Available potential energy (APE) is a quantity first derived in Edward Lorenz in1955 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  to deal with non–hydrostatic states.
Graph of morning sounding with a large hydrolapse followed by an afternoon sounding showing the cooling which occurred in the mid-levels resulting in an unstable atmosphere as surface parcels have become negatively buoyant. Red line is temperature; green line is the dew point; yellow line is the lifted air parcel. Source: Creative Commons
In the field of atmospheric meteorology, the term convective available potential energy is invoked to indicate the is the energy content of air parcel, if that parcel were elevated to a certain altitude in the atmosphere. The meteorological unit most commonly utilized is Joules per cubic meter. The existence of a positive value for the convective available potential energy is an indicator of vertical instability in the lower atmosphere, and can be a good predictor of thunderstorm activity.
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