Theta-e

(or Equivalent Potential Temperature) - The temperature a parcel of air would have if
a) it was lifted until it became saturated,
b) all water vapor was condensed out, and
c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars.
Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.