BEIJING, Dec. 2 -- China's top observatory on Monday issued yellow alerts for fog in most southern regions and heavy smog in some northern and eastern areas.

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

From Monday night to Tuesday, heavy fog will reduce visibility to less than 1,000 meters in the provinces of Sichuan, Guizhou, Hunan, Yunnan, Jiangsu and Fujian, as well as the municipality of Shanghai, the National Meteorological Center said in a statement.

During the same period, moderate smog will shroud some areas in eastern and northern China, while Shanghai, central and southern parts of Jiangsu, and the central and southern parts of Hebei will be hit by heavy smog.