has pure spectral colours

Almost parallel sun rays enter the top face of a hexagonal plate crystal. There they are refracted and pass through the ice to leave through a near vertical side face. The path is equivalent to passage through a 90° prism in a spectrometer.