3. Stability

Determining Stability » Potential Errors Non-Adiabatic Warming and Cooling

Non-adiabatic heating and cooling processes

We use the parcel method to assess stability because the results correlate nicely with the observed weather. However, because the parcel method relies solely upon adiabatic processes, it fails to account for many processes that affect stability. For example, if we look at a convective environment, we can find several processes that result in the transfer of heat and moisture between the rising updraft and the surrounding environment. These processes include the following:

  • Horizontal mixing of the convective updraft or cumulus cloud with its environment. Mixing the saturated updraft with the drier surrounding air cools the updraft through evaporation and reduces its water content. This decreases the buoyancy of the updraft, particularly in the outer parts of the convective column.
  • Vertical mixing, both within the updraft and between the updraft and the environment at the top. In cumulus clouds this leads to downdrafts, which redistribute condensed water and heat. Consequently, lapse rates may depart from moist (saturation) adiabatic.
  • Latent cooling from melting of falling precipitation. For instance, the lapse rate in rising saturated air that is being cooled by melting of falling snow or hail can substantially exceed the moist (saturation) adiabatic lapse rate.