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How It Works
The heat index combines air temperature and relative humidity into a single number that represents how hot it actually feels. When humidity is high, sweat evaporates more slowly, reducing the body's ability to cool itself.
This calculator uses the Rothfusz regression equation, the standard formula adopted by the U.S. National Weather Service. Enter the air temperature in °F and relative humidity as a percentage to get the apparent temperature.
Example Problem
The air temperature is 96 °F and the relative humidity is 65%. What does it feel like outside?
- Enter T = 96 °F and R = 65% into the Rothfusz equation
- The nine-term polynomial evaluates to approximately 121 °F
- This falls in the “Danger” category — heat cramps, exhaustion, and heat stroke are probable
Heat Index Health Effects
| Heat Index | Classification | Possible Health Effects |
|---|---|---|
| 80–90 °F | Caution | Fatigue possible with prolonged exposure and physical activity. |
| 90–105 °F | Extreme Caution | Heat cramps and heat exhaustion possible. |
| 105–130 °F | Danger | Heat cramps, heat exhaustion, and heat stroke probable. |
| > 130 °F | Extreme Danger | Heat stroke highly likely. |
Frequently Asked Questions
What is the heat index and why does it matter?
The heat index is the “feels like” temperature that accounts for humidity. It matters because heat-related illness risk depends on apparent temperature, not air temperature alone. At 90 °F with 70% humidity, the heat index reaches about 106 °F, putting people at serious risk.
How do you calculate heat index from temperature and humidity?
The NWS uses the Rothfusz regression, a nine-term polynomial in air temperature (T) and relative humidity (R). The equation includes squared and cross terms to capture the nonlinear relationship between humidity and perceived heat.
What heat index is dangerous for outdoor activity?
The NWS classifies heat index values above 105 °F as “Danger” and above 130 °F as “Extreme Danger.” At the Danger level, heat cramps, heat exhaustion, and heat stroke become probable with prolonged exposure.
Does the heat index work at any temperature?
The Rothfusz equation is designed for air temperatures of 80 °F and above. Below that threshold, the heat index approximation is less meaningful because humidity has minimal effect on comfort at lower temperatures.
What is the difference between heat index and wet bulb temperature?
The heat index is an empirical “feels like” number for general public use. Wet bulb temperature is a direct physical measurement of evaporative cooling potential. Both account for humidity, but wet bulb is used more in occupational safety and climate science.
Related Calculators
- Wind Chill Calculator — calculate how cold it feels when wind is factored in.
- Water Vapor Pressure Calculator — find vapor pressure at a given temperature using the Antoine equation.
- Relative Humidity Calculator — compute relative humidity from temperature and dew point.
- Temperature Conversion Calculator — convert between Fahrenheit, Celsius, and Kelvin.
- Cloud Base Calculator — estimate cloud ceiling from temperature and dewpoint.