Heat Index Calculator
Enter the air temperature in Fahrenheit and the relative humidity to find the heat index, the temperature it actually feels like when humidity is factored in. This uses the National Weather Service Rothfusz regression.
How to calculate the heat index
- Enter the air temperature in degrees Fahrenheit.
- Enter the relative humidity as a percent from 1 to 100.
- Read the heat index in Fahrenheit and Celsius.
Examples
Hot and humid
T = 90 F, RH = 70%
Heat index = 105.9 F (41.1 C)
Warm with very high humidity
T = 85 F, RH = 90%
Heat index = 101.8 F (38.8 C)
Frequently asked questions
What is the heat index?
The heat index, or apparent temperature, is how hot the air feels to the human body when relative humidity is combined with the actual air temperature. High humidity slows sweat evaporation, so it feels hotter.
What formula does this use?
It uses the National Weather Service Rothfusz regression, with a simpler averaged form for cooler conditions and correction terms for very low or very high humidity.
Why does the heat index need a high temperature to matter?
When the averaged simple estimate is below 80 F, humidity has little effect, so the calculator returns that value. The full regression applies once it feels at least that warm.
When is the heat index dangerous?
The NWS flags caution from about 80 F, extreme caution near 90 F, danger near 103 F and extreme danger at 125 F and above, where heat stroke becomes likely.
Does the heat index account for sun or wind?
No. It assumes shade and light wind. Direct sunlight can add up to about 15 F, and strong wind or sweating can lower how hot it feels.
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