They are invisible, but formidable. Heat blows kill much more than hurricanes or floods, however, they rarely make the front page. THE Thermal stressthis physical phenomenon still too little known, would be responsible for the death of approximately 500,000 people per year According to the World Meteorological Organization (OMM). But the real figure could be up to 30 times higher. This week, LA france s “Applies to experience its first heat wave of the year. However, displaying “40 ° C in the shade” is not enough to reflect the extreme danger of these episodes. So how do we really measure this invisible heat that kills?
Why the temperature in degrees Celsius is not enough
On paper, 35 or 40 ° C already seem formidable. In reality, this figure alone says almost nothing about real risk for the human body. What the thermometer displays does not take into account the humidity, the wind, the capacity of the body to sweat, nor the immediate environment.
“”The heat is a silent killer, because the symptoms are not so obvious “underlines Alejandro Saez Reale, of the OMM. When The body can no longer coolit can go from simple headaches to a Multivisceral failure in a few hours.
HAS 35 ° C with a humidity rate of 100%six hours are enough for cause the death of a healthy personrecall researchers in 2023. Under these conditions, sweat no longer has any effect: it no longer evaporates, the heat accumulates in the body until stopping.
What clues to assess the heat really felt?
To compensate for the insufficiency of Celsius degrees, several indicators were developed to translate the Heat felttaking into account various environmental parameters. Among the most used:
- Universal thermal climate index (UTCI) : adopted by Copernicus, it combines temperature, humidity, wind, sunshine and thermal radiation. He defines a extreme thermal stress from 46 ° C felt;
- Heat index (Heat Index) : developed by the United States, it meets temperature and humidity, with a color scale ranging from yellow (“prudence”) to bright red (“extreme danger”);
- Humidx : used in Canada, it calculates a temperature felt By crossing heat and humidity, with progressive discomfort thresholds;
- Wet thermometer : Ancient but effective method, it considers from what level of humidity and heat, perspiration can no longer regulate body temperature.
These tools make it possible to better understand the human tolerance Faced with heat. Because this threshold varies: at 35 ° C drythe desert remains more bearable than tropical forest at 30 ° C and 90% humidity.
Persistent limits according to experts
But these clues, as sophisticated as they are, are not infallible. John Nairn, specialist in WMO heat waves, warns against a Useless use of some indicators. “”UTCI is a very bad measure in chronically hot regions “he said. Originally developed for Germany, this index remains less relevant in countries where heat is daily and extreme.
Another limit: these tools are concentrated almost exclusively on the human healthregardless of Systemic heat effects. However, a heat wave can block transportation, have electrical networks disjurrent or make air -conditioned hospitals ineffective. “”If the heat reaches a level such that it prevents the operation of an infrastructure, there will be a consequence on the passengers who will no longer be protected “insists John Nairn.
Facing a intensification of global heat wavesmeasuring thermal stress is therefore no longer just for the weather forecast, but Vital risk management. The Celsius alone have lost their alert power: it is now necessary to read between the degrees.