27 June 2021

The Fire Rainbow: Beautiful and Rare Circumhorizontal Arc

If you are very lucky you may see a fire rainbow once or twice in your life. It sounds almost as if it could be the title of one of a series of children’s books – Harry Potter and the Fire Rainbow has a certain ring to it: but this phenomenon is not fiction. If you are in the right place and at the right time then a fire rainbow is something that you will remember.

To name it properly, a fire rainbow is a circumhorizontal arc. It is also known as a circumhorizon arc but whichever you chose, scientists (and aficionados) call it a CHA. It is given its name because it looks as if a rainbow has spontaneously combusted as it made its way across the sky. One might even suspect that some malign fairy or goblin has blown the rainbow up to stop some errant human discovering that elusive pot of gold at its end...

However, the real explanation behind a fire rainbow lies more in science text books than in a Brothers Grimm tale. A CHA is a kind of halo – which is an optical phenomenon. These appear around the moon – or in this case the sun. You have probably seen a halo around a strong light source – take a look at street lights in the fog for example.

Although there are many different types of optical halos, a CHA is caused by the refraction, though ice crystals in cirrus clouds, of light from the sun. Refraction happens when the speed of light is reduced inside a particular, slower medium. This particular refraction happens when light goes from air without cloud to air containing cloud. In this case it is vital that the cloud is cirrus in shape.

A cirrus cloud is one of those thin, wispy ones, often with tufts sticking out like disheveled hair. They can be huge – covering so much of the sky that you cannot see where one ends and another begins. When they appear as a massive sheet they are called cirrostratus. They are formed at enormous heights – over eight thousand meters. There is very little moisture at those heights and that’s why they are so slim.

So, what happens when light hits a cirrus cloud and what special conditions are needed to form a fire rainbow? The refraction of the light causes it to separate from its “white” form to its different components (which are called wavelengths). You could say that the light is bent out of shape and split up in to all the different colors that make it up. In other words, a rainbow – or in this case, a fire rainbow.

So, why don’t we see fire rainbows as often as (its now more mundane) cousin, the rainbow? For a start the sun has to be at least fifty eight degrees above the horizon for one to occur – and you have to be lucky enough to have cirrus clouds around at the same time. As a result of the necessary height of the sun you will not see a fire rainbow north of fifty five degrees – and likewise further south of the magic fifty five degrees. You may occasionally see one if you are high up on a mountain further south or north, but it is unlikely.

There is more. The ice crystals in the cirrus cloud have to be horizontal to refract the sun. If they are then an  arc may be formed - if not, forget it. Ice crystals are hexagonal, meaning that they have six sides. The light goes in through one of the side faces of the crystal - let's say the top one. Then it is refracted and it leaves through the bottom horizontal face.

It is vital that the crystal is aligned just so as otherwise the light will not separate in to the rainbow-like colors we expect. If the alignment is correct then the whole cirrus cloud will “explode” in to a flaming, fire rainbow.

So, if you are lucky enough to see this phenomenon, perhaps it is a good idea to make a wish. After all, the conditions for a fire rainbow are so exact that it means that coming across one is a rare sight indeed. When nature gifts us this sort of favor, then perhaps it is only right that we make a wish when we encounter it. It may not be very scientific, but to be honest, if you saw one, would you care?

Image Credit htomren
Image Credit edrabbit
Image Credit athoshun
First Image Flickr User Carplips