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What causes inversion?

In the winter months, that yucky inversion comes to visit the Salt Lake Valley. What is it anyway?

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Smog blankets the Salt Lake Valley during an inversion, as seen from Little Cottonwood Canyon on Thursday, Jan. 20, 2022.

Smog blankets the Salt Lake Valley during an inversion, as seen from Little Cottonwood Canyon on Thursday, Jan. 20, 2022. An inversion is forecasted for the valley on the weekend of Dec. 16, 2022.

Kristin Murphy, Deseret News

Remember learning that hot air rises and cool air sinks in science class?

That’s the basis of a temperature inversion that makes the Salt Lake Valley look a little, well gloomy at times.

Normally in the troposphere — the closest layer of the atmosphere to the Earth — the warmer air stays close to the ground and gradually changes to cooler temperatures the farther away from earth it gets.

It’s more or less like a gradient from warm to cold.

When that normal order changes and cold air drops to the earth’s surface, it’s called “inversion.” In the colder months, a layer of colder air can get trapped closer to the ground, and warmer air gets above it, as a lid to keep the cold air from rising, per the Utah Department of Environmental Quality.

Along with the cold air, pollutants become trapped, which can make the air quality poor and unhealthy too.

With the Wasatch Mountain range to the east, Traverse Mountains to the south, and Oquirrh Mountain to the west, the Salt Lake Valley is a perfect place where inversion can sit, trapping in the cold air and pollutants, just like a bowl.

The only way for an inversion to disperse and return the valley to normal airflow and order is for a high-pressure system — strong enough to push the trapped air around — to push through the area and normalize the gradient again, per the National Oceanic and Atmospheric Administration.

That can sometimes take only minutes or other times days.

This is where the Salt Lake Travelwise initiative comes into play. It encourages lower pollution — especially during inversion — by encouraging residents to work from home or carpool to keep the air clean and trap fewer pollutants in the valley.