When a stacked spectrum of colors stretched across the sky above parts of central and western Pennsylvania in early July 2021, people wondered how it was happening. There was no rain and the sky was clear blue. That’s because it wasn’t really a rainbow. The stack of colors is known as a circumhorizon arc — also known as a “firebow.” Though the word “arc” implies a degree of sweeping lines, 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 in front of you. Firebows, on the other hand, form when the sun is fairly high in the sky and sunlight refracts off of the ice crystals in high-altitude clouds. These ice crystals behave like tiny little prisms, scattering light and producing the not-quite-fiery show. The likelihood of laying eyes on one varies by time of year and location. In order for sunlight to strike the ice crystals at the right angle, the sun has to be in a particular position, one that’s more likely in months when the sun climbs the highest. In the northern hemisphere, that makes the phenomenon most likely in the summer. Firebows sometimes stick around longer than a rainbow, especially when a thunderstorm's thick clouds don’t sweep past and cover them up.
