While France is going through a late heat wave, it is not uncommon to hear about “felt temperature”. In Bordeaux, for example, the temperature this Monday was 37°C, felt like 38. But what do these indications really refer to? If in meteorology, the air temperature is that which is measured with a thermometer placed 1.5 meters from the ground in an openwork shelter, the physiological perception of temperature varies from one individual to another, explains Météo -France, and depending on atmospheric conditions such as wind, rain or sun.
The felt temperature is indeed an index, devoid of unit: the felt temperature is not calculated in degrees Celsius, and in no way corresponds to the temperature that would be felt if it were real. Concretely, a felt temperature of 70 does not refer to a temperature of 70°C, because there are no instruments capable of measuring sensations.
“The thermal comfort indicators take into account different factors, depending on the time of year, develops Grégory Fiffre. In winter, as the feeling of cold is accentuated by the wind, the Windchill index, or wind chill index, is based on the conjunction of cold and wind. In summer it is different: the heat is made more difficult for the human body to bear by the degree of humidity in the air. We are therefore currently calculating the Humidex index, which takes into account temperature and humidity. »
The algorithms behind the calculations therefore vary, as they are based on different variables and scales. A “perceived temperature” cannot therefore be understood on its own: it must be related to risk thresholds in order to grasp its meaning. For the Humidex, which reports the correlation between heat and humidity, it is considered that from 40, the human body is exposed to an uncomfortable situation. Beyond 45, it is in danger, and after 54, in extreme danger, exposed to heatstroke.
For Windchill, an index invented by the Canadians, which applies to winter weather conditions combining wind and temperatures below 10°C, the threshold of very strong discomfort is rather located around -27, which corresponds, illustrates the forecaster, to “a risk of frostbite”, and “a temperature of -15°C coupled with a wind at 50 km/h”.
“Without explaining this scale, the temperature felt does not mean much,” says Grégory Fiffre. It is moreover more of an indication intended for public authorities and health authorities, so that they can adapt their responses, rather than for the general public. »
And especially since we are not all equal when it comes to the “feeling” of cold and heat. A person from Nice, for example, will see his tolerance threshold for heat, even humid, higher than a person from Lille… Whereas, conversely, a person from the north of France will probably take a little longer than a southerner to suffer from the cold. Not to mention the state of health and age, which modify the perceptions of individuals.
