Fire in the Sky: The Curious Case of a Circumhorizontal Arc

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• What a Fire Rainbow Really Is
• Why the Geometry Is Rare
• A Sky Surprise in West Virginia

Fire in the Sky: The Curious Case of a Circumhorizontal Arc

What a Fire Rainbow Really Is

What’s happening to this cloud? Ice crystals in a distant cirrus cloud are acting like little floating prisms. Known informally as a fire rainbow for its flame-like appearance, a circumhorizontal arc appears parallel to the horizon. For this celestial trick to work, the Sun has to be at least 58 degrees high in a sky where cirrus clouds linger below—specifically cirrus fibratus in this case.

Why the Geometry Is Rare

Here’s the neat physics in bite-sized terms: the cloud is made up of countless flat, hexagonal ice crystals. When sunlight hits them just so, the light is refracted (bent) through the crystals and spread into a vivid, horizontal arc that mirrors the horizon. The alignment has to be uncanny—these ice crystals must be oriented more or less perfectly horizontally to refract light in a coordinated, rainbow-like display. That’s why circumhorizontal arcs are relatively rare: the geometry has to be just right, the Sun has to be sufficiently high, and the right cloud type has to be in the right place at the right time.

A Sky Surprise in West Virginia

The featured fire rainbow was photographed in 2021 near North Fork Mountain in West Virginia, USA. A momentary meteorological MOAB of color and geometry, it’s the sort of optical wonder that makes the weather feel a little more magical than your average Tuesday. If you’re chasing one of these, a clear horizon, high summer sun, and cirrus clouds at mid-to-high altitudes are your best bets. And if you miss it, don’t worry—the sky keeps a few tricks up its sleeve for those who know where to look.

Image via NASA

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