BEIJING, Nov. 2  -- Smog will gather in China's northern areas from Wednesday night, bringing poor visibility, the national observatory said.

The National Meteorological Center (NMC) issued a yellow alert at 6:00 p.m. for moderate smog in the Beijing-Tianjin-Hebei region in the next 24 hours, with some parts of Henan, Shanxi and Shaanxi provinces likely to be affected.

The situation is expected to worsen on Friday as there will be no wind to disperse the pollution.

Residents are advised to take protective measures, and those with respiratory diseases should reduce outdoor activity.

Beijing's Air Quality Index reached 150, indicating mild pollution, on Wednesday evening, with hazardous fine particulate matter PM2.5 as the major pollutant.

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