What you couldn’t see Thursday afternoon was a sign of deteriorating air quality in the Kelso-Longview area as a temperature inversion traps pollutants in the region.

Even from the foot of the Lewis and Clark Bridge, the Rainier Hills were almost completely hidden behind a haze of smog and plain old river fog.

The National Weather Service has issued an air stagnation advisory for Cowlitz County, Clatskanie and St. Helens lasting until Monday. Low wind levels and a temperature inversion over the region have led to air stagnation troubles, said Portland-based Weather Service meteorologist David Bishop.

A temperature inversion is a reversal of the normal behavior of the atmosphere, in which a layer of cool air at the surface is overlain by a layer of warm air. (In normal conditions air temperature usually decreases with elevation.) Warmer air aloft traps air pollution.

“With an inversion, imagine if you put a cover on pot of boiling water, how it holds everything in,” Bishop said.

As temperatures drop, “People start burning their stoves or fire places, and that smoke accumulates,” said Uri Papish, executive director of Southwest Clean Air Agency.

Air quality was at a “moderate” level in Longview and Chehalis Thursday afternoon, but no burn bans or advisories are in effect yet, Papish said.

Papish said the Southwest Clean Air Agency will continue monitoring the air quality and likely decide Friday whether or not to issue any warnings or bans.

Stagnant air typically makes foggy conditions overnight last longer into the day, Bishop noted.

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