An unusual weather effect greeted Sydneysiders on Saturday when a seemingly biblical ring wrapped around the sun.

The phenomenon, called a 'sun halo', is fairly common, but has appeared less frequently than usual over summer due to the dry weather.

A sun halo appears in the sky over western Sydney on Saturday. Credit:Nathan Emery

It occurs when light interacts with ice crystals suspended in clouds high up in the Earth's atmosphere.

Those crystals bend the light, appearing to give the sun a halo.