During daylight hours, the sun warms the ground which warms the air near the Earth's surface. The warm air rises as more dense cool air displaces it. Any pollution in the air normally disperses in the turbulence caused by the mixing of the air. When a layer of cool air traps the layer of warm air near the Earth's surface, then the situation is known as a temperature inversion. In this case, pollutants are not dispersed but continue to build up in the still, stagnant air.

Temperature Inversions

Subsidence Inversion