China warns of widespread smog, Beijing issues second 'red alert'

China warned residents across a large part of its north on Dec. 18 to prepare for a wave of choking smog arriving over the weekend, the worst of which is expected over Beijing, prompting the capital to issue its second ever "red alert."

The National Meteorological Center said the smog would stretch from Xian, home to the world-famous Terracotta Warriors, across part of central China, through Beijing and up into Shenyang and Harbin in the frigid northeast. 

The air pollution would begin rolling in from about the evening of Dec. 19 and last until Dec. 22, with visibility in the worst affected areas such as Beijing likely to fall to less than 1 kilometer, it said. 

The pollution index would probably exceed 500 in Beijing and parts of Hebei province, which surrounds the capital, it said.  Residents are encouraged to remain indoors at levels higher than 300, according to government guidelines. 

In Beijing, a red alert means around half the vehicles are removed from the roads with an odd-even license plate system enforced. Schools are recommended to close and outdoor construction is banned. 

The Beijing city government issued its first "red alert" last week following criticism that previous bouts of smog had failed to trigger the highest warning level. 

Beijing's second red alert comes after a landmark climate agreement was reached in Paris earlier this month, setting a course to move away from a fossil fuel-driven economy within decades in a bid to arrest global warming. 

A red alert is triggered when the government believes air quality will surpass a level of 200 on an air quality index that measures various pollutants for at least three days. The U.S. government deems a level of more than 200 "very unhealthy...