Circumhorizontal arc

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Circumhorizontal arc photographed in Coeur d'Alene, Idaho on June 3, 2006
Circumhorizontal arc photographed in Coeur d'Alene, Idaho on June 3, 2006

A circumhorizontal arc, also known as a fire rainbow, is an optical phenomenon similar in appearance to a rainbow, caused by the refraction of light through the ice crystals in cirrus clouds. It occurs only when the sun is high in the sky, at least 58° above the horizon, and can only occur in cirrus clouds.

The phenomenon is quite rare because the ice crystals must be aligned horizontally (instead of the more typical vertical alignment) to reflect. It is often thought of as the rarest natural occurrence in the world.

One particularly fine example was photographed over northwestern Idaho on June 3, 2006, and was reported in both the New Scientist and the Daily Mail (the latter under the caption "flaming rainbow").


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