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🌡️ Heat Index Calculator
Calculate how hot it feels based on temperature and humidity
☀️ Weather Conditions
📊 Heat Index Results
Feels Like Temperature
100°F
Actual Temperature
90°F
Relative Humidity
60%
Temperature Difference
+10°F
Heat Risk Level
Caution
📚 Understanding Heat Index
What is Heat Index?
The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with air temperature. High humidity reduces the body's ability to cool itself through evaporation of sweat.
Heat Index Formula
The National Weather Service uses the Rothfusz regression equation:
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R²
Where T = temperature (°F) and R = relative humidity (%)
Heat Risk Categories
- Caution (80-90°F): Fatigue possible with prolonged exposure and physical activity
- Extreme Caution (90-103°F): Heat cramps and heat exhaustion possible
- Danger (103-125°F): Heat cramps and heat exhaustion likely; heat stroke possible
- Extreme Danger (125°F+): Heat stroke highly likely with continued exposure
Safety Precautions
- Stay hydrated by drinking plenty of water
- Avoid strenuous outdoor activities during peak heat hours
- Wear light-colored, loose-fitting clothing
- Take frequent breaks in air-conditioned or shaded areas
- Never leave children or pets in parked vehicles
- Check on elderly neighbors and relatives
Frequently Asked Questions
Why does humidity make it feel hotter?
Your body cools itself through evaporation of sweat. When humidity is high, the air is already saturated with moisture, making it harder for sweat to evaporate. This reduces your body's ability to cool down, making you feel hotter.
At what heat index should I avoid outdoor activities?
When the heat index reaches 90°F or higher, you should limit outdoor activities, especially strenuous exercise. At 103°F and above, outdoor activities become dangerous and should be avoided or limited to short periods with frequent breaks.
Is heat index the same as "feels like" temperature?
Yes, heat index is commonly referred to as the "feels like" temperature in hot weather. In cold weather, the "feels like" temperature is called wind chill. Both describe how weather conditions affect human perception of temperature.
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