Australia Weather News

Perth has been blanketed with thick fog, prompting warnings for motorists to use caution in the conditions.

The fog could be seen as early as 4am local time on Thursday and persisted into peak hour, with visibility extremely limited along some roads.

Humidity reached 99 per cent at 1am Thursday, and was still at 93 per cent by 8am, triggering the foggy conditions.

The Bureau of Meteorology said fog may be present in parts of WA's South West Land Division, with the high humidity stretching from the northern Wheatbelt across to the edge of the Goldfields.

Main Roads has urged commuters to use caution on parts of the Tonkin Highway, the Narrows Bridge, and the Mitchell and Graham Farmer Freeways.

People texting in to 102.5 ABC Perth reported fog across the city including City Beach, Mt Helena, Malaga and Banksia Grove.

What causes fog?

Fog forms when air cools overnight to the point it can no longer hold all its moisture.

As temperatures drop, humidity reaches 100 per cent and excess moisture condenses into tiny droplets.

These droplets remain suspended in the air, creating mist and fog, most commonly during the night as the surface loses heat.

The Bureau of Meteorology said fog was most common in valleys and formed when skies were clear and there was no wind.

Chittering crash

A man believed to be in his 60s was taken to Royal Perth Hospital following a crash north of Perth.

Authorities say a car and a truck collided at around 7.15am near the Chittering Roadhouse on the Great Northern Highway.

The man was left trapped in the wreckage, with rescue crews using hydraulic equipment to free him.

Main Roads reported reduced visibility in the area of the crash.