BEIJING, Dec. 28 (Xinhua) -- China's national observatory on Thursday issued a yellow alert for fog in some northern and eastern regions.

On Friday morning, thick fog will be seen in the Beijing-Tianjin-Hebei region as well as provinces of Anhui, Hubei, Jiangsu and Shandong, reducing visibility of some areas to less than 200 meters, the National Meteorological Center said.

The center said drivers in affected regions should slow down to safe speeds, 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.