China's top observatory on Sunday renewed a yellow alert for thick fog, which forced the temporary closure of expressways in large areas of northern, central and eastern China.

In the worst-affected areas, visibility was reduced to less than 50 meters.

On Sunday, thick fog affected Tianjin, as well as some parts of Hebei, Henan, Anhui, Hubei, Shandong and Jiangsu provinces, reducing visibility in some areas to less than 200 meters, the National Meteorological Center said on its website.

The center warned that drivers in affected regions should slow down, while airports, freeways and shipping ports should take appropriate safety measures.

China has a four-tier, color-coded warning system for severe weather, with red being the most serious, followed by orange, yellow and blue.

With visibility reduced to less than 200 meters in more than 10 cities in Shandong, dozens of toll stations on more than 40 highways were either closed or forced to restrict the number of vehicles on Sunday morning.

Shandong's meteorological bureau upgraded the alert for heavy fog from yellow to orange.

Apart from road closures in Tianjin, Binhai International Airport was also affected, with eight flights delayed and five canceled in the morning.

According to national weather authorities, plains areas and regions along the Yellow, Huaihe and Hanjiang rivers also experienced light to moderate smog on Sunday.

Starting on Monday night, cold fronts will move slowly southward. The fog and smog in affected areas is expected to ease gradually when the cold front arrives, bringing better weather conditions to disperse pollutants, the center said.

The cold air will also bring temperature falls of 6 C to 10 C to some parts in northern, central and eastern China. While Northeast China will experience daily highs of -10 C from Tuesday, the daily high temperature in the Beijing-Tianjin-Hebei province cluster will dip below 0 C, according to forecasts.