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.
As you know, such situations in which temperature increases with height are called inversions, since they are opposite to the normal situation of temperature decreasing with height. Using the capabilities on the SnowWatch web site, here is the temperature variation with height at 5 PM on Thursday--around 50F at 2000 ft and around freezing at sea level. These are the kind of days I love to hike up Tiger Mountains or other peaks in the Issaquah Alps.....frigid in clouds at the start and low-fifties and bright sun after a short hike. Maybe this weekend!
Here is the plot of temperature with height around 7 AM Friday morning. Heights are in meters. Mama Mia that's an inversion. There is a lower cool layer about 200 m (600ft thick), topped by a very sharp inversion to 400 meters, in which the temperature increases about 9C (16F)!
Here is a picture from a home in Bellevue at 1170 ft at 8 AM Friday..they are just above the low clouds:
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.
I should note that the bad air quality in Beijing also has occurred under cold high pressure, but their readings are MUCH, MUCH worse.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.
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