Ghost fire in the sky near Franklin, WV. Photo by Christa Harbig
Fire Rainbows: A Rare Cloud phenomenon
“Fire Rainbows” or “rainbow clouds” are neither fire, nor rainbows, but are so called because of their brilliant pastel colors and flame like appearance.
Technically they are known as circumhorizontal arc – an ice halo formed by hexagonal, plate-shaped ice crystals in high level cirrus clouds. The halo is so large that the arc appears parallel to the horizon, hence the name.
Brightly colored circumhorizontal arc occur mostly during the summer and between particular latitudes. When the sun is very high in the sky, sunlight entering flat, hexagon shaped ice crystals gets split into individual colors just like in a prism. The conditions required to form a “fire rainbow” is very precise – the sun has to be at an elevation of 58° or greater, there must be high altitude cirrus clouds with plate-shaped ice crystals, and sunlight has to enter the ice crystals at a specific angle.
This is why circumhorizontal arc is such a rare phenomenon.