An inversion keeps ice fog on the ground across Faribanks on Monday, Dec. 17, 2012. The National Weather Service's seven-day forecast is calling for temperatures to range between 20 and 40 below most of this week.

FAIRBANKS — If you were to ascend through the lower atmosphere in a balloon in most of the world, the temperature would get colder as you got higher. But not in Fairbanks during an inversion.

Inversions are reversals of the usual atmospheric order. Usually, temperature decreases with height because the sun heats the Earth’s surface, making the air closest to the surface warmer.

Here in the Tanana Valley, a mixture of our high latitude and our topography makes conditions especially ripe for inversions in the winter. Because of our inversions, residents on the hills enjoy comparatively balmy weather when the temperatures bottom out in town. However, inversions also trap pollution in the area, giving the Golden Heart City an air quality problem that’s disproportionate to the pollution the community produces.

University of Alaska Fairbanks atmospheric science professor Nicole Mölders has studied the Tanana Valley’s inversions to better understand the link between the pollution and the inversions. As she explains it, inversions are bad for air quality because they stop a process that usually cleans the air.

“Inversions suppress vertical mixing and hence the exchange with the clean air aloft,” she said.

In a 2010 paper in the scientific journal “Atmospheric Research,” Mölders concluded that unhealthy levels of pollution are more common on days when there’s an inversion and that it doesn’t matter how deep or strong the inversion is or how many days it lasts. The study also concluded that the unhealthy air days tend to be drier and colder.

Mölders used data on inversions from 2004 to 2009. The study was confined to the Northern Hemisphere’s darkest months: November, December and January.

In those months, Fairbanks had a weak inversion 39 percent of the time. At that level of inversion, the air at a height of 100 meters above the valley floor was

4 degrees Fahrenheit warmer than the temperature at the surface.

About 11 percent of the time there was a strong inversion, a temperature difference of more than

14 degrees Fahrenheit was recorded at 100 meters.

Understanding inversions

Fairbanks and other high-latitude areas are prone to inversions in the winter because of the short daylight hours and the presence of snow cover.

“Once the sun goes down, the snow surface cools much faster than the air higher above,” Mölders said. “Because the surface cools so fast, it basically cools the air that’s closest to the surface too.”

Other weather, such as high pressure systems, can also make inversions, but in Fairbanks the rapid cooling of the ground makes inversions especially common.

The horseshoe shape of the Tanana Valley also plays a role in the area’s winter air quality. Because it’s close to the cold surface, the air along hills cools faster than other air at the same altitude. Because cold air is denser, it sinks.

“The cold air flows down the slopes and so you get the cold air sitting on the bottom of the valley,” Mölders said.

The Goldstream Valley serves like a clean air reservoir for the Tanana Valley during inversions. Because the Goldstream Valley is slightly higher, cold air flows down into the Tanana Valley during inversions. That’s a good thing because the Goldstream Valley’s air is relatively clean, Mölders said.

Once they’re established, inversions can stay in place for days. Mölders likens it to the way vinegar and oil don’t mix because of their different densities.

Contact outdoors editor Sam Friedman at 459-7545. Follow him on Twitter: @FDNMoutdoors.