Wind Chill Calculator (°C and °F)

Wind chill is the temperature that still air would have to be for exposed skin to lose heat as fast as it is losing it now. Enter the air temperature and the wind speed and the calculator returns that equivalent, along with how many degrees colder it feels, using the North American formula adopted in 2001 — the metric form from Environment Canada, the imperial one from the National Weather Service. The model is defined at or below 10 °C and 4.8 km/h of wind, and says so when you go outside that range.

The effect is larger than most people's intuition. At −5 °C, a 20 km/h wind brings the equivalent down to about −11.6 °C; at −20 °C with 40 km/h it is roughly −34 °C. That is fourteen degrees of difference driven entirely by air movement, which is why forecasts quote it at all rather than leaving people to read the thermometer.

The single most useful thing to know is what wind chill does not do: it cannot make anything colder than the air. Skin feels colder because moving air strips away the thin warm layer it keeps around itself and replaces it faster than the body can reheat it. A water pipe, a car radiator or a parked bicycle has no such layer to lose and no heat of its own to give up, so wind makes it reach air temperature sooner and never takes it below. A pipe will not freeze at −20 °C wind chill if the air is at +2 °C, and planning winter precautions around the wind chill number rather than the actual temperature gets that backwards.

Two further limits. The current formula replaced a 1945 one derived from how fast water froze in plastic cylinders on an Antarctic expedition, which produced considerably colder figures — so a wind chill chart from an old book will not agree with a modern forecast, and neither is a mistake. And the model assumes an adult walking into the wind at night with no sun, so bright sunshine makes it pessimistic while wet skin or wet clothing makes it optimistic by a wide margin, since evaporation removes heat faster than convection ever will.