Optical phenomenon: Unusual upside-down rainbow

Half of an upside-down rainbow was seen in Norfolk recently, but do you know what it is?

Weather & Radar user Nathan spotted this optical phenomenon in the sky in Norwich, Norfolk, and although it looks like a rainbow, is actually a circumzenithal arc, part of the halo family.

The effect is the result of refraction of the sun’s light hitting ice crystals suspended in the atmosphere, rather than hitting raindrops, which creates the traditional rainbow. It only occurs when the sun is no higher than 32° above the horizon.

These arcs are not particularly rare, but can be easily missed due to their appearance high in the atmosphere.

In the image above, an upper tangent arc is also visible. These touch the halo at a point directly above the sun and form when rod-shaped hexagonal ice crystals have their long axis aligned horizontally.

If you spot any of these optical phenomenon, be sure to snap a photo and send it to us using our dedicated image uploader.