Our weather has finally done a complete 180 with this latest ridge of high pressure. It has been almost a week since we’ve seen any new snow, and it looks like we still have a couple more days of this stagnant weather, before the pattern begins to change again. As I mentioned in last week’s column, the winter ridge of high pressure is not necessarily a welcome pattern – though in this year’s case a long dry spell probably hasn’t inspired too many complaints. We have been stuck in some locally dense fog for days.
With temperatures below freezing, black ice along with severely reduced visibilities have moved in to replace the previous hazards of snow packed roads and weighted down roofs.
One of the other hazards of this high-pressure pattern, though you might not initially recognize it as a problem, is the stagnant air. The National Weather Service puts out an “air stagnation advisory” during such conditions. Under this weather pattern, not only is there very little horizontal movement or air, there is little vertical movement as well. In a rural area, the lack of air movement might not be an issue, but when you factor in the output of exhaust and other pollutants from automobiles, fireplaces, and local industry in the more populated areas, you’ll see how stagnant air could lead to a marked decrease in air quality.
On a typical day, we have an atmosphere where the temperature decreases with height. During the day, whatever sunshine we get heats the ground, which in turn heats the air directly above it. This warmer air is less dense than the cooler air above it and begins to rise (think of the hot air balloon concept). This rising air leads to areas of lower pressure at the surface, which causes air to move in toward it from the sides. A circulation develops which leads to a “mixing” of the atmosphere. Pollutants released near the ground get carried away by these vertical and horizontal movements of the air, and are eventually dispersed.
Under a strong ridge of high pressure like what we’ve been under, however, we often have a temperature inversion. This means that temperatures increase with height. The dense, colder air at the surface is in effect “trapped”. Low clouds and fog severely limit any sunshine from reaching the ground to warm the air, perpetuating the temperature inversion. An extensive ridge of high pressure also means that there is little pressure change across the area – which leads to mainly calm winds. The lack of significant vertical or horizontal movement of air means that any pollutants injected into it are stuck there, only to build up each day that weather conditions remain the same. Those who are especially sensitive to reduced air quality (those with asthma for example) can find daily air quality conditions and forecasts for Spokane and Coeur d’Alene areas at this government EPA site:
http://cfpub.epa.gov/ airnow/index.cfm?action +airnow.local
It does look like the ridge should break down by this coming Tuesday, bringing us not only a chance of precipitation, but also a breath of “fresh” air!
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