Wind chill

After seeing Heat-Index and Humidex, which measure the influence of humidity on the way we support high temperatures, I propose you today to dedicate ourselves to the wind chill, that concerns low temperatures.

The wind chill is an index which makes it possible to demonstrate the lowering of the temperature locally on the surface of the skin (for mammal, hot-blooded, therefore), under the effect of the wind at low temperatures. Indeed, in the absence of wind, only the natural convection eliminates the air heated locally by the surface of the skin. This is no longer the case when the wind blows: a non-negligible part of this hot air is eliminated by the effect of the wind, and the resulting cold temperature sensation can quickly become worse.
This index has an interest when considering the absolute wind – when one considers the meteorological observation, therefore – but also when considering the relative wind: a cyclist who rides at 30 km/h when there’s no wind, suffered a relative wind of 30 km/h and the wind cooling therefore applies fully as soon as the atmospheric temperature becomes low!

Initially developed in the early 1940s by Paul Siple, an American scientist who participated in numerous expeditions to Antarctica, the calculation of this wind chill was improved in the early 2000s by Canadian and American physicians.

Weather Station uses the following formulas to compute wind chill:

\(\left\{\begin{array}{ll}{R}_{(1)}=13,12+0,6215{T}+(0,3965{T}-11,37)\times{V}^{0,16}\\{V}>4,8km/h\end{array}\right.\)
 

\(\left\{\begin{array}{ll}{R}_{(2)}={T}+0,2(0,1345{T}-1,59)\times{V}\\{V}\leq4,8km/h\end{array}\right.\)
 

with \({T}\) the atmospheric temperature in °C and \({V}\) the wind speed expressed in km/h.

It should be noted that the calculation of this index does not take into account the effect of atmospheric pressure on the calorific capacity of the air and the ambient vapor pressure or the air humidity (which have a direct influence on transpiration and therefore on the regulation of body temperature) nor the effect of moisture which modifies the thermal resistance of the skin nor the solar radiance.
This is more of an index determined by a statistical projection, which does not take into account factors specific to the individual such as corpulence, lipid film of the skin (natural or cosmetic), food, etc.

Danger levels

As with Heat-Index or Humidex, you will find on the Internet a large amount of tables indicating the danger levels associated with the wind chill values. I reproduce one here to give you your own idea:

(*): it is generally considered that the effect of wind chill only makes sense for temperatures below 10 °C. This is partly explained by the fact that, above this temperature, other phenomena (such as perspiration) are no longer negligible factors.