GREENSBORO, NC -- It might look like rainbow and you might have even heard it called a fire rainbow, but what some people saw in the sky last Friday was a circumhorizontal arc.
A rare sighting that can only be seen in the Triad from March through September a circumhorizontal arc is a phenomenon caused by the sunlight hitting ice crystals just right in the atmosphere. This occurs when sun- or moonlight is refracted in a plate-shaped ice crystal that is suspended in the atmosphere, typically in high cirrus clouds.
The process that causes the circumhorizontal arc is just like the effect of when you shine light into a prism.
In ideal conditions, the arc appears as a large, brightly spectrum-colored band that appears parallel to the horizon located directly below the sun or moon.