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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