August 23, 2015: Fire rainbow

This rare weather phenomenon appeared above the Isle of Palms outside Charleston, South Carolina last Sunday. People call it a "fire rainbow" but its actual term is "circumhorizontal arc". The Verge wants you to call it that instead of calling it fire rainbow.

Hell with that. It's a fire rainbow.



The vivid rainbow colors result when the sunlight hits hexagonal ice crystals, at a particular angle, in wispy cirrus clouds. Only cirrus clouds, which float 20,000 feet (6,096 meters) above the ground, are cold enough for ice crystals to develop.

That is wicked cool.

The Verge can bite my non-shiny non-metal...

Yes, "circumhorizontal arc" is correct. "Fire rainbow" is not only a vivid two-word description that is very easy to remember, it's a heck of a lot easier to say! The "fire" part comes from the fact that it only shows up on wispy, streaky clouds that appear to be vertically oriented from ground level. When the rainbow colors are added, the visual similarity to flame is strong.

Water, light, and air really seem to enjoy getting freaky together in some places, and I live in one of them. Attached is a photo I shot just back in June (scaled down to 25% of original size). I read Earth Science Photo of the Day every day, and it's responsible for everything I (barely) know about atmospheric refraction and reflection effects. Rainbows can only be seen when there is a 45-degree angle involving the sun, the viewer, and the point where we see rainbows. This one was shot when the sun was either already behind a mountain or about to set, which is why the red-orange-yellow side is all vivid pink and the blue-green-violet side seems nearly absent.

I've also got a shot of a "low-bow", shot from almost the same position but on the other side of the valley. It was nearly noon, so I'm not sure I should have technically been able to see the rainbow, but in the pic it's clear what's visible is, instead of the "foot" end of a rainbow, the very top of the arc hovering just above ground level. I have submitted both for consideration on EPoD, but I don't think I know enough about the technical camera details to get published.

Yeah, rare sighting, hard to photograph, thanks for sharing, Snake.