Rainbow Arcs

It is a circumzenithal arc (CZA) formed not by raindrops but by ice crystals drifting in high, freezing clouds. "CZAs are the most beautiful of all the ice halos," declares atmospheric optics expert Les Cowley. "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."

The best time to see a CZA is when the sun is fairly low. "Look straight up especially if sundogs are visible," advises Cowley. "The center of the bow always sunwards and red is on the outside."