🌡️Heat Index Calculator
Best for T > 27°C / 80°F
40–100%
The heat index (also called apparent temperature or feels-like temperature in summer) estimates how hot it actually feels based on air temperature and relative humidity. High humidity prevents sweat from evaporating effectively, reducing the body's ability to cool itself. The Rothfusz (1990) regression equation used by NOAA and the NWS is the most widely accepted formula.
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Tip: Heat index assumes shaded conditions. Direct sunlight can add an additional 8–15°C to the apparent temperature. Drink water before you feel thirsty — thirst indicates you are already mildly dehydrated.
- 1Based on Rothfusz (1990) regression of physiological studies
- 2HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR ...
- 3T = air temperature (°F), R = relative humidity (%)
- 4Applies when T ≥ 80°F (27°C) and RH ≥ 40%
- 5Accuracy: ±1.3°F under typical conditions
35°C, 60% humidity=Heat index ≈ 42°C — feels significantly hotterHealth risk threshold
40°C, 80% humidity=Heat index ≈ 60°C — extreme dangerCan be fatal without cooling
| Feels-Like Temperature | Category | Health Effects |
|---|---|---|
| 27–32°C (80–90°F) | Caution | Fatigue with prolonged exposure |
| 33–39°C (91–103°F) | Extreme Caution | Possible heat cramps/exhaustion |
| 40–51°C (104–124°F) | Danger | Heat cramps/exhaustion likely; heat stroke possible |
| > 52°C (125°F) | Extreme Danger | Heat stroke highly likely |
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