Rainbow of fire | The provinces

The rainbow is an optical phenomenon that we are used to. But today we talk about another much more exceptionally optical effect: the rainbow of fire. In the first place, it must be said that it is not a fire, nor a real rainbow. The scientific name is that of a circum arch and means horizontal for the earth’s surface. Let’s look at the circumstances that have to be given, so that if you are lucky one day you are lucky to be in the right place and moment, you can enjoy this weird but beautiful, optical phenomenon.

It happens when sunlight is broken by ice crystals present in two very high types of clouds: Cirrus and Circus, which are above 6 kilometers high. The first question is that the cloud only contains flattened hexagonal ice crystals and, all, horizontally aligned. The second question is that the sun is an angle larger than 58º with the horizon. From this position the light rays enter the vertical face of hexagonal ice crystals and the horizontal go down. In a more lax language it can be said that crystals behave like a gigantic optical prism that breaks down the light in its seven colors. Under these circumstances, the cloud is illuminated with all the colors of the rainbow. It is as if it burned with a fire of colors, hence is denomination, popular and incorrect, of rainbow of fire. Because it occurs in a cloud, it does not give rise to a complete arch in the air, as is the case with the usual rainbow. As we see, it is a refractive (directional) phenomenon that only occurs when exceptional circumstances are given. In addition, when the sun is at an altitude of approximately 58º on the horizon, the rainbow of fire cannot occur in both hemispheres on wide grabs above 55º. In them the sun is never at a height larger than 58º on the horizon. The colored image adapts to the shape of the cloud. This is the reason why different types of circumhorizontal arches can be created: rainbow of braided fire, wavy fire, stripes, rivers and more. This diversity enables us to enjoy an incredible variety of rainbow. In widths in which it is possible to see the rainbow of fire, the chance to see it depends on the number of days a year in which the sun reaches the correct height. For example, In Los Angeles, with 670 days a year with the sun above 58, it is much more likely to see than in London, where only this circumstance is given 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: Cycenital arches, iridescent clouds, infrachhalateral arches, etc. The first are based on a very similar effect. They need the same clouds and hexagonal crystals of ice in a vertical position. That is why the height of the sun has other requirements. In others the type of clouds is different and instead of ice there are drops of water.