A dazzling arc of psychedelic colour reminiscent of the Cheshire Cat's grin in Lewis Carroll's Alice in Wonderland.
But this is no fantasy or trick of the light, it is known as a
circumzenithal arc. Seen here shimmering in the sky over Cambridge in the afternoon sunshine, it is often mistaken for a rainbow hanging upside down.
Rare: An astronomer caught this unusual upside-down rainbow on camera near her home in Cambridge
Rare: An astronomer caught this unusual upside-down rainbow on camera near her home in Cambridge
But unlike a rainbow, the sky has to be clear of rain and low level clouds for it to be seen.
Relatively rare in Britain, the arc only appears when sunlight shines at a specific angle through a thin veil of wispy clouds at a height of around 20,000 to 25,000 feet.
At this altitude the cirrus clouds are made of ice crystals, the size of grains of salt.
Meteorologists say the clouds must be convex to the sun with the ice particles lined up together in the right direction to refract the light.
This results in the sunlight bouncing off the ice crystals high in the atmosphere, sending the light rays back up and bending the sunlight like a glass prism into a spectrum of colour.
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.
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.
