Sweat Index Calculator

The SWEAT (Severe Weather Threat) index combines moisture, instability, wind speeds, and directional shear into one score.

• Td850: 850 hPa dewpoint (°C).
• TT: Total Totals index (dimensionless). If computed: TT = (T850 − T500) + (Td850 − T500).
• V850, V500: wind speeds at 850 and 500 hPa (knots).
• DD850, DD500: wind directions (degrees, meteorological).
• Directional term: emphasizes veering wind with height, often linked with warm advection and favorable shear.

Many training tools apply screening rules: if winds are weak or directions do not show a classic veering profile, the shear term is set to zero.

1. Choose whether you will enter TT directly or compute it from T850 and T500.
2. Enter Td850, wind speeds at 850 and 500 hPa, and wind directions at both levels.
3. Keep Apply screening rules enabled for a conservative operational-style result.
4. Press Calculate to see the score and term-by-term breakdown above the form.
5. Use Download CSV or Download PDF to export the latest result.

1) What the Sweat Index measures

The Sweat Index, commonly called SWEAT, is a composite number that summarizes several severe-storm ingredients from upper-air data. It blends low-level moisture, instability, wind speed, and directional shear into one score that is easy to compare between soundings and forecast hours.

2) Inputs you need and where they come from

This calculator uses 850 hPa dewpoint (Td850), Total Totals (TT) or temperatures to compute TT, plus 850 and 500 hPa winds (speed and direction). These values typically come from radiosonde observations, model soundings, or analyzed upper-air charts.

3) Moisture term: why Td850 is weighted strongly

The moisture contribution is 12×Td850, so each 1°C rise in dewpoint adds 12 points. For example, Td850 of 10°C contributes 120, while 18°C contributes 216. This reflects how deeper boundary-layer moisture supports stronger buoyancy and heavier precipitation cores.

4) Instability term: Total Totals as a quick proxy

The instability part is 20×(TT−49). Values below 49 are commonly screened to zero because they often indicate weak lapse rates or limited convective depth. A TT of 55 adds 120 points, which can dominate the total when moisture is also high.

5) Wind speed terms: organization and storm longevity

Wind speed enters as 2×V850 + V500, using knots. Faster flow supports storm ventilation and upscale growth. A profile with 30 kt at 850 hPa and 50 kt at 500 hPa contributes 110 points from speed alone, even before direction is considered.

6) Directional shear term: the role of veering

The directional term uses 125×(sin(DD500−DD850)+0.2). Positive veering with height often boosts this contribution, while backing flow can reduce it. The optional screening rules set the term to zero unless winds are at least 15 kt and directions fall in common veering windows.

7) How to interpret the score in practice

Many operational guides treat 150–300 as slight severe potential, 300–400 as severe possible, and 400+ as a tornadic-favorable environment when forcing and storm mode cooperate. Use the term breakdown to see whether the score is driven by moisture, instability, or shear. Scores above 500 are uncommon and usually signal multiple ingredients aligning locally and simultaneously.

8) Limitations and best-use workflow

SWEAT is not a guarantee of storms. It does not explicitly include CIN, storm-relative helicity, boundary placement, or convective triggers. Treat it as a screening tool: compare nearby soundings, toggle screening rules, then confirm with radar trends, mesoscale analyses, and local expertise.

Q1) What is a typical SWEAT range in everyday weather?

Many fair-weather soundings produce values below 150. Ordinary thunderstorm environments often fall between 150 and 300. Higher values usually require both rich low-level moisture and stronger midlevel flow.

Q2) Can I use relative humidity at the surface instead of Td850?

No. SWEAT is defined with 850 hPa dewpoint because it represents deeper low-level moisture. If you only have surface data, first estimate an 850 hPa dewpoint from a model sounding or upper-air analysis.

Q3) Why does the calculator set the instability term to zero for TT below 49?

Lower TT values often indicate limited convective depth or weaker lapse rates. Many operational implementations screen this term so the index does not overstate risk when instability is marginal.

Q4) What do the screening rules change?

When enabled, the directional shear term is removed unless winds are at least 15 kt and directions show a classic veering profile with height. This produces a more conservative result that better matches common training guidance.

Q5) Do wind directions use meteorological or mathematical convention?

Use meteorological wind direction: the direction the wind is coming from, in degrees. For example, 180° indicates a southerly wind, while 270° indicates a westerly wind.

Q6) Is a SWEAT value above 400 a tornado forecast?

Not by itself. A value above 400 suggests a favorable ingredient mix, but tornado potential also depends on boundaries, storm mode, inhibition, and forcing. Always combine SWEAT with mesoscale analysis and real-time observations.

Q7) Why are knots used for wind speed?

The original index was developed with knots for upper-air winds, and the coefficients assume that unit. If you have m/s or km/h, convert to knots before entering values to keep the score consistent.