Circumzenithal arc

A circumzenithal arc is an optical phenomenon similar in appearance to a rainbow, but belonging to the family of halos produced by the refraction of sunlight through ice crystals in the atmosphere rather than through raindrops. It forms when light enters the flat top face of horizontally oriented hexagonal ice crystals and exits through a side face, producing a brightly colored arc roughly forty six degrees above the Sun. The arc requires specific conditions to appear, including a Sun altitude below about thirty two degrees and a layer of ice crystals with a common horizontal orientation undisturbed by turbulence.