Stagnant Air Keeping Parts of the West Cool

By Matt Alto, Meteorologist
January 20, 2013; 5:55 AM ET
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As a brutal surge of cold air invades much of the East over the coming days, a much different weather pattern will be found across the West.

A persistent area of high pressure centered over the Great Basin will continue to be responsible for promoting more dry weather. In fact, this center of high pressure has been the strongest so far this winter.

Although the majority of the West will be enjoying warm and sunny weather into next week, not everyone will be experiencing mild air and copious amounts of sunshine.

Areas of low clouds, fog and stagnant air will continue to be found throughout many of the interior valleys of the Northwest into early next week. Freezing fog will also be possible in locations that have below-freezing temperatures.

A strong push of warm air aloft in combination with weak winds in the valleys due to high pressure has been responsible for creating a meteorological phenomena known as a temperature inversion.

Typically in the lower atmosphere, the air near the surface of the earth is warmer than the air above it, largely because the atmosphere is heated from the ground up.

A temperature inversion, like the one occurring over the West now, refers to an increase in temperature with height. The warmer air aloft acts as a lid and holds the cold and stagnant air in the lower levels of the atmosphere.

As is the case with the Northwest, mountains can increase the strength of the inversions in the valleys.

The light winds in the lowlands will create very little to no movement of the air throughout the region, resulting in deteriorating air quality, below-normal temperatures, and areas of persistent low clouds and fog.

As pollutants from vehicles, wood burning and industrial factories continue to be emitted into the air, the inversion will continue to trap these pollutants near the ground, leading to poor air quality.

Residents with respiratory problems, young children and the elderly in many of the valley locations from western Washington and Oregon into northern Idaho should limit the amount of time they spend outdoors through early next week.

Locations above the inversion in the higher terrain will experience plenty of sunshine and mild temperatures into early next week.

On Wednesday, the strong ridge of high pressure will begin to break down as an upper-air area of low pressure moves into the Pacific Northwest. This system will break down the inversion leading to improved weather conditions in the lowlands.

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