Fire Rainbow

Claim:   Photograph shows a “fire rainbow” over Idaho.

TRUE

Example:   [Collected via e-mail, 2006]

THIS IS A FIRE RAINBOW — THE RAREST OF ALL NATURALLY OCCURING ATMOSPHERIC PHENOMENA.

THE PICTURE WAS CAPTURED ON THE IDAHO/WASHINGTON BORDER. THE EVENT LASTED ABOUT 1 HOUR.

CLOUDS HAVE TO BE CIRRUS, AT LEAST 20K FEET IN THE AIR, WITH JUST THE RIGHT AMOUNT OF ICE CRYSTALS AND THE SUN HAS TO HIT THE CLOUDS AT PRCEISELY 58 DEGREES.

Origins:   This image of a “fire rainbow” is a real photograph of a brightly colored atmospheric phenomenon known as a circumhorizon(tal) arc. The

example shown above was captured on camera as it hung for about an hour across a several-hundred square mile area of sky above northern Idaho (near the Washington border) on 3 June 2006.

In general, a circumhorizontal arc (or “fire rainbow”) appears when the sun is high in the sky (i.e., higher than 58° above the horizon), and its light passes through diaphanous, high-altitude cirrus clouds made up of hexagonal plate crystals. Sunlight entering the crystals’ vertical side faces and leaving through their bottom faces is refracted (as through a prism) and separated into an array of visible colors. When the plate crystals in cirrus clouds are aligned optimally (i.e., with their faces parallel to the ground), the resulting display is a brilliant spectrum of colors reminiscent of a rainbow.

Last updated:   21 July 2015

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