NOAA / US National Weather Service, wind chill and cold-weather risk

Official guidance on cold-weather risk. Frostbite of exposed skin is governed by wind chill, not air temperature alone, and exposure time: at around minus 15°C with light wind, frostbite can occur within about 30 minutes, faster at lower wind-chill values, and not at all above freezing. Hypothermia (core below 35°C) is a real risk when wet, exposed to wind, or slowing or stopping, since heat production falls while heat loss continues. Strong.