How the Wind Chill & Heat Index Calculator Works
Wind chill and heat index both answer the same underlying question — how does moving air or humid air change how a given temperature actually feels on skin — but they cover opposite ends of the thermometer and use two completely different formulas. Wind chill describes how much colder cold air feels once wind is added, because moving air strips away the thin layer of warm air your body maintains next to your skin. Heat index describes how much hotter warm air feels once humidity is added, because humid air slows the evaporation of sweat, the body's main cooling mechanism. This calculator computes whichever one actually applies to the numbers you enter, using the exact formulas published by the U.S. National Weather Service, and tells you honestly when neither one is defined for your conditions rather than forcing a number out of a formula that was never built for that range.
Wind chill uses the NWS/Environment Canada formula adopted in 2001 after human trials in a chilled wind tunnel: WC = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275TV^0.16, with air temperature T in Fahrenheit and wind speed V in miles per hour. That formula is only valid at or below 50°F with wind faster than 3 miles per hour — the NWS found calm air below that speed has no measurable chilling effect on exposed skin, so the calculator shows an explicit "not defined" message rather than a number outside that band. Heat index uses the Rothfusz regression, a nine-term polynomial in temperature and relative humidity first published by the NWS in 1990, refined from earlier work by Robert Steadman. Below about 80°F the full regression is replaced by a simpler averaging formula, and two separate correction terms are added for unusually low humidity (below 13%, between 80–112°F) or unusually high humidity (above 85%, between 80–87°F) — both terms are copied exactly from the NWS's own published equation page, not approximated.
To see both formulas checked against the NWS's own published examples: at 0°F with a 15 mph wind, this calculator computes a wind chill of −19°F, matching the exact example the NWS gives on its own wind chill safety page, where exposed skin can freeze within 30 minutes at that reading. At 95°F with 70% relative humidity, the raw Rothfusz polynomial computes a heat index around 123°F; the officially printed NWS heat index chart shows 124°F for that same cell, a one-degree gap that comes from light rounding built into how the original 1990 chart was drawn rather than a mistake in either number — this is disclosed openly in an FAQ rather than quietly adjusted to force a "nicer" match.
Beyond the two headline numbers, this tool adds the context that turns a raw temperature into something you can act on. Wind chill results are matched against the NWS's own frostbite exposure chart, which sets three thresholds — 30 minutes at −18°F or colder, 10 minutes at −32°F or colder, and 5 minutes at −48°F or colder — so a −25°F reading isn't just a number, it's a clock. Heat index results carry the four-tier NWS classification used by OSHA's own Heat Safety Tool: Caution from 80–90°F, Extreme Caution from 91–103°F, Danger from 104–124°F, and Extreme Danger above 125°F, each with the specific heat-illness risk that band represents. As a bonus, the same temperature and humidity you've already entered also produce a dew point using the Magnus formula (with the standard a=17.27, b=237.7°C constants), a genuinely useful third number meteorologists watch alongside both indices.
It's worth being explicit about what these formulas assume and don't cover. Both indices model a person in the shade with light wind, average clothing, and no direct sun; the NWS itself notes that full sunshine can add up to 15°F to how heat actually feels, and wind chill assumes a clear night sky with no solar warming at all. Neither formula accounts for physical exertion, individual acclimatisation, or hydration, and neither should be confused with Wet Bulb Globe Temperature, a separate index used by sports-medicine and OSHA guidance that also factors in solar radiation and requires different instruments to measure. Used within its stated range — cold and windy, or hot and humid — this calculator reproduces the same numbers the National Weather Service publishes; outside that range, it says so instead of guessing, and every calculation runs entirely in your browser with nothing you type ever uploaded anywhere.
