In Meteorology, what is an Inversion?

Inversions can cause severe weather in the American Midwest.

An inversion is a situation where 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 temperature inversion, although inversions can take other forms as well. Essentially, an inversion can be thought of as a change in the natural order of things, suppressing convection and other processes that allow air to circulate around the Earth.

Inversions force smog and pollution to be trapped close to the ground.

Under normal conditions, warm air close to the ground slowly rises, pushing a layer of cooler air. When a temperature inversion occurs, cooler air accumulates close to the ground, with a layer of warm air pressing down on its top. This forces clouds, smoke and pollution to be trapped close to the ground because they cannot float upwards, and a temperature inversion can sometimes break explosively, with heavy storms or tornadoes.

Weather inversions can often create fog.

A classic form of temperature inversion is the marine inversion, caused by cold air from the ocean surface being pushed towards the shore. Marine inversions explain why many coastlines around the world are hazy. Inversions also often appear in valleys, where warm air presses against cooler valley air. As many urban areas are in valleys or close to the ocean, they often suffer from extreme pollution compounded by inversions.

Weather inversion doesn’t just affect the weather in the surrounding area. Inversions can also affect human health, as in the case of an inversion that traps pollution, and can also impair visibility by forcing heavy cloud cover closer to the ground. Inversions can also play tricks on sounds and radio signals; radio signals are often stronger during an inversion, for example, and the heavy haze characteristic of marine inversions can do peculiar things to noises, making them appear further away or closer than they actually are.

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Ultimately, the inversions resolve on their own, sometimes quite abruptly, and sometimes they appear and disappear several times throughout the day. In other cases, an inversion can hover for several days, often leading to concerns about air quality and potentially dangerous weather conditions.

Roof inversions, in which a layer of warm air traps a layer of cooler air, are notorious in the Midwest because when the cap finally breaks, a lot of energy can be released, resulting in harsh weather. The “cold” air in these inversions is often quite hot, so these inversions can feel very oppressive and tense until they finally disappear.

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