Showalter Index Calculator

Formula Used

Showalter Index (SI) compares the 500 hPa environment temperature to a lifted parcel temperature at 500 hPa.

SI = T₅₀₀ − T_p,500
The parcel starts at 850 hPa with T₈₅₀ and Td₈₅₀.
Lift the parcel dry-adiabatically to the LCL, then moist-adiabatically to 500 hPa.

Negative SI typically indicates increasing instability and higher thunderstorm potential, while positive SI suggests more stable conditions.

How to Use This Calculator

Enter the 850 hPa temperature and dew point from a sounding.
Enter the 500 hPa temperature from the same sounding time.
Select the temperature unit used by your source data.
Press Calculate to view SI and intermediate steps.
Use Download CSV or Download PDF to save results.

Example Data Table

These sample inputs demonstrate typical values for a warm-season sounding.

Case	T(850) °C	Td(850) °C	T(500) °C	Expected SI (°C)	General Meaning
A	18	12	-6	~ -2 to 2	Marginal to unstable
B	22	18	-10	~ -4 to -1	Unstable, storms favored
C	10	2	-12	~ 2 to 6	More stable

Exact SI depends on the thermodynamic path and rounding.

Interpretation Guide

SI Range (°C)	Category	Typical Signal
≤ -6	Very unstable	Strong to severe storms possible
-6 to -3	Unstable	Thunderstorms likely with forcing
-3 to 0	Marginal	Thunderstorms possible
0 to 3	Slightly stable	Showers, isolated storms
> 3	Stable	Deep convection suppressed

Use SI with CAPE, CIN, shear, and moisture depth for better guidance.

Professional Guide

1) What the Showalter Index Measures

The Showalter Index (SI) is a stability indicator designed to flag thunderstorm potential using two mid‑level pressure surfaces. It compares the observed 500 hPa temperature to the 500 hPa temperature of a parcel lifted from 850 hPa. Because it focuses on the lower troposphere and mid‑levels, it is often used as a quick screening metric when full sounding analysis is not available.

2) Required Sounding Data

You need three values from the same upper‑air sounding time: temperature at 850 hPa, dew point at 850 hPa, and temperature at 500 hPa. These levels are commonly present in radiosonde data and many model soundings. A small difference of 1–2 °C in any input can shift SI noticeably, so use quality‑controlled values where possible.

3) Parcel Path Assumptions

The parcel is lifted dry‑adiabatically until it reaches the lifting condensation level (LCL), then it follows a moist ascent to 500 hPa. This calculator estimates the LCL with a widely used analytic approximation and then integrates a pseudo‑moist‑adiabatic path in small pressure steps. Smaller step sizes generally reduce numerical error but require more computation.

4) Reading Index Values

SI is reported in degrees Celsius (and also converted to Fahrenheit for convenience). Values near 0 °C are often considered marginal. Negative values commonly indicate increasing instability, while positive values suggest greater resistance to deep convection. Many practitioners treat SI ≤ −3 °C as a stronger signal that storms are favored, especially with sufficient lift and moisture depth.

5) Regional and Seasonal Context

Thresholds are not universal. Humid warm‑season environments may produce thunderstorms with only slightly negative SI, while elevated mixed layers or dry mid‑levels can complicate interpretation. In colder seasons, 850 hPa may sit above shallow surface inversions, and SI can under‑represent near‑surface stability. Always consider the broader thermodynamic profile.

6) Comparing with Other Stability Metrics

SI is most useful as a rapid check, not a complete diagnosis. For decision‑making, compare with CAPE and convective inhibition, low‑level moisture depth, and vertical wind shear. An environment can show modest SI yet still support severe convection if CAPE is large and shear is strong. Conversely, a very negative SI can fail to produce storms without triggering mechanisms.

7) Quality Control Tips

Verify that dew point does not exceed temperature at 850 hPa; measurement noise or rounding can cause small inconsistencies. If you are reading values from gridded model output, ensure the 850 and 500 hPa levels are present and not extrapolated. If you see extreme results, rerun with a smaller integration step and double‑check unit selection.

8) Practical Use Cases

Use this tool to compare multiple soundings across a day, rank instability between locations, or document training examples. The CSV export is convenient for archiving, and the PDF export supports reporting workflows. For operational forecasting, treat SI as one input among many, especially when mesoscale boundaries, terrain forcing, or dry air intrusions are present.

FAQs

1) What does the Showalter Index represent?

It represents the temperature difference between the observed 500 hPa air and a lifted 850 hPa parcel at 500 hPa. More negative values generally indicate greater potential for deep convection.

2) Why are 850 hPa and 500 hPa used?

These levels are standard in upper‑air datasets and capture low‑level moisture/heat (850 hPa) and mid‑level cooling (500 hPa). The contrast helps identify environments supportive of thunderstorms.

3) Can I use surface temperature instead of 850 hPa?

Not for the classic SI definition. If you want a surface‑based parcel, consider indices or calculations designed for surface parcels. SI is specifically defined from 850 hPa to 500 hPa.

4) Why does the calculator limit dew point to temperature?

Dew point cannot physically exceed temperature. Small measurement or rounding issues can violate this, so the calculator enforces consistency to prevent unrealistic parcel calculations.

5) What SI value suggests thunderstorms are likely?

Many users interpret SI ≤ −3 °C as a stronger signal for storms, with SI between −3 and 0 °C as marginal. However, lift, moisture depth, and shear strongly affect outcomes.

6) How accurate is the parcel temperature estimate?

It is an estimate based on an analytic LCL approximation and a pseudo‑moist‑adiabatic integration. It is suitable for screening and comparisons, but not a substitute for full thermodynamic sounding tools.

7) When should I rely on other diagnostics instead?

If storms are driven by surface boundaries, strong shear, or complex profiles, SI alone can mislead. Combine SI with CAPE, inhibition, moisture depth, and wind profiles for better guidance.