Wind Chill (2001 North American Formula)

Wind chill answers one question: how cold does bare skin lose heat, expressed as the still-air temperature that would strip it at the same rate. The 2001 North American formula replaced the 1945 Siple and Passel index, which had been calibrated by measuring how fast water froze in plastic cylinders hung from a pole in Antarctica and which produced numbers most people found implausibly severe. The replacement came from a joint Canadian and American working group that modelled heat loss from a human face, then validated it on volunteers walking in a chilled wind tunnel at the Defence and Civil Institute of Environmental Medicine in Toronto.

The details in the coefficients are worth knowing. Wind speed enters as $V^{0.16}$, a strongly flattening power, which is why the first 20 km/h of wind does most of the damage and the next 20 does much less. The speed is measured at the standard 10 m anemometer height and internally scaled down to face height, so the number already accounts for the fact that your face is not at the top of a mast. And the index is defined only at or below 10 °C with winds above 4.8 km/h, which is why this page refuses to compute outside that window rather than returning a confident-looking answer from a fit that was never tested there.

One thing it does not tell you is how cold an object gets. Wind chill cannot push anything below the true air temperature; a windy −10 °C night will not freeze your pipes any harder than a calm one, it will merely get them there faster. What the index does track well is frostbite risk on exposed skin, which is the reason it is broadcast.