The circumzenithal arc, CZA, is the most beautiful of all the ice halos. The first sighting is always a surprise, an ethereal rainbow fled from its watery origins and wrapped improbably about the zenith. It is often described as an "upside down rainbow" by first timers. Someone also charmingly likened it to "a grin in the sky".
Look straight up near to the zenith when the sun if fairly low and especially if sundogs are visible. The centre of the bow always sunwards and red is on the outside.
The CZA is never a complete circle around the zenith, that is the exceptionally rare Kern arc.
The halo, not a rainbow, is produced by hexagonal plate ice crystals ranging from 0.05mm across to a millimetre or so. The crystals drift downwards relative to air currents within cirrus cloud and become aerodynamically aligned. CZA forming rays enter a large top face and leave via a near vertical side face. The almost parallel incident sunlight generates a pure non-overlapped spectrum and the refraction through facets 90° to one another separates the colours widely.
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