Circumhorizontal arc in the Nepal Himalayas
A circumhorizontal arc (bottom) in relation to a circumscribed halo (top), Oregon.
A circumhorizontal arc is an optical phenomenon – an ice-halo formed by the refraction of sun- or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in cirrus or cirrostratus clouds. In its full form, the arc has the appearance of a large, brightly spectrum-coloured band running parallel to the horizon, located below the Sun or Moon. The distance below the Sun or Moon is twice as far as the common 22-degree halo. Red is the uppermost colour. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen. As with all halos, it can be caused by the Sun as well as (but much more rarely) by the Moon.
Other currently accepted names for the circumhorizontal arc are circumhorizon arc or lower symmetric 46° plate arc. The misleading term “fire rainbow” is sometimes used to describe this phenomenon, although it is neither a rainbow, nor related in any way to fire. The term, apparently coined in 2006, may originate in the occasional appearance of the arc as “flames” in the sky, when it occurs in fragmentary cirrus clouds.