Weather

Weather and the lay of the land (for example, hills around a valley, high mountains between a big industrial city and suburban or rural areas) help determine where ground-level ozone goes and how bad it gets. When temperature inversions occur (warm air stays trapped near the ground by a layer of cooler air) and winds are calm, high concentrations of groundlevel ozone may persist for days at a time. As traffic and other sources add more ozone-forming pollutants to the air, the ground-level ozone gets worse.