✅ What Is Heat Index?
The heat index, also known as humiture, represents how hot the weather feels to the human body when humidity is combined with the actual air temperature.
Just like the wind chill factor makes cold weather feel colder, the heat index makes hot weather feel hotter.
🔥 Example
If the air temperature is 83°F with 70% relative humidity, the weather can feel like 88°F.
This happens because humidity slows down the evaporation of sweat — the body’s natural cooling system.
⭐ Why Perceived Heat Increases
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Higher humidity → slower sweat evaporation
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Slower evaporation → reduced cooling
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Reduced cooling → higher apparent temperature (heat index)
Although the heat index can be used indoors, it is mainly used for outdoor temperature assessments.
✅ How to Calculate Heat Index?
The U.S. National Weather Service (NWS) uses a mathematical equation developed in 1978 by George Winterling.
It is valid when:
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Temperature ≥ 80°F (27°C)
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Relative humidity ≥ 40%
The full equation includes many assumptions about:
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Body weight
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Height
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Clothing
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Fitness level
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Blood thickness
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Wind speed
Because of these personal differences, the heat index is only an estimate, not an exact measurement for every individual.
Most people use:
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A heat index chart
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A heat index calculator
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A weather app
✅ Heat Index Health Effects
The heat index can impact human health significantly. When humidity is high, sweat cannot evaporate effectively, and the body struggles to cool down. This may lead to heat exhaustion, heat cramps, or even heat stroke.
Heat Index Levels & Effects
| Celsius | Fahrenheit | Notes |
|---|---|---|
| 27–32°C | 80–90°F | Caution: Fatigue possible with prolonged exposure; risk of heat cramps. |
| 32–41°C | 90–105°F | Extreme caution: Heat cramps or heat exhaustion possible. |
| 41–54°C | 105–130°F | Danger: Heat cramps/exhaustion likely; heat stroke probable with continued activity. |
| Over 54°C | Over 130°F | Extreme danger: Heat stroke imminent. |
🌞 Direct sunlight can add up to 14°F to the heat index.
🚸 Who Is at Higher Risk?
Children and individuals with specific conditions are more vulnerable.
Children
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Higher skin surface area relative to body size
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Sweat less efficiently than adults
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Often forget to rest or hydrate
Higher-risk individuals include those with:
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Obesity
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Diabetes
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Heart disease
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Cystic fibrosis
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Developmental disabilities
💧 Important Hydration Note
Thirst is a late sign of dehydration.
Hydrate before, during, and after outdoor activity, especially in hot and humid weather.