Temperature Inversion

Detailed Explanation

A temperature inversion occurs when the normal atmospheric temperature pattern is reversed — instead of air getting cooler with altitude, a layer of warm air sits above cooler air near the ground. This warm layer acts as a lid, trapping pollutants close to the surface where people breathe. Under normal conditions, warm air near the ground rises and carries pollutants upward, where they disperse. During an inversion, this vertical mixing is suppressed, and pollutants accumulate to dangerous concentrations. Inversions are particularly common in valley cities, where surrounding terrain limits horizontal air movement as well. Salt Lake City, Denver, Los Angeles, Phoenix, and cities in California's Central Valley regularly experience inversions, especially in winter when cold air pools in valleys overnight. These events can cause AQI to spike from "Good" to "Unhealthy" or worse within hours. AirHistory's data shows that cities prone to inversions often have higher variability in their annual AQI readings and more unhealthy air days than their average AQI alone would suggest. Inversions also trap wildfire smoke, compounding the effects of both phenomena. Understanding inversion patterns is critical for interpreting why some geographically sheltered cities consistently receive lower Air Quality Grades despite relatively low emission levels.