BEIJING, March 14 (Xinhua) -- China's national observatory issued a yellow alert Wednesday as fog lingered in central, eastern and southern China.

Heavy fog will affect parts of Henan, Shandong, Anhui, Hubei, Hunan, Guizhou and Sichuan provinces from Wednesday evening to Thursday morning, according to the National Meteorological Center.

In some areas of Henan, Shandong, Hubei, Hunan and Sichuan, visibility will be reduced to less than 200 meters.

The center said drivers should slow down to safe speeds, while appropriate safety measures should be taken at airports, freeways and ferry terminals.

In the meantime, the center issued a blue alert for a cold wave, which is expected to hit north China, and the northwestern and northeastern parts of the country between Wednesday night and Friday, bringing down temperatures.

The cold wave is expected to disperse the heavy air pollution that has been lingering in Beijing-Tianjin-Hebei region and nearby areas since last Friday, the Ministry of Environmental Protection said Wednesday.

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