SGU Science Picture Of The Week: A Fire Rainbow-Circumhorizontal Arc
There’s not many of them, but this is one of those things that, if I saw it in the sky before last week, I would have said out loud…”What…The Fuck…Is That?”.
What you’re looking at is an optical atmospheric phenomenon called a circumhorizontal arc. This particular picture was seen last August 16 in South Carolina in the U.S.
It is caused by a very special and rare set of circumstances. Primarily, you need to be in a part of the world where the sun can get at least 58 degrees high in the sky. That means that this wonderful sight could never been seen if you lived north of 55°N or south of 55°S. There also needs to be hexagonal plate crystals in the atmosphere. For these to form, you need cirrus (or cirrostratus) clouds which typically float around at 20,000 feet (6,096 meters). It’s only at this altitude is it cold enough for these beautiful crystals to exist. The crystals also have to be oriented properly such that their faces are parallel to the ground.
Only when all these specific conditions exist could the light from the sun (or the full moon) hit the crystals just right to refract their light, splitting it into its component colors, prism-like, and send it down to the waiting retinas of gaping humans.
There also seems to be a lot of recent talk about what this phenomenon is called. All over the web it’s called a Fire Rainbow. Many people, including meteorologists are saying this is a misnomer. For example, meteorologist at the US Naval Academy, Raymond Lee said the term is a…
“complete misnomer, akin to identifying a conventional gasoline engine as an electric motor”
Usually I’m a stickler for naming conventions but this one doesn’t bother me. Sure, this isn’t a classic rainbow, nor is fire involved in any way. It certainly is rainbow-like in appearance though. It is also created in a similar way with light being refracted by particles in the air causing colorful light dispersion. In this case it’s an ice crystal instead of a raindrop. The fire part of the name was apparently coined in 2006 when the phenomenon looked fire-like due to the disconnected nature of the cirrus clouds at that time.
Also, “fire rainbow” just sounds cool. What would you rather shout to your friends?
“Look…A Fire Rainbow”or “Look…A Circumhorizontal Arc”
Image Credit: Tiffany B Jenks*
Attribution has been debate over who exactly took this picture