What is "circumzenithal arc"

The circumzenithal arc, also called the circumzenith arc, (CZA), upside-down rainbow, and the Bravais' arc, is an optical phenomenon similar in appearance to a rainbow, but belonging to the family of halos arising from refraction of sunlight through ice crystals, generally in cirrus or cirrostratus clouds, rather than from raindrops. The arc is located at a considerable distance (approximately 46°) above the sun and at most forms a quarter of a circle centered on the zenith. It has been called "a smile in the sky", its first impression being that of an upside-down rainbow. The CZA is one of the brightest and most colorful members of the halo family. Its colors, ranging from violet on top to red at the bottom, are purer than those of a rainbow because there is much less overlap in their formation.

Contrary to public awareness, the CZA is not a rare phenomenon, but it tends to be overlooked since it occurs so far overhead. It is worthwhile to look out for it when sun dogs are visible, since the same type of ice crystals that cause them (plate-shaped hexagonal prisms in horizontal orientation) are responsible for the CZA.