The Harbour Bridge was staying warm under a blanket on Thursday morning, as Sydneysiders woke up to fog across the city.

Photographs showed the city covered in the low fog, which was more dense in the western suburbs.

The fog is expected to hang around until just after 9am, with some remnants in the west.

There are three factors needed for morning fog, Bureau of Meteorology forecaster Olenka Duma said: moisture in the air, light winds and clear skies.