Showalter Stability Index

The Showalter Stability Index is a measure of the stability of the atmosphere in a local region. It is determined by lifting an air parcel to 500 millibars and then comparing its temperature to that of the environment. If the parcel is colder than its new environment, then the atmosphere is more stable. If the parcel is warmer than its new environment, then the atmosphere is unstable and the potential for thunderstorm development and severe weather increases.

Showalter Index values used at NASP are the following:

Showalter Index (SI)

3 - 1 = Thunderstorm possible, strong trigger needed

0 (-3) = Unstable, Thunderstorms probable

-4 (-6) = Very Unstable, heavy thunderstorm potential

< (6) =Extremely unstable, heavy to strong thunderstorm potential

Concept Mapping Toolkit
Insitute for Human and Machine Cognition
The University of West Florida