A method of testing for instability in which a displacement is made from a steady state under the assumption that only the parcel or parcels displaced are affected, the environment remaining unchanged. Although this method has been applied to various problems (e.g., inertial instability), its most familiar context is with vertical displacements from hydrostatic equilibrium, in which the parcel displaced is assumed to undergo adiabatic temperature changes, and the buoyant force resulting from its contrast with the unchanged environment leads to the criterion for stability, γ < Γ, where γ is the lapse rate of virtual temperature with height and Γ is the dry- or saturation-adiabatic lapse rate, according to the condition of the parcel. Compare slice method.