History and definition
Wind chill refers to the sensation of cold produced by the wind on a body releasing heat, while the actual ambient air temperature does not drop. Paul Siple and Charles Passel were the first to develop the concept of wind chill factor around the 1940's during Antarctic experiments.
The concept then gradually spread thanks to the meteorological service of the United States. The initial calculation of the windchill index was based on the energy loss of the human body in Watt/m² of skin. This method was difficult to interpret by the general public. It has been greatly enhanced by Environment Canada to meet the needs of Canadians.
Today, a new index close to the reality felt by the human body is expressed by a number without unit similar to the temperature. It prevents cold injuries in the vicinity and below 0°C or 32°F, especially by dressing more warmly to avoid severe frostbite by sample.
A motionless person knows that the sensation of cold is all the more important so since the wind is violent. The cooling sensation is caused by combined effect of wind and temperature on the human body. It is wind chill, also called "felt temperature". We recall that this is not a temperature but a index likened to temperature for a good understanding and especially a better use of the general public. The wind chill index or "felt temperature" is thus without units.
Our body is isolated from the outside ambient temperature by a thin layer of air close to the skin, called boundary layer. A man or an animal forms a layer of moist air on the surface of his skin through his internal metabolism. The air is a good thermal insulation and this layer trapped in clothing or fur maintains a constant skin temperature.
This layer disappears by convection with the effect of the wind, exposing the skin to the ambient temperature of the air, and therefore to the cold. The body must produce sufficient energy for recreating this protective layer. If the wind blows the layers one after the other, the skin temperature decreases. The air moves the humidified air and brings drier air, promoting in that way evaporation and thus cooling: we feel the cold. Metabolism decreases faster as temperatures become low, potentially leading to hypothermia.
At the same air temperature, the more violent the wind is, the lowest the skin temperature feels. Similarly, the wind causes a evaporation process of moisture from the skin that removes even more heat from the body. The loss of body heat is therefore more rapid and important with wet skin than dry skin.
Display on AWEKAS instrument panel
With the conditions of validity written below, the wind chill index is strictly calculated from the negative temperature of -45.55°C or -50°F to 10°C or 50°F. Above, the "felt temperature" displayed is identical to the ambient air temperature or without display with the indication "n.a" for not applicable.
Calculation formulae
During the 2000's, scientific and medical experts worked to develop the current new wind chill index. This index is mostly useful during the winter period, volunteers dressed in winter uniforms have been exposed to several combinations of temperature and wind speed in a refrigerated wind tunnel. To define and finalise the new index, the researchers noted the heat loss rates of their faces, the only exposed parts, under both dry and wet conditions.
All these results have led to the equation of a model near the reality. Above 10°C or 50°F, the heat losses became very low and the induced risks negligible. This upper threshold of calculation has therefore been retained. For low temperatures, activity without rapid depletion of his caloric reserves has been taken into account. The value is thus set at -45,55°C or -50°F in quiet time.
Therefore formulae are verified and only valid between 10°C or 50°F and -45.55°C or 50°F. According to the WMO guides, the ambient temperature T is measured under shelter from wind, sun and precipitation at 1,5 m of grassy soil. The wind speed V is measured by an anemometer normally installed at 10 m height (WMO standards).
Metric system to have the WC index (number without units) from temperatures T in degrees Celsius and wind speed V in km per hour
0 km.h-1 ≤ V ≤ 5 km.h-1: WC = T + 0.2 (0.1345 T -1.59) × V
5 km.h-1 < V ≤ 177 km.h-1: WC = 13.12 + 0.6215 TC + (0.3965 TC -11.37) × V^0.16
Imperial system to have the WF index (number without units) from temperatures in degrees Fahrenheit and wind speed V in mile per hour
0 mph < V ≤ 3,107 mph: WF = TF + 0,2 (0,21646 TF - 11,5326) × V
3,107 mph <V ≤ 117 mph: WF = 35.74 + 0.6215 TF + (0.4275 TF - 35.75) x V ^ 0.16
Conversion factors used: V mph = 0.621371 V km/h V km/h = 3.6 V m / s T°F = 32 + 1.8 T°C
Effects and risks
The index was determined by a statistical projection based on an experiment based on measure of the energy loss by the skin on the unprotected parts of the volunteers' faces. Therefore, it is not taken into account the effects peculiar to each individual, such as the nature of the skin, hid aging, his lipid film, the presence of beard and the thickness of the horny layer and subcutaneous fat. Protection by cosmetic products can slow down the wastage. The weight/height ratio of individuals also affect their resistance to cold as well as their ability to regulate body heat and their own calorific capacity. A stocky person will be more resistant than a tall, slim person. Finally the diet, hydration by drink and the muscular and metabolic activity of the individual determine his own production of heat: his resistance to hypothermia. Age and fitness are also important factors: the elderly and children have less muscle mass and thus generate less body heat.
The wind chill index can be a useful indication to runners, cyclists and motorcyclists for visualising the cooling to which their body is exposed depending on their speed of movement. A marathon runner at about 20 km /h at an ambient temperature of 10 C will experience a temperature similar to 7.4°C. A biker traveling at 90 km / h at 0°C would have a feeling of -10.2°C, hence the quality and importance of protective equipment.
In order to improve the wind chill index, the heating effect produced by direct exposure or solar radiance should be taken into account, which makes it possible to withstand low temperatures for longer periods in the absence of significant wind but which can also cause burns not immediately felt. In bright and sunny weather, the temperature could also be increased from 5 to 10°C (10 to 18°F). The effect of atmospheric pressure on the heat capacity of the air and the ambient vapor pressure and the relative humidity of the air is not taken into account, in grams of steam per kilogram of air also determines the dry vapor pressure already present in the air. This pressure limits evaporation by breathing or sweating. This evaporation is the main source of reduction and regulation of body temperature. Finally, the effect of moisture from precipitation or the environment is not taken into account. It significantly modifies the thermal resistance of the
skin.
By way of conclusion
The lower the temperature is, the more the impact on the temperature perceived by the human body is big. Consequently, the Nordic countries being colder, they are more sensitive on this subject than temperate or warm countries.
Surveys were conducted to ensure that the new wind chill index would meet the needs of the population. The new index is expressed by a number similar to the temperature, according to the preferences of most users. However, since the wind chill index represents the sensation of cold on the skin, it is not a real temperature. It must be given without the symbol of degree. For example, "Today the temperature is -10°C, and the wind factor is -20".
