Circumzenithal Arc

Ice crystals can come in many different shapes and sizes. High up in the atmosphere, water can remain in its liquid phase well below 0°C (super-cooled liquid water), and the temperature at which it freezes has a great effect on the shape of the resulting ice crystal.

Circumzenithal arcs (CZA) are caused by light being refracted through plate crystals (flat, hexagonal crystals). The light enters through the top, horizontal face of the crystal, and leaves through a vertical side face. CZA’s usually only form when the sun is less than 32.3° high in the sky.  The arc appears as an ‘upside down rainbow’, and will be seen above the sun from the point of view of the observer.

The photo below shows a good example of a circumzenithal arc. 

Photo taken by Robyn D Smith, of Ardees Photography.