November 9, 2009
Posted by Sam Jackson
So, I've felt pretty awful the last week, and the 'weather' -- or, more accurately, the pollution -- has been part of the problem. Twitter's @BeijingAir has been a constantly depressing reminder, nearly always showing "Hazardous" or at best, "Unhealthy" in the last week.
Why has it been so bad? Well, because of a specific weather pattern that has trapped pollution, one that is especially perilous in the winter when people start to burn coal to stay warm.
A picture's worth a thousand words here, thanks to NASA:
This photo represents most of Eastern China - Beijing is in the middle-northern part of the smog cloud. Here's an explanation from NASA's team on why this smog may have built up so thick:
Thick haze and fog settled over much of China on October 28, 2009. In this photo-like image captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite, the thickest of the gray-brown haze conforms to the low-lying contours of the Yellow River Valley and the western half of the North China Plain near the Luliang Mountains.
A temperature inversion may be responsible for the build up of pollution over eastern China. Normally, air cools with altitude, but occasionally, a layer of cool air will be trapped beneath a layer of warm air. Since the cool air is more dense than the air above it, the two layers don’t mix and pollutants build up in the cool air near Earth’s surface.
Temperature inversions develop most often during the winter, when long, cool nights chill the ground. The cold land cools the air nearest the ground, leaving the air at higher altitudes warmer. The two layers of air do not easily mix, and the temperature inversion can last for days if winds are calm.
Today the sky has finally shifted to actual clouds, instead of smog, and it's started to rain and let down some gross wet snow. Maybe that will help to clear things up...
By the way, using tools like NASA's WorldWind, which I have long been a fan of, you can track weather patterns like this in near-real time, and make your own animations using MODIS. Very cool stuff.