3. Stability

Determining Stability » Potential Errors Other Effects that Modify Vertical Motion

Non-thermodynamic effects that retard or enhance the buoyancy-driven vertical motion of air parcels

Several non-thermodynamic effects occur that retard or enhance the buoyancy-driven vertical motion of parcels, particularly in and around convective storms. The parcel method fails to account for these processes. Among those processes are the following:

  • Friction and drag between the rising thermal or cloud and the surrounding winds. These processes have their greatest effect when there is strong vertical wind shear in the environment.
  • A reduction in buoyancy due to the weight of condensed water. This effect retards updrafts and enhances downdrafts.
  • Drag from falling precipitation. This is another process that retards updrafts and enhances downdrafts.
  • Compensatory subsidence near a convective updraft. Some of the surrounding air is drawn downward to replace the rising and expanding updraft.