Circumzenithal arc

The arc is located a considerable distance (approximately 46°) above the observed Sun and at most forms a quarter of a circle centered on the zenith.

[5] The CZA is brightest when the sun is at 22° above the horizon, which causes sunlight to enter and exit the crystals at the minimum deviation angle; then it is also about 22° in radius, 1.5° in width.

When the Sun is observed above 32.2°, light exits the crystals through the bottom face instead, contributing to the almost colorless parhelic circle.

Because the phenomenon also requires that the ice crystals have a common orientation, it occurs only in the absence of turbulence and when there is no significant up- or downdraft.

Illuminating the top air-water interface of a nearly completely water-filled cylindrical glass under a shallow angle will refract the light into the water.

A circumzenithal arc in Salem, Massachusetts, Oct 27, 2012. Also visible are a supralateral arc , Parry arc (upper suncave), and upper tangent arc .
From top to bottom: a circumzenithal arc on top of a 46° halo , on top of a Parry arc , on top of a tangent arc , on top of a 22° halo , on top of the actual sun .
A Circumzenithal Arc. Seen in Mount Vernon, Washington. : June 10, 2024
Analogous refraction demonstration experiment for the Circumzenithal Arc. [ 3 ] Here, it is mistakenly labelled as an artificial rainbow in Gilberts book [ 9 ]