Fighter Index of Thermal Stress Calculator

Inputs

Use any of the three input modes. If wet-bulb isn’t available, the calculator can estimate it from humidity or dew point.

Input mode Dry-bulb + Relative Humidity Dry-bulb + Dew Point Dry-bulb + Wet-bulb

Temperature unit °C °F

Decimal places 0 1 2 3 4 5 6

Dry-bulb temperature

Ambient air temperature (reported as dry-bulb).

Wet-bulb temperature

Natural wet-bulb (if measured).

Relative humidity (%)

Used to estimate wet-bulb when not measured.

Dew point temperature

Used to compute humidity, then wet-bulb.

Calculate Reset

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Formula used

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How to use

1. Select an input mode based on the weather values you have.
2. Choose the temperature unit you will enter.
3. Enter the dry-bulb temperature, plus humidity, dew point, or wet-bulb.
4. Click Calculate to show results above the form.
5. Use the download buttons to export a calculation report.

Example data

Examples below use dry-bulb (°C) and relative humidity to estimate wet-bulb, then compute FITS (°F).

Tip: increasing humidity raises wet-bulb temperature and can push FITS into higher risk zones.

Article

The sections below explain what the index means, what the inputs represent, and how to interpret results for operational planning.

1) What the fighter heat-stress index represents

The fighter index of thermal stress (FITS) is a quick cockpit heat-stress estimate derived from ambient weather. It combines dry-bulb temperature and wet-bulb temperature to approximate how hard it is for the body to shed heat. Higher values indicate reduced cooling, faster fatigue, and greater risk during ground waits and sortie cycles.

2) Dry-bulb temperature: the baseline heat load

Dry-bulb (Tdb) is the standard air temperature measured in shade and ventilation. It sets the baseline thermal load from the environment and warmed surfaces. In the FITS formula, Tdb carries a coefficient of 0.35, reflecting that air temperature matters, but moisture-driven cooling limits matter more.

3) Wet-bulb temperature: humidity and evaporation combined

Wet-bulb (Twb) reflects evaporative cooling potential and rises sharply with humidity. FITS weights Twb at 0.83, so a small Twb increase can move the final index into a higher risk zone. That is why the same dry-bulb temperature can feel very different between dry and humid climates.

4) How the calculator estimates Twb when missing

If Twb is not available, the tool estimates it from dry-bulb plus either relative humidity or dew point. For reliable planning, keep inputs within typical meteorological ranges and treat estimates as guidance. Estimation uncertainty grows at extreme humidity values and outside common temperature ranges.

5) FITS equation used in this tool

The calculator converts inputs to Fahrenheit for the operational equation: FITS (°F) = 0.83×Twb (°F) + 0.35×Tdb (°F) + 5.08. The constant 5.08 shifts the index to match the historical scale used in cockpit heat-stress guidance.

6) Interpreting risk zones with practical thresholds

This calculator labels zones using common operational breakpoints: Normal below 90 °F, Caution from 90–99 °F, and Danger above 100 °F. As a planning flag, values above 115 °F suggest strongly limiting exposure and reducing non-essential ground time outside cooling.

7) Data examples: how conditions change the index

Small changes in humidity can produce large index swings. For example, at 32 °C with 50% RH, the estimated Twb is about 24.0 °C and FITS about 98.9 °F (Caution). At 35 °C with 60% RH, Twb rises near 28.5 °C and FITS near 107.5 °F (Danger). At 38 °C with 70% RH, FITS can reach ~116.2 °F (high Danger).

8) Using results to plan hydration and recovery

Treat FITS as a “schedule and exposure” indicator. In Caution, prioritize hydration, reduce preflight exertion, and keep cool air access close. In Danger, shorten uncooled exposure, extend recovery between sorties, and stage shade or cooling where delays occur. Always follow local medical and operational policy.

FAQs

1) What is FITS in simple terms?

FITS is a quick heat-stress number that blends air temperature and wet-bulb temperature. Higher FITS means reduced evaporative cooling and higher operational heat risk.

2) Which input mode should I choose?

Use measured wet-bulb when available. Otherwise, pick relative humidity if you have it. Dew point works well too, because it can be converted into humidity for estimating wet-bulb.

3) Is FITS the same as WBGT?

No. WBGT includes radiation and wind effects more explicitly. FITS is a simplified operational estimate designed around dry-bulb and wet-bulb conditions, so it is best for fast screening and planning.

4) Why does humidity raise FITS so much?

Humidity limits sweat evaporation. That increases wet-bulb temperature, and FITS heavily weights wet-bulb (0.83). So humid air reduces cooling even when dry-bulb stays the same.

5) Can I enter temperatures in Fahrenheit?

Yes. Select °F in the unit selector and enter values directly. The tool converts internally as needed and still reports FITS in °F, which matches the operational scale used for thresholds.

6) How accurate is estimated wet-bulb temperature?

It is an approximation based on typical atmospheric behavior. It works best in common ranges and can drift at extremes. If you can measure wet-bulb directly, use that for the most dependable index.

7) What should I do in the Danger zone?

Minimize time outside cooling, extend recovery between sorties, and avoid intense exertion before flight. Use shade, cooling vests, and planned hydration. Follow unit procedures and medical guidance for heat-stress control.