Thick fog shrouds Tian'anmen Square in Beijing February 21, 2007. A dense fog covering Beijing cut visibility to a few dozen metres and paralysed flights in and out of the national capital on Wednesday. [Xinhua]

Firecrackers during the Chinese New Year may have affected the weather but were not the cause of Wednesday's thick fog in Beijing, said Director of the Beijing Meteorological Observatory Guo Hu on Thursday.

Guo made the remarks in response to local media reports, which pointed the blame for Wednesday's fog that canceled flights and closed down highways at exploding firecrackers.

"We had actually forecast a haze in Beijing for Wednesday, although it developed into a fog due to an increase in humidity after being affected by fog from Shandong Province and vapor from the Bohai Sea," Guo said.

It had been windless in Beijing since Tuesday and particles in the air had been accumulating, resulting in Wednesday's thick fog, explained weather forecasters with the Beijing Meteorological Observatory.

The Capital International Airport was forced to cancel 190 flights, while sections of seven Beijing and 10 Tianjin highways were also forced to close temporarily.

It finally became clear on Thursday after the fog dissipated with strong winds.

Fireworks are part of traditional celebrations marking Spring Festival, the most important holiday for Chinese, which fell on Feb. 18 this year.

Since Beijing lifted a 12-year ban for last year's Spring Festival, fireworks have made a loud return to the Chinese capital.

The sales of fireworks by two major suppliers have reached 380,000 boxes in Beijing this year, compared with 240,000 boxes last year, according to a municipal public security official.