Trick of light: What is a circumzenithal arc?

An optical phenomenon known as a circumzenithal arc is often sent in to us, do you know what they are?

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.

This type of halo is also called a circumzenithal arc. However, it only occurs when the sun is no higher than 32° above the horizon. Circumzenithal arcs are not rainbows, because rainbows are caused by water droplets.

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

A very similar phenomenon are circumhorizontal arcs, where a coloured band of light runs parallel to the horizon.

If you spot any of the more than 50 types of arcs, or a traditional rainbow, be sure to snap a photo and send it to us using our dedicated image uploader.