What the number is measuring
Wind chill answers one question: how fast is bare skin losing heat right now?
Still air next to your skin warms up and forms a thin insulating layer. Wind strips that layer away and replaces it with cold air, continuously, so heat leaves your skin faster. Wind chill expresses that rate as a temperature — the still-air temperature that would cool you at the same speed.
That makes it a comparison, not a measurement. There is no place where the air is at the wind chill temperature. A thermometer in a gale reads the air temperature, because wind cannot cool anything below the temperature of the air doing the cooling. What wind changes is how quickly things arrive at that temperature.
The chart
Air temperature down the side, wind speed across the top, wind chill in the cells. Both scales, because the chart is only useful in the unit you think in.
| Air temp | 5 mph | 10 mph | 15 mph | 20 mph | 30 mph | 40 mph |
|---|---|---|---|---|---|---|
| 40 °F (4 °C) | 36 (2) | 34 (1) | 32 (0) | 30 (−1) | 28 (−2) | 27 (−3) |
| 30 °F (−1 °C) | 25 (−4) | 21 (−6) | 19 (−7) | 17 (−8) | 15 (−9) | 13 (−11) |
| 20 °F (−7 °C) | 13 (−11) | 9 (−13) | 6 (−14) | 4 (−16) | 1 (−17) | −1 (−18) |
| 10 °F (−12 °C) | 1 (−17) | −4 (−20) | −7 (−22) | −9 (−23) | −12 (−24) | −15 (−26) |
| 0 °F (−18 °C) | −11 (−24) | −16 (−27) | −19 (−28) | −22 (−30) | −26 (−32) | −29 (−34) |
| −10 °F (−23 °C) | −22 (−30) | −28 (−33) | −32 (−36) | −35 (−37) | −39 (−39) | −43 (−42) |
| −20 °F (−29 °C) | −34 (−37) | −41 (−41) | −45 (−43) | −48 (−44) | −53 (−47) | −57 (−49) |
Two patterns are worth reading off it. The first few miles per hour of wind do most of the damage — going from calm to 10 mph costs more than going from 30 to 40. And the colder the air, the more each mile per hour takes off: at 40 °F, a 40 mph wind costs 13 degrees; at −20 °F, it costs 37.
How the number is calculated
The current North American formula dates from 2001 and replaced a 1945 version that produced dramatically colder figures. If you remember wind chills that sounded worse, you are remembering the old formula, not a colder decade.
With temperature in °F and wind speed in mph:
WC = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275T × V^0.16
The V^0.16 term is why the first few mph matter most: it rises steeply at low wind speeds and flattens out at high ones. The formula assumes an adult face at walking pace, in the dark, out of the sun — which is worth knowing, because bright sunshine can offset a meaningful part of it.
It is defined for air temperatures at or below 50 °F (10 °C) and winds above about 3 mph. Outside that range the equation returns values that are not wrong so much as meaningless, which is why apps stop showing wind chill in mild weather rather than showing a small number.
The frostbite times behind the warnings
The reason forecasters lead with wind chill is that it maps onto how long uncovered skin can be exposed. The National Weather Service publishes three thresholds:
- −18 °F (−28 °C) wind chill: frostbite on exposed skin in about 30 minutes.
- −33 °F (−36 °C) wind chill: about 10 minutes.
- −48 °F (−44 °C) wind chill: about 5 minutes.
They assume dry, uncovered skin on an adult. Wet skin fails faster, and so does skin on a child, whose surface area is large relative to their body mass. The practical response to a low wind chill is not another jumper — it is covering the face and hands that the number is actually about.
What wind chill does not apply to
This is where the number is most often misused. Wind chill is a heat-loss rate for human skin. It says nothing about:
- Pipes. Wind brings a pipe to air temperature faster, but never below it. Use the air temperature and the exposure time — see what temperature pipes freeze at.
- Plants. Frost damage is about the plant tissue reaching freezing point, which the air temperature governs.
- Cars. A battery cares about the temperature of the battery, not about how quickly the wind got it there.
- Paint, concrete, adhesives. Every specification is written against air and surface temperature.
- Dogs and other animals. The formula is calibrated on human skin. Wind genuinely matters to a short-coated dog, but the number on your screen was not calculated for one — how cold is too cold to walk a dog is the better guide.
The rule of thumb: if the thing you are worried about is not a person's exposed skin, look at the air temperature instead.