When we looked up high in the sky there was a rainbow that looked almost as if it was upside down. I've never seen this before and wondered what was going on. It looked like this..
Wow! What a wondrous vision! The sky is smiling! How delightful but how to answer, that's a difficult decision! Not a matter of small disambiguation, this heavenly grin can captivate a nation.
Our quest for the answer led us through the Arts, Literature and Sciences. Mentioned in mythology; Greek, Hindu and Norse, penned in poetry by Keats and Wordsworth, studied in science by Descartes, Newton and Young, and sang in songs by Kermit the Frog and Dorothy in the Wizard of Oz. All of it fascinating, but the answer was proving as elusive as trying to reach the end of a rainbow itself.
However, undeterred in our ephemeral endeavours we believe we have found the answer to understanding this happy expression. It is called a Circumzenithal Arc! No, we're not having a lark.
The circumzenithal arc is a quarter circle, pointing toward the setting sun and the "rainbow'" is much brighter and more concentrated than a rainfall rainbow. The colours are in reverse order from a rainbow, with violet on the top and red at the bottom. The arc usually vanishes quickly because the cirrus clouds containing the ice crystals shift their position. Ice particles in high cirrus clouds occur all year round, but circumzenithal arcs are usually obscured by lower level clouds.
Rather than being caused by raindrops, it is the result of freak atmospheric conditions rarely seen outside the North and South Poles. While normal rainbows are formed when light penetrates raindrops and emerges on the other side without changing direction, this smile is formed when sunlight shines through millions of tiny ice crystals in cirrus and cirrus stratus clouds.
Because the crystals are flat and hexagonal, they invert the light and create an upside-down curve. The phenomenon relies on the sun being low in the sky, normally less than 32 degrees from the horizon. The arcs can appear at any time of the year, hovering in the sky only fleetingly because clouds tend to move quickly near the zenith.
Because the crystals are flat and hexagonal, they invert the light and create an upside-down curve. The phenomenon relies on the sun being low in the sky, normally less than 32 degrees from the horizon. The arcs can appear at any time of the year, hovering in the sky only fleetingly because clouds tend to move quickly near the zenith.
Circumzenithal arcs are so named as they go around the zenith - the point in the sky directly above the observer- rather than the sun.
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2 comments:
What a wondrous journey you have been on to answer our FAQ! The sun was indeed low in the sky on that beautiful evening. What a perfect smiley way to end the day.
Thankyou.
Jo x
Just wish we could have seen it too but you're very welcome.
Indeed, it was a pleasure of no small measure. It's not every day one gets the opportunity to seek enlightenment.
Thank you for your feedback!
:-) x
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