A thick blanket of fog over Perth affected domestic and international flights at the airport earlier today.
The fog descended on the city early last night and extended to the Wheatbelt and parts of the South West.
Perth Airport said 45 flights were affected, including 19 international flights and 26 domestic.
Most of those were delays.
A spokeswoman said some incoming flights were forced to circle the airport until they could land, while others were diverted.
She said operations returned to normal about 4:30am (AWST), however, there would be delays as the backlog is cleared.
Qantas said two flights from Melbourne and Brisbane to Perth had been diverted to Adelaide.
It is understood a Cathay Pacific flight was diverted to Alice Springs and a Singapore Airlines plane diverted to Adelaide.
Perth Airport has advised passengers to check its website for updated information.
© ABC 2014
15:08 EST Alice Springs is currently experiencing its coldest August to date in 25 years, and the fourth coldest on record.