A citizen rides a bike in the heavy smog in Taizhou city, East China's Zhejiang province on Feb 24, 2013. (File photo: Xinhua)

BEIJING, Nov. 25 (Xinhua) -- China's national observatory on Sunday issued an orange alert for thick fog in some northern, eastern and southwestern regions.

From Sunday night to Monday, thick fog will be seen in the provinces of Jiangsu, Anhui, Shandong, Hebei, and Sichuan, reducing visibility in some areas to less than 50 meters, the National Meteorological Center said.

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