These astonishing visual displays appear to be fiery rainbows, yet are very icy indeed – and not rainbows in the traditional sense at all.
Fire Rainbows are the rarest of all naturally occurring atmospheric phenomena, so if you thought you understood beauty in the magical awe that a rainbow can inspire, then this kind could still manage to delight and surprise you. The thing is that it is only rarely that weather conditions line up perfectly to provide these visual feasts, which are always frustratingly fleeting in nature.
That is why one has to be very quick to catch sight of these undeniably magical experiences, which occur only when cirrus clouds – and no other type – are at 20,000 ft at least, with just the correct percentage of ice crystals present, added to which sunlight has to hit the cloud precisely 58 degrees, if the effect is to emerge.
These astonishing visual displays appear to be fiery rainbows, yet the truth is that they are very icy indeed – and known to students of such weather events as a circumhorizontal arc. In truth, these stunning sights are not rainbows in the traditional sense, but actually caused by light passing through those wispy, high-altitude cirrus clouds, as long as the hexagonal ice crystals within them are shaped like thick plates, positioned so that their faces are parallel to the ground, which obviously rarely is the case.
The physics of this work in that when the light enters via the vertical side face and leaves via the bottom face, it then refracts, or bends in the same way as if passed through a prism, so provided those crystals are aligned just right, the entire cloud lights up in a colourful spectacular, those horizontal crystals refracting the beams of the sun.
The circumhorizonta arc forms as light rays exiting through the horizontal bottom face at that 90 degree inclination creates those well-separated rainbow colours that turn the entire cirrus cloud into a very short-lived spectacle rarely surviving beyond half an hour. You are most likely to witness these events in the USA, where it must be truly difficult to tear your eyes away.
How gratifying, though, is the knowledge that such wondrous displays are absolutely free of charge – should you be lucky enough to ever see them – our unique planet offers an incredible diversity of natural experiences, and natural beauty waits all around us, if only we open our eyes and look.