Inversion Season Is Here

Don't be fooled by the lack of snow and sunny skies.  Inversion season is here and this morning a fairly strong one exists over the Salt Lake Valley.

The term inversion has different meanings for meteorologists and the public.  Meteorologists consider an inversion to be a situation in which instead of temperature decreasing with height, it increases.  We have such an inversion in place this morning.  As shown in the sounding below, the temperature at the surface this morning at the Salt Lake City airport was -0.30˚C, but as you went up, it climbed to 4.2˚C at 767 mb (about 8000 feet).  That is a meteorological inversion.  

Source: NOAA/NWS/SPC

To the public, an inversion is a situation with poor air quality, typically occurring the winter.  These events often feature high PM2.5 concentrations and in some cases fog.  

However, meteorological inversions and what the public calls an inversion are equivalent.  It is possible to have a meteorological inversion, but good air quality, and it is possible to have a poor air pollution episode without a meteorological inversion.  

For example, the meteorological inversion present this morning formed late yesterday and last night as high pressure built over the area and temperatures aloft increased.  As a result, there hasn't been sufficient time to build up high PM2.5 concentrations and cause an air quality event with PM2.5 levels at unhealthy for sensitive groups.  Although a meteorological inversion is present, the air quality is good to moderate.  A look at purple air data shows PM2.5 concentrations generally less than 5 at many east bench locations and 12-20, with a few outliers, along the valley floor.  

On the other hand, some poor air quality events are not bonafide meteorological inversions.  During winter, the air in the Salt Lake Valley can become highly stagnated even if a true meteorological inversion is not present.  This is because the stability of the air depends not only on temperature, but also pressure.  Situations in which the temperature does not increase with height, but instead decreases very slowly with height, are still very stable, and prone to poor air quality.

Remember that the air quality isn't bad because of the "inversion."  The inversion is a naturally occurring phenomenon.  The air quality is bad because of emissions related to fossil fuel combustion and other activities occurring in the Salt Lake Valley (click here for more information).