14 DAY TRIAL // At times, those who are lucky enough can stand in awe at the spectacular beauty of a fire rainbow, which makes the sky look like it is aflame. Going by the scientific name of circumhorizontal arc or circumhorizon arc (CHA), nature surely throws in a lot of effort for one of these rainbows to occur. Basically, it is a halo created by the refraction of sunlight in the ice crystals of the cirrus clouds. But wait, there's more to it than that.
A halo is a phenomenon from the field of optical physics, caused by the refraction of light in a denser medium. You have surely observed such phenomena when looking at the moon, for example, or at strong light sources during night time – it's that slightly different in “texture” circle that surrounds them. The refraction is the process that takes place whenever the speed of light is slowed down by the aforementioned denser medium, which splits the “white” light into its color spectrum.
The cirrus clouds are the thin ones that look like the filaments of a canvas and spread over a large portion of the sky. At an altitude of over 8 kilometers (26,000 feet), water vapors turn into small icy crystals. However, the mere presence of these elements is not enough for a CHA to appear. The Sun, for example, has to be positioned at least 57.8° above the horizon in any direction, while the maximal intensity of its elevation has to be of 67.9°.
Furthermore, the sunlight must fall and also exit on a certain angle towards the hexagonal ice crystals in order for the refraction to yield the expected fiery result. This means that the crystals need to be oriented perfectly horizontally towards the direction of the incoming light, so that their 90° inclination can split it effectively, creating a well-separated palette of vivid colors. But when all these conditions are met, the effect creates the portents for a rare, unforgettable experience.