BEIJING - China's meteorological authority upgraded its smog alert from yellow to orange, the second most serious level, on Sunday evening, as smog worsened in many parts of the country.

Parts of Beijng, Tianjin, Hebei, Henan, Shanxi and Shaanxi will be covered with medium-levels of smog for 24 hours from 8 pm Sunday, while some regions will experience heavy pollution with visibility less than 500 meters, said the National Meteorological Center (NMC).

The NMC issued a yellow alert for smog Sunday morning, expecting a cold front to help disperse the smog Monday night.

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

When the orange alert is in force, outdoor activities in schools will be canceled and construction projects suspended.

Despite government efforts to fight pollution, frequent outbreaks of smog have become increasingly common in winter in northern China where cold weather conditions and the burning of coal for heating combine to exacerbate the situation.

In a five-year plan on environmental improvements released December 5, the State Council ordered strict controls on the consumption of coal, a major source of pollution in China.

Beijing, Tianjin, Hebei, Shandong, the Pearl River and Yangtze River delta regions, and the 10 cities with the worst air quality must all see a reduction in coal consumption, according to the plan.