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.

How to forecast Snow/Sleet/Freezing Rain

The approaching winter storm will likely start as snow and transition to freezing rain and then to all rain. So how do meteorologist forecast precipitation type?  The main thing that determines the precipitation type is the temperature aloft.

Snow is just ice crystals aloft that fall and never unfreeze because the temperature at different levels it falls through is always below freezing, this includes the surface being at or below freezing.

Rain starts as ice crystals aloft but when it falls into warmer air a loft that is above freezing it unfreezes and turns into rain. The rain never falls into temperatures below freezing again so it stays in liquid form until it reaches the surface.

Sleet starts as ice crystals aloft like snow, but then falls into warmer air that is above freezing and melts into rain. The liquid then falls into a deep cold pool, where temperatures are below freezing and refreeze into ice crystals before falling to the surface which is also at or below freezing. (Sleet is NOT hail. The difference is how they are made a loft)

Freezing rain, like sleet, starts as ice crystals aloft and falls into warm air and unfreezes. The difference between the two is that the freezing rain doesn't fall until a deep cold pool of air below freezing, so it does not get the chance to refreeze. So the liquid reaches the surface, which is at or below freezing, as rain and freezes on contact.

                           Above is a diagram to show how each form of precipitation occurs.

So we now know warm air a loft is important in forecasting between snow, sleet, and freezing rain but how do we determine what the temperatures up in the sky will be? For this we look at a thermodynamic diagram, which we call a skew-T or a sounding.

Pictured above is a skew-t diagram which shows us altitude, temperature, dew point, wind speed and wind direction. We get these diagrams from launching weather balloons twice a day and then inputting the data from the balloons into the weather models that are run. The models then make their own forecast soundings, which are graphs like above for how they think the atmosphere will look like vertically at a particular place and time in the future.

The sounding above is a freezing rain sounding. The blue line is the 0 degree Celsius line, everything to the left of it is below freezing and everything to the right is above freezing. The red line is the temperature line and the green is the dew point line. Start at the top of the graph and pretend you're an ice crystal and follow the red line down. You will see you are above freezing until about 800 mbs up in the atmosphere. Then the red line goes to the right of the freezing line which means the air temperature is above freezing and you would melt into liquid. As you continue following the red line you go back to the left of the freezing line but do not have a chance to refreeze before you hit the surface which is also below freezing so you hit the ground as rain but freeze on contact.

Hopefully this gives you a little insight on how we forecast for the different types of winter weather!