The rainbow is an optical phenomenon that we are used to seeing. But today we are talking about another much more exceptional optical effect: the rainbow of fire. First of all, it should be said that it is neither fire nor a true rainbow. Its scientific name is the circum-horizontal arc and it means horizontal to the earth’s surface. Let’s see the conditions that must be met so that, if one day you are lucky enough to be in the right place and at the right time, you can enjoy this rare, but beautiful, optical phenomenon.
It is produced when sunlight is refracted by ice crystals present in two types of very high clouds: cirrus clouds and cirrus-strata, which are over 6 kilometers high. The first requirement is that the cloud contains only flattened hexagonal ice crystals, all of them aligned horizontally. The second requirement is that the sun is at an angle greater than 58º with the horizon. From this position, light rays enter the vertical face of the hexagonal ice crystals and exit the horizontal downward. In looser language, it can be said that crystals behave like a gigantic optical prism that breaks down light into its seven colors. Under all these conditions the cloud glows with all the colors of the rainbow. It is as if it burned with a fire of colors, hence its popular and improper denomination of rainbow fire. Because it occurs in a cloud, it does not produce a full arc in the sky, as with the usual rainbow. As we can see, it is a phenomenon of refraction (directional) that occurs only when exceptional conditions are met. Furthermore, by requiring that the sun be at a height of about 58º above the horizon, the rainbow of fire cannot be produced in both hemispheres at latitudes above 55º. In them the sun is never at a height higher than 58º above the horizon. The colored image adapts to the shape of the cloud. This is why various types of circum-horizontal arcs can be created: braided fire rainbows, wavy fire rainbows, striped, river rainbows, and more. This diversity allows us to enjoy an incredible variety of fire rainbows. At latitudes where it is possible to see the rainbow of fire, the probability of seeing it depends on the number of days per year in which the sun reaches the appropriate height. For example, in Los Angeles, with 670 days a year with the sun above 58º, it is much more likely to be seen than in London, where this circumstance only occurs 140 days a year.
There are other optical phenomena that can be confused with the rainbow of fire because they also give rise to beautiful colored clouds: circumcenital arches, iridescent clouds, infralateral arches, etc. The former are based on a very similar effect. They need the same clouds and the hexagonal ice crystals in an upright position. Therefore, the height of the sun has other requirements. In the others, the type of clouds is different and, instead of ice, there are drops of water.