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Clear skies, cold air, moderate pollution

Tue., Dec. 6, 2011, 7:22 a.m.

Morning temperatures have dropped into the middle 20s today, and air quality has deteriorated to the moderately polluted category.

Spokane International Airport was reporting 24 degrees and some clouds. It was 23 in Coeur d’Alene and 20 at Deer Park.

An air stagnation advisory remains in effect fhrough 4 p.m. Friday, which is causing air pollution levels to rise overnight.

A stage 1 burn ban remained in effect in the Spokane urban area today as well as in semi-rural areas surrounding the city. Wood burning is still allowed with federally certified wood stoves and fireplace inserts. All outdoor burning is banned in the county.

Cold air is trapped underneath higher air pressure, which contains a warm layer of air that acts like a lid over the valleys of the region, preventing pollution from dispersing.

The problem is widespread across the Pacific Northwest because of the size of the current high pressure pattern. Pasco is reporting ice fog this morning, and the Tri-Cities area has a thick layer of fog covering it.

This morning’s atmospheric sounding in Spokane showed a shallow layer of milder air just above the ground and another deeper layer of milder air starting about 5,000 feet in elevation.

Air quality should improve later today as ground temperatures are warmed by sunlight, allowing for more vertical movement in the atmosphere. But pollutants may build again tonight.

Today’s high in Spokane is expected to reach 36 degrees under mostly sunny skies with lows tonight returning to the lower 20s. The forecast remains largely unchanged through Saturday with a mix of sun and some clouds at times.

Fog may develop by Wednesday in northern valleys, forecasters said.

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