Circumzenithal arc
The circumzenithal arc (CZA) is a bright halo in the sky that looks like an upside‑down rainbow. It forms when sunlight refracts through ice crystals in high clouds, not through raindrops. The arc sits high overhead, about 46 degrees away from the Sun, and is centered on the zenith (the point directly above you). It’s often described as a “smile in the sky.” The CZA is one of the most colorful halos, with colors from violet at the top to red at the bottom, appearing purer than a rainbow because the colors don’t overlap much.
The arc is produced by plate‑shaped hexagonal ice crystals lying horizontally. Sunlight enters through the crystal’s flat top and exits through a side face, bending as if through a 90‑degree prism. This geometry gives a wide, vividly colored arc. The CZA is brightest when the Sun is about 22 degrees above the horizon. It lies about 22 degrees from the zenith and is roughly 1.5 degrees wide. If the Sun climbs higher than about 32.2 degrees, the arc fades or disappears.
The CZA can also appear with Moonlight, though that lunar version is much rarer because the Moon must be especially bright. For a simple demonstration, you can create an artificial CZA with a nearly full glass of water. Shine light at a shallow angle on the water’s surface at the edge of a table; the light then refracts twice in a way that mimics the halo, producing a colorful arc projected on the floor. Other halos can be produced in similar ways.
This page was last edited on 3 February 2026, at 18:22 (CET).