— Subsidence inversion

The atmosphere has large-scale, organized movements throughout the world.  Examples of this appear on weather maps as surface-level pressure “highs” and “lows” that can be hundreds of miles wide, and which muddle along on marches across the globe.  Weather “fronts” are also examples of large-scale atmospheric organization.  We are all familiar with these examples, and we know that they move horizontally along the earth.


But there are also large-scale organized flows of the atmosphere that move vertically.  The atmospheric pressure highs and lows have vertical flows associated with them.  Air moves upward in a low pressure region, and air moves downward in a high pressure region.  In fact, there is a cyclic flow of air.  The air that flows up in low pressure cells ultimately tops out, and flows horizontally over to feed the downward flow of air in high pressure cells.


When air descends from on high in our troposphere, there is the potential for a temperature inversion to occur.  These inversions are called subsidence inversions. 


For example, the air flows coming out of the tops of low pressure cells might be at 20,000 feet asl.  They converge at the top of a high pressure cell from varying directions.  The air masses entering this “collision zone” turn and go downward.  As the subsiding air moves closer to the earth, it encounters higher pressure, and compresses.  Compression warms the subsiding air.  The upper portion of Figure 5 shows a subsidence region of the atmosphere and the increasing temperature at lower altitudes.

Meanwhile down adjacent to the earth, there is a band of air called the planetary boundary layer (PBL).[6]  In Figure 5, the PBL is shown extending up to 1.5 kilometers from the surface.  Air parcels in this layer are mixed with one another by winds, surface features, and (during the day) solar heating of the ground.  This layer has the normal temperature variation with altitude that we discussed at the beginning of the Temperature inversions webpage.  In the example shown in Figure 5, the temperature at the surface is 25°C.  The air temperature decreases at the rate of 6.5°C per kilometer increase in altitude. 

The thickness of the PBL varies from perhaps a hundred meters at night to a couple of thousand meters during the day.  If the temperature of air at the top of the PBL is lower than that of the subsiding air at the same altitude, a temperature inversion will occur there.  


Figure 5 shows a smooth transition from the PBL temperature over to the subsidence branch.  Note that the rate at which the temperature changes in the PBL region is different from the rate of change in the subsidence region.  The rate of change in the subsidence region is the larger.  This is a consequence of different processes that operate on the air in these two regions.


Subsidence inversions are unlikely to play much of a role in light propagation on the Marfa plain simply because subsidence inversions require a high pressure cell for their creation.  If you examine weather maps over time, you’ll discover that Marfa is not close to any of the famous persistent weather highs, like the Pacific High and the Bermuda High. 


The closest surface high pressure that appears regularly in the southwest tends to form over the four corners region of Arizona, New Mexico, Utah, and Colorado.  This high-pressure center is more frequent in summer than in winter time.  But it’s over 500 mile away from the Marfa plain. 

You just don’t see surface pressure highs hanging around the Marfa plain.  And as a result, you don’t see subsidence inversions.