The rarest of all naturally occurring atmospheric phenomena.
The picture was captured this week on the Idaho / Washington border, the event lasted about one hour.
Clouds have to be cirrus, at least 20k feet in altitude, with just the right amount of ice crystals and the sun has to hit the clouds at precisely 58 degrees.
Stunning!
"The arc (sometimes called a circumhorizontal arc) is usually produced by plate oriented crystals and is a close relation to the circumzenithal arc. Light rays enter the almost vertical crystal side faces and leave via the lower horizontal face (ray path 3-1). The refraction of the almost parallel sun's rays through faces inclined at 90° produces pure, bright and well separated prismatic colours ~ purer than those of the rainbow. The colours are at their best when the crystal tilts are smallest. Large crystal tilts produce more pastel hues." [Source: Atmospheric Optics]
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