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Winter inversions can impact Montana air quality

Posted at 10:35 AM, Dec 28, 2018
and last updated 2018-12-28 12:37:24-05

HELENA – During summer wildfire season, the smoke can turn skies brown for days on end — but wintertime air quality isn’t always perfect, either.

High pressure weather systems in the winter ofter bring air quality. The inversions can be persistent, causing frigid temperatures and hazy skies. But what exactly is an inversion?

“In the summertime, temperature decreases with height, so when you climb up a mountain the temperature is lower at the top than it is at the bottom,” explained Air Quality Meteorologist Kristen Martin.

“But in the wintertime, for a variety of reasons, that flips, and it’s colder at the surface sometimes and warmer above.  And that warm air acts as a lid, and doesn’t allow much mixing,” she added.

The warm air rises, but cold air becomes trapped in valleys and high pressure also worsens inversions, because a stable atmosphere means less winds and less air mixing.

“You end up with pollution building up in the valleys, and sometimes- depending on the weather- this can last for days,” Martin said.

“The valleys kind of act like a bowl to continue to trap that pollution, and also because there’s so little sunlight, the ground tends to be colder here,” she continued. “And also with our snowfall, that also helps promote inversions.

Along with lower temperatures comes lower air quality and often the haze that you see during a particularly strong inversion comes from wood-burning stoves.

The pollutants measured in the winter are the same as what’s measured in the summer during wildfire season.

“We’re measuring particulate matter 2.5 [in Helena]. That’s what’s recorded on our Today’s Air website. And it does a good job of measuring both our summer wildfire impacts and our winter woodstove impacts that we see in our valleys as well,” Martin concluded.

The air quality as of mid-morning on Friday was primarily listed as “good” across much of the Treasure State. However, “moderate” air quality was being reported in the Flathead Valley, Hamilton and Libby.

Click here to check out the latest air quality readings from across Montana.

-Katie Alexander reporting for MTN News