What Is a Temperature Inversion?
Normally, air temperature decreases with altitude -- warm air near the ground rises, carrying pollutants upward where they disperse. A temperature inversion reverses this: a layer of warm air sits above cooler air at the surface, acting like a lid that traps everything below. This is especially common in valleys and basins during winter.
An inversion acts like an invisible lid over the valley, preventing pollutants from rising and dispersing.