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What Is Winter Inversion?

Can the atmosphere literally be flipped on its head? Find out!

Love winter or hate it, one thing is sure, there’s nothing like the smell of crisp, clear, cold mountain air to make a person feel truly alive.

Air quality is often best in the winter time, when the weather works to “clean” the air, trapping ozone and other pollutants in precipitation or blowing them off with sharp, gusty winds.

But in some North American cities, including Los Angeles, Boise, Fairbanks, Salt Lake City, Vancouver, and others that are tightly penned in by mountains, winter can mean an increase in the amount of pollution in the air, thanks to an effect called winter inversion.

Most of the time, air is warmer near the surface and cooler higher in the atmosphere. In an inversion, though, the normal atmospheric state is flipped on its head, or inverted.

Inversions often follow on the heels of a snowstorm, when fresh snow pack on the ground reflects the Sun’s warmth back up into the atmosphere, resulting in colder temperatures near the surface and warmer air above. The higher pressure layer of warm air sitting atop the colder layer acts like the lid on a pot, trapping the cold air below.

As the inversion goes on, this trapped air begins to stagnate, and the concentration of fine particulate pollutants in the air, emitted by the consumption of fossil fuels in vehicles, power plants, and factories, increases. An extended period of inversion sometimes results in the formation of a brownish fog.

Fine particulate pollution is dangerous to breathe, and can even be deadly at high enough concentrations. The tiny particles are so small they can get deep into the lungs, and eventually the bloodstream, causing serious health problems.

In the short term, it can irritate the lungs, making it difficult to breathe. Over time injure the lungs, causing or aggravating serious respiratory diseases, including asthma, emphysema, or bronchitis, lung cancer, as well as heart conditions such as congestive heart failure and coronary artery disease. Premature delivery and birth defects have also been attributed to high levels of atmospheric pollution.

These effects are most pronounced in children, the elderly, those with compromised respiratory function, and anyone who works outdoors.

In rarer cases, severe air-pollution events caused by inversions have resulted in immediate death. The Great Smog of 1952, also known as The Big Smoke, descended on London, England in December of that year. Blamed for as many as 12,000 premature deaths, and 100,000 cases of acute respiratory distress, the four-day inversion was caused by period of cold weather combined with an anticyclone that formed around the area, trapping air inside.

Airborne pollutants from burning coal, which fueled and heated much of the city at the time, caused a thick layer of smog to form over the city in what has since come to be known as the worst air-pollution event in the history of the United Kingdom.

The inversion event was directly responsible for the creation of Britain’s Clean Air Act of 1956.

So if you’re having a bad “air day” due to inversion, be sure to heed air quality warnings and alerts.

Jaime McLeod is a longtime journalist who has written for a wide variety of newspapers, magazines, and websites, including MTV.com. She enjoys the outdoors, growing and eating organic food, and is interested in all aspects of natural wellness.

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