Rob Kerr / The BulletinKC Chamberlain, 58, front, balances while walking on a log during an outing with her son Reece Pressman, 18, Tuesday morning as sunlight is filtered through a dissipating fog at Smith Rock State Park in Terrebonne. The forecast calls for inversion weather to continue. The stagnant air and freezing temperatures associated with the inversion have caused poor air quality and freezing fog warnings.

An inversion has put a lid on Central Oregon for a week.

And it will likely continue until the weekend, possibly into the start of next week, said Diann Coonfield, a forecaster for the National Weather Service in Pendleton.

“There is really no change until Monday night or Tuesday,” she said Tuesday afternoon.

The Weather Service issued a stagnant air advisory for towns around Central Oregon last Wednesday. The advisory remains in effect until Friday, and Coonfield said the federal agency may extend it. During the advisory the Weather Service warns of degraded air quality, periods of low visibility and slick surfaces resulting from freezing fog.

High pressure holding over Central Oregon is causing the inversion, in which cold air flows down from the mountains and then lingers in valleys and basins. The inversion may cause fog to form, as it did in Bend Monday morning and night.

The inversion may also keep air pollution such as wood smoke and car exhaust close to the ground, said Frank Messina, environmental specialist for the Oregon Department of Environmental Quality in Bend.

“It just sort of seals it in place,” he said.

How bad the air gets during an inversion depends on a combination of factors, including the strength of the high pressure system, the amount of pollution and location.

“A lot of it depends on our topography,” he said.

Even during an inversion, Bend may have clear air because it is close to the mountains but not in a confining valley. Just 35 miles to the northeast, Prineville is in a different situation. The town sits in a bowl, which may hold in pollution during an inversion.

Figures from a DEQ air monitor near downtown Bend show air pollution spiking in the evening in recent days but remaining in the range considered good by the state agency.

Data from an air monitor at Davidson Park in downtown Prineville showed levels deemed unhealthy for sensitive groups by the DEQ on Saturday and Sunday nights.

Extended inversions can cause some of the worst air quality problems in Central Oregon, Messina said, aside from wildfire smoke during fire season. Air pollution capped in by an inversion may be widespread while wildfire smoke may be localized.

Inversions are a normal occurrence here during wintertime, Coonfield said, when the low angle of the sun and the long nights create a pool of cold air.

During an inversion, DEQ officials ask people to try to limit the amount of pollution they create, particularly from open burning, said Mark Bailey, eastern region air quality manager for the DEQ in Bend.

“We just ask (people) to not burn at all and wait until it passes,” he said.

—Reporter: 541-617-7812, ddarling@bendbulletin.com