Calculate the wind chill ("feels like" temperature in cold, windy conditions) from air temperature and wind speed, using the official NWS formula.
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Wind chill measures how much colder moving air makes exposed skin feel compared to the actual measured air temperature, because wind strips away the thin layer of warmer air that naturally forms right next to the skin, accelerating heat loss from the body. This tool uses the official wind chill formula adopted by the U.S. National Weather Service and Environment Canada in 2001, based on modern research into how the human face loses heat in cold, windy conditions. The formula is only meaningful within its intended range — air temperatures at or below 50°F with wind speeds above about 3 mph — since wind has little meaningful cooling effect on perceived temperature outside those conditions. This same jointly developed US/Canadian formula is what appears in official winter weather advisories and cold-weather warnings today, replacing an older, less accurate formula that had been in use since the 1970s.
Anyone who has stepped outside on a cold, windy day knows that the wind makes it feel noticeably colder than the thermometer reading alone would suggest. Wind chill is the number that quantifies exactly how much colder it feels, and it plays the same role in winter that the heat index plays in summer: turning a raw temperature reading into a more accurate sense of what your body actually experiences.
The physical mechanism behind wind chill is heat loss through convection. Your body naturally warms a thin layer of air right next to your skin, which acts as a small buffer slowing further heat loss. Moving air continuously strips this warmed layer away and replaces it with cooler air, forcing your body to keep re-warming a new layer and losing heat faster in the process. The stronger the wind, the faster this cycle repeats, and the colder exposed skin feels relative to the still-air temperature.
The specific formula used by this calculator isn't an old approximation — it's the modern wind chill index formally adopted jointly by the U.S. National Weather Service and Environment Canada in 2001, replacing an older formula from the 1970s. The update was based on updated research using human trials that measured actual facial heat loss under controlled wind and cold conditions, making the current formula considerably more accurate to how exposed skin, particularly on the face, genuinely responds to cold and wind together.
Wind chill isn't meaningful in every condition — it specifically applies to air temperatures at or below 50°F with wind speeds above about 3 mph, since outside that range wind simply doesn't meaningfully change how cold the air feels. Within its intended range, though, it's a genuinely important safety figure: frostbite risk increases substantially as wind chill drops, becoming a real concern below roughly -18°F wind chill (with frostbite possible in as little as 10 to 30 minutes of exposed skin) and becoming extremely dangerous, with frostbite possible within minutes, at -40°F wind chill and below. Checking wind chill rather than just air temperature is one of the simplest ways to judge real cold-weather risk before heading outdoors.
Because the wind chill formula is specifically calibrated for cold conditions (50°F and below) with wind speeds above roughly 3 mph — outside that range, wind has too little effect on heat loss from skin for wind chill to be a meaningful separate figure from the actual air temperature.
Frostbite risk generally becomes a serious concern below roughly -18°F wind chill, with risk of frostbite in as little as 10-30 minutes of exposure, and becomes extremely dangerous (frostbite possible within minutes) at -40°F wind chill and below.
No — they use completely different formulas based on different physical mechanisms; heat index accounts for humidity's effect on sweat evaporation in hot weather, while wind chill accounts for wind's effect on convective heat loss from skin in cold weather.
The earlier formula dated to the 1970s and was based on older research; the 2001 formula jointly adopted by the US and Canada is based on updated human-trial research into how the face actually loses heat in cold, windy conditions, making it considerably more accurate.