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: the NWS chart places the 30-minute frostbite threshold at a wind chill of around -29°C (approximately -20°F), well below -15°C; risk escalates sharply as wind chill drops further, and does not occur 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.