photo #4619196

No, it’s not an upside-down rainbow. This colourful smile in the sky is a circumzenithal arc. It is formed by the sunlight shining through ice crystals, rather than raindrops, shaped like tiny hexagonal plates, which must be making up these Cirrus clouds spotted by Jonathan Palombo over Burnley, Lancashire, England. The colours of circumzenithal arcs are actually brighter and purer than those seen in rainbows. And in many parts of the world, circumzenithal arcs actually appear more frequently than rainbows do. So why, you may wonder, do so few people ever seem to spot one?The reason must be to do with where in the sky a circumzenithal arc appears. Rather than forming within easy view in the lower part of the sky like a rainbow does, a circumzenithal arc appears directly overhead. The light effect is positioned like a segment of a circle around the zenith, the point directly up above your head. You have to choose actively to look straight up to have any chance of noticing one. How, then, will you know when to do so? The optical effect is formed by the same ice-crystal shapes and orientations as sun dogs, the bright spots of light that appear level with a low Sun off to one or other side of it. Next time you spot a sun dog level with the Sun near the horizon, take it as a sign that a circumzenithal arc might be appearing too. Look directly upwards and, if you’re lucky, you’ll spot a sky smile overhead that’s hiding in plain sight.Circumzenithal arc (with a faint supralateral arc intersecting it) forming in Cirrus fibratus spotted by Jonathan Palombo over Burnley, Lancashire, England.