Inversion creates foggy aggravation in the Treasure Valley

What's behind the current weather pattern - and when will it end?

kmoeller@idahostatesman.comJanuary 24, 2014 

Inversion seen from Bogus Basin Road

The inversion, seen Tuesday from Bogus Basin Road, keeps the Treasure Valley dark and gray, while skies above it are clear.


It's been almost two weeks, but for some it feels like two years since Boise basked under sunshine and blue skies.

Inversion conditions set up Jan. 12, throwing a suffocating gray blanket over the Treasure Valley. And they'll be here through next week.

What is an inversion?

Basically, it means that cold air — along with fog and air pollution — is trapped in the Valley, when it would normally mix out and blow away.

Temperatures in the mountains around Boise are lower than those in the Valley for most of the year, but between Thanksgiving and Valentine's Day, the situation can become inverted, or turned upside down, with warmer air trapping colder air near the ground.

This happens every year, but some years the inversion is stronger than others, making the temperature differential more extreme.

How much of a differential?

The high at the Boise Airport on Wednesday was 27 degrees. Just a few miles away but at least 3,000 feet higher at Bogus Basin Mountain Recreation Area, the high was 47 degrees.

Meteorologist Jay Breidenbach said he felt overdressed for the relatively balmy temperatures at Bogus last weekend: "I had to get down to a short-sleeved shirt," he said.

What creates inversions?

Several things: heat loss due to short winter days and snow on the ground; geography (cold air that can't easily escape the Valley, hemmed in by mountains); or a high-pressure ridge aloft that prevents any storms or stirring of the atmosphere.

When will we see relief?

The current inversion isn't expected to budge until next week. "Two models are showing that in seven or eight days, the pattern will change," Breidenbach said.

What's with the ever-present fog this year?

Snow that fell and melted — or sublimated, going from solid to gas without becoming liquid — before the inversion set in probably contributed to all the moisture in the air. "It didn't take much cooling to reach the dew point and a saturated atmosphere, which is how fog forms," Breidenbach said.

Why isn't our air quality even worse, given two weeks of stagnant air?

Meteorologists speculate that some of the pollution is getting washed out by precipitation, including snow flurries coming out of the fog.

Pollution gets trapped in the Valley with the cold air, creating a dirty haze.

Air quality has diminished over the past couple of weeks, and the Idaho Department of Environmental Quality has issued yellow air quality alerts five of the past seven days.

Michael Toole, regional airshed coordinator for the Idaho Department of Environmental Quality, could not be reached for comment.

How does this compare to previous inversions?

The weather service doesn't officially track the length of inversions, but Boise meteorologists said there was a similar pattern last year from Jan. 12 through Jan. 24.

Any good news?

Strong inversions are unusual after mid-February because the sun angle is higher, allowing more surface heating and better mixing of air.

Katy Moeller: 377-6413

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