BEIJING, Jan. 13 -- China's national observatory on Saturday morning renewed a yellow alert for thick fog in some parts of the country.

From Saturday morning, heavy fog is expected to affect parts of Tianjin, Hebei, Henan, Shandong, Jiangsu, Anhui and Hubei, according to the National Meteorological Center.

Visibility in some affected areas will be reduced to less than 500 meters, and in some instances, even below 50 meters, the center said.

Motorists are advised to control their speed to ensure safety, while precautionary measures should be taken at airports, expressways and ferry terminals to ensure traffic safety, according to the center.

China has a three-tier color-coded warning system for thick fog, with red being the most serious, followed by orange and yellow.