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.
Low cloud over the Swan River early on Thursday morning. (Supplied: Anna Fantasia)
Humidity reached 99 per cent at 1am Thursday, and was still at 93 per cent by 8am, triggering the foggy conditions.
The Swan River was blanketed in fog on Thursday morning. (Supplied: Vicki Garrett)
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.
South Perth’s skyline is usually visible from Elizabeth Quay. (Supplied: Vicki Garrett)
People texting in to 102.5 ABC Perth reported fog across the city including City Beach, Mt Helena, Malaga and Banksia Grove.
The fog was widespread. (ABC News: Gian De Poloni)
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.
Fog seen at Meltham train station. (ABC News: Gian De Poloni)
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.
The fog set across suburban streets, pictured here in Dianella. (Supplied)