The intricacies of instabilities

In response to Sherwood's comments and in an attempt to restore proper usag e of terminology associated with moist instability, the early history of mo ist instability is reviewed. This review shows that many of Sherwood's conc erns about the terminology were understood at the time of their origination . Definitions of conditional instability include both the lapse-rate defini tion (i.e., the environmental lapse rate lies between the dry- and the mois t-adiabatic lapse rates) and the available-energy definition (i.e., a parce l possesses positive buoyant energy; also called latent instability), neith er of which can be considered an instability in the classic sense. Furtherm ore, the lapse-rate definition is really a statement of uncertainty about i nstability. The uncertainty can be resolved by including the effects of moi sture through a consideration of the available-energy definition (i.e., con vective available potential energy) or potential instability. It is shown t hat such misunderstandings about conditional instability were likely due to the simplifications resulting from the substitution of lapse rates for buo yancy in the vertical acceleration equation. Despite these valid concerns a bout the value of the lapse-rate definition of conditional instability, con sideration of the lapse rate and moisture separately can be useful in some contexts (e.g., the ingredients-based methodology for forecasting deep, moi st convection). It is argued that the release of potential (or convective) instability through layer lifting may occur in association with fronts, rat her than with isolated convection, the terminology "convective'' being an u nfortunate modifier. The merits and demerits of slantwise convective availa ble potential energy are discussed, with the hope of improving diagnostic m ethodologies for assessing slantwise convection. Finally, it is argued that , when assessing precipitation events, undue emphasis may appear to be plac ed on instability, rather than the forcing for ascent, which should be of p rimary importance.