BEIJING, Dec. 25 -- Beijing has issued a yellow alert for heavy pollution on Friday morning, according to local authorities.

A blue alert was issued Thursday and that night the air quality hit grade six, the highest of a six-grade system.

The smog worsened in Beijing on Friday and Beijing air pollution emergency management headquarters upgraded its alert from blue to yellow.

Beijing Education Committee issued an urgent notice calling for kindergartens, primary schools and middle schools to ban outdoor activities on Friday.

The heavy smog led to over 200 flights delayed by Friday noon, according to the official microblog of Beijing Capital International Airport.

PM2.5 stood at an average of 180 micrograms per cubic meter in Beijing as of 2:30 p.m Friday, but hit 360 micrograms per cubic meter in the southwest part of the city, data from Beijing Municipal Environmental Monitoring Center showed (BMEMC).

A cold front will disperse the smog on Saturday, and the air quality will drop to grade two on Sunday, according to BMEMC.

China has a four-tier warning system for weather condition, with red as the most severe, followed by orange, yellow and blue.