BEIJING, Dec. 26 -- The smog plaguing much of north China is expected to disperse on Saturday afternoon thanks to a cold front, the weather observatory forecast.

Cold air will arrive in Beijing and its neighboring provinces of Hebei and Shandong along with Tianjin Municipality on Saturday afternoon and help clear the smog, according to the National Meteorological Center (NMC).

Beijing issued a blue alert for air pollution on Thursday, but upgraded it to yellow on Friday. China has a four-tier warning system, with red as the most severe, followed by orange, yellow and blue.

The NMC said unfavorable weather and pollutant emissions had led to poor air quality in Beijing since Thursday.

The general air quality index readings in Beijing hit 500 by Friday noon. PM2.5 stood at an average of 477 micrograms per cubic meter as of 8 p.m., data from Beijing Municipal Environmental Monitoring Center showed.

Beijing educational authorities issued an urgent notice early Friday morning, calling for kindergartens, primary schools and middle schools to restrict outdoor activities.

Over 200 flights at Beijing's Capital International Airport were canceled due to poor visibility.