Smoggy weather that forced dozens of flights to be canceled and delayed yesterday is forecast to continue into the weekend.

Shanghai woke up to chocking smog again yesterday with an orange heavy fog alert issued by the city's weather bureau.

The city's PM2.5 density soared to 468 micrograms per cubic meter by midnight, more than six times the nation's limit of 75. The highest density of 602.4 micrograms per cubic meter was recorded in Xuhui District around midnight.

The air quality index had reached 305 by midnight, or severely polluted, the highest level. The highest AQI at midnight was recorded in Qingpu District — 361.

The Shanghai Environmental Monitoring Center forecast that air quality would be bad again this morning with an AQI of over 305, but that it would lessen to heavily polluted in the afternoon.

Visibility yesterday was between 100 and 200 meters in most districts, causing problems for drivers and affecting almost 100 flights.

The orange alert, the second highest of a three-color warning system and the first this winter, was removed at 9:55am as visibility improved but was reinstated at 7:30pm.

The bureau issued a yellow haze alert at 1:40pm and upgraded it to orange at 5pm.

The bureau said the heavy fog was the result of increased humidity but it dispersed as temperatures rose after the sun came up.

Similar conditions are expected to continue throughout the weekend before possibly being alleviated from Monday with the arrival of a cold front.

At Pudong International Airport, 45 flights had been canceled or delayed by 11am and 50 flights due to land diverted to other airports.

Air traffic gradually returned to normal after 10am.

Hongqiao International Airport escaped the worst of the fog and only one flight was delayed. Thirteen flights diverted from Pudong landed at Hongqiao.

The city's main elevated roads were heavily congested yesterday morning, with vehicles moving extremely slowly.

But there were no reports of an increase in accidents as drivers took extra care.

The Yangtze Tunnel Bridge released vehicles intermittently during the morning due to poor visibility but gradually resumed normal operations after 10am. The Donghai Bridge was also severely congested during the morning.

Six highways — the G40, S32, G1501, G15, S4, S32 — were closed or partially closed yesterday morning due to the fog. Several highways in neighboring Jiangsu and Zhejiang provinces were also affected.

Downtown ferry services were hit, with 18 canceled, early yesterday although 17 had been restored by 10am. Traffic police opened the Fuxing Road, Dalian Road and Xiangyin Road tunnels to bicycles, mopeds and motorcycles while ferry services were suspended.

The Shanghai Port was closed yesterday morning, affecting more than 200 boats, when visibility fell to less than 100 meters in some areas.

More than 800 long-distance buses were canceled or delayed at the city's two main long-distance bus stations.

"Haze in Shanghai usually occurs in winter, when cold air is more common," said Wu Rui, a forecaster at the bureau. "After colder air pushes through the city, the reduction in air pressure and wind gusts will produce a stable atmosphere for the formation of haze."

Man Liping, another forecaster, said the consecutive dry days were also a factor. "Precipitation can wash the city's air and let particles settle," Man said.

A weak cold front will arrive today but only bring an increase in clouds and won't do much for the smog, forecasters said.

Temperatures will reach the mid teens at the weekend and bounce back up to 17 degrees Celsius on Sunday, while lows will range between 8 and 9 degrees.

On Monday, highs will drop to 12 degrees with a low of 6.