An unusual but spectacular sight, known as a fire rainbow, was spotted near Hazard, Kentucky, on Sunday.
Circumhorizontal arc
© Michael Herald
This phenomenon, which resembles a piece of a brightly colored rainbow, is also known as a circumhorizontal arc.

The name fire rainbow comes from its bright rainbow colors and almost flame-like shape.

Unlike rainbows, circumhorizontal arcs occur from the refraction of sunlight through hexagonal, plate-shaped ice crystals when the sun is 58 degrees above the horizon and no rainfall is involved. Sunlight enters these ice crystals and splits into individual colors, like a prism.

Circumhorizontal arc
© Michael Herald
Circumhorizontal arcs appear primarily where cirrus clouds are located. Cirrus clouds are thin and wispy high-altitude clouds and when fire rainbows are observed it almost appears that these wispy cirrus clouds take on a rainbow palette.

Fire rainbows are most often spotted in the spring and summer in the mid-latitudes since the sun is not sufficiently high enough above the horizon in higher latitudes.

Circumzenithal arcs are a variation of this phenomenon and appear as a quarter-circle upside-down rainbow. In this case, the sun has to be 5 to 32 degrees above the horizon.