circumhorizon

For light to enter the cirrus cloud crystals at just the right angle to make a circumhorizon arc, the sun must be at an angle of at least 58 degrees above the horizon.

Celestial Phenomena 2008

A circumhorizon arc only forms when the clouds contain a specific, hexagonally shaped ice crystal.

Celestial Phenomena 2008

Several very specific conditions must exist in tandem for a circumhorizon arc to occur.

Celestial Phenomena 2008

The answer is that it isn’t a rainbow. The stack of colors is known as a circumhorizon (or circumhorizontal) arc—or, sometimes, a “firebow.” Though the word “arc” implies a degree of swoopiness, a firebow looks more like a flat band than a rainbow’s signature semicircle. These stunners form differently, too. You might see a rainbow when sunlight rushes past you from behind and strikes water droplets (from rain, fog, or a hose) in front of you. Firebows, on the other hand, form when the sun is fairly high in the sky—more than 58 degrees above the horizon—and sunlight refracts off of the ice crystals in high-altitude clouds. Arranged just so, the ice crystals in faraway cirrus clouds behave like “tiny little prisms,” according to NASA, scattering light and producing the not-quite-fiery show.

Meet 'Firebows,' the Flat, Rainbow-Like Stacks That Appear in Sunny Skies Jessica Leigh Hester 2021