Sep. 17, 2008 - Smiling Rainbow
In the Nook...
This rare spectacle - which curves up rather than down - was caught on camera at the weekend by an astronomer near her home in Cambridge.
The "smiling" band of brilliant colours is known as a circumzenithal arc. It occurs when sunlight bounces off ice crystals high in the atmosphere, sending the light rays back up towards the sky.
It differs from a normal rainbow where light penetrates raindrops and re-emerges on the other side heading towards earth, causing a drooping effect.
Upside down rainbows are rarely seen outside the polar regions and need a particular set of atmospheric conditions in order to occur.
Dr. Mitton, a fellow of the Royal Astronomical Society who has written astronomy books for children, spotted the phenomenon near her home.
"I've never seen anything like it before - and I'm 60," she said. "The conditions have to be just right: you need the right sort of ice crystals and the sky has to be clear.
"It's quite surprising for this to occur somewhere like Cambridge, usually it is in places that are colder.
"We're not sure how big an area it was visible over, but it was certainly very impressive. It was just an amazing combination of factors that happened at the right time."
The colours in the inverted rainbow appear all the more intense because of the position of the sun, which was at its optimum point of 22º above the horizon.
Dr. Mitton's husband Simon, 62, a fellow astronomer with a degree in physics, said the curve was pointing towards the setting sun.
"The 'rainbow' is much brighter and more concentrated than a rainfall rainbow," he said.
The colours in the inverted curve run from red nearer the horizon, through yellow and green, to blue near the zenith.
A spokeswoman for the Met Office said upside down rainbows were an unusual sight in British skies.
She said: "It is convex to the sun and is formed by refraction in suitably-oriented ice crystals and may show vivid rainbow colouring, as this one does."
Smiling Rainbow
http://www.homeschoolblogger.com/nancysnook/591201/
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