Shanghai's air quality will return to normal slowly as fresh winds blow away pollution that has stifled the city, an official with the city's environment watchdog said yesterday.

The Shanghai Meteorological Bureau issued an orange fog alert at 6:50am Monday.

The city's air was still classified as "lightly polluted" yesterday but improved gradually in the afternoon as mist cleared.

However, the foggy weather grounded more than 70 flights at the city's two airports from Sunday night to yesterday morning as visibility dropped to less than 400 meters.

The Shanghai Meteorological Bureau issued an orange fog alert at 6:50am yesterday, the second highest of a three-stage system, when visibility over most areas of the city fell to 100 meters. The alert was lifted at 9:15am.

According to Lin Chenyuan, a forecaster with the Shanghai Environmental Monitoring Center, the air will be clear today and tomorrow, with fresh air blowing in from the sea, and will become good up to Thursday, when rainfall is expected.

Inhalable particulates are expected to decline to an index of about 60 today from yesterday's 161, while sulfur and nitrogen dioxide, the two other main measures of air quality, will drop by half.

"Light breezes and drizzle are expected to dispel air pollutants over the next few days, resulting in improved air conditions," said Lin.

Meanwhile, temperatures should rise this week, with mostly cloudy and drizzly conditions as a result of wet and warm air streams from the south, the city's meteorological bureau said yesterday.

Today will be sunny with cloud, with temperatures ranging from 13 to 19 degrees Celsius. Tomorrow will be cloudy and drizzly and the high should reach 20 degrees. The maximum temperature this week is expected to be 23 degrees on Thursday and Friday, the bureau said.

The city's air quality was classified as slightly polluted over the weekend and worsened rapidly. Pollution reached a peak on Sunday night, when the level of inhalable particulates rose to more than seven times normal, mainly because strong wind from the northwest brought polluted air from inland, said Lin.

The city also experienced high humidity on Sunday night, so that particles adhered to water molecules, making the sky look dusty, he said.

So far this year, the city has had 23 days of air pollution - including 20 lightly polluted days, one at "medium level" and two "seriously polluted" - five more than in the same period last year, said the center.