The crap earlier in the week was smog, not fog. Not a time of year BJ gets real fog. On a train down south on Monday, hazy and smoggy all the way through Hebei and Shandong. All the power plants and factories in these areas, when the wind is from the south, blows this crap up to Beijing. Then add Beijing's vehicle pollution to the mix. Which gets trapped against the mountains on the north and west. Expect more bad air as we head from spring into summer, and we get no more north/northwest winds.

The Chinese/Beijing gov't despise the US Embassy for putting this information out there. It used to be on the home page of the Embassy website in BJ, but they had to take it off and twitter it instead, where it is blockable to the (majority of) Chinese public.