The top image shows normal conditions when warm air rises from the surface, carrying pollutants from industry and homes up into the atmosphere where the air temperature is cooler. The bottom image shows thermal inversion, or when cool air and pollutants from the surface are inhibited from rising by a layer of warm air.

Credit: EPA

Rising, warm air often helps disperse pollution from near the surface (top), but in a temperature inversion the warm air cannot rise, trapping pollution at the surface (bottom).