Thick haze and fog settled over much of China on 28 October, 2009. The thickest of the grey-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
Photograph: Aqua/MODIS/NASA
The Guardian
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