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   Circumzenithal Arc  

Circumzenithal arc, Atacama Desert, Chile.   Imaged at the European Southern Observatory by Sylvain Rondi (site) on the morning of 21st March '02 when the sun was 22° high, the ideal altitude for CZAs. Image©2002 Sylvain Rondi, shown with permission.

The circumzenithal arc, CZA, is the most beautiful of all the 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 and only recently photographed Kern arc.