BEIJING, Jan. 13 -- Yellow alerts for fog in China's southern regions were issued on Monday by the country's central observatory, as continuous rainfall increased humidity.

From Monday night to Tuesday morning, heavy fog will reduce visibility to less than 500 meters in some areas of Anhui, Jiangxi, Hunan, Chongqing and Guizhou, according to the National Meteorological Center.

Smog will shroud middle and southern parts of north China, as well as the areas along the Yellow River and Huaihe River.

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