The sky is the nature's canvas, on which she paints beautiful and ever-changing pictures. Light itself is her color and she uses it in various ways to create beautiful optical phenomena. One such rare masterpiece of her creation, is the fire rainbow.
If you see a splash of colors in the sky, with the whole thing looking like a multicolored flame, then you have probably witnessed a fire rainbow.
Its sighting, unlike a rainbow, is not so common. It is supposed to be a very rare phenomenon that a person could see once or twice in his lifetime. That too, if he has his eyes trained at the sky, to appreciate the masterpieces put up there, by nature.
If you are on a photography hunt to capture that rare sighting of a fire rainbow, then you need to know when and how it occurs. Like the hunter who must know when and where to look for his prey, the photographer must know when and in what conditions can this phenomenon occur. During such sightings, photography skills are actually tested.
What is this optical phenomenon?
In the language of the meteorologists and physicists, a fire rainbow is known as a circumhorizontal arc, circumhorizon arc, lower symmetric 46 degree plate arc, or just CHM in short. In scientific language, it is classified as an 'Ice Halo'. It is generated by refraction (slowing down of light) in ice crystals, placed in cirrus clouds.
Its full halo is an extensive multicolored range of colors, seen parallel to the horizon, in presence of cirrus clouds. The center of this halo is always observed to be below the Sun. It is observed that red is always the first in the band of colors. If the conditions are not ideal, only fragments of the arc are seen.
How is it Formed?
Through years of observation and study, meteorologists have discovered the exact conditions that give rise to a CHM. When it forms, the Sun is always high up in the sky; often observed to be at an elevation angle of 58 degrees or slightly more.
Also involved are cirrus clouds, which have a large number of plate-shaped ice crystals. It is important that you are in the right location, to be able to see it, as the position of the Sun relative to you and its altitude, decides it.
It is a known fact, that fire rainbows are not visible at positions north of 55 degree north latitude or south of 55 degree south latitude. Sightings are more probable around the summer solstice. Elaborate timetables of where these phenomena are likely to occur and be seen, have been made available online.
If you go down to the physics of how this type of optical halo occurs, then you must understand it in terms of refraction of light through ice crystals of specific types. An optical halo occurs when a sun ray enters a horizontally oriented ice crystal of flat hexagon shape, from a vertical face on the side and exits through the near-horizontal face at the bottom.
This refraction event creates an angle of 90 degrees, between the entering and exiting ray. This produces a band of dispersed colors, that are well-separated and clearly visible. A collection of such typical sized and shaped crystals, oriented at specific angles with respect to the Sun and at the right elevation, create a fire rainbow. Knowing how such a beautiful sight is created, doesn't undermine its beauty but only adds to a deeper appreciation of it.