In the troposphere, the bottom layer of the atmosphere, low-lying air warmed by the ground is usually at a higher temperature than the air above it. When the warm air rises and the cooler upper air descends, the lower atmosphere naturally mixes, a process that can disperse pollution concentrated near the ground. Sometimes, though, air next to the ground cools faster than the air above it. Warmer, lighter air then lies above cooler, heavier air and the freshening effects of mixing are suppressed. Meteorologists call that phenomenon a surface-based temperature inversion.
Early warning of such an event can be extremely beneficial to areas prone to strong or persistent inversions. At the county health department where I work, in Pittsburgh, Pennsylvania, advance notice of an inversion allows us—pollution inspectors, engineers, health professionals, emergency responders, and others—to better prepare for deterioration of air quality and its potential effects: malodors, asthma attacks, increased...
