A woman puts a mask on her dog in Beijing yesterday. Choking smog blanketed Beijing and much of northern China yesterday as the national meteorological center upgraded its alert level from yellow to orange, the second-most serious level. Smog will continue in most parts of North China today and tomorrow. China has a four-tier color-coded weather warning system, with red representing the most severe weather, followed by orange, yellow and blue. Light smog also hit Shenzhen yesterday, with the local weather authority issuing a yellow-level warning.China News Service