The Queen of Halos
A particularly demure circumzenithal arc flickers in passing streaky cirrus miles high over California. Pictured by Orion Elenzil.?
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" although rainbow it is not. Exactly shaped ice crystals fashion it rather than mere raindrops. ?Look straight up near to the zenith when the sun is fairly low and especially if sundogs are visible. The arc�s centre points sunwards. Red is on its outside. ?The CZA is one of Nature�s two spectrometers (the related circumhorizon arc is the other). 90� ice prisms split the sun�s almost parallel rays to show us an almost pure spectrum. Rainbows, coronae nor glories can ever match its purity.??
Parallel sun rays enter the top face of icy hexagonal plate crystals composing the cirrus cloud. They leave through a side face split into a spectrum.??A laboratory prism spectrometer works in the same way. ?