A heavy fog that has blanketed Shanghai since late Jan. 7 has forced the cancellation of some flights.

Shanghai Meteorological Station issued the first fog warning at 9:46 p.m. on Jan. 7 and upgraded it to a color-coded "red" warning, the highest level at 1:41 a.m. on Jan. 8.

The fog reduced visibility in the city's Pudong and Hongqiao airports to 100 meters and 900 meters respectively. At Pudong, the worst hit area, more than 100 domestic and international flights were delayed and a few had been canceled since late night on Jan. 7.

China Civil Aviation Administration's (CAAC) regulations stipulated that an airport's standard visibility for plane's taking off and landing was 550 meters and 800 meters respectively.

It was not immediately known how many passengers were stranded at the Pudong International Airport last night. The latest information said the flights at the airport were starting to take off as the fog dispersed.

The heavy fog appeared after a warm air mass from the sea encountered cold air in Shanghai. The fog was expected to dissipate in the afternoon, according to the meteorological station.