BEIJING - The Beijing municipal weather bureau said zonal circulation, relatively warm temperatures and high humidity have contributed to the smog that has blanketed the city for the past week.

Zhang Yingjuan, a meteorologist, said the average temperature in Beijing from mid-December to Thursday was minus 2.4 degrees Celsius, 1.6 degrees higher than the same period of an average year.

She said there were also nine foggy days during the period, substantially more than the average of 0.9 days in reference data.

Head of the weather bureau Qiao Lin said zonal circulation helped to keep and increase fog and smog by blocking cold air from entering the city.

Qiao said the static atmosphere prevented pollutants from being dispersed vertically and reduced visibility.

High humidity also provided conditions for particulate matter to absorb moisture and swell, he added.

Beijing is surrounded on three sides by mountains, which keeps cold fronts from entering the city from the west and north, while the south, where most outside pollutants come from, is left vulnerable.

The bureau warned the smog will continue till the weekend and be dispersed by cold air starting Sunday.