One of a range of interesting and beautiful kinds of aerial spectra (
rainbows, in a loose sense) that most people have never heard of and will probably never see because they don't look for them, a circumzenith arc is an
arc partway around the
zenith of the sky (ie. directly up), on the same side as the sun. It can be as striking and saturated as your basic rainy rainbow.
Along with sun dogs, the circumzenith arc is well worth looking out for when there are a lot of cirrus clouds about, and the sun is not too high in the sky (specifically, it is brightest when the sun is at an elevation of 22.1°, and doesn't appear at all if it is higher than 32.3°). Like sun dogs, a circumzenith arc is caused by refraction through flattish, horizontally aligned hexagonal ice crystals; being caused by the exact same kind of cloud, the two kinds of spectra are often seen at the same time or within a few hours of each other. Circumzenith arcs are said to be visible on about twenty to thirty days of the year.