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NWS wind chill and Rothfusz heat index formulas. Works in F or C.
Wind Chill and Heat Index Calculator
The thermometer tells you the air temperature. This calculator tells you how it feels: wind chill for cold, windy days and heat index for hot, humid ones, each with a plain-language safety note.
-Feels like
-Actual temperature
-Difference
What wind chill measures
Wind chill is how cold exposed skin feels when wind carries heat away from the body. The National Weather Service formula applies at temperatures of 50F or below and wind speeds of 3 mph or more. Outside those limits the formula is extrapolated, and the result should be read as a rough guide.
What heat index measures
The heat index combines air temperature and humidity into what the air feels like to the human body, because humid air slows sweat evaporation. The simplified Rothfusz equation is used here, and it is most reliable at 80F and above.
Worked example
Air temperature 35F with a 15 mph wind.
- Wind chill = 35.74 + 0.6215(35) - 35.75(15)^0.16 + 0.4275(35)(15)^0.16 = 25.4F.
- That feels about 10 degrees colder than the thermometer reads.
- The same 35F with 40% humidity gives a heat index close to the actual temperature.
Take the safety bands seriously
Wind chill near or below 0F can cause frostbite within 30 minutes. A heat index above 103F can cause heat exhaustion during activity. These bands are conservative by design: when in doubt, take shelter and stay hydrated.
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Frequently asked questions
Why does the feels-like temperature differ from the thermometer?
Wind speeds up heat loss from the skin, making it feel colder. Humidity slows sweat evaporation, making it feel hotter. A thermometer measures air temperature, not how your body experiences it.
Can I enter Celsius?
Yes. The calculator converts Celsius to Fahrenheit internally, runs the formula, and converts the result back to your unit.
What if my inputs are outside the formula range?
The calculator still shows a result, but adds a note that the formula was extrapolated. Treat out-of-range results as a rough guide, not a precise forecast.
Which is more dangerous, wind chill or heat index?
Both can be. Wind chill below 0F risks frostbite in minutes, while a heat index above 115F risks heatstroke. Read the safety note the calculator shows for your conditions.