Anyone who looked at the sky yesterday was sure to see a peculiar sight as a large halo of prism-like coloured lights surrounded the sun.

Halos are an optical phenomenon similar in concept to a rainbow but also very different.

They form when sunlight or moonlight is refracted or bent by ice crystals associated with thin-high level clouds, like cirrus or cirrostratus.

The most common type of halo is the 22-degree halo. Hexagonal (six-sided) ice crystals project a ring of light 22 degrees from the sun or moon. As light passes through the crystal, it is bent or refracted twice and the two refractions bend the light by 22 degrees from its original direction, producing a ring of light.

A halo is often an indicator of cloudy or rainy weather as high-level cirrus and cirrostratus clouds that cause halos tend to drift ahead of frontal systems that produce rainfall.

If a wind stirs from the southwest, the sky becomes hazy, a halo forms around the sun which is then dimmed and finally obscured by increasingly thick cloud then rain is likely within a few hours.

However, to see one is not a reliable sign of bad weather and they are visible all over the world and throughout the year. Keep an eye out whenever the sky is wisped with thin cirrus clouds which have ice crystals in even the hottest climates.

The halo is always the same diameter regardless of its position in the sky and sometimes only parts of the complete circle are visible.