It occurs only when the sun is high in the sky, at least 58° above the horizon, and can only occur in the presences of cirrus clouds. It can thus not be observed at locations north of 55°N or south of 55°S, except occasionally at higher latitudes from mountains.[1]
To be visible the sun must be at an elevation of 57.8° (90 -32.2°) or more and if cloud conditions are right it is seen along the horizon on the same side of the sky as the sun. It reaches its maximum intensity at a sun elevation of 67.9° .
The phenomenon is quite rare because the ice crystals must be aligned horizontally to refract the high sun. The arc is formed as light rays enter the horizontally-oriented flat hexagonal crystals through a vertical side face and exit through the horizontal bottom face. It is the 90° inclination that produces the well-separated rainbow-like colours and, if the crystal alignment is just right, makes the entire cirrus cloud shine like a flaming rainbow.[2][1]
A circumhorizontal arc can be confused with an infralateral arc when the sun is high in the sky; the former is however always oriented horizontally where the latter is oriented as a section of a rainbow, e.g. as an arc stretching upwards from the horizon.[2]
One particularly fine example was photographed over northwestern Idaho on June 3, 2006, and was reported in both the New Scientist[3] and the Daily Mail (the latter under the caption "flaming rainbow"). As the event was eventually featured on National Geographic News[4], the news quickly spread over the internet.
References
^ ab "Circumhorizontal arc" (in English). Arbeitskreis Meteore e.V.. Retrieved on 2007-04-22. (Including a photo featuring a circumhorizontal arc over Mont Blanc in 1988.)
