Four Reasons Why Shanghai Just Had Its Worst Smog Day Of All Time

A reader sent us the above before-and-after photo from his office in Shanghai. The clear picture on the right is from mid-October. The photo on the left is from today.

According to various measures, the city's smog levels hit record highs.

What makes it get so bad?

The reader who sent us in the photo describes a combination of factors:

- Pressure And Wind: If there's no wind, and the pressure system is right, the smog can just hang out for a long time without going anywhere.
- The Cold: When the temperature collapses, a lot more coal is burned. More coal = more smog.
- Cars: The number of cars on the road in Shanghai keeps growing, and this obviously creates ongoing upward pressure on smog readings.
- Christmas Season: Factories are cranking overtime right now to meet demand for Christmas products.

Put 'em all together, and voila, the worst smog day in Shanghai history.