Rare upside-down rainbow spotted in UK

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.
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. Renowned astronomer and writer Dr Jacqueline Mitton was lucky enough to capture the optical phenomenon on camera near her home in Cambridge last Sunday. The 60-year-old who has a doctorate in astrophysics from Cambridge University 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.’  
According to Dr Mitton, the colours in the rainbow were intensified by the position of the sun, which was at the optimum spot in the sky of 22 degrees. The vision was made even more dazzling by the presence of “sun dogs� - gleaming spots on a halo around the sun. Dr Mitton added: ‘It was just an amazing combination of factors that happened at the right time.’