Yep, it's similar over here in Hong Kong too, though a bit more complicated and the directions are different. When you go up north of the Kowloon Peninsula into

Yep, it's similar over here in Hong Kong too, though a bit more complicated and the directions are different. When you go up north of the Kowloon Peninsula into the mountains, there's a lot of commuter towns built in valleys, where you can find surprisingly cheap housing. Now, the thing about valleys is, wind direction depends on time of day. In the daytime, the sides of the valley get hotter than the centre, so winds blow out of the valley, making the air nice and clean --- but no one's home to enjoy it because they're all off at their jobs in other districts. And at nighttime when the sides of the valley cool faster than the centre, winds blow down into the valley. Just in time for the people coming home to enjoy everyone else's pollution, PLUS whatever they generate locally. Car exhaust is one obvious source, but cooking exhaust is a surprisingly big problem too --- you'd never think of it until you live in a place full of 50-story apartments, closely packed together, where everyone comes home at about the same time and cooks up a stir-fry for dinner. Conversely, the most expensive houses (aside from the ones up on hilltops with nice views) are generally on the south of the main island. You've got mountains to your north and the ocean to your south. So in the summer, when prevailing winds come from the south, you get a clean ocean breeze. In the winter, when prevailing winds come from the north, you get all of mainland China's factory pollution plus all of Hong Kong's car and power plant exhaust blowing in your direction --- but you're protected by the mountains, so most of it goes around you and blows out to sea.