Circumhorizontal arc

Circumhorizontal arc over the Nepalese Himalaya
A circumhorizontal arc (bottom) in relation to a circumscribed halo (top), Oregon.
Circumhorizontal Arc over peak 12225 in the Sierra Mountains of California

A circumhorizontal arc is an optical phenomenon that belongs to the family of ice halos formed by the refraction of sunlight or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in actual cirrus or cirrostratus clouds. In its full form, the arc has the appearance of a large, brightly spectrum-coloured band (red being the topmost colour) running parallel to the horizon, located far below the Sun or Moon. The distance between the arc and the Sun or Moon is twice as far as the common 22-degree halo. 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) the Moon.[1]

Other currently accepted names for the circumhorizontal arc are circumhorizon arc or lower symmetric 46° plate arc.[2] 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,[3] may originate in the occasional appearance of the arc as "flames" in the sky, when it occurs in fragmentary cirrus clouds.[4]

  1. ^ "OPOD - Moonlight Circumhorizon Arc".
  2. ^ Tape, Walter and Moilanen, Jarmo - 'Atmospheric Halos & the search for angle x', American geophysical Union,2006 - pp196-7
  3. ^ Les Cowley. "Circumhorizon arc". atoptics.co.uk. Retrieved 2012-05-19.
  4. ^ "Fire Rainbows". UC Santa Barbara Department of Geography. August 29, 2009. Archived from the original on May 12, 2011. Retrieved May 19, 2012.