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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
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