By Brian Clark Howard, National Geographic
A colorful “fire rainbow” lit up the sky, and the Internet, this week. But despite its name, the rare phenomenon isn’t related to recent wildfires. Instead, it’s caused by a unique alignment of forces in the atmosphere.
Technically called a circumhorizontal arc, fire rainbows are caused by light passing through wispy, high-altitude cirrus clouds. This one was seen over South Carolina Monday for about an hour. It was photographed and uploaded to Instagram.
Fire rainbows occur only when the sun is very high in the sky (more than 58° above the horizon). What's more, the hexagonal ice crystals that make up cirrus clouds must be shaped like thick plates with their faces parallel to the ground.
When light enters through a vertical side face of such an ice crystal and leaves from the bottom face, it refracts, or bends, in the same way that light passes through a prism. If a cirrus cloud’s crystals are aligned just right, the whole section lights up in a spectrum of colors.
The phenomenon is similar to the iridescent clouds, also called “rainbow clouds” —confusingly, they’re sometimes also called fire rainbows—that form on top of cumulus clouds after thunderstorms.
Those form after air is pushed up rapidly, causing it to cool and expand. Water droplets condense that can then act as prisms, forming a rainbow cap over the cumulus cloud. (See photos of sun dogs and other atmospheric delights.)