Beijing was blanked by heavy smog yesterday, prompting the authorities to issue a yellow alert for the first working day after the week-long National Day holiday. The capital's heavy air pollution response office said it expected the dense pollution to continue until Saturday, with little sign before then of the strong winds needed to disperse the pollutants. The statement said weather conditions would remain "still and stable" in the city and the capital would be affected by pollution coming from outside Beijing. The smog cut visibility in urban areas to two to three kilometres. Red is the highest pollution alert on the mainland, followed by orange, yellow and blue. Meteorologist Guo Jianxing told The Beijing News that the heavy smog would improve slightly tomorrow before worsening again on Saturday. Guo said a cold front was expected to arrive in Beijing after Saturday night and disperse the pollutants. Visibility would improve as well. The conditions were compounded yesterday when a thunderstorm hit the capital between 7am and 10am. The humidity prevented the pollutants from dissipating as morning rush hour got under way on Beijing's already congested roads for the first day back at work. Heavy smog was also choking Langfang , in neighbouring Hebei province, where the authorities started imposing emergency traffic restrictions yesterday that will continue until at least Saturday, according to The Mirror . Cars will be kept off roads on alternate days depending on the last digits of each vehicle's number plate. The authorities said an end to the restrictions would depend on air quality in Beijing. Many other cities were also battling pollutants. The National Meteorological Centre said Tianjin , central and southern Hebei province, northern Shandong province, central and southern Henan province and parts of Shanxi province registered moderately heavy smog. Some parts of Tianjin, Hebei and Henan had heavy smog. The China News Service quoted unidentified meteorologists as saying satellite images revealed numerous fires in rural Hebei and Henan. Smoke was visible in those areas and believed to have been caused by farmers burning off crop stubble. The report said smoke from the burn-offs was contributing to the region's smog, and winds in central and eastern China were not strong enough to blow the pollutants away. In Xian , Shaanxi province, environmental monitors said that by noon yesterday air in the city had reached heavily polluted levels, with an air quality index reading of 230. Visibility was low and many residents were wearing face masks. The reading was in sharp contrast to the gains Xian has made in air quality this year. The level of PM2.5 pollutants - particularly fine and hazardous particles - dropped nearly one-third in the first eight months compared with the same period last year.