On November 1, 2001, the National Weather Service implemented a new Wind Chill Temperature (WCT) index for the 2001/2002 winter season, designed to more accurately calculate how cold air feels on human skin. The former index used by the United States and Canada was based on 1945 research of Antarctic explorers Siple and Passel. They measured the cooling rate of water in a container hanging from a tall pole outside. A container of water will freeze faster than flesh. As a result, the previous wind chill index underestimated the time to freezing and overestimated the chilling effect of the wind. The new index is based on heat loss from exposed skin and was tested on human subjects.

Humans do not sense the temperature of the air directly. When we feel that it is cold, we are actually sensing the temperature of our skin. Because our skin temperature is lower when it is windy (we lose heat from our skin faster than our body can warm it), we feel that it is colder when there is wind. This sensation is what the wind chill index attempts to quantify. Although the wind chill index is expressed on a temperature scale, it is not a temperature: it only expresses a human sensation.

The new wind chill index is now being used in Canada and the United States. Specifically, the new WCT index:

•  calculates wind speed at an average height of five feet (typical height of an adult human face) based on readings from the national standard height of 33 feet (height of an anemometer)
•  is based on a human face model developed in clinical trials in a wind tunnel using volunteers
•  incorporates modern heat transfer theory
•  lowers the calm wind threshold from 4 mph to 3 mph (the speed of a typical pedestrian)
• uses a consistent standard for skin tissue resistance
• assumes no impact from the sun (i.e. clear night sky).

What the wind chill "temperature" indicates

First, some principles:

• Wind chill concerns loss of energy through the processes of conduction and evaporation;
  
• conduction occurs when two objects of differing temperature are brought into contact (e.g. your skin and the air, or your car and the air);
• the speed of conduction depends on the temperature difference -- the greater the difference, the faster the cooling (or heating);
• therefore, the colder the air temperature, the faster the temperature of the object will drop;
• or the higher the temperature of the object, the faster the temperature will drop;
• also, the higher the wind speed, the faster the temperature will drop because air flow increases evaporation and conduction.

Wind chill accounts for the effect of wind speed on conduction and evaporation losses.

Bearing these ideas in mind: Wind chill temperature indicates the temperature at which the rate of cooling of human facial skin on a walking pedestrian would be the same if there was no wind. It is said to indicate how cold the air actually feels on the skin. Although the wind chill index is expressed on a temperature scale, it is not a temperature: it only expresses a human sensation.

It follows that no object (your car, your skin, etc.) can actually end up colder than the air temperature; and if an object is already at air temperature, then wind will not cause its temperature to fall any further.

The calculator below gives the rate of cooling in watts/sq.meter if you input your data in Celsius and km/hr: