What is θe?

Equivalent potential temperature.

Equivalent potential temperature, written θe, is the temperature an air parcel would have if all of its water vapor condensed, the released latent heat warmed the parcel, and the parcel were then brought adiabatically to a pressure of 1000 hPa. Figure 3 shows the construction for one parcel.

Because it is conserved in both dry and moist adiabatic motion, θe is a standard tracer of air masses and a measure of the energy available to convection. It combines the two quantities, temperature and moisture, that most of our work is about.

Temperature-pressure diagram showing a parcel lifted dry-adiabatically to its condensation level, then along a moist adiabat, then brought down dry-adiabatically to 1000 hPa, where its temperature is the equivalent potential temperature.
Figure 3. Graphical definition of θe on a temperature–pressure diagram. A parcel at 900 hPa with a temperature of 24 °C and a dew point of 16 °C is (1) lifted dry-adiabatically to its lifting condensation level (LCL, 800 hPa), (2) lifted along a pseudo-adiabat until nearly all of its water vapor has condensed, the released latent heat warming it, and (3) brought down dry-adiabatically to 1000 hPa, where its temperature is θe = 344 K. Bringing the parcel straight down to 1000 hPa (dashed) gives its potential temperature, θ = 306 K. Thin gray lines: dry adiabats (solid) and pseudo-adiabats (dashed).