A SPOOKY fog creeping slowly across Melbourne is a rare freak of nature, the weather bureau says.

The dense fog covered large sections of Melbourne and was affecting flights at Melbourne Airport.

Passengers are advised to check with their airlines or the Melbourne Airport website after a number of diversions and delays.

The fog was expected to cover much of the metropolitan area tonight, after visibility was reduced to 500m at Moorabbin Airport earlier this evening.



"So much fog, I can't even see if the tram is coming," one Twitter user wrote.



Another tweeted that the eerie sight looked "like Armageddon rolling in".



The low visibility was expected to cause traffic problems, but it is unclear whether air traffic will be affected.



Victoria Police has urged drivers to "please take care driving on the Westgate Freeway and Bolte Bridge areas".



"Slow down and use your fog lights as necessary," authorities said in a tweet.



Senior forecaster Dean Stewart said the dense blanket covering the city had drifted back in from Port Phillip Bay this afternoon on a light southerly wind.



He said the fog had originally drifted out to sea on very light winds, but the sun and gentle breezes had been unable to properly disperse it.



"Basically it's been over the bay for most of the day, and has just drifted back," he said.



"It's not unprecedented. It probably happens every three years or so, when you get this combination of conditions."



The bureau predicts the fog will be widespread this evening, but will disperse much more quickly tomorrow morning as a result of northerly winds.

For more pictures and local updates visit the Herald Sun.