Why Temperature Inversion Is Dangerous

Inversions can choke all the fun out of winter in some places (looking at you, Salt Lake City)

A temperature inversion traps and fills the Salt Lake Valley with thick smog. (George Frey/Getty)
Photo: George Frey/Getty weather

Not only do we have to worry about large-scale weather patterns affecting our daily lives in the winter, but local geography can also dictate the atmospheric flavor of the day, leaving parts of the country racked with gloomy skies and stagnant air.

Inversions make for many a problem in some areas during the colder months, including ice storms, air pollution that results in health issues, and even enhanced effects of explosions and other loud noises. A temperature inversion occurs when a layer of warm air develops on top of a layer of cooler air. This warm layer, the inversion, acts like a cap that seals the cooler atmosphere beneath it. We sometimes hear about a “cap” in thunderstorm forecasts, since air in an updraft has a hard time rising through the inversion to produce thunderstorms.

If the fast-moving air of an updraft has a hard time breaking through a temperature inversion, imagine how much harder it is for the atmosphere to churn itself up when one of these events occurs during the winter months.

There are several different kinds of inversions you can encounter when it gets chilly. Each type of inversion comes with its own set of inconveniences and hazards.

Here’s what you need to know about each one.

Valley Inversions

Cold air is dense. It drains from higher elevations to lower elevations and hugs as close to the ground as possible. This makes valleys especially vulnerable to inversions in the winter. And few areas in the United States more susceptible to valley inversions than Utah’s Salt Lake Valley.

The bowl-shaped topography that surrounds Salt Lake City’s metro area exposes the region to several hazardous inversion events every winter. These inversions occur when cold air gets trapped in the valley and a layer of warm air seals it from above. The region’s terrain prevents that bubble of cold air from draining away, and the mountains stop winds from easily scouring it out and ushering in fresher air.

The end result is a spell of nasty, stagnant air that can be extremely hazardous to the health of anyone in the region. The stale air of an inversion allows for the buildup of pollutants created by vehicles, factories, fireplaces, and wildfires. These pollutants most often affect those with health problems such as asthma, but particularly unhealthy air can lead to respiratory problems even in folks without preexisting conditions.

Salt Lake City has some of the worst air quality in the United States during the winter as a result of these inversions. Officials encourage people to carpool and use public transportation to reduce the amount of pollution when these inversions take place.

Valley inversions can also lead to some unintended consequences. Blasting at quarries and munitions testing at military bases usually come to a halt during inversions due to the risk of causing damage and injuries to people many miles away. The capping effect of an inversion in the atmosphere can reflect the blast wave from an explosion back down toward the surface, spreading the effects of an explosion farther than intended.

A famous instance of this occurred about ten years ago Esparto, California, during the filming of an episode of Mythbusters that required a huge explosion. The larger-than-expected blast shattered windows in the town more than a mile away from the blast as a result of an inversion overhead.

Cold-Air Damming

Mountains are a significant source of inversions when it’s cold outside. Not only can valleys trap cold air and force cities to choke on their own exhaust, but mountains can trap cold air blown by the wind. This can lead to significant cold snaps and even ice storms for folks who live in the foothills of certain mountain ranges.

Cold-air damming occurs when winds blow cold air up against the side of a mountain range. The surge of cold, dense air can’t ride up and over the ridges of the mountains, forcing it to pool up, instead, at the base and hang around until the wind changes direction.

It’s frequently cold, gloomy, and drizzly in states like Virginia and North Carolina during the fall and winter as a result of easterly winds blowing cold air up against the eastern side of the Appalachian Mountains. This setup can lead to more than just a raw drizzle. A low-pressure system approaching the mid-Atlantic from the south can force warm air into what would otherwise be a snowstorm, sometimes leading to a surprise ice storm in the region’s Piedmont.

The world’s most infamous example of poor air quality is also a result of cold-air damming. Beijing, which is home to more than 20 million people, sits at the horseshoe-shaped confluence of two mountain ranges in northeastern China. Prevailing winter winds from the south and east subject the city to days-long inversions through cold-air damming. The shape of the terrain prevents incoming weather systems from easily scouring out the stagnant air, exacerbating its already-treacherous pollution problem. The city’s air can spike the air-quality index off the charts during the winter, threatening the health of even an otherwise healthy person.

Marine Layer

The famed marine layer that bathes cities like San Francisco is another example of an inversion. The Pacific waters off the coast of San Francisco are quite cold, often significantly colder than the temperature of the air moving over the region. The cold water chills the air immediately above the ocean surface through conduction, leading to an inversion.

Warm air moving over cooler water is a recipe for fog, which is the most notable impact of an inversion caused by the marine layer. San Francisco’s fog is so famous that it even earned its own name and Twitter account. But fog isn’t the only impact this inversion can have. Concerts in San Francisco can be notoriously loud due to sound waves bouncing off the cap above the city and reflecting back down toward the surface. This was a particular problem during a 2014 BeyoncĂ© concert, which was made so loud by an inversion layer that residents miles away from the stadium where it took place complained about it.

Learning to Adapt to Inversions

Unfortunately, there’s nothing we can do to prevent inversions from developing during the winter months, so we have to adapt to the challenges they present. Places like Salt Lake City can reduce the air-pollution risk posed by inversions through emissions-reduction efforts like investing in public transit, stricter emissions standards on factories, and an overall shift toward clean energy. People with respiratory problems should try to avoid going outside on high-pollution days and wear respiratory masks if they can’t stay inside.

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