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Updated Wednesday, January 16, 2013 at 12:16 AM

Drier weather brings stagnant air

By Jack Broom
Seattle Times staff reporter

The good news: The Puget Sound area is expected to stay dry for days, possibly into next week.

The bad: Stagnant air is trapping pollutants, triggering a burn ban and health concerns.

The weird: Thanks to a widespread “temperature inversion,” it was 40 degrees at the 5,000-foot level of Mount Baker at 4 a.m. Tuesday while it was only 18 degrees at 4,000 feet.

“The high pressure has created a lid of warm air, and it’s staying over us,” said Johnny Burg of the National Weather Service, “There’s no wind coming from anywhere.”

The Weather Service warned that a “stagnant air mass” had formed over much of Western Washington and is expected to build over the next few days, trapping pollutants from sources such as vehicle exhaust and wood smoke.

The situation eased somewhat through the day Tuesday, prompting the Puget Sound Clean Air Agency to scale back the burn-ban it ordered Monday.

Fires are still not allowed in fireplaces or uncertified woodstoves, and outdoor fires, including bonfires, campfires and fire pits are also banned in King, Pierce and Snohomish counties.

But the use of EPA-certified woodstoves or fireplace inserts is allowed.

The agency said recent studies have tied wood-smoke exposure to health problems such as aggravated asthma, increased heart attacks, acute bronchitis and chronic lung disease.

Another side effect of cool air pooling in the lowlands is that even though no precipitation is forecast for the next few days in the Puget Sound area, freezing fog could form on roads and highways, particularly in the early morning.

In a normal winter-weather pattern, Burg said, storms spread though Western Washington, moving pollutants up and out of the area.

“When you have no wind, there’s no mixing of the atmosphere,” he said.

Although Tuesday was expected to be the fifth consecutive dry day at Seattle-Tacoma International Airport, the month is actually still slightly wetter than normal — due largely to a 1.51-inch soaking last Wednesday.

Through Monday, January rainfall at the airport was 2.71 inches, .07 inches more than normal at this point in the month.

That will likely change in the next few days, and this will become a drier-than-normal January, Burg said.

Overnight low temperatures just below freezing are forecast for the Seattle area for the next five days, with daytime highs in the high 30s to 40.

Dr. Michael Kennedy of the Northwest Asthma and Allergy Center said air pollution can be a significant hazard, particularly if children with asthma play outside strenuously.

Even people who don’t have asthma, but have a respiratory infection such as cold, may get worse if they’re exposed to smoke and air pollution, he said

Jack Broom: jbroom@seattletimes.com


The Seattle Times
The bottom half of Mount Rainier disappeared behind a layer of stagnant air on Tuesday afternoon. The Weather Service has warned that a stagnant air mass had formed over much of Western Washington and is expected to build over the next few days.




The Seattle Times
The downtown Seattle skyline is visible behind a layer of stagnant air in this view looking south along Interstate 5 on Tuesday afternoon.




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