Conserved variable diagram
From Glossary of Meteorology
conserved variable diagram
One conserved variable representing heat or temperature is usually plotted along the abscissa, while the another conserved variable representing the conservation of water is plotted along the ordinate. The variables are chosen to be those that are conserved for both saturated and unsaturated motion. Examples of conserved variables are equivalent potential temperature θe, liquid water potential temperature θL, moist static energy se, liquid water static energy sL, total water mixing ratio rT, saturation point pressure PSP, and saturation point temperature TSP. These variables are not conserved for processes such as precipitation, radiative cooling, and mixing. When a third variable representing height z or pressure P is also indicated in the diagram, then there is enough information to define completely the thermodynamic state and water content of the air. Examples of popular sets of variables are (P, θe,rT), (P, θL, rT), (P, se, rT), (P, sL, rT), and (P, PSP, TSP).
Bohren, C. F., and B. A. Albrecht 1998. Atmospheric Thermodynamics. 402 pp.