File:Sichuan Haze.jpg
Haze in this region tends to worsen in the winter, when cold, heavy air traps pollutants near the surface. In this case, the haze was likely trapped in the Sichuan Basin by a temperature inversion. Normally, air is warmest near the surface of the Earth. Occasionally, a mass of warm air will move over cooler air so that the atmosphere actually warms with altitude. Since the cool air does not have the energy to rise through the warm air, vertical circulation slows and air becomes trapped near the surface.
Any pollutant that enters the air gets trapped as well, and haze builds up over time. Temperature inversions often form in basins and valleys because surrounding higher-elevation land helps contain cold air. Daily images of the Sichuan Basin show that that the pollution has persisted for several days.