Circumhorizon Arc
Imaged by Matthew Adams (more images) at Granite Bay, California on May 21 with the early afternoon sun 62° high..
  ©Matthew Adams, shown with permission.

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This immense and colourful ice halo is to be found at two outstretched hand distances beneath the sun and parallel to the horizon.

Summer is the season to look because the sun must be at least 58° high for it to appear.

Contrary to some very misleading web site accounts, it is not particularly rare and in the United States might be sighted several times a year.   Higher latitude Europe is quite another matter because the sun is rarely high enough and the skies more cloudy.   See visibility dates for your locale.

We do not always see the complete halo. As here, it often completely 'lights' smaller cirrus cloud fragments making them glow with spectral hues.

Hexagonal plate-shaped ice crystals drifting in high and cold cirrus cloud produce the halo. Air resistance aligns them so that their large hexagonal faces are nearly horizontal.

Sunlight enters a side face and leaves through the bottom horizontal face.

The refraction of almost parallel rays through faces inclined at 90° gives a wide colour separation and high colour purity.

The circumhorizon arc is often accompanied by another halo half the distance to the sun - a 22 degree halo or a circumscribed halo.