12 January 2013
Temps in Beijing are running around -5C and visibility is less than 200 meters in places. There is a large high pressure system over the region that is causing a strong temperature inversion. This means the air aloft is warmer than the air at the surface so smoke and soot from factories and trucks gets trapped over the city. A blob of warm smoke from a factory smoke stack will begin to rise because it is warmer than the air around it, but it suddenly reaches a level where the air around it is actually warmer and therefore it sinks back down over the city.
The BBC has an update HERE.