Heat Index Calculator
Units:
The heat index (feels-like temperature) combines air temperature and relative humidity to determine the apparent temperature. High humidity impairs the body's ability to cool itself through sweating, making hot conditions feel significantly hotter than the thermometer reads.
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Tip: During heat advisories, stay hydrated (drink before you feel thirsty), avoid outdoor activity during peak hours (10 AM–4 PM), and check on elderly neighbours who are most vulnerable.
- 1Uses the Rothfusz regression equation (NWS, USA)
- 2Only applicable when temperature > 27°C (80°F) and humidity > 40%
- 3Below this threshold, the actual temperature is used directly
- 4HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541TH − 0.00683783T² − 0.05481717RH² + ...
35°C (95°F), 70% humidity=Feels like 51°C (124°F)Extreme danger level
32°C (90°F), 50% humidity=Feels like 38°C (100°F)Caution — heat cramps possible
| Heat Index (°F) | Category | Risk |
|---|---|---|
| 80–90 | Caution | Fatigue possible with prolonged exposure |
| 90–103 | Extreme Caution | Heat cramps, heat exhaustion possible |
| 103–124 | Danger | Heat cramps/exhaustion likely, heat stroke possible |
| 125+ | Extreme Danger | Heat stroke highly likely |
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Fun Fact
The human body can only cool effectively when sweat evaporates. At 35°C with 100% humidity, sweat cannot evaporate at all — the body's cooling system effectively shuts down.
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