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Circumhorizontal arc
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.〔http://www.atoptics.co.uk/fz643.htm〕
Other currently accepted names for the circumhorizontal arc are circumhorizon arc or lower symmetric 46° plate arc.〔Tape, Walter and Moilanen, Jarmo - 'Atmospheric Halos & the search for angle x', American geophysical Union,2006 - pp196-7〕 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.〔(【引用サイトリンク】title=Fire Rainbows )〕
==Formation==
The halo is formed by sunlight entering horizontally-oriented, flat, hexagon ice crystals through a vertical side face and leaving through the near horizontal bottom face (plate thickness does not affect the formation of the halo). In principle, Parry oriented column crystals may also produce the arc, although this is rare. The 90° inclination between the ray entrance and exit faces produce the well-separated spectral colours. The arc has a considerable angular extent and thus, rarely is complete. When only fragments of a cirrus cloud are in the appropriate sky and sun position, they may appear to shine with spectral colours.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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