Cooler weather means temperature inversions

We are lucky to live in an area where, for the most part, we breathe nice, clean air. Occasionally there are some air quality issues during the warm season due to smoke from field burning and/or wildfires. But we are much better off than Atlanta, Houston and Los Angeles, three cities that reside in the top 10 list for most polluted air. In 2003, Spokane actually made it on a list of 20 metro areas with the least ozone air pollution. The list also included such cities as Bellingham, and Portland, Salem and Eugene, Ore.

During the winter, air quality sometimes takes a turn for the worse. The amount of pollutants does not necessarily increase, but weather conditions can play a large part in determining the quality of air that we breathe.

Someone once said, “The solution to pollution is dilution.” Exhaust from vehicles, smoke from wood burning, and industrial emissions all contribute to polluting our air.

Dilution and/or dispersion of dirty air particles can happen in two ways. A good breeze can carry pollutants away from the area (not so good for folks downstream) while also mixing, and therefore diluting, the pollutants into cleaner air.

Mixing also occurs as the sun heats the earth’s surface. Pockets of warm air begin to rise (think of a hot air balloon), and these rising currents can carry pollutants high into the atmosphere, where we don’t have to inhale them. The turbulent rising and falling motions of the air again serve the purpose of diluting the dirty air.

Two things can happen in the winter that interfere with the dilution and transporting away of pollutants. One is the lingering presence of high pressure across the Inland Northwest. On one hand, the sunny skies and calm winds might feel good compared with the normally gray and blustery winter weather. In the absence of storm systems, however, winds can remain calm for several days. The lack of horizontal movement of air means that dirt, exhaust and other harmful chemicals released into the air stay where they are.

Secondly, and of greater impact, is the occurrence of snow-cooled air or shallow arctic air at the surface coupled with warmer air aloft. This creates a temperature inversion. The denser cold air becomes trapped at the surface, capped by the warmer air above it. With little winter sun to heat the surface, inversions can persist for days. Without vertical air currents, pollutants become trapped, as if there were a lid. This is especially a problem in deep valley.

When conditions such as these are expected to last for several days, the National Weather Service will issue an air stagnation advisory, which alerts the public to weather conditions that can lead to unsafe air quality. The advisory in itself is not an indicator of poor air quality, but dirtier air is a good possibility under such stagnant conditions.

Winter outlook

Meteorologists at the Climate Prediction Center recently issued their latest winter outlook, valid for December through February. With El Niño continuing to strengthen, it still looks like the warmer and drier forecast across the Inland Northwest over the next three months is on track. So far, however, we’re in the middle of quite a wet weather pattern. Normal rainfall for the month of November is 2.97 inches. We’ve already received more than 5 inches of precipitation (2.2 inches as snow) as of Nov. 21. It looks like the wet, and sometimes snowy weather will carry us through to the end of this month.