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Abstract

A generalized temperature function, ϑ is introduced to help clarify similarities and differences among various moist `potential' temperatures. The equivalent potential temperature, θe, the liquid water potential temperature, θℓ, and the entropy potential temperature, θs are shown to be specific instances of ϑ, that are selected by the choice of reference state to which ϑ is referred. Contrary to the common assumption that θe,θℓ, and θs are all different flavors of the same thing, only θℓ satisfies the strict definition of a 'potential temperature', as corresponding to a reference temperature accessible by an isentropic and closed transformation of a system in equilibrium; only θe measures difference in air-parcel enthalpy; and only θs measures differences in air-parcel entropy. None mix linearly, but all reduce to the dry potential temperature, θ in the limit as the water mass fraction goes to zero. As is well known, θ does mix linearly and inherits all the favorable (entropic, enthalpic, and potential temperature) properties of its various -- but descriptively less rich -- moist counterparts. Of the three moist `potential' temperatures, θs is the least familiar, but the most well mixed in the broader tropics, which merits further study as a basis for constraining mixing processes.