It's an upside-down rainbow... but you can see there's no pot of gold at the end of this one

Seen just a handful of times this stunning phenomena is known as the 'smile in the sky.'

Freak weather conditions created this reverse rainbow in Leicestershire where it was pictured by antique dealer William Freeman.

But while the sight looks like an upside-down rainbow, it is actually caused by light shining through tiny ice crystals in the clouds.

Shimmering: The reverse rainbow is caused by tiny ice crystals in the clouds

Rather than being caused by raindrops like normal rainbows, it is the result of freak atmospheric conditions, rarely seen outside the North and South Poles.

This is one of only a handful of occasions where an arc has been spotted in the UK.

And only a small group in the garden of a restaurant saw the shimmering arc for a few minutes in the sky in the early evening on Friday.

Mr Freeman, 35, who owns an antique and fine art gallery, said: 'I had just finished dinner when I went outside and saw what appeared to be an upside-down rainbow.

'Everyone was stunned - I've never seen anything like it. It was a very bizarre sight on such a lovely sunny evening with no rain.

'I grabbed my phone and took a photo but within minutes it had completely disappeared.

'I feel very lucky to have seen this weather wonder and to have captured a picture of it.'

But unlike a rainbow, the sky has to be clear of rain and low level clouds for a circumzenithal arc 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 tiny ice crystals.

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.

The arc is generally only seen in the artic circle - and this is one of just a handful of instances where the arc has been spotted in the UK.

The 'rainbow'' is also 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.

The arc usually vanishes quickly because the cirrus clouds containing the ice crystals shift their position.

William said: 'It was literally only there for about eight minutes before it disappeared completely. 'Everyone in the garden was starring up in the sky just admiring it.

'Everyone in the pub was talking about it afterwards, no one had ever seen anything like it before, and I don't think I would have believed it if I hadn't seen it with my own eyes!'

Ice particles in high cirrus clouds occur all year round, but circumzenithal arcs are usually obscured by lower level clouds.

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.

'It is quite rare to see an arc as clearly as this in the UK. Visibility of these arcs can vary greatly with someone ten miles away not being able to see it.'

Source: DailyMail