Temperature Inversions

The plume from Buffalo Trace Distillery can’t rise vertically because of the inversion layer

Driving into work this morning, the plume rise from the Buffalo Trace Distillery caught my attention because it was being trapped by an inversion layer in the atmosphere. Usually the atmosphere cools with increasing height, but a temperature inversion, or inversion layer, is when the air gets warmer with increasing height. Typically the sun heats the Earth’s surface and air around it, warm air is less dense than cold air, so that air rises until it cools to its surrounding temperature. This rising motion is called convection. With a surface inversion, the air at the surface is cooler than the air above it so it cannot rise. The consequence of this is that air pollution can get trapped near the surface, and have a serious heath effect on people. For example in London in 1952 a significant amount of air pollution got trapped near the Earth’s surface and caused 4,000 premature deaths and made up to 100,000 people sick. Luckily because of the EPA and Clean Air Act we do not see events like this in the US, but China and India have major air pollution problems that cause health issues.

 How do Inversions Form?

There are different types of inversions but this is an example of a radiation inversion. Radiation temperature inversions are typically at night when there are clear skies and calm winds. The Earth cools by putting off terrestrial radiation. To form an inversion there needs to be clear skies because clouds reflect a portion of the terrestrial radiation back to the surface which keeps the boundary layer warm. Calm winds are important so there isn’t mixing. When you combine clear skies, calm winds, and the outgoing radiation you get maximum cooling in the boundary layer so the air there is cooler than the air higher up in the atmosphere and an inversion forms.

 This morning is an example of a strong inversion. We use a thermodynamic diagram, called a sounding, to look at the vertical profile of the atmosphere. The red line is the temperature and green line is the dew point. When the temperature line moves right it indicates warming. As we can see at the surface the red line sharply goes right, that indicates the inversion layer.  

Inversions and Thunderstorm development 

Capping inversions are important in the development of severe thunderstorms. A capping inversion is when there is a layer of warm air in the atmosphere that acts to shut off convection (upward movement). When the cap is broken, either by extreme convection or a lifting mechanism like a front, the sudden release of the built up convection under the cap causes rapid, and usually severe thunderstorm to development. 

Pictured above is an example of a capping inversion. The red line is the temperature as you rise up in the atmosphere. The blue shaded area is the stable layer, The green area is the capping inversion. Notice how the red line turns right, that is how you identify an inversion.