Wind Chill Calculator

Wind chill is not a temperature

It is reported in degrees, printed next to the real temperature, and called a "feels like" figure, so everyone reasonably assumes it is a temperature. It is not, and the misunderstanding causes real confusion every winter.

Wind chill is a rate of heat loss, dressed up in the units of temperature so that humans can act on it. What it actually says is: "still air at this temperature would pull heat out of your skin as fast as the wind is doing right now." It is an equivalence, not a measurement. Nothing outside is at the wind chill temperature. Put a thermometer out in a gale at 20°F and it reads 20°F, not −5.

Once that clicks, everything else about wind chill makes sense: why it only applies to living things, why your car's radiator ignores it, and why the number is still worth knowing. It is a statement about you, not about the air.

What the wind is actually stealing

Stand still in cold, calm air and your body does something clever. It warms a paper-thin layer of air right against your skin, and that layer becomes a small, free insulating blanket. You are not really touching the cold air; you are touching your own warmed-up microclimate.

Wind removes that layer. Continuously. Every gust strips off the air you just spent energy warming and replaces it with fresh cold air, which you then warm, which is then stripped away again. You are heating the entire county, one thin layer at a time.

That is the whole mechanism, and it explains why wind chill only matters for things that generate heat. You have a boundary layer because you are warm. So does a dog, a horse, an exposed hand. A brick does not have one to lose.

The formula, and the treadmills that produced it

This tool uses the current index, adopted by the US National Weather Service and Environment Canada on 1 November 2001:

WCT = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275T × V^0.16

with T as air temperature in Fahrenheit and V as wind speed in mph. It looks like it was fitted rather than derived, because it was.

What is nice about it is where it came from. Rather than theorising, researchers put human volunteers in a refrigerated wind tunnel in Toronto, at the Defence and Civil Institute of Environmental Medicine, walked them on treadmills, and measured how fast their faces actually cooled at various temperatures and wind speeds. The formula is a curve fitted to real human skin.

Two details baked into it that most people never hear. It calculates wind speed at 5 feet, the typical height of an adult face, rather than at the 33-foot height where weather stations put their anemometers, since wind is slower near the ground. And it assumes a clear night sky, meaning no sun. Both of those choices matter, and the second one shows up later on this page.

A worked example

Air temperature 0°F, wind 15 mph.

V^0.16 = 15^0.16 = 1.542
WCT = 35.74 + (0.6215 × 0) − (35.75 × 1.542) + (0.4275 × 0 × 1.542)
WCT = 35.74 − 55.14 = −19°F

Which matches the published NWS chart exactly. Worth noticing that at 0°F the temperature terms drop out entirely and the whole thing collapses to 35.74 minus the wind term, which is a neat accident of the arithmetic rather than anything meaningful.

A few more, all of which reproduce the official chart: 0°F with 30 mph gives −26°F. 15°F with 30 mph gives −5°F. −10°F with 20 mph gives −35°F.

Your car does not care

This is the practical payoff of the "not a temperature" point, and the National Weather Service is unusually blunt about it: wind chill applies only to people and animals.

For an inanimate object, the only thing wind does is get it to air temperature faster. It will never take the object below the actual air temperature. Wind is not a refrigerator; it cannot cool something past the temperature of the air doing the cooling.

So, concretely:

• Your pipes will not freeze at 35°F just because the wind chill says 20°F. Water freezes at 32°F. The air is 35°F. The pipes will get to 35°F rather quickly and then stop, and nothing freezes.
• Your car's radiator, your windscreen, your outdoor tap: same story. Wind chill is irrelevant to all of them.
• But your dog is an animal, and animals have the same boundary layer you do. Wind chill applies to pets, and to livestock, and this is where it gets forgotten.

The one caveat: if the air temperature is below freezing, wind gets your pipes down there faster, which shortens the time you have to do something about it. The wind changes the speed, never the destination.

Why it stops working above 50°F or under 3 mph

Enter a warm temperature or a still day and this tool declines to give you a wind chill. That is correct behaviour, and it is worth explaining rather than treating as a fault.

The index is defined only for air temperatures at or below 50°F and wind speeds above 3 mph. Those are the boundaries of the conditions the wind tunnel trials actually covered, and outside them the formula is extrapolating into territory nobody measured.

The 3 mph floor is the more interesting one. Below about that, the air is barely moving relative to you, and your natural boundary layer survives. There is nothing for the formula to describe. Push the equation below 3 mph anyway and it starts producing wind chills warmer than the air temperature, which is nonsense: at 1 mph the arithmetic claims you feel warmer than you are. That is a fitted curve running off the end of its data, and it is exactly why the guard exists.

Above 50°F, wind still cools you, but the effect is small and the relationship no longer holds the shape the formula assumes. That is what heat index and other indices are for, at the other end of the year.

The first 10 mph do most of the damage

Look at the V^0.16 in the formula. That exponent is doing something important and counterintuitive: wind chill follows a power curve with sharply diminishing returns.

At 0°F air temperature:

The first 10 mph of extra wind costs you nearly 9 degrees. The jump from 35 to 45 mph, a far more dramatic-sounding change, costs 2.6.

The physics is that your boundary layer gets stripped away almost as soon as there is any real wind at all. Once it is gone, more wind cannot take it away twice. So the difference between a still day and a breezy one is much bigger than the difference between windy and howling, which is not how anybody's intuition works.

Practically, this is an argument for shelter over speed. Getting behind a wall, or into the lee of a building, buys you more than you would guess, because dropping from 15 mph to 3 mph of exposure recovers most of the loss.

The number that actually matters is frostbite time

Wind chill in degrees is a comparison. Frostbite time is a decision, and it is what the official charts add next to the numbers.

Roughly, on exposed skin:

At the colder end of the official chart, the times fall to 10 minutes and then to 5. Five minutes is less time than it takes to clear a windscreen properly.

Which reframes the whole thing. The question is not "how cold does it feel", it is "how long can my face be out here". And the answer is about exposed skin specifically, so the response is not usually a warmer coat, it is covering the bits that are showing: face, ears, fingers, wrists. Frostbite starts with numbness and a change in skin colour, and numbness is a bad early-warning system, because the thing that would tell you to go inside is the thing that stops working.

Worth remembering that cold is dangerous with or without wind. Hypothermia does not require a breeze, and a still −20°F day is not safe just because the wind chill line is missing.

The old formula was worse, and cans of water are why

The 2001 index replaced one that had stood since 1945, and the story of the old one explains why the new one exists.

Paul Siple and Charles Passel were Antarctic explorers. To quantify how cold the wind made things, they hung plastic containers of water outside and timed how quickly the water froze at various wind speeds. It was resourceful, and for 1945 it was genuinely good science.

It was also measuring the wrong thing. A plastic cylinder of water is not a human face. It does not generate heat, does not have circulation adjusting to the cold, and is not covered in skin. The resulting index substantially overstated how cold the wind made people feel, and at high wind speeds the old formula reads roughly 10 to 15 degrees colder than the modern one.

Which is a small thing worth knowing if you are comparing old records or listening to someone reminisce about a −70 wind chill in their youth. Some of that difference is the weather. Some of it is that the number used to be calculated from a freezing can of water on a roof in Antarctica.

Sunshine is worth 10 to 18 degrees

Remember that the formula assumes a clear night sky and no sun. That assumption has a big consequence that almost nobody accounts for.

Bright sunshine can raise the apparent temperature on exposed skin by roughly 10 to 18°F, because direct solar radiation warms you even while the thermometer stays put. So the same reported wind chill is a meaningfully different experience at noon on a bright day than at 4 AM.

Which means the reported figure is, in effect, the worst-case version. A sunny −5°F wind chill is a walk. The same −5°F wind chill after dark is a different proposition entirely, and the number on your phone will not have changed.

About the output unit

One quirk of this tool worth flagging so it does not catch you out. You can enter your temperature in Celsius or Kelvin, and your wind in km/h or metres per second, and it will handle the conversion correctly. But the answer always comes back in Fahrenheit.

So if you enter −10°C and 30 km/h, the result reads about −3, and that is −3 Fahrenheit, not Celsius. The Celsius answer is around −19.5°C. Read a small negative number after entering Celsius and it is easy to conclude the wind is doing nothing, when in fact it has taken nearly ten degrees Celsius off you.

To convert back: subtract 32, multiply by 5, divide by 9. Or, for a rough mental version, take 30 off and halve it.

Questions people ask

Will my pipes freeze if the wind chill is below 32°F?

Not if the actual air temperature is above freezing. Wind chill applies only to people and animals. For an object, wind just brings it to the air temperature faster and cannot take it any lower. If the air is 35°F, your pipes reach 35°F and stop, whatever the wind chill says. If the air is below freezing, wind will get them there sooner, so it shortens your window rather than changing the outcome.

What is the wind chill at 0°F with a 15 mph wind?

About −19°F, and frostbite becomes possible on exposed skin in roughly 30 minutes. Double the wind to 30 mph and the wind chill only falls to about −26°F, but the frostbite time drops to under 15 minutes, which is the more important half of that sentence.

Why does it refuse to answer at 60°F?

Because the index is only defined at or below 50°F and above 3 mph, which is the range the original human trials covered. Outside it, the formula is extrapolating rather than describing, and below 3 mph it starts claiming you feel warmer than the air actually is. The tool declining is the tool being correct.

I entered Celsius but the answer looks wrong. Why?

The result is always in Fahrenheit, whichever unit you enter. So −10°C with 30 km/h returns about −3, meaning −3°F, which is roughly −19.5°C. Convert back with (F − 32) × 5 ÷ 9.

Does twice the wind mean twice the chill?

Not remotely. The formula uses wind speed to the power of 0.16, which produces steeply diminishing returns. At 0°F, going from 5 to 15 mph costs you nearly 9 degrees; going from 35 to 45 mph costs 2.6. Your insulating layer of warmed air is stripped away early, and once it is gone more wind has less left to take. Practically: getting out of the wind helps a lot more than you would expect.

References

Where the figures come from. The wind chill formula this tool uses, its restriction to temperatures at or below 50°F and wind speeds above 3 mph, the calculation of wind speed at the 5-foot height of an adult face, the assumption of no solar heating, the 10 to 18°F effect of bright sunshine, the frostbite times quoted above, and the statement that wind chill applies only to people and animals and cannot cool an object below the air temperature all come from the National Weather Service. The original index, derived from measurements of how quickly water froze in plastic containers in Antarctica and later found to overstate the cooling of human skin, is Siple and Passel's.

1. National Weather Service. Wind Chill Chart and Understanding Wind Chill. https://www.weather.gov/safety/cold-wind-chill-chart
2. Siple PA, Passel CF. Measurements of dry atmospheric cooling in subfreezing temperatures. Proceedings of the American Philosophical Society. 1945;89(1):177-199.