This is not a rainbow: Circumhorizon arcs spotted in the Valley

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Check out this really cool photo that Kim Johnson sent us from Arab, at approximately 11:30am Monday.

Kim described it as a “straight rainbow just hovering in the sky at Arab City Hall.”

Cindy Bryan saw something similar, too, in Fort Payne -- about 45 miles east as the crow flies.

While it looks like a rainbow and it belongs in the atmospheric optics family, this phenomenon is actually a circumhorizon arc. It is a halo that is located beneath the sun, parallel to the horizon.

Circumhorizon arcs occur when sunlight passes through plate-oriented ice crystals suspended in the sky; these ice crystals are what make up high, thin clouds like cirrus and cirrostratus clouds, located about 16 thousand to 50 thousand feet above the ground.

As the sun rises higher in the sky, sunlight enters a portion of the ice crystal that is nearly vertical before exiting out the flat, horizontal bottom. As it does so, the light refracts and produces bright, prismatic colors. Often, the colors are more vibrant and pure than those seen in a rainbow.

Circumhorizon arcs are not particularly rare in the United States, including the Tennessee Valley. They are more frequent in the late spring through summer, because the sun must rise higher than 58 degrees from the horizon in order for the arcs to form.

Did you see a circumhorizon arc too? Share your photo with us by emailing photo@whnt.com. Be sure to include your location!