Two citizens do morning exercise in Xuhui District, Shanghai, on Jan. 23. [dfdaily.com]

Shanghai is experiencing another round of serious air pollution this week with the spread of cold and hazy weather, forecasters said.

Rain has departed from the city and the weather should turn clear and stable, mostly sunny and partly cloudy, in the next five days.

But pollutants that have come with the cold air from the north are worsening the air quality, according to the Shanghai Observatory.

The city's air quality index (AQI) reached 203 by 11pm yesterday, described as "heavily polluted."

That's the fifth-highest of the new, six-level air pollution scale, officials said.

High pollution levels could remain in the atmosphere for the next three to four days as a result of stable weather conditions, officials said.

The city's air quality was expected to improve by this morning, according to the observatory, though the AQI was rising last night.

"With the expansion of the cold ... the haze these days may reoccur in the next three to five days," said Man Liping, a chief service officer of the bureau.

Today is forecast to be partly cloudy with some light haze and the mercury should range from a low of 3 degrees Celsius to a high of 11 degrees, according to the observatory.

The city should get colder tomorrow, with a low of 1 degree Celsius and a high of 8 degrees, and sunny skies.

On Saturday, the low is forecast to be 2 degrees Celsius and the high 9 degrees, again with sunny skies.