From AMS Glossary
An arc centered on the zenith and found at least 46° above the sun (or moon).
It is produced by the refraction of light through the 90° prisms of ice crystals, having entered through the horizontal bases and passed out through the vertical sides. Normally, the crystals are large, oriented, hexagonal plates. The circumzenithal arc is the high sky counterpart of the circumhorizontal arc. The circumhorizontal arc is low in the sky when the sun is high (above 58° elevation); the circumzenithal arc is high in the sky when the sun is low (below 32° elevation). The majority of circumzenithal arcs are short-lived, short in length and faint or moderately bright. However, very colorful circumhorizontal arcs can occur when the refraction is close to the minimum angle of deviation, corresponding to a solar elevation angle of about 22°.