A dirty layer of winter air is hovering over many northern Utah cities as an inversion sets in that is expected to last for days, if not weeks.

National Weather Service meteorologist Pete Wilensky said Friday that a high-pressure system over much of the U.S. West is leading to the murky winter air that causes problems for people with asthma and other respiratory illnesses.

Wilensky says the inversions are expected to last through all next week with air quality gradually deteriorating. The earliest possible break is the weekend of Dec. 16-17.

The winter inversions are a phenomenon in Utah's urban corridor caused by weather and geography. Cold, stagnant air settles in the bowl-shaped mountain basins, trapping tailpipe and other emissions that have no way of escaping to create a brown, murky haze the engulfs the metro area.