ReckonBoxWhat the wind chill number means
Wind chill is the temperature it feels like on exposed skin once wind is factored in, because moving air strips away the thin layer of warmth your body heats around itself. At 20F with a 15 mph wind, it feels like about 6F.
The wind does not actually make the air colder. It carries heat away from you faster, so your skin cools toward the air temperature more quickly and the cold bites harder. That is why wind chill is always at or above the true air temperature, never below it, and why it describes a rate of heat loss rather than a new temperature the world has dropped to.
The formula, and why it changed in 2001
The current wind chill formula, adopted by the US National Weather Service and Environment Canada in 2001, is 35.74 + 0.6215T minus 35.75 times V to the 0.16 power, plus 0.4275 times T times V to the 0.16 power, where T is the air temperature in Fahrenheit and V is the wind speed in mph.
An older 1945 formula, based on how fast water froze in a can, ran too cold and overstated the danger. The 2001 revision models heat loss from a human face at about five feet off the ground, assuming a walking pace, and it is the version every US and Canadian forecast now uses. Working in Celsius and km/h, the constants become 13.12, 0.6215, 11.37, and 0.3965. This tool uses the Fahrenheit and mph version, so enter US units.
When the formula applies, and when it does not
The National Weather Service defines wind chill only for air temperatures at or below 50F and wind speeds above 3 mph. Push past either bound and the number stops meaning anything: above 50F the wind is not pulling meaningful warmth off you, and at or below 3 mph the air counts as calm, so it feels like the plain temperature. Plenty of calculators will still hand you a figure at 70F. This one blanks the result and says why, because a wind chill the formula was never built to produce is worse than no number at all.
Frostbite is the reason the number matters
The National Weather Service warns that exposed skin can freeze in about 30 minutes at a wind chill of -19F, and the time drops fast as the wind chill falls further. This tool flags that -19F threshold. The number is a floor for caution and no promise, since bare skin, damp clothing, and poor circulation all speed frostbite up. At a wind chill of -30F, which is a -30F you reach at about 5F air with a stiff 25 mph wind, the margin for a bare hand or ear is measured in minutes.
What wind chill does not touch
Wind chill only describes heat loss from a warm body, so it applies to you and to animals with exposed skin, and to nothing else. Your car engine, your water pipes, and the thermometer on the porch all cool toward the actual air temperature no matter how hard the wind blows, and never below it. A -20F wind chill will not burst a pipe that a true 15F night would leave intact. Treat wind chill as a personal safety read, and use the real air temperature for anything mechanical.
These results are estimates of environmental exposure, not medical advice. If someone shows signs of frostbite or hypothermia, warm them gradually and seek medical care.
Frequently asked questions
What is wind chill? Wind chill is the temperature it feels like on exposed skin once wind is factored in, because moving air strips away the thin warm layer your body heats around itself. At 20F with a 15 mph wind it feels like about 6F. Wind chill never drops below the actual air temperature, and it only speeds up how fast you lose heat, it does not cool objects below the air temperature.
How is wind chill calculated? The current formula, adopted by the US National Weather Service and Environment Canada in 2001, is 35.74 + 0.6215T minus 35.75 times V to the 0.16 power, plus 0.4275 times T times V to the 0.16 power, where T is the air temperature in Fahrenheit and V is the wind speed in mph. It replaced a 1945 formula that overstated the effect. The metric version uses 13.12, 0.6215, 11.37, and 0.3965 with Celsius and km/h.
When does wind chill apply? The National Weather Service defines wind chill only for air temperatures at or below 50F and wind speeds above 3 mph. Above 50F, wind does not make it feel meaningfully colder, and at or below 3 mph the air counts as calm, so it feels like the plain air temperature. This tool blanks the result outside that range instead of returning a number the formula was never built to give.
When does wind chill cause frostbite? The National Weather Service warns that exposed skin can freeze in about 30 minutes at a wind chill of -19F, and the time shortens quickly as the wind chill drops further. This tool flags that threshold. Frostbite risk is not just about the number: bare skin, wet clothing, and poor circulation all speed it up, so treat the flag as a floor for caution, not a countdown.
Does wind chill affect cars, pipes, or pets differently? Wind chill only describes heat loss from a warm body, so it applies to people and animals, not to objects. A car engine or a water pipe will cool toward the actual air temperature no matter how hard the wind blows, and never below it, so a -20F wind chill will not freeze a pipe that the true 15F air would leave alone. Pets with exposed skin or thin coats do feel wind chill much as people do.