The fire rainbow is a phenomenon of cold color involving meteorology. Contrary to its nickname, the fire rainbow has nothing to do with flames, and, if you see one, it will appear far above you, high in the atmosphere. The appearance of this lovely sight requires some specific conditions to exist for the elements to come together: cold cirrus clouds, sunlight, a bit of science, and an observer at the right latitude.
A pretty thing by another name
The actual name of what has been popularly labeled “fire rainbow” is circumhorizontal arc, much less exciting but more accurate with regard to where it appears. The sun must be 58 degrees high in the sky or higher, and the arc will appear above the horizon as you perceive it.
Why 58 degrees? The reason you are witnessing this meteorological event in the first place is that hexagonal ice crystals with a flat face, arranged end to end parallel to the Earth’s surface, are refracting the sun’s light. These crystals appear as cirrus clouds to the ground observer. If you are seeing the arc, you are somewhere in the line of their prismatic refraction.
Fire rainbows appear both north and south of the equator. The only real limitation on your chances of seeing them is the science of refraction, as this forum writer discussed it for Australia. “It reaches its maximum intensity at a sun elevation of 67.9°.” There likely are many people who will never see a fire rainbow because, for them, the sun will never rise above 58 degrees in the sky. On the other hand, those people who live in colder climates may have a better chance to see circumhorizontal arcs, simply because they will experience more cirrus clouds containing those all-important ice crystals.
When a rainbow is not a rainbow
While the circumhorizontal arc is technically not a rainbow, like any output of a prism refracting sunlight it will produce the familiar red-to-blue spectrum of color. There is a difference between the fire rainbow and the normally occurring rainbow that will help you to tell them apart easily: the fire rainbow’s color spectrum is not displayed neatly as orderly concentric light paths. Instead, most appearances will remind observers of an actual fire because at least one part of the structure resembles the randomness of the flames in a campfire.
Where fire rainbows are absent
Despite the fact that the other conditions might be present, your location as the observer is equally crucial to seeing circumhorizontal arcs. Since the sun’s position must reach at least 58 degrees in the sky, an observer living above 55 degrees north latitude or below 55 degrees south latitude (check a globe for these areas) is unlikely to witness such formations. It is not impossible, but the observer may need to be in an unusual location to spot any arcs at all.
