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: