Sky watchers were treated to a rare sight on Saturday evening as an 'upside down rainbow' was spotted over Somerset.

The circumzenithal arc, sometimes known as an upside down rainbow, was photographed from Wills Road, Bridgwater at about 8.40pm on Saturday evening.

The optical phenomenon is similar in appearance to a rainbow, but it belongs to the family of halos arising from refraction of sunlight through ice crystals, generally in cirrus or cirrostratus clouds, rather than from raindrops.

It forms no more than one-quarter of a circle centered on the zenith and on the same side of the sky as the sun. Its colors are from blue on the inside to red on the outside of the arc.

It is one of the brightest and most colorful halos. Its colors are purer than those of the rainbow because there is much less color overlap in its formation.

The circumzenithal arc has been called "a smile in the sky", its first impression being that of an upside-down rainbow. It is rarely noticed because it occurs so far overhead.

Wikipedia