Circumhorizontal Arc, (sometimes known as Fire Rainbow for its flame-like appearance) is the rarest of all naturally occurring atmospheric wonder. It is an optical phenomenon, an ice-halo formed by plate shaped ice crystals in high level cirrus clouds.
For a circumhorizon arc to be visible, the sun must be at least 58 degrees high in a sky. Cirrus clouds containing relatively large plate shaped ice crystals must also be p
resent. The arc is produced by this plate oriented crystals. Sunlight enters a near vertical side face and leaves via the lower near horizontal face (check the source for diagram). The refraction of the almost parallel solar rays through faces inclined at 90 degree produces the pure, bright and well separated prismatic colors. The colors are purer than those of the rainbow.Source: http://www.atoptics.co.uk/halo/chaform.htm
