Commuters and plane passengers faced disruptions this morning as fog and a mechanical problem caused flight delays, train cancellations and a road traffic alert.
The fog delay some early morning departures and arrivals from Perth Airport.
Driving conditions were hazardous.
Weather Bureau spokesman Neil Bennett said Perth experienced fog about 14 times a year but an event as widespread as this happened only about once a year.
A maintenance mishap caused the cancellation of trains between the Esplanade station and Cockburn.
Up to 15 inbound flights were put into holding patterns and three flights were diverted to Kalgoorlie because aircraft were short of fuel.
Qantas flight QF593 from Adelaide was diverted to Kalgoorlie, along with Virgin Australia flight VA 9491 from Ravensthorpe and Qantaslink 1821 from Newman.
Perth Airport only has a CAT 1 instrument landing system which significantly restricts operations in fog.
Under CAT 1, pilots must be able to see the runway from a height of 61m with forward visibility of 800m.
A new system to be installed reduces that visibility height to just 15m and forward visibility to only 75m, almost eliminating diversions.
Perth Airport has said it would install that system but no time line has been given.
Weather Bureau senior forecaster Adam Conroy said yesterday’s cold front brought about the fog, which extends over much of WA’s South West.
“It is through the South West of the State as far south as Albany and east to Esperance,” he said.
“We had a cold front move through yesterday, which caused rain and a left a fair bit of moisture.
“Overnight the sky cleared and winds were light and that caused fog to fall.”
Mr Conroy said fog was particularly thick, causing less than 50 per cent visibility in parts of Perth.
The bureau expects the air to clear by about 9am.
Trains are now running between Perth and Cockburn after they were cancelled this morning due to a blockage on the tracks.
Perth Transport Authority spokesman David Hynes said a rail grinding machine broke down on the southbound line, north of Cockburn.
Trains between Mandurah and Perth were not affected and normal services resumed at 7.45am.
In a separate incident, a car crashed through the railings near Canning Bridge on Kwinana Freeway.
Emergency Services were alerted to the crash at 6.25am, which occurred on northbound lanes of the freeway.