via ZME Science

When you read the words ‘fire rainbow’ you may imagine a fantastic arch of fire lighting up the sky like something out of a sci-fi movie with dragons. But, actually, fire rainbows are far from being that or even to be related to the real burning fire.

Fire rainbow’s real name is circumhorizontal arch and is a rare optical phenomenon that creates the illusion of a bright colorful halo in the sky. Generally when they appear, they are produced by the Sun, but on a few occasions they have been made by the Moon.


Why Do They Happen?

Via High Brow

The cause of this manifestation isn’t as difficult to understand as it sounds. They are similar to regular rainbows or iridescent clouds, which are commonly called rainbow clouds. However, they need some specific and special conditions to get them to light up.

The clouds work as a natural prism thanks to the water inside them. Cirrus clouds are the ones who have the most important part in this event, they are the highest clouds, composed of ice crystals, and locate as long band clouds in the sky. The crystals inside the cirrus must be shaped like thick plates and aligned perfectly until they face down the ground.

Once the crystals are in the perfect location, it is time for the Sun to act. When the Sun is very high and the cirrus is in position, the light goes through the infinite number of crystals at a perfect 90° angle. The sunlight enters through the upper face, but when it leaves down the bottom face, it refracts.

The refraction of the light is a basic optical phenomenon that causes the wavelength to suffer a change of direction while it goes obliquely through another surface that allows the phenomenon. This would be something like the light ‘bends’ and it can reflect different colors according to the direction it has.

The 90-degree angle formed allows the colors in the light spectrum to separate almost flawlessly. Fire rainbows are rarely complete, and sometimes appear just glances of it as fragments of other colors in the blue sky.

This event is generally rare, but it can be seen occasionally in the United States. It all depends on how high the Sun is, which changes according to the geographic location. As it needs to be elevated 58 degrees higher than the horizon, it can commonly appear in Los Angeles, during the summertime, however, in Europe, it is hard to admire, due to the latitude of the continent.

Perhaps after reading this now you wonder why then are they popularly called fire rainbows? The term popped up since sometimes the fragments – or the whole halo – looks like an arch of ‘flames’ in the sky, but this only happens when the event takes place through a fragmentary cirrus cloud.