Wind chill and heat index — feels-like temperature
Air temperature alone does not describe comfort. Wind chill explains how cold exposed skin feels when wind strips away the insulating boundary layer of warm air. Heat index explains how hot it feels when humidity slows sweat evaporation. This wind chill calculator uses U.S. National Weather Service formulas: wind chill for cold windy days (temperature ≤ 50°F, wind ≥ 3 mph) and heat index for hot humid days via the Rothfusz regression. Enter air temperature in degrees Celsius; results show °C with °F shown for reference.
Wind chill formula
The U.S. NWS formula uses Fahrenheit and mph internally: WC = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275T×V^0.16. This calculator converts your °C input, applies the formula, and converts the feels-like result back to Celsius. Example: about −4°C with 15 mph wind yields a much lower wind chill. Wind chill applies at 10°C and below with wind ≥ 3 mph.
Heat index formula
Heat index combines temperature and relative humidity. The Rothfusz equation adds correction terms for very low humidity below 80°F and very high humidity near 87°F. When heat index exceeds roughly 41°C (105°F), heat illness risk rises—hydrate, limit exertion, and seek shade. Results display in Celsius first.
When to use each mode
- Wind chill mode: winter forecasts, ski trips, dog walks, construction in cold climates.
- Heat index mode: summer humidity, athletic training, outdoor events in the Southeast U.S.
Frostbite and exposure time
Wind chill estimates heat loss rate for exposed skin—it is not an actual thermometer reading. At extreme wind chill values, frostbite can occur in minutes on uncovered skin. Cover extremities, limit time outdoors, and watch for numbness or waxy skin. Hypothermia risk rises when core temperature drops; wet clothing accelerates cooling.
Heat index safety bands
NWS categorizes heat index into caution, extreme caution, danger, and extreme danger. Above roughly 103°F heat index, heat cramps and exhaustion become more likely; above 125°F is rare but extremely dangerous. Schedule outdoor labor for morning/evening, use shade, and never leave children or pets in parked cars—even moderate outdoor temps create lethal cabin heat.
Limitations
Formulas assume shade, light wind exposure for heat index, and standard clothing assumptions for wind chill. Sun, rain, individual health, and altitude change perceived comfort. Always follow official NWS advisories during extreme weather.
Convert base temperatures with Fahrenheit to Celsius on Toolsle.