This calculator requires the use of Javascript enabled and capable browsers. This script calculates the Showalter Index from the difference of an 850 millibar parcel (air mass) temperature and the 850 millibar environmental temperature. The Showalter Index is the simplest measure of the local static stability of the atmosphere. Positive values indicate the lifted parcel is colder than its new environment, and thus the atmosphere is stable. Thunderstorms are likely to develop in regions where the stability indices are at critical levels AND an initial lifting mechanism exists. The lifting mechanism can be solar heating or air flow patterns. The Showalter Index (SI) is very similar to the Computed Lifted Index (LI) except that it always uses the parcel characteristics at the 850 mb level. It is a stability index used to gage the severity of potential storms given that the initial lifted parcel begins at 850 mb. The Glossary of Meteorology states that cumulonimbus cloud structures do not usually form when SI is greater than +4, but that showers and thunderstorms become more likely as the value of SI decreases from +4. Our information source for the Showalter Index is the Glossary of Meteorology; Huschke, R. E., Ed., 1959.
SI is useful when shallow, cool air below 850mb conceals grater convective potential aloft. SI is useful when tornadic potential is high, because the moist layer almost always extends above 850mb during violent outbreaks. These are the defined levels.
Showalter Index greater than 0: Stable, but weak with convection possible for SI = 1-2 if strong lifting is present.
Showalter Index between 0 and -3: Moderately unstable.
Showalter Index between -4 and -6: Very Unstable.
Showalter Index between less than -6: Extremely Unstable.
