Wind chill calculator
Wind chill is how cold exposed skin feels once moving air strips off the thin insulating layer of warm air over it. The 2001 North American formula is defined only at 10 °C or below with wind above about 5 km/h, and returns nothing meaningful outside that.
How to use the wind chill calculator
The formula is an empirical fit rather than physics from first principles: air temperature and wind speed raised to the power 0.16, with coefficients set by the 2001 JAG/TI trials, in which volunteers walked a treadmill in a chilled wind tunnel with heat-flux sensors taped to their faces. It replaced a 1945 index derived from how quickly a plastic cylinder of water froze on an Antarctic expedition, which overstated the effect badly — the old scale reported around −45 where the new one reports −30.
The range limits fall out of that fit. Below roughly 5 km/h the boundary layer over the skin is not being disturbed enough to matter, and above 10 °C the trials never went, which is why this reports whether your inputs are inside the range instead of extrapolating. Wind speed is taken as a met-station reading at 10 metres and scaled down to face height at 1.5 metres inside the calculation, so a handheld anemometer held at head height will give you a milder number than the forecast does, and both are right.
The one thing wind chill never does is lower the temperature of an object. A pipe, a car battery or a bottle of water cools toward the air temperature faster in wind and never past it. Only something generating its own heat — you, a dog, a running engine — experiences the chill at all, which is why the number is quoted with a frostbite time rather than as a temperature something will reach.
What people use it for
- Deciding how long a child can wait outside at a bus stop
- Judging whether a run or a ride is worth doing today
- Estimating how quickly exposed skin freezes on a ski day
- Reading a broadcast feels-like figure against the raw temperature
- Settling whether wind chill can actually freeze an outdoor tap
Questions
How cold exposed skin feels once wind removes the warm boundary layer over it. It is a measure for living things, not for objects.
No. Wind speeds the cooling of anything warmer than the air, but nothing ever gets colder than the air itself. A pipe at −5 °C stays at −5 whatever the wind chill reads.
At 10 °C or below with wind above about 5 km/h. Outside that the fit was never made, and this tool says so rather than extrapolating.
The 1945 index came from how fast a cylinder of water froze and overstated the chill by around 15 degrees at the extremes. The 2001 replacement is fitted to human trials in a wind tunnel.
The formula assumes a station reading at 10 metres and scales it to face height at 1.5 metres. A handheld reading is already near face height, so it is not the same input.
Around −27 exposed skin can freeze in about 30 minutes; at −40 it is closer to 10.