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STBS5 - Stability Index

As defined within the OFCM Federal Meteorological Handbook #3:

The stability index is a quantitative measure of the potential energy existing in the atmosphere. It is found by determining the temperature a parcel of air, initially at a selected level, would have if brought from its condensation level to the 500hPa surface by a moist adiabatic process, and then subtracting that temperature from the ambient air temperature at 500hPa.
In more general terms, the stability index indicates the degree of stability of the layer of air extending from a selected level near the surface to the 500hPa surface. Positive values denote stable conditions, while negative values denote unstable conditions. The degree of stability (or instability) is indicated by the magnitude of the value.

In selecting the level near the surface to be used as the base level for the stability index computation, the following criteria is generally used:

If the station elevation is... then the base level is...
Less than 1000 gpm 850 hPa
1000 to 1400 gpm 800 hPa
1401 to 2000 gpm 750 hPa