Photo taken on Jan. 21 shows a general view of fog-shrouded Shanghai. [News365.com.cn]

Dense fog should not hang over the city again like it did yesterday, when it helped spoil Shanghai's air quality, authorities said.

The sky should remain overcast and drizzly today while the mercury is expected to drop by a few degrees from Monday with a low of 4 degrees Celsius and a high of 6, according to the Shanghai Observatory.

The weather is forecast to be partly cloudy tomorrow, with a low of 2 degrees Celsius and a high of 8. Thursday is expected to be sunny with a low of 3 degrees Celsius and a high of 10.

Dense fog shrouded the city yesterday morning with light winds and high humidity accompanying a trough of low pressure, explained Fu Yi, a chief service officer of the observatory.

"It's not possible for such weather to come back tomorrow," Fu said.

The fog also helped spoil the excellent air quality carried on sea breezes on Sunday, environmental authorities said.

The city's PM2.5 rose sharply from around 40 micrograms per cubic meter at 5am, to a peak of 263 micrograms per cubic meter at 9am, due to the accumulation of local pollutants, according to Shanghai Environmental Monitoring Center, which issued a warning to people with heart diseases and respiratory syndromes at 10:47am.

The hourly density of PM2.5 - airborne particles smaller than 2.5 micrometers in diameter - dropped to 130 micrograms per cubic meter during the afternoon thanks to the winds. China's acceptable daily limit for PM2.5 is 75 micrograms per cubic meter.