WRN – Wind Chill Temperature (WCT) index

In late 2001, the National Weather Service implemented a new Wind Chill Temperature (WCT) Index. The reason for the change was to improve upon the old index used by the NWS and the Meteorological Services of Canada (MSC) which was based on the 1945 Siple and Passel Index.

The new Wind Chill/Temperature Index makes use of advances in meteorology, biometeorology and computer modeling to provide a more accurate, useful formula for calculating the dangers from winter winds and freezing temperatures. In addition, clinical trials have been conducted and the results of those trials have been used to verify and improve the accuracy of the new formula.

Led by the NWS, implementation of the standarized WCT Index took place throughout the nation and involved the entire meteorological community, and the media, which plays an important role in explaining the index to the general public. In this way, an accurate and consistent measure has been provided to help the public protect itself against the dangers of frostbite and hypothermia. The index has been implemented in Canada and the United States in order to have a consistent WCT Index for North America.

Specifically, the new WCT Index:

  • uses calculated wind speeds at an average height of five feet (typical height of a human face) based on readings from the national standard height of 33 feet (typical height of an anemometer);
  • is based on the latest heat transfer theory, i.e, .heat loss from the body to its surroundings, during cold and breezy/windy days ;
  • and uses a standard factor for skin tissue and assumes a no sunlight scenario.

Twc= 35.74 + 0.6215T – 35.75(V0.16) + 0.4275T(V0.16)


V = Wind Speed in mph
T = Air Temperature in oF
Twc= windchill

The NWS Wind Chill Temperature (WCT) index uses advances in science, technology, and computer modeling to provide an accurate, understandable, and useful formula for calculating the dangers from winter winds and freezing temperatures. The index does the following:

Calculates wind speed at an average height of 5 feet, the typical height of an adult human face, based on readings from the national standard height of 33 feet, typical height of an anemometer

Is based on a human face model

Incorporates heat transfer theory based on heat loss from the body to its surroundings, during cold and breezy/windy days

Lowers the calm wind threshold to 3 mph

Uses a consistent standard for skin tissue resistance

Assumes no impact from the sun, i.e., clear night sky.

The word “Wind Chill” has been coined for the first time in an article by Dr. Saypel in 1939 named as “Adaptation of South Pole discoverers with cold weather“. Later in the process of discovering polarregions, Saypel and his colleague, Pasel, conducted several studies regarding quantifying and estimating the values of wind chill. They presented a parameter based on their findings which is still considerable in the background of this discussion.

Feeling temperature by human body changes based on variations in atmospheric conditions. In order for the body temperature to remain stable, the temperature absorbed and produced by the body should be the same as the lost temperature. If the lost temperature is more than the absorbed and produced temperature, the human body feels cold and vice versa. Therefore, there is a constant exchange of temperature between human body, specifically at skin surface, and the surrounding environment. In a cold day, a thin layer of hot weather molecules covers the skin and protects the body against the surrounding cold weather, and decrease the speed of heat transmission to the environment.

When wind blows, the thin layer of hot weather molecules get away from body and the body heat is quickly transmitted to the environment, and the body feels cold. The faster the wind blows, the faster the body loses heat and feels cold. The fact that how wind can cause human body feels colder is described through an index called the wind chill-producing. In other words, the common effect of wind and temperature in loss of heat in human body and other organisms is called wind’s chill-producing or wind chill.

This parameter is presented to express the relationship between the lack of human comfort and the common effect of wind and temperature. The above parameter is not an actual temperature, however; this is the feeling of temperature by body that is measured based on the simultaneous effect of wind and the temperature on the body. This parameter converts the frost-chill-producing power of the wind to temperature equivalence in a clear day (in which the wind speed is equal to or less than 6.4 km/hr.

Intense blow of wind at temperatures lower than zero could emit heat from body quickly that might lead to skin frostiness. In a cold weather, skin humidity can also be an important factor in the way cold is felt since water on body skin directs the loss of heat in a better way. The high speed of energy loss by body might lead its temperature to under normal level and cause a condition that is called hypothermia. This condition leads to body tiredness and faintness and if body temperature gets lower than 26° C (degree centigrade) it causes death. Temperature alone cannot provide us enough information to decide on the type and number of clothes we wear. Other climatic factors specifically wind have significant role in this regard. The importance of wind chill index (WCI) is that it can be used as an appropriate criterion for wearing clothes in cold weather.

Although this index has not taken into account some environmental factors such as relative humidity of weather and sun radiation, the researchers working on and conducting research on this index believe that using this index is sufficient in order to protect human health in cold weather. For example, meteorology organizations of many countries presents the wind chill-producing degree to people through public media and internet sites in order to choose covering and warm clothes. Besides, the US army uses this index for developing and reforming the production of warm uniforms for soldiers.

Wind chill index has wide range of applications in countries located in cold regions, in a way that an opinion poll performed in Canada shows that 82% of the people of this country use wind chill index in order to plan their daily outdoor activities in winter. Furthermore, many organizations and schools consider this index while planning and making decisions. Even people who make living outdoors like constructional workers, hockey club workers, or operators of telecabins, stop the work when this index is very low.

Table 1 presents the economic loss of the world due to natural disasters from 1900 to 2001. According to this table, wind chill which is very similar to cold wave in its nature, is among the first eight economically destructive events in the world.

Research background related to innovative formulas to estimate WCI: Different formulas for estimating wind chill have been presented so far that some of them are pointed out here. Variables related to WCI are subjected to some changes in different tables which are due to using different formulas for estimating heat loss or different conceptions of heat loss from body or slow wind speed.

In the 1930’s, Sypel and Pazel presented a formula for estimating the heat loss from human body based on their investigation in Southern Pole,

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