Perth Airport Chief Executive Officer Brad Geatches said the fog visibility was down to about 300m when the sun rose around 7am.
He said numerous flights had been disrupted but there was nothing the airport could do about it until the fog lifted.
"It doesn't matter what sort of airport infrastructure you have when visibility is down to 300 metres nothing happens,'' Mr Geatches said.
Mr Geatches said when the flog lifts things "will get back to normal fairly quickly'' but it could take three to four hours to clear backlogged flights.
