Introduction
Nearly all the procedures routinely used to analyze the stability of the atmosphere employ the "parcel" method. The stability is determined by lifting or lowering a hypothetical air parcel and comparing the resulting parcel conditions to the conditions of the surrounding environment. Here the surrounding environment and associated environmental lapse rate is that measured by a sounding device (e.g. rawinsonde) and plotted on the thermodynamic diagram. We assume ascending or descending parcels of air experience temperature and moisture changes associated with two primary processes: pressure change and the release or uptake of latent heat due to condensation or evaporation. We further assume that the parcel does not interact with its surroundings. Therefore the temperature of a parcel changes adiabatically as the parcel is displaced a small distance vertically from its original position. As a consequence, the virtual temperature of an unsaturated parcel changes at the dry adiabatic rate. For saturated conditions, an ascending parcel experiences latent heat of condensation and will therefore change at the moist adiabatic rate, while a descending parcel warms and immediately become unsaturated, warming at the dry adiabatic lapse rate
Using parcel theory, there are three basic conditions of atmospheric stability.