France experienced the hottest May day on record on May 25, 2026. In Nantes, the mercury was close to 35°C. In Brest, 33°C, more than ten degrees above seasonal norms. In total, monthly records were shattered in 352 weather stations, mainly in the west of the country according to Météo France.
Behind these spectacular figures, a more discreet piece of data triggered the official alert: the national thermal indicator exceeded 24°C on average in the country. This is a first for the month of May, breaking a record dating from 1944. Few people know this tool. And yet, it is he who officially defines, since 1947, whether or not France is experiencing a heat wave.
The national thermal indicator, or ITN in meteorologists’ jargon, is an average. Concretely, every day, Météo France records the minimum and maximum temperatures in 30 reference stations spread across the entire metropolitan territory: from Brest to Strasbourg, from Lille to Perpignan, via Clermont-Ferrand or Poitiers. The average of the highest and lowest temperatures recorded over 24 hours gives a single figure which then summarizes the situation over the whole of France.
These 30 stations are not chosen at random. They are located in plain areas, away from local microclimates such as coasts, mountains and large metropolises which could distort the national reading. The idea is to obtain the most faithful possible reflection of the “average” climate of the country. This indicator was created jointly by Météo France and EDF in 1947, originally for very practical reasons: anticipating electricity needs according to temperatures. It has since acquired a much more central role.
Seventy years of a curve that doesn’t lie
Calculated since 1945, the ITN today constitutes an irreplaceable climate archive. From 1950 to the 1980s, the indicator oscillated relatively stable, around 11.5 to 12°C on annual average. Cold years (10.6°C in 1956, 10.7°C in 1963) alternate with milder years, without any marked trend. Then everything changes. At the end of the 1980s, the curve began to climb. In 1989, the annual ITN exceeded 13°C for the first time. In 1990, again. The milestone has been reached and it will never go back down permanently.
From the 2000s, the movement accelerated. The ten hottest years since 1945 all occurred after the year 2000. In 2022, the annual ITN reached 14.5°C, an absolute record since measurements began, or almost 3°C more than the average of the 1950s. Since then, the curve has not gone down. The year 2023 shows 14.4°C and 2024 stabilizes at 13.9°C: a year described as “abnormally not very sunny” but which is still among the five hottest on record. 2025 rises to 14.1°C.
France is warming faster than the planet
These numbers are not natural fluctuations. They reflect a transformation of the French climate. The warming observed in mainland France reaches +2.2°C over the period 2015–2024 compared to the pre-industrial era. This is more than the global average, which shows +1.3°C on the same basis.
The concrete consequences are already there. Over the decade 2015-2024, the number of heat wave days increased six-fold in France compared to the period 1961-1990, going from two days per year on average to 13. At the same time, cold waves went from six days per year to less than one. The summer of 2025 included two heat waves and ranked as the third hottest summer since 1900, behind 2003 and 2022, recalls Météo France in its assessment of the summer of 2025.
By 2050, France could experience warming of +2.7°C compared to the pre-industrial period. In practice, this means that the annual temperature of 13.9°C recorded in 2024 by the national thermal indicator will be exceeded more than every other year.
