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Thom Brommerich
Circumhorizontal Arc, 2005

Natural Science Photograph; direct digital capture

Thom-B-Foto, Orangevale, California, United States

A circumhorizontal arc is a rare optical weather phenomenon that appears as a horizontal rainbow. It is caused by the refraction of light through ice crystals in cirrus clouds, which only occurs when the sun is high and at least 58 above the horizon. The ice crystals must be aligned horizontally to refract the sun presenting this unusual pattern. When light rays enter the hexagonal crystals in a vertical orientation, they will exit through the horizontal bottom face causing the light rays to refract or bend. An example of this happens when light passes through a prism. The 90 refraction of the light produces the stacked rainbow effect, which is also known as a flaming or fire rainbow.

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