Tuesday, September 16, 2008

Upside down Rainbow

The sky is smiling @ Cambridge Evening News

EXCEPTIONAL atmospheric conditions created a rare and stunning display in the skies above Cambridge. At 4.45pm on Sunday, a circumzenithal arc - which looks like a bright, upside down rainbow - was visible above the city.

Cambridge-based astronomer Jacqueline Mitton captured the stunning sight, caused by sunlight being refracted through ice crystals high in the atmosphere, with her camera.

The phenomenon is rarely seen outside the polar regions.

She said: "I've never seen anything like it before - and I'm 60." "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."

The intensity of the colours in the rainbow was heightened by the sun being at the optimum spot in the sky - 22 degrees. And the sky was made even more dazzling with the presence of "sun dogs" - gleaming spots on a halo around the sun - appearing near the phenomenon in the afternoon sky.

Dr Mitton said: "It was just an amazing combination of factors that happened at the right time."

Jacqueline's husband Simon, an astronomy writer, said: "The circumzenithal arc is a quarter circle, pointing toward the setting sun." "The 'rainbow' is much brighter and more concentrated than a rainfall rainbow."

A Met Office spokeswoman said: "They are fairly rare." "It is convex to the sun and is formed by refraction in suitably-oriented ice crystals and may show vivid rainbow colouring, as in this case."