BEIJING, Jan. 23 (Xinhua) -- China's national observatory on Wednesday issued a yellow alert for thick fog that will hit various parts of the country.

From Wednesday night to Thursday morning, thick fog will be seen in the cities of Tianjin and Shanghai, northern parts of Xinjiang Uygur Autonomous Region, as well as the provinces of Hebei, Shandong, Henan, Anhui, Jiangsu, Zhejiang, Hubei, Hunan, Jiangxi and Guizhou, reducing visibility in some areas to less than 500 meters, the National Meteorological Center said.

In some of these regions, visibility may be reduced to less than 200 meters.

The center said drivers in affected regions should slow down, while airports, freeways and 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.