Cold air and foggy conditions have dominated the lowlands on both sides of the Cascades, as shown by visible satellite imagery this afternoon. The irony of all this is that while near sea level the temperatures were in the 20s and 30sF , a few thousand feet up it was sunny and in the lower 50s.
Inversions are the natural result of a having high pressure over us in winter. High pressure is associated with sinking air that eliminates upper and middle level clouds, allowing the earth to radiate heat to space. Thus, the earth can cool effectively and that cools the nearby air. So we have our refrigerator at the surface. The sinking of air aloft associated with the high pressure produces warming, since air is warmed as it is compressed as it travels from lower pressure aloft to higher pressure closer to the surface. A virtual hot plate aloft. Cool the surface and warm aloft and you eventually get an inversion.
Inversions tend to strengthen in time as long as high pressure remains aloft. Inversion layers are associated with great atmospheric stability---think of them as atmospheric lids. Cold air is dense and heavy and likes to stay under the less dense air aloft.
The result of this stability is lots of low clouds, fog, cold temperatures, black ice, and unfortunately air pollution. In fact, for much of the last week the Puget Sound Clean Air Agency has had a burn ban in effect and during the last day air quality has really declined in places. Here us a plot of the particulates in Seattle Duwamish Valley. Not good. In fact, it is bad enough the PSCAA is declaring the air as unhealthy for sensitive groups.
The poorest air quality in our region tends to be in valley's or bowls where the cold air can settle and winds are weak: some of the drainages in Lake Forest Park can be very bad, as can the terrain bowl near Darrington. Southern Puget Sound is also an air quality problem area.
The local high-resolution forecasts made at the UW also predict the amount of mixing or "ventilation" of the lower atmosphere for the use of local air quality agencies. Here is the prediction for Saturday morning...not good.
NW air quality has been actually improving for several reasons. First, a LOT less people burn wood today--it wasn't a long time ago that the Seattle Times had pages of ads for wood burning stoves. Many folks now have gas fireplaces or inserts instead. Second, the burn bans are very effective tools, discouraging folks from burning wood when the atmosphere is stable. So some of you might not be happy with the Puget Sound Clean Air Agency, but their efforts (including burn bans) have a very significant impact on reducing air pollution during the winter. Third, cars and trucks are considerably cleaner now...and more.
The forecast.....cold and low clouds through early next week. And then on Tuesday we make the transition to our normal weather. Warmer with rain sounds ok, after sun, fog, and cold. But first, Tiger Mountain.