Katarina Kovacevic, a meteorologist with Bureau of Meteorology, said Cirrus clouds, which hold the ice crystals, often appear ahead of fronts and were far from mystical.

"It's basically just a cloud high up in the atmosphere with ice crystals, that's pretty much all there is to it," Ms Kovacevic said.

"That's distinct from some clouds lower in the atmosphere with liquid water in them."

The sun halo is similar to a corona, which is a similar luminescent phenomenon surrounding a stellar object that is often seen at night.

Ms Kovacevic said despite the apparent similarity between the two phenomena, they tended to have different causes.

A corona is usually caused by light interacting with water droplets, and often appears around the moon.

Sydney was forecast to have a dry evening on Saturday, with temperatures of about 20 degrees.