The perceived temperature takes into account atmospheric conditions which can make the cold or the heat more painful. It should not be confused with the actual temperature.
The French have no choice but to endure an extreme heat wave since August 17, 2023. According to Météo-France, the country is heading for the hottest episode of the summer, with an intensification of this late heat wave expected. between Tuesday 22 and Wednesday 23 August. 40°C could be reached locally. A few concrete tips can help with this heat. It is particularly difficult to fall asleep when it is so hot.
While this heat wave is raging in France, the notions of real temperature and felt temperature reappear, without us always understanding very well what they mean. Here’s how to stop confusing them.
What is the felt temperature?
The temperature felt is an index. This is also referred to as the “wind chill index”. The perceived temperature takes into account the air temperature and the wind speed, because ” the physiological perception of temperature varies from one individual to another and according to atmospheric conditions (wind, rain, sunshine) “, recalls Météo-France.
The notion of perceived temperature is mainly used in winter, when it is cold. But, it also becomes more and more useful in summer, with the frequency of episodes of high heat which increases due to climate change.
Humidity influences the perceived temperature. When the body is too hot, it sweats to cool down. But, in very humid conditions, perspiration evaporates less easily and the body struggles to evaporate perspiration (and therefore to cool down). Heatstroke can occur in the most severe cases.
What is wind chill?
Wind chill refers to a cooling sensation, caused by both temperature and wind.
When there is no wind, a thin layer of air heated by the body forms on contact with the skin. It is humidified by the evaporation of the water present on the surface of the body. ” When there is wind, this insulating film is continuously blown away “, explains Météo-France. The skin then finds itself in contact with drier and colder air, which it will warm to recreate the initial balance (which cools the body).
This concept is particularly widely used in Canada, where it allows the population to better anticipate the effects of cold on the body. Canada has also developed another measure of heat, the Humidex, to describe ” the intensity of heat felt by people “.
In the United States, the heat index is used (heat index), which corresponds to what we call apparent temperature in France. The heat index has no international value, it is different according to geographical areas.
What is the difference between real and felt temperature?
The actual temperature is the air temperature (or shade temperature). It is measured with a thermometer or a probe installed 1.5 m above the ground in an open shelter.
This temperature cannot account for the physiological perception of temperature, because it does not take into account other factors: wind, rain, sunshine. We therefore need the temperature felt to express a stronger feeling of cold or hot depending on these other factors.
The felt temperature does not mean that it is hotter than the weather forecast displayed on a map. It makes it possible to take into account the sensation of hot or cold, which can cause even more discomfort for the body.
How is the perceived temperature calculated?
The calculation of the felt temperature or the wind chill index is carried out using a ” empirical mathematical relationship, which takes into account air temperature and wind speed », Indicates Météo-France. There is no unit for this index.
Le Monde provides a converter to calculate the perceived temperature. The United States Weather Service also offers a Heat Index Calculator.
You can also use temperature tables, like this one:
Let’s take an example. ” With an air temperature of -10° C and a wind of 30 km/h, it will be -20, for example. This means that the sensation on the skin will be close to that experienced under a temperature of -20° C on a windless day. “, according to Météo-France.
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