Determining Stability » The Parcel Method Introduction

Nearly all techniques routinely used to analyze the stability of the atmosphere employ the "parcel" method. In this method, stability is determined by lifting or lowering a hypothetical air parcel and comparing its resulting characteristics to those of the surrounding environment. Here the surrounding environment and associated environmental lapse rate are defined by measurements from a sounding device (e.g., rawinsonde), which are then plotted on a thermodynamic diagram.
Using the parcel method, we assume ascending or descending parcels of air experience temperature and moisture changes associated with two primary processes:
- Pressure change
- The release or uptake of latent heat due to condensation or evaporation
We further assume a parcel does not interact with its surroundings. Therefore, the temperature of a parcel changes adiabatically as it is displaced a small distance vertically from its original position. As a consequence, an unsaturated parcel's virtual temperature changes at the dry adiabatic rate. For saturated conditions, an ascending parcel experiences latent heat of condensation and will cool at the saturated adiabatic rate, while a descending parcel warms and immediately becomes unsaturated, warming at the dry adiabatic lapse rate.
The parcel method defines three basic states of atmospheric stability: stable, neutral, and unstable. We will examine them next.