BEIJING, Oct. 26 (Xinhua) -- The heavy fog blanketing the Chinese capital Beijing is expected to linger overnight, but it may clear on Saturday morning with the help of rainfall, the Beijing Meteorological Observatory told Xinhua.

"The fog will linger until tomorrow morning, as the humidity remains high and the wind is not strong enough to disperse the fog during the night," said Guo Jinlan, chief weatherman with the municipal observatory.

"There will be rainfall early on Saturday morning, which will help to clear the fog tomorrow morning," Guo said.

The municipal observatory increased the fog warning level from yellow to orange at 5:20 a.m. on Friday as the fog grew thicker. The highest fog warning level is red.

Thousands of passengers were left stranded and a demonstration flight of the new Airbus A380 superjumbo was among dozens of flights delayed or cancelled when thick fog blanketed Beijing's main airport.

Beijing Capital International Airport delayed more than 30 flights and canceled other 40 this morning as the mist cut visibility at the airport to a few hundred meters.

However, flights gradually resumed after 9:00 a.m. as visibility steadily increased.

The visibility in the downtown area was less than 200 meters this morning but by 8 p.m., the visibility in the downtown increased to 1,000 meters, with the eastern and southern parts more seriously affected, according to the municipal observatory.

There were no serious traffic jams in the morning and evening peak hours as residents had been informed of the bad weather and the municipal traffic management bureau took measures to clear traffic, the information office of the bureau told Xinhua.

"Usually, it takes me 50 minutes during the morning rush to drive to office. This morning, the traffic was a little slower and it took me an hour. During the evening's peak hours, the traffic is pretty like normal," said Liu Jia, a lady who lives near the city's eastern third ring road and drives to work on weekdays to the city's southern fifth ring road.

There were no casualties caused by traffic accidents due to the fog in Beijing on Friday, according to the office.

Dense fog was also reported in the city's neighboring Hebei Province and Tianjin Municipality and the bureau closed major highways linking them to Beijing and the capital's eastern sixth ring road for safety reasons, according to the office. They will remain closed until the fog clears.

The thick haze exacerbated the city's pollution as the municipal observatory predicted that the air pollution index from 20:00 p.m. Friday to 20:00 p.m. Saturday would stand between 120 to 150, meaning the air is slightly polluted.

Experts warned the elderly and children to stay indoors to avoid contracting respiratory diseases.