January 18, 2013

Super Inversion and Bad Air

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.


  1. Hi Cliff,

    I remember reading a few years back that fog, typically fog created during inversions, can scrub or clean the dust and pollutants from the air. Is this true?

  2. Good question, Bubba. I was just wondering the same thing: what are the environmental benefits of fog?

  3. Hi Cliff,

    Hope you're well. We're suffering terribly from the inversion here in the Gorge. Any chance you can talk a bit about what causes the low clouds to form during the inversion? I'm going to link to this blog post, and I'm sure everyone trapped in the Gorge gloom (we call it the Nothing) would love to understand the mechanism of their misery.

    Thank you!

  4. Cliff ~ Can you explain this?....Last night around 9:00 p.m. here in Eatonville it was 30 degrees, calm winds, everything was getting frosty. Within half an hour it was 42 with winds 15-20. This morning at 6:00 a.m. it was 41 degrees, breezy (15-20 mph), no frost on the roads or cars. But I drove not even a mile away and it's calm, 31, and frosty. Really wierd! Currently it's still breezy, clear, sunny, and 42. We are only at 850 feet elevation. Obviously there's no fog here!

  5. On your broadcast today, you mentioned that east of the Cascades, "they" have crop fans to stir up the air and thus break up the inversion. This statement might give people the wrong impression about crop fans (BTW, actually referred to in farming as "wind machines") - that they might be used to directly improve the air quality for people.

    Well, they are extremely expensive to operate (because of fuel costs) and would not be very effective in improving the air quality after all.

    The wind machines are specifically used to slightly raise the air temperature under very limited atmospheric conditions in close proximity to the machine itself, much less than 10 acres per machine effectively.

    The machines are only run to save crops from frost damage while blossoms or fruit (for the most part) are present.

    If you really wanted to do this on a large-scale and effect the Puget Sound air quality...could you imagine 10's of thousands of wind machines humming away, they themselves spewing out fossil-fuel emissions. A fallacy of thought, to be sure!

    Love your broadcast. Keep up the good work!

  6. rI went for a 60 minute run last night around Ballard/Phinney Ridge. It was fun to run in the think fog but I smelled like I had been sitting next to a campfire when I got home. I didn't smell it all during the run. Without a burn ban I imagine it would have been much worse.

  7. With the discussion here of winter WA weather, some of you might be interested in the frost and ice trends. I just posted some analyses going back to 1894 here:


  8. I'm living in Redmond Ridge, I assume this is part of the Sammammish plateau so the air quality should be good?

  9. Does anyone know why PSCAA often bans wood fires in Pierce and Snohomish counties, while allowing them in King County at the same time? I hope it's not because THEY are based there!

    From a meteorological point of view it does not make sense to me (at least on first analysis) to allow fires in King Co., which has MORE cars, people, homes, etc., but not allow fires in the two neighboring counties... smacks of playing favorites to the casual observer! Or is there some special ventilation that King County gets?

  10. Thank you for explaining why high pressure in the winter results in inversions, but not high pressure in the summer. That question has been bugging me ever since this last inversion started.


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