Circumhorizontal arc.

This rare rainbow, a circumhorizontal arc seen in Idaho June 3 (above), "isn't a rainbow in the traditional sense—it is caused by light passing through wispy, high-altitude cirrus clouds. The sight occurs only when the sun is very high in the sky (more than 58° above the horizon). What's more, the hexagonal ice crystals that make up cirrus clouds must be shaped like thick plates with their faces parallel to the ground." Other shots of the phenomenon at Flickr.

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