Tuesday, in the village of Puget-Théniers (Alpes-Maritimes), the thermometers showed 41.8 ° C. A priori a new absolute record for the department. But no, the station is located near concrete surfaces, which invalidates the figure, thus illustrating the drastic criteria applied by Météo-France to establish measurements faithful to international standards.
However, the stakes are high, because the measurements of the more than 500 stations of Radome, the real-time ground observation network of Météo-France, then feed that of the International Meteorological Organization (OMM) and its some 10,000 stations located in the four corners of the globe. And according to WMO standards, the French meteorological service applies an “environmental classification” for each of its stations. Therefore, wind, precipitation, temperature, humidity, radiation, etc. are taken into account in its nomenclature.
Extremely precise criteria
Thus, a class 1 site will be considered as a reference site, while a class 5 site will be a site where nearby obstacles create an inappropriate framework for establishing measurements that are supposed to be representative of a large area (of at least a few tens of square kilometres).
For temperature and humidity, “the sensors inside the shelter must be installed at a standard height of 1.5 m”, we remind Météo-France. “A class 1 site must be located on flat and horizontal ground, with ground covered with low natural vegetation (less than 10 cm) representative of the region”, it is added.
Other criteria to obtain the best classification: “the measurement point must be located more than 100 meters from artificial heat or reflective surfaces (buildings, concrete areas, etc.), more than 100th from bodies of water (unless they are significant in the region), away from any shadow cast when the height of the sun is greater than 5°”.
Thus, the station of the village of “Puget-Théniers is not part of the list of reliable stations due to the presence near concrete surfaces and rocks” which can cause a slight overheating.
46.0°C, the absolute record set in France
This does not mean that values should be shelved. The famous Parisian station of Montsouris is class 4 but “is one of the stations on which we communicate given the longevity of the station (1872) and its location (Paris intra muros)”, explains the national weather service.
“The choice of the reliability of a station is made partly according to the environmental class, but also other factors: station network (Météo-France or partner), analysis by forecasters, photographic study, etc. “, we specify.
“If a site has more than 30 years of continuous temperature measurements, then Météo-France can communicate on a possible record” (monthly or “absolute”, that is to say all months combined), also taking into account “the spatio-temporal consistency of the data with neighboring measurements”. For Météo-France, the highest temperature ever recorded in France was 46.0°C in Vérargues (Hérault) on June 28, 2019.
But for the main world records, the criteria are even more drastic before any approval by the OMM, which has the last word after lengthy checks. Thus the 54.4 ° C measured at Furnace Creek (California) on August 16, 2020 and July 9, 2021, potentially the third strongest heat ever measured on Earth, are still under examination. “The 2020 and 2021 Death Valley sensors have been disassembled and shipped to an independent testing and calibration laboratory. One of the tests has been carried out and we are awaiting the second,” WMO rapporteur on weather and climate extremes, Randy Cerveny, explained on Monday.