Why do we always say, "Bundle up," when there is weather like this?
We describe it as wind chill but when looking that up in the Glossary of Weather and Climate you are told to see Wind Chill Factor. There you will find it described as the cooling effect of any combination of temperature and wind expressed as the loss of body heat in Kilogram-calories per hour per square meter of skin surface.
When we show you a wind chill graphic, that is not a specific temperature but instead a temperature that is an approximation of what if feels like due to the loss of heat on exposed skin from the wind and cold temperatures.
The wind chill has not always been the same. There was a switch on November of 2001 when a new index was implemented. The new wind chill temperature attempts to better calculate how the wind and cold temperatures will affect humans. Approximating wind speed at 5 feet instead of 10 meters, using the knowledge of a better understanding about human flesh and heat transfer, etc.
Just like any extreme weather, there are advisories, watches, and warnings that can be issued when the wind chill temperature is forecast to drop below a certain point. A wind chill of 10 below zero would call for a wind chill advisory while 25 below zero would call for a warning.
Although wind chill extremes are not recorded and kept like high/low/precipitation records are, we can approximate the lowest wind chill ever felt here in Philadelphia. From the Philadelphia Weather Book by Glenn Schwartz and Jon Nese, they came to the conclusion that the lowest wind chill in Philadelphia was probably between 40 and 50 degrees below zero. They determined that by looking at the lowest recorded temperature in the city (-11) and wind speeds of 20 to 25 mph at the time of that temperature.
The new wind chill chart can be found at the National Weather Service page in Philadelphia.