An inversion is a situation in which the layers of the atmosphere do not act normally, inhibiting normal weather processes and often trapping smog, smoke, and clouds close to the ground. The most common form of inversion is a temperature inversion, although inversions can take other forms as well. Essentially, an inversion can be thought of as a flip in the natural order of things, suppressing convection and other processes which allow air to cycle across the earth.
In normal conditions, hot air close to the ground slowly rises upwards, pushing through a layer of cooler air. When a temperature inversion occurs, cooler air gathers near the ground, with a layer of hot air pressing down on top of it. This forces clouds, smog, and pollution to be trapped near the ground, because they cannot waft upwards, and a temperature inversion can sometimes break explosively, with severe thunderstorms or tornadoes.
One classic form of temperature inversion is the marine inversion, caused by cool air from the surface of the ocean being pushed onto shore. Marine inversions explain why many coastlines around the world are foggy. Inversions also commonly appear in valleys, where warm air presses down on cooler air in the valley. Since many urban areas are in valleys or near the ocean, they often suffer from extreme pollution made worse by inversions.
A weather inversion does not just impact the weather in the surrounding area. Inversions can also affect human health, as in the case of an inversion which traps pollution, and they can also impair visibility by forcing heavy cloud cover close to the ground. Inversions can also play funny tricks with radio signals and sounds; radio signals are often stronger during an inversion, for example, and the heavy fog characteristic of marine inversions can do peculiar things to noises, making them seem further or closer than they really are.
Inversions ultimately resolve themselves, sometimes quite abruptly, and sometimes they appear and disappear several times over the course of a day. In other instances, an inversion may hover for several days, often leading to concerns about air quality and potentially dangerous weather conditions.
Capping inversions, in which a layer of hot air traps a layer of cooler air, are notorious in the Midwest, because when the cap finally breaks, a huge amount of energy can be released, resulting in severe weather. The “cool” air in such inversions is often actually quite warm, so these inversions can feel very oppressive and tense until they finally disappear.