The wind chill temperature is how cold people feel when outside. Wind chill is based on the rate of heat loss from exposed skin caused by wind and cold. As the wind increases it draws heat from the body, driving down skin temperature and eventually the internal body temperature. Therefore the wind makes it FEEL much colder.
Frostbite can occur on exposed skin when the wind chill is at -15 degrees in just 30 minutes. That's why wind chill warnings are issued at -15. Wind chill advisories are issued when the wind chill is forecast to be around 0.
Wind chills are only calculated when the temperature is 50 degrees or less and the wind speed is at least 3mph.