“Fire rainbows” are neither fire, nor rainbows. They’re referred to as fire rainbows because of their brilliant pastel colors and flame-like appearance. Technically, they’re known as circumhorizontal arcs — ice halos 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 arcs 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 are 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. That's why circumhorizontal arcs are such a rare phenomenon.
