Showalter Index

Estimate Showalter 850-hPa-origin parcel index via explicit supplied LCL and caller piecewise dry/moist lapse approximation; differs from supplied surface-parcel lifted index, not a full pseudoadiabatic solver or validated thunder forecast.

Description

The Showalter index compares the environment with a parcel lifted from 850 hPa to 500 hPa.

The Showalter index compares the environment with a parcel lifted from 850 hPa to 500 hPa. This approximation separates dry ascent below the lifting condensation level from moist ascent above it.^{[1, 2]}

When to use Showalter Index

Use this tool to estimate Showalter 850-hPa-origin parcel index via explicit supplied LCL and caller piecewise dry/moist lapse approximation; differs from supplied surface-parcel lifted index, not a full pseudoadiabatic solver or validated thunder forecast.

Prepare the observed sounding, cloud records or forecast verification data. Check pressure and height coordinates, wind conventions and the stated units.

How to use Showalter Index and understand its results

Prepare the observed sounding, cloud records or forecast verification data. Check pressure and height coordinates, wind conventions and the stated units.
Enter temperature850ParcelC, environment500HpaTemperatureC, observed850HeightM, observed500HeightM, supplied850ParcelLclHeightM, callerDryLapseCPerKm, callerMoistLapseCPerKm using the field table and calculation rules, then run the tool.
Read piecewiseLifted850Parcel500TemperatureC, calibratedShowalterIndexC. The explanation below defines the units, signs, ordering and any flags or placeholder values.
If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.

Input field	Meaning	Value type; see rules below	Requirement
temperature850ParcelC	Temperature850 parcel c	number	Required
environment500HpaTemperatureC	Environment500 hPa temperature c	number	Required
observed850HeightM	Observed850 height m	number	Required
observed500HeightM	Observed500 height m	number	Required
supplied850ParcelLclHeightM	Supplied850 parcel lcl height m	number	Required
callerDryLapseCPerKm	Caller dry lapse c per km	number	Required
callerMoistLapseCPerKm	Caller moist lapse c per km	number	Required

Enter ordered 850/500-hPa heights, an LCL no lower than the origin and nonnegative lapse rates in °C/km. Split the ascent at the supplied LCL, allowing an LCL above the destination, and report the parcel temperature and signed index in Celsius.

SI = environment500 − [origin850 − (dry distance × dry lapse + moist distance × moist lapse)/1000]

Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.

Output field	Meaning	Value type
piecewiseLifted850Parcel500TemperatureC	Piecewise lifted850 parcel500 temperature c	number
calibratedShowalterIndexC	Calibrated showalter index c	number

json

{
  "input": {
    "temperature850ParcelC": 20,
    "environment500HpaTemperatureC": -10,
    "observed850HeightM": 1000,
    "observed500HeightM": 5000,
    "supplied850ParcelLclHeightM": 2000,
    "callerDryLapseCPerKm": 10,
    "callerMoistLapseCPerKm": 5
  },
  "output": {
    "piecewiseLifted850Parcel500TemperatureC": -5,
    "calibratedShowalterIndexC": -5
  }
}

Limitations and assumptions

The LCL and lapse rates are supplied. This piecewise calculation does not solve a pseudoadiabatic parcel trajectory or establish thunderstorm likelihood.

Numeric inputs and returned numeric values must be finite; an unrepresentable returned value produces an error. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.

Alternative or Complementary methods

Use Lifted Index: Difference measured environmental and caller independently lifted surface-parcel temperatures at 500 hPa; parcel lift is not computed and index alone does not classify storm occurrence. For a complementary task, use Foehn Chinook Warming Rate: Model windward dry/saturated ascent then dry lee descent with supplied crest/LCL/lapse rates; explicit rainout/unsaturated descent idealization, not actual Chinook event or environmental profile fit.

Further background: Atmospheric stability index glossary.

References

Showalter Index — tool.io
Atmospheric stability index glossary — National Weather Service

Similar or alternative tools

Lifted Index
Difference measured environmental and caller independently lifted surface-parcel temperatures at 500 hPa; parcel lift is not computed and index alone does not classify storm occurrence.
Foehn Chinook Warming Rate
Model windward dry/saturated ascent then dry lee descent with supplied crest/LCL/lapse rates; explicit rainout/unsaturated descent idealization, not actual Chinook event or environmental profile fit.