English: Team V received a glimpse of a rare optical phenomenon here May 18. The circumhorizontal arc, more popularly known as a “fire rainbow,” appeared in the skies above Vandenberg approximately 11 a.m. to 1 p.m. The arc is formed when sunlight rays interact with the ice crystals of cirrus clouds, producing well-separated rainbow-like colors. If the crystal alignment is just right, this interaction makes the entire cirrus cloud appear to shine like a fiery rainbow. (Photo credit: Wikipedia)
When you think you understand the beauty in nature, especially in the magical awe that a rainbow can inspire, then something will appear that still manages to both surprise and delight you in equal measure. Just occasionally, the weather conditions will line up perfectly to provide us with a visual feast that we need to be very quick to catch sight of. At times reminding us of the aurora borealis or simple rainbows, these events are niether, but a magical experience in thier own right. These are Fire Rainbows — the rarest of all naturally occurring atmospheric phemonema.
The clouds needed for these rare events have to be cirrus, and at least 20,000 feet in the air, with just the right amount of ice crystals. The sun also has to hit the clouds at precisely 58 degrees.
It looks like a rainbow on fire, but this phenomenon is as cold as ice. Known in the weather world as a circumhorizontal arc, this isn’t a rainbow in the traditional sense—it is caused by light passing through wispy, high-altitude cirrus clouds. What’s more, the hexagonal ice crystals that make up cirrus clouds must be shaped like thick plates with their faces parallel to the ground.
When light enters through a vertical side face of such an ice crystal and leaves from the bottom face, it refracts, or bends, in the same way that light passes through a prism. If a cirrus’s crystals are aligned just right, the whole cloud lights up in a spectrum of colours.
The phenomenon is quite rare because the ice crystals must be aligned horizontally to refract the high sun. The arc is formed as light rays enter the horizontally-oriented flat hexagonal crystals through a vertical side face and exit through the horizontal bottom face. It is the 90 degrees inclination that produces the well-separated rainbow-like colours and, if the crystal alignment is just right, makes the entire cirrus cloud shine like a flaming rainbow.
The absolute wonder of these spectacular events is that they can be very short-lived, rarely surviving beyond half an hour, simply because all of the factors involved must be just so before it can happen. The following series of pictures were all taken in the USA, where you are most likely to witness these events.
It is truly difficult to tear your eyes away from these specactular images, and gratifying is the knowledge that this wondrous display comes absolutely free should you be lucky enough to ever see it. We live on a unique planet in the universe, and should be immensely grateful for the incredible diversity of natural experiences that surround us. Beauty is waiting for all of us in the great outdoors. All we need do is go there and open our eyes.
I have been writing for pleasure for half a century, but only started writing for money around 1994. Since then I have had a few thousand articles, stories and poems published on line and in print all round the world.
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