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.