Cold-Pool Season Is Coming

Although we've seen a substantial decrease in smoke in recent days, a look at the sky sometimes shows a bit of smog due to pollution along the Wasatch Front.  Here's an example from yesterday afternoon.

Inversion season is coming, and while it isn't here yet, on some days we're starting to see that the decrease in daytime heating due to the shorter days and lower angle sun is resulting in decreased mixing of the lower atmosphere and a bit more particular matter buildup. 

During the day, energy from the sun warms the ground and in turn the lower atmosphere, leading to turbulence and mixing.  The layer in which this occurs is known as the convective boundary layer, which I'll abbreviate as CBL.  The maximum depth of the CBL, typically achieved in the afternoon, depends on many factors, one of which is the amount of incoming energy from the sun.  

During the summer, the CBL in Utah can be very deep, sometimes extending to more than 10,000 feet above the valley floor.  Vigorous mixing in the summertime CBL prevents a buildup of particulate matter (although we can still get high ozone concentrations).  

During the winter, however, the energy available from the sun is much lower.  On many days, the CBL is very shallow, sometimes as little as a few hundred feet deep.  On these days, we see a build up of particulate matter.  Unless a storm comes through to stir things up, the situation tends to worsen with time.

We're not there yet, but we are seeing some of the effects of the decrease in solar energy.  Yesterday, for example, the CBL reached a depth of only about 3000 feet.  You can see the CBL in the graphic below, which is a thermodynamic diagram used by meteorologists and known as a Skew-T.  It shows the vertical profile of temperature (red), dewpoint (green), and wind (barbs to the right) above Salt Lake City from the surface to the upper atmosphere.  The y-axis is pressure, which decreases with elevation (meteorologists typically use pressure as a coordinate rather than height for a variety of reasons).

The CBL is the layer near the ground that I've indicated as "well mixed."  The surface temperature is 63˚F and above the surface the temperature decreases at a rate of about 10˚C per kilometer.  Such a temperature decrease is consistent with the air density being constant with height.  The air can mix vigirously through such a layer.  

SkewT source: NOAA/NWS/SPC

At the top of the CBL, however, there's a stable layer.  In this layer, the temperature increases with height.  Such a layer is stable and resistant to mixing. Additionally, it is capped by a weakly stable layer.  Together, this puts a lid on the atmosphere over the Salt Lake Valley, limiting mixing of the valley atmosphere at about mid-mountain level.  In turn, those mountains limit horizontal transport of the atmosphere, and we see a bit of pollution.  In the summer, the energy of the sun would be sufficient that the lower atmosphere would have warmed more and mixing would extend through a deeper layer.  However, it's now October, and there simply isn't enough solar heating.

The meteorological definition of inversion is a layer in which the atmospheric temperature increases with height.  In Utah, it's used differently in popular culture to describe a number of situations in which the air is trapped at low levels and pollution is building up.  Not all of these situations feature an inversion, but they do feature relatively dense low-level air is trapped near the valley floor.  A better meteorological name for such air is cold pool. 

Cold pool season is coming.  Cold pools are natural.  They have been a common cool-season occurrence in the valleys and basins of the mid latitudes, including Utah, for millions of years.  What is not natural are emissions due to human activity, one source of which is fossil fuel combustion.  Thus, this is another situation where we have met the enemy and it is us.