The sky held a fire rainbow over the New River foothills North of Phoenix today. It lasted for about half an hour. I was at 5 Acre Arts, Lynda was giving me a ride in her ATV through some pretty uncharted terrain when I saw it. I didn’t think it would show up on my iPhone camera, but it did, and quite nicely, too.
Technically, it is not called a fire rainbow, as it has nothing to do with fire and isn’t really a rainbow, it’s a halo. I like the name, though, so I’m going to be less precise and call it that.
Victoria Gilman, a writer for National Geographic, explained the phenomenon in a 2006 article:
It looks like a rainbow that’s been set on fire, but this phenomenon is as cold as ice.
Known in the weather world as a circumhorizontal arc, this rare sight . . .isn’t a rainbow in the traditional sense—it is caused by light passing through wispy, high-altitude cirrus clouds. The sight occurs 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’s crystals are aligned just right, the whole cloud lights up in a spectrum of colors.
You can see the cirrus clouds in the photo–those thin row-like clouds. They carry ice crystals and generally signify a change in weather.
These halos are rare, and not visible north of 55 degrees N (Northern Europe) or South of 55 degrees South (the Southern end of South America). I could have easily been inside, or not paying attention. It was a gift, and one I’m profoundly grateful for.