 |
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
rainbow colors in the sky, with the whole thing looking like a multicolored
flame, then you have probably witnessed a fire rainbow. It is a rainbow type
phenomenon, in the sense that it has the colors of a rainbow but it is shaped
quite differently. It looks like a rainbow colored flame and hence the name
'fire rainbow'. When people see a picture of such a rainbow, some think that
it's a hoax and is a computer generated picture! Nothing could be far from the
truth. Let such people know that it is not an illusion created by photoshop,
but an actual documented meteorological event. The facts that I talk about here
will illuminate the actual nature of this optical phenomenon.
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 art pieces 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. It is just 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 a Fire Rainbow?
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 for 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 a typical type of
clouds called '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. For it to form, the Sun is always high up in the sky; often
observed to be at a elevation angle of 58 degrees or slight more. Also required
are cirrus clouds which have a large number of plate shaped ice crystals. It is
important that you are at the right location to be able to see the 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 crystals 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 entering ray and exiting ray which produced a band of dispersed
colors that are well separated and visible clearly. 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.
