Wind chill quantifies how wind increases the rate of heat loss from exposed skin. At −10 °C with 30 km/h wind, it feels like −20 °C. The current formula (NWS/Environment Canada, 2001) is based on facial cooling rates.
Danger thresholds
Wind chill of −28 °C: frostbite risk in 30 minutes. −40 °C: risk in 10 minutes. −55 °C: risk in 2 minutes. In Spain, extreme wind chill occurs on summits (Pyrenees, Sierra Nevada, Picos de Europa) and on the northern Meseta during cold waves with wind. It complements the heat index at the opposite extreme.