Temperature Inversions

I’m sure most of you have heard the term inversion, but how many really know what an inversion is? Inversions occur in the atmosphere all the time and are responsible for a variety of weather.  An inversion is just that, an inversion of the normal temperature profile going up in the atmosphere.  Normally the air cools at 5 degrees (f) for every 1000 feet you go up in the atmosphere; hence it is colder as you go up in elevation.  A good example of this was the other day when Denver hit 97 degrees, while up in Conifer a few thousand feet higher it was only 83 degrees.  If there had been an inversion present the temperature would have been lower in Denver than areas in the foothills.  So, an inversion layer is the warm layer of air above the cooler lower levels.  This has important aspect here along the Front Range both in the winter and in the summer.

First, some basic atmospheric thermodynamics need to be explained.  If you take a stable layer of air and heat it, it will expand and become lighter than the surrounding air.  As a result it will begin to rise.  As long as it continues to be warmer than the surrounding air it will continue to rise higher and higher in the atmosphere.  Now if there is an inversion over this layer as the air tries to rise higher and  it enters this warmer layer, it is no longer warmer that the surrounding air (it is now cooler) so it now begins to sink. The inversion acts as a cap on the lower cooler layer trapping that air beneath it.  On most days the sun will heat the lower layer to the temperature of the inversion and the two different layers mix and the inversion is no longer there. But we see many days in Colorado where the cap is never mixed out or broken.

Looking at a winter scenario, a cold arctic air mass has just moved into Denver via a Canadian cold front.  The cold air is dense and shallow (only a few thousand feet thick) and undercuts warmer air in place creating the inversion in temperature.  This is a likely scenario:  Denver is a rather cold, 18 degrees today while Evergreen in the foothills is a toasty 42.  We have an inversion at about 2500 feet above the ground level.  During the winter months the main concern is air pollution that is being trapped in the cold layer of air at the surface.  Until the cold surface layer warms to 42 or above the surface air will not mix with the warmer layers above it and we wind up with a brown cloud over Denver until winds or warming temperatures break the inversion.

During the summer months we are not so much concerned about pollution as we are about thunderstorms.  Thunderstorms depend on strong rising current of air to build and develop.  As long as these rising currents of air are warmer than the surrounding air they will continue to rise and feed the thunderstorms.  However, there are many days during the summer when warmer air usually moving in from the desert southwest will move in over a slightly cooler layer at the surface.  When this happens we say that the atmosphere is capped.  In other words as air rises it meets this layer of warmer air and is forced back down.  This effectively stops any thunderstorms from developing.  We’ve had 2-3 days in the past week of July where we have had a cap over the Front Range and thunderstorms just could not get going.

I hope this helps you the next time someone talks about an inversion or a cap and why the pollution is particularly bad that day or when you have a clear sunny summer day without seeing any thunderstorms.