fire rainbow / circumhorizontal arc

A colloquial expression used to describe the circumhorizontal arc. This photometeor belongs to the halo family and consists of a luminous arc, similar to a small fraction of a rainbow, that appears in the sky in an imaginary plane parallel to the horizon, provided the sun is high enough. This latter condition prevents its observation in the polar regions, due to the low height the sun reaches there. Its formation is a consequence of the refraction of light as it passes through hexagonal ice crystals contained in cirriform clouds, oriented such that the light strikes their lateral prismatic faces and emerges through the horizontal bases. This somewhat uncommon circumstance makes the fire rainbow a difficult photometeor to see. When it occurs, it is very striking, as it produces on a cirrus cloud an effect resembling a brightly colored flame, hence its name.