On Moist Instability*

An argument is made that the concepts of conditional instability and conditional symmetric instability need to be revisited. Confusion in the profession has led to two extant definitions of conditional instability that are superficially similar but fundamentally inconsistent. Only one definition corresponds to a real instability. Further, spillover of this confusion into the analysis of slantwise instability mechanisms appears to be producing inappropriate diagnosis of conditional symmetric instability. Concepts from hydrodynamic stability theory are helpful in discussing the situation.