Circumzenithal arc

CZA is caused by ice crystals that form plate-shaped hexagonal prisms, in horizontal orientation. The light that forms the CZA enters an ice crystal through its flat top face, and exits through a side prism face. The refraction of almost-parallel sunlight through what is essentially a 90-degree prism accounts for the wide color separation and the purity of color. The CZA can only form when the sun is at an altitude lower than 32.2°. 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. The CZA radius varies between 32.2° and 0°, getting smaller with rising solar altitude. It is best observed with solar altitudes of about 15°-25°; towards either extreme, it is vanishingly faint. 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. ==Lunar circumzenithal arc==