Information provided by the National Weather Service:
The National Weather Service Wind Chill Temperature index is designed to accurately calculate how cold air feels on human skin. This index is based on heat loss from exposed skin.
The Wind Chill Chart (see below) includes a frostbite indicator that shows the points where temperature, wind speed and exposure time will produce frostbite on people. The chart includes three shaded areas of frostbite danger. Each shaded area shows how long (30, 10 and 5 minutes) a person can be exposed before frostbite develops.
For example, a temperature of 0 degrees F and a wind speed of 15 mph will produce a wind chill temperature of -19 degrees F. Under these conditions, exposed skin can freeze in 30 minutes.