Shanghai’s air quality improved yesterday afternoon following a wave of moderate pollution, while today’s temperatures will decide whether winter has officially arrived.

The city’s air was moderately polluted by PM2.5 particles blown in from northern China and the air quality index hit 199 at 9am, almost reaching the heavily polluted level of 200, according to the Shanghai Environmental Monitoring Center.

But the index had fallen to 94 as by 5pm.

Air quality is forecast to be good to excellent today, and that will continue tomorrow and Saturday, the center said.

Today should be cloudy to overcast, with temperatures between 13 and 6 degrees Celsius and stronger winds, the Shanghai Meteorological Bureau said.

Winter becomes official when the average temperature is below 10 degrees for five consecutive days after beginning of the lidong solar term, or the start of winter according to the traditional Chinese calendar.

As of yesterday, four days had met the standard so today’s temperature will decide if winter has formally arrived.

A weak cold front is on the way and tomorrow’s high will plummet to below 10 degrees, according to the bureau, and the weather should be overcast and cloudy.

Temperatures will rebound over the weekend, reaching highs of 11 to 15 degrees and lows between 5 and 7 degrees, forecasters said. The weekend should be sunny to cloudy.