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Rare upside-down rainbow spotted in UK
Published: September 19, 2008- Digg
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Her husband Simon, 62, 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.’ Rainbows are formed when sunlight is refracted in a raindrop.
But in a circumzenithal arc, the colours are in reverse order from a rainbow, with violet on the top and red at the bottom.
" Daily Mail
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.
Circumzenithal arcs are so named as they go around the zenith - the point in the sky directly above the observer- rather than the sun.
A spokesman for the Met Office said: “Circumzenithal arcs are seen relatively rarely in Britain because they can only be seen at the right combination of atmospheric conditions.
‘The height, depth and position of the ice clouds must be right as the cloud needs to be at a specific angle convex to the sun.
‘Circumzenithal arcs actually occur quite commonly as these types of clouds occur throughout the year.
‘But the vision is usually obscured by clouds underneath which means circumzenithal arcs are much rarer than rainbows and halos
‘It is quite rare to see one as vivid as this. It depends on the position of the observer because its visibility can vary greatly, someone ten miles away would probably not be able to see it.’
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