Inversion Layers


In a typical situation, the atmosphere (with regard to the troposphere) becomes cooler as elevation increases.  An “inversion” occurs when a section of the atmosphere becomes warmer as the elevation increases.  Inversion layers are a significant factor in the formation of smog in Los Angeles because they create stable atmospheric conditions.  Inversions act to prevent mixing in the lower regions of the troposphere, so pollutants become trapped quite easily and contribute to the formation of smog.  (When the atmosphere cools with elevation, an unstable condition is created which allows pollutants to more easily disperse.)  The following illustrations show the three different types of inversion layer structures that are most common in the L.A. basin. (Illustrations are after those in Richard P. Turco’s book, Earth Under Siege, an excellent resource for further reading on this topic.)


The first diagram (Fig. 1) shows what is known as a Marine Inversion.  This occurs when cool, moist air that originates over the ocean is blown onto land by our prevailing westerly winds.  The cool temperature of this air makes it more dense, so it readily flows underneath the warmer, drier air that is present over the basin.  This type of air flow is also accentuated by the fact that land surfaces are heated more rapidly during the day than the ocean. This extra heating creates rising warm air, which then draws in the cooler ocean air.  L.A.’s coastal location makes it an ideal setting for this type of inversion.


  Fig. 1.




The second diagram (Fig. 2) shows a Regional Subsidence Inversion. This occurs when air flows down from a higher location to a lower elevation.  As air sinks, or descends, it’s warmed because of the compression it undergoes.  Typically, this air is also dry.  Not only has it originated over a dry land mass, but any moisture it may have contained at one time was likely condensed and precipitated out as it rose over our local mountains prior to its descent into the basin.  Mixing of the air masses can occur on occasion, but once the warm, dry air (appearing in L.A. as Santa Ana winds) meets the cooler ocean air, it typically slides over the top of it to create a temperature inversion. The Angeles Crest mountains to the north and the San Bernardino mountains to the east of us make this scenario possible.


  Fig. 2




The third diagram (Fig. 3) shows a High Pressure Inversion.  Because high pressure systems are a function of descending air, we again see a situation of warm, dry air sinking to act as a cap over our cooler marine air.  This is a regular occurrence for Southern California in the summer as we are located near the eastern edge of the Pacific High, which is a component of the Hadley global circulation cell.


  Fig. 3


Written by Anna Huber.

Illustrations from Earth Under Siege, © R. Turco, 2001

Last updated, Aug. 26, 2004.