Enter Sounding Inputs
Worked Example Table
| Parameter | Example Value | Meaning |
|---|---|---|
| 850 mb dew point | 16 °C | Low-level moisture term input |
| 850 mb temperature | 18 °C | Used to derive Total Totals |
| 500 mb temperature | -8 °C | Used to derive Total Totals |
| Total Totals | 50 | Computed as 18 + 16 - 2(-8) |
| 850 mb wind | 25 kt from 170° | Lower-level wind contribution |
| 500 mb wind | 45 kt from 260° | Mid-level wind contribution |
| Directional shear term | 150.00 | Criteria satisfied and term remains active |
| Final SWEAT Index | 457.00 | Illustrative tornado-threshold example |
SWEAT Equation
SWEAT = 12(Td850) + 20(TT - 49) + 2(f850) + f500 + 125(sin(Dir500 - Dir850) + 0.2)
Where:
- Td850 = 850 mb dew point in °C
- TT = Total Totals index
- f850 = 850 mb wind speed in knots
- f500 = 500 mb wind speed in knots
- Dir500 - Dir850 = wind-direction difference in degrees
TT = T850 + Td850 - 2(T500)
Operationally, no SWEAT term is allowed to stay negative. The shear term is also forced to zero unless all directional and minimum-speed requirements are satisfied.
How to Use This Calculator
- Select whether you want to derive Total Totals from temperatures or enter TT directly.
- Enter the 850 mb dew point in degrees Celsius.
- Provide either T850 and T500, or the direct TT value.
- Choose the wind-speed unit you have available.
- Enter 850 mb and 500 mb wind speeds and directions.
- Press Calculate SWEAT Index to display the result above the form.
- Review the breakdown table, graph, and notes for term behavior.
- Use the export buttons to save the result as CSV or PDF.
FAQs
1) What does the SWEAT index measure?
It estimates severe-weather potential by combining low-level moisture, instability, wind speed, and directional shear into one composite value. It is meant for severe storm diagnosis, not ordinary shower forecasting.
2) What values suggest stronger severe-weather potential?
Values near 250 can support strong convection. Around 300, severe thunderstorms become more concerning. Near 400 or higher, tornado-favorable environments become more plausible, especially when lift and storm initiation are present.
3) Why does the calculator convert wind speed to knots?
The operational SWEAT equation uses 850 mb and 500 mb wind speeds in knots. Converting entered speeds keeps the output consistent with the standard meteorological form of the index.
4) Why can the shear term become zero?
The directional shear contribution is only used when wind directions fall in the accepted sectors, the 500 mb direction exceeds the 850 mb direction, and both wind speeds are at least 15 knots.
5) Can I enter Total Totals directly?
Yes. Choose the direct TT mode when you already know the Total Totals value. Otherwise, let the calculator derive it from 850 mb temperature, 850 mb dew point, and 500 mb temperature.
6) Is a high SWEAT value enough to guarantee severe storms?
No. A high value shows potential, not certainty. Storm initiation still depends on lifting mechanisms, mesoscale boundaries, capping strength, moisture depth, and timing.
7) Why might the dew point or TT term be zero?
The equation does not keep negative contributions. If the 850 mb dew point is negative, that moisture term becomes zero. If TT is below 49, the instability term becomes zero.
8) Is this calculator useful outside storm forecasting?
It is mainly a sounding-based severe-weather screening tool. It can support atmospheric analysis, forecasting exercises, and classroom work, but it should be combined with other stability and shear diagnostics.