Friday, November 27, 2015

Surface-Based Inversion Forms

An inversion occurs when temperature warms with height, which is the reverse (inversion) of the normal situation (temperature cooling with height).   A surface-based inversion has its bottom (base) at the ground, so temperatures immediately increase with height as you ascend.

One can secure a good view of the development of the inversion last night looking at the vertical temperature distribution over North Seattle (NOAA Sand Point) produced by the vertical temperature sounder there.  First, here are the vertical temperature profiles starting at 4 PM (yellow line) through 10 PM (black line) on Thursday.  We start with a normal situation (temperature decreasing with height) and rapidly move to a low level inversion.  Heights are in meters, so the inversion is only 300 meters deep (about 1000 ft)

We can then look at the changes during the next 6 hours (10 PM to 4 AM), below.  Inversion doesn't change much at low levels.

To get a strong surface inversion around here (and in most places), you want to start with clear skies....and as shown by the visible satellite picture below we had that last night.  Clear skies allow the surface to radiate infrared energy to space, cooling the surface.  Why doesn't the atmosphere above cool as much?   Because it is not as effective an emitter of infrared energy as the surface.

Why did we have clear skies?  Because we had high pressure over our region, or to be more exact, high pressure that was centered a bit east of the Cascade crest.   This positioning is important as we will see.  The map below shows the sea level pressure distribution at 10 PM Thursday night.  Notice there was a weak offshore pressure gradient:  lower offshore, higher inshore.  Very important.

High pressure is associated with sinking air and thus little middle and upper clouds.   Clouds stop the surface from radiating infrared energy to space (or at least slows it down).  Thus, clouds are bad for inversion formation.  

High pressure is generally associated with weak pressure differences and thus light winds. Winds cause turbulence that mixes the lower atmosphere:  bad for inversion formation.   Here are the winds over our area at 4 AM Friday.  Light winds, but with a weak offshore (towards the west) component.

Weak offshore winds are good for inversions over western Washington.  If the winds are very light, there is a tendency for fog to form and fog can weaken inversions by reducing the loss of infrared energy to space.  A weak offshore flow brings dry air to low levels and greatly reduces the chances of low-level fog,  Offshore flow also produces warming aloft (as air sinks down the western slopes Cascades), which is good for inversions as well.

The bottom line is that high pressure is the parent of cool season inversions around here.   Inversions that can bring frost to the surface and poor air quality.   Why bad air?  Because inversions are very stable structures:  they suppress mixing in the vertical with dense cold air below warm less dense air. So inversions can keep low-level pollution near the surface.  In fact, the National Weather Service has put out an urgent air stagnation advisory this AM:


Gpacharlie said...

I noticed this last night. At ~ 500' the outside temp was 31°f. As we ascended to ~ 1000' the temp increased to 37°f which was the opposite of the norm.

Yasmine Galenorn said...

Yeah, my asthma has been bad the past few days. I'm counting the days till wind or rain...

Ansel said...

Cliff, I have 2 questions, both relating to inversions.

First, why does King county- the population center- have better air, and fewer burn bans, that both Pierce and Snohomish? It doesn't make sense to me. If it's the wind, why would there be more wind?

Second, why does southern New England, where I come from, have plenty of wind during clear winter weather (as I remember, the wind pretty much howls all winter, clear or not, therefore it's not inversion-prone) whereas here in WA we usually seem to go stagnant as soon as the high pressure forms over us?

Thanks, Ansel

Gpacharlie said...

No fun ☹️

John McBride said...

Usually when an inversion affects the area I can see atmospheric evidence of it in our Wedgwood (NE Seattle) neighborhood. Not this time though. Fog did form south and south west of here but didn't quite make it over the ridge just west of us.

For now I'm focused on the possibility of a wind event in the Thursday / Friday frame, although the weather models are dueling that one out. Probably just a 25 - 30% chance. I'll wait for you to weigh in Mr. Mass as to that likelihood. Cyclogenesis is right up your alley.

Rick said...

Having lived in PS, watched its weather a long time, and wondering if I'll ever again see snow like we had 30 years ago this week, I've come to dread December, or the point when November's rainstorms cut off (usually just before Thanksgiving) and high pressure clamps down. The stagnations can last for weeks. Storm forecasts abound, always placing activity at 5 or more days away. Such as we have going on right now. Fog, burn bans, inversions, and cold, dead air at the bottom dominate. It happens during El Nino, La Nina, and everything in between. It takes a huge bite out of rainfall, and especially snow in the mountains. Meanwhile there are breathless headlines of mud slides in southern California or blizzards such as those that came in almost every day last year from Boston. There was an episode we had a couple of years ago that nearly brought us a rainless December. January and February are often no better. At least Cliff called this one right when it started last week. I hope it isn't still parked over us at Christmas.