Wisconsin residents stunned by rare 'fire rainbow'

By Weather.com

You've all seen rainbows, but have you seen a rainbow in a stripe across the sky? That's what residents of western Wisconsin saw and photographed over the weekend.

What you see in the photos is called a "circumhorizontal arc" -- or CA -- or what appears to be a fragment of a rainbow.

Typically you associate rainbows with rain and sunlight. However, CAs result from the refraction of sunlight through plate-shaped ice crystals.

For a CA to form, the sun has to be 58 degrees above the horizon. This is a rare sight at higher latitudes, where the sun is not sufficiently high above the horizon. In the middle latitudes, however, the spring and summer months offer the best chance to see this.

CAs show up primarily where cirrus clouds are located. It's as if wispy cirrus clouds take on a rainbow palette.

Another variation of this is called a circumzenithal arc, which appears as a quarter-circle rainbow. For this to form, the sun has to be 5 to 32 degrees above the horizon. An angle of about 22 degrees above the horizon gives the optimal brightness, arc length and width for viewing. It will always be on the same side of the sky as the sun.

We would love to see your amazing photos of these phenomena. Share them with us either at weather.com/photos, or on Facebook, Twitter or Google+.

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