BEIJING -?Smog continued on Wednesday in many parts of China, delaying flights and closing highways.

Smog continues in north China

More than 3,000 passengers were stranded early Wednesday at Tianhe Airport in Wuhan, capital of the central Hubei province due to heavy fog.

Visibility was less than 150 meters at the airport and 30 flights were delayed, said Tianhe Airport sources. The airport resumed normal operations shortly after 10 a.m. as the fog began to disperse.

In the eastern province of Shandong, more than 120 expressway toll stations were temporarily shut down on Wednesday and ten flights at the airports in Qingdao and Jinan were delayed. The provincial meteorological station issued a yellow alert for the smog, which is forecast to continue until Thursday night.

Smog set in on Monday and persists in north and central China, including Beijing. The National Meteorological Center (NMC) issued a yellow alert on Wednesday morning.

Beijing and Tianjin, as well as parts of Hebei and Shandong provinces expect "moderate" air pollution from Wednesday to Thursday morning, heavy in some places, according to the NMC forecast.

China's meteorological alerts are categorized as blue, yellow, orange and red as the severity ascends.