The Spellbinding Phenomenon Of Fire Rainbow
This optical phenomenon is more commonly referred to as a fire rainbow, rather than the more mathematical-sounding "circumhorizontal arc," which is more commonly encountered in geography textbooks and less commonly used in everyday conversation.
In August of 2015, residents of South Carolina were treated to a sight in the sky that was both weirdand lovely. At the time, a photograph of a "multi-colored angel" with a rainbow-like look went viral on the internet. So, what exactly was this angel with a rainbow of colors? Is this a heavenly message? No, it was an optical phenomenon known as a circumhorizontal arc, which is a technical term for what happened. However, this was not the first or only instance in which a circumhorizontal arc has been observed in the past. It has been observed on rare occasions in other locations of the United States as well. So, what exactly is this fire rainbow, and how does it come to be?
Prof. Steve Ackerman from the University of Wisconsin-Department Madison of Atmospheric and Oceanic Sciences thinks that multiple elements must come together for this magnificent phenomenon to occur. A fire rainbow is one of the few forms of halos generated by the refraction of lightin plate-shaped ice crystals intermingled in the atmosphere. It is most commonly seen in cirrus clouds and is one of the few types of halos that may be seen in the sun.
Low-level clouds are distinguished from middle-level clouds and high-level clouds, which are distinguished by their altitude levels. Those clouds located below 6,000 feet in altitude are known as low-level clouds. Clouds in the lower atmosphere are primarily cumulus clouds, which are fluffy and have an appearance that is reminiscent of a cauliflower.
A smattering of cumulus clouds signals good weather—the kind of weather you'd like to be out enjoying a picnic. Middle-level clouds, which are between 6,000 and 20,000 feet above the surface of the earth, follow next. The majority of them are made up of cumulus clouds, which are also known as altocumulus. Alternative cumulus clouds are white and less fluffy than cumulus clouds, and they are frequently patchy in nature. Those who live over 20,000 feet will experience high-altitude clouds. Among the high-level clouds, cirrus clouds are the most numerous, and they are of particular importance to us.
When certain conditions are met, fire rainbows or circumhorizontal arcs appear. To begin, there is a certain angle at which light passes through these high-altitude cirrus clouds. Following that, they have the Sun's position. An occurrence of a flaming rainbow is only possible when the Sun is at least 58 degrees above the horizon. Light disassembles into constituent colors through ice crystals, which must be hexagonal in shape with their faces running parallel to the ground in order for this to happen.
It is refracted, or bent, when light passes through the vertical side face of these ice crystals and departs through the bottom face, which means that its direction of progression is altered (i.e., it bends). When light bends in this manner, it is analogous to when light bends through a prism.
As a result, if the crystals in this area of the cirrus cloud are perfectly aligned, the entire section acts as a prism, giving the cloud a look similar to that of a rainbow. Because cirrus clouds are wispy, they have the appearance of flames, which is why this phenomenon has been known as the "fire rainbow."
Fire rainbows, as previously stated, are extremely rare sightings. Unless the Sun (or Moon) is at least 58 degrees above the horizon, it is very difficult to observe a fire rainbow in latitudes north of 55 degrees North or south of 55 degrees South. The frequency and length of its occurrences in locations where it has been seen also vary widely.
- The Sun is only high enough in the sky for a paltry 140 hours between May and July, as may be observed, for example, in London.
- While the Sun is high enough for 670 hours each year in Los Angeles, the same is not true for the rest of the world.
- In addition, the weather in Europe is frequently overcast.
- Seeing a fire rainbow in Europe would be significantly more difficult due to the presence of low and middle-level clouds, which would make seeing a fire rainbow even more difficult.
It is common to mistake cloud iridescence for fire rainbows. However, unlike a flaming rainbow, cloud iridescence is caused by light diffractionrather than refraction and hence appears to be multi-colored rather than refracting light.
Diffractionis the curving of light when it comes into contact with an obstruction that it is unable to travel through without being bent.
Aside from the technical differences, there are also apparent distinctions to be made. Unlike cloud iridescence, fire rainbows always occur at the same location in reference to the Sun or the Moon, whereas cloud iridescence can occur at any point in the sky. Aside from that, the color bands in a fire rainbow always run horizontally, with red from the VIBGYOR color spectrum, red at the very top, and violet at the very bottom of the spectrum.
However, cloud iridescence does not always exhibit this fixed series of hues; rather, the color sequence in them is unpredictable.
When a fire rainbow appears in the sky, it is considered one of the most beautiful natural phenomena on the planet. A flame-like look is created by perfectly matching the hues of the rainbow. When ice crystals are present in the atmosphere, refraction, one of the most fundamental laws of optics, is at work to determine how much light is reflected.
As already said, flaming or fire rainbows are quite rare. The source of light, which is the Sun (or Moon), must be at least 58o above the horizon in order for a fire rainbow to appear, making it very hard to observe in locations north of 55oN or south of 55oS. Furthermore, in areas where it has been observed, the frequency with which it occurs and the length of time it lasts differ significantly.
Iridescent cloud versus 'fire rainbow'
There is a discussion on Reddit going on about the phenomenon of fire rainbow. Check out some of the comments.
"I've seen milder versions of this, but it has been much more noticeable through polarised glasses."
"I heard than the Northern Lights are not so noticeable with a naked eye. For some reason they are much more clear through a lens (camera, glasses). Maybe this phenomenon is similar in terms of visibility."
People in the USA are really lucky because they can observe the unusual phenomenon of fire rainbow more frequently than many others in other parts of the world, residents in the United States should count their blessings!