3. Stability

Determining Stability » The Parcel Method Introduction

Photo of thunderstorm at night

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.