A circumhorizontal arc is an optical phenomenon in which an ice halo forms by hexagonal, plate-shaped ice crystals in high level cirrus clouds.
More about cirrus clouds:
http://en.wikipedia.org/wiki/Cirrus_clouds
The halo is so large that the arc appears parallel to the horizon, hence the name.
When the sun is very high in the sky, sunlight entering flat, hexagon shaped ice crystals gets split into individual colors just like in a prism.
Source and further reading:
http://www.amusingplanet.com/2013/05/fire-rainbows-rare-cloud-phenomenon.html
Image credit: Lisa Gonnelli
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