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 POTENTIAL TEMPERATURE AND EQUIVALENT POTENTIAL TEMPERATURE

METEOROLOGIST JEFF HABY

Many students are curious in the operational importance of potential temperature and equivalent potential temperature. Potential temperature can be used to compare the temperature of air parcels that are at different levels in the troposphere. Temperature tends to decrease with height. This fact makes it more difficult to note which regions in the troposphere are experiencing WAA and CAA. Therefore, bringing air parcels adiabatically to a standard level (1000 millibars) allows comparisons to be made between air parcels at different elevations. If the potential temperature of an air parcel at one pressure level is colder than air parcels at other pressure levels, a forecaster can infer cold air advection or a cold pocket exists at the pressure level with the lowest Theta (potential temperature).

Finding the potential temperature at a constant pressure level over an area produces one type of Theta chart. The term Theta and potential temperature are synonyms. Higher Theta represents warmer air while lower Theta represents colder air. For example, Theta can be found at 700 millibars. Each location at 700 millibars drops a parcel from the 700 to 1000 millibar level and the temperature is read off at 1000 millibars and thus this is the 700 millibar Theta temperature (Theta is always given in degrees Kelvin). A vertical cross section of Theta can be produced by finding the areal distribution of Theta at many pressure levels, then connecting the points of equal Theta. At this point, sloping constant Theta surfaces can be plotted (see Chaston's Weather Maps book P. 167-8). Air parcels tend to travel along constant Theta surfaces. This makes sense because constant Theta surfaces represent "constant density" surfaces. The path of least resistance on an air parcel that is advecting is for it to remain at the same density as its environment. The term that describes this process is isentropic lifting / descent. Isentropic lifting / descent occurs whenever WAA, CAA or flow of one air mass over another occurs. Less dense air will tend to glide up and over more dense air (thus low level WAA leads to rising air) when less dense air advects toward more dense air. You will hear Theta and isentropic lifting referenced to often in forecast discussions. The trajectories that wind vectors take over Theta surfaces determine how much lifting or sinking will take place due to advection. NWS forecasters are experts on these processes and use them as a major part of their forecasting process. All the different ways of graphing Theta can be quite complex. Key points to remember are that (1) air parcels in a convectively stable environment tend to advect along constant Theta surfaces and (2) low level WAA produces isentropic lifting and uplift while CAA produces isentropic downglide and sinking.

While potential temperature can be used to compare temperatures at different elevations and the trajectory air parcels will take (rising or sinking), equivalent potential temperature can be used to compare BOTH moisture content and temperature of the air. The equivalent potential temperature (or Theta-e as it is usually called) is found by lowering an air parcel to the 1000 mb level AND releasing the latent heat in the parcel. The lifting of a parcel from its original pressure level to the upper levels of the troposphere will release the latent heat of condensation and freezing in that parcel. The more moisture the parcel contains the more latent heat that can be released. Theta-e is used operationally to map out which regions have the most unstable and thus positively buoyant air. The Theta-E of an air parcel increases with increasing temperature and increasing moisture content. Therefore, in a region with adequate instability, areas of relatively high Theta-e (called Theta-e ridges) are often the burst points for thermodynamically induced thunderstorms and MCS's. Theta-e ridges can often be found in those areas experiencing the greatest warm air advection and moisture advection. For more information on Theta-e, consult Chaston's book "weather maps" (starting on page 127).