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Heat Index Calculator

🌡️Heat Index Calculator

Temperature (°C)

Best for T > 27°C / 80°F

Relativní vlhkost (%)

40–100%

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Detailed Guide Coming Soon

We're working on a comprehensive educational guide for the Heat Index Calculator in your language. The content below is shown in English.

What is Heat Index Calculator?

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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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How to Heat Index Calculator

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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 + ...

Worked Examples

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Example 1

Given:35°C (95°F), 70% humidity

Výsledek:Feels like 51°C (124°F)

Extreme danger level

Example 2

Given:32°C (90°F), 50% humidity

Výsledek:Feels like 38°C (100°F)

Caution — heat cramps possible

Heat Index Risk Categories (NWS)

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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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Pro 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.

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Did you know?

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.

References

Mathematically verified

Reviewed October 2026

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Heat Index Calculator