A quantity first derived in Lorenz (1955) in an investigation to discover what portion of total potential energy could be transformed into kinetic energy under the constraint of quasi-hydrostatic and adiabatic processes. Available potential energy (APE) was defined as the difference between a system's mass integrated total potential energy and the total potential energy of a hydrostatic reference state, i.e. the difference in potential energy between the actual physical state and the reference state, where the latter is defined as the state of minimum potential energy that can be reached through reversible adiabatic processes. In the reference state, all density surfaces are level. This was extended in Van Mieghem (1956) to deal with non-hydrostatic states.