Since May, this is already the second time that France has been swept by an episode of intense heat. Several terms then circulate in the media: heat peak or heat wave and, of course, the fatal heatwave. But what exactly do these terms cover? (Apart from knowing that you are going to be very hot) And how to use both?
Heat peak, heat wave, heatwave: three different concepts
Contrary to what one might believe, these terms are not synonymous.
- The shortest is heat peak. It designates an episode of very high temperatures but of short duration, generally limited to 24 or 48 hours. The temperatures clearly exceed seasonal norms, but the episode remains punctual;
- The heat wave constitutes a broader term. It corresponds to a period during which temperatures remain abnormally high for several consecutive days. This is a climatological definition used nationally;
- Finally, the heatwave represents the upper stage. It designates a prolonged period of at least three days and three nights during which temperatures remain very high both day and night, to the point of constituting a health risk for the population.
In other words, all heat waves are heat waves, but not all heat waves are heat waves.
How does Météo-France determine a heat wave?
To identify a heat wave, climatologists rely on a very specific tool: the national thermal indicator. This indicator corresponds to the average of minimum and maximum temperatures recorded in 30 meteorological stations spread across the entire territory. Created in 1947, it makes it possible to monitor changes in heat across the country.
Two criteria must be met simultaneously:
- The national thermal indicator must reach at least 25.3°C for a day;
- It must remain greater than or equal to 23.4°C for at least three consecutive days.
Conversely, the heat wave ends when this indicator drops below 23.4°C for two days in a row or below 22.4°C.
To give an idea, the hottest days ever recorded in France since 1947 remain July 25, 2019 and August 5, 2003, with a national thermal indicator of 29.4°C.
When do we really talk about a heatwave?
On the other hand, heat waves are not based on the same criteria everywhere in France. For what ? Because the population of Lille, Brest or Toulouse is not used to the same temperatures and does not react in the same way to heat.
Thus, for each department, Météo-France and the health authorities have therefore defined specific thresholds taking into account local history, the acclimatization of residents and the health consequences observed during previous episodes.
In Toulouse, for example, an episode is considered to be heatwave when maximum temperatures exceed 36°C and minimums 21°C for at least three consecutive days.
The key element is often the nighttime temperature. When the thermometer no longer drops sufficiently at night, the body is no longer able to recover the heat accumulated during the day, which considerably increases health risks.
Why do alerts change color?
When temperatures rise, the French often turn to Météo-France’s vigilance map. But what do the different colors actually mean?
- The green level corresponds to a normal situation, without any particular risk linked to heat;
- The yellow level may indicate a heat spike or persistent heat episode. Temperatures are high and can already pose a risk for fragile people, infants, the elderly or exposed workers;
- The orange level corresponds to a real heatwave. Departmental health thresholds are reached or exceeded for several days and nights. The entire population can then be affected by the risks linked to heat;
- Finally, the red level is reserved for exceptional situations. We then talk about extreme heatwavecharacterized by an unusual duration, a very high intensity or a significant geographical extension. This level of vigilance can cause major disruptions to daily life and public services.
What are the health risks?
Whether it is a heat peak or a heatwave, the recommendations remain the same: drink regularly, avoid physical exertion during the hottest hours, keep your home cool and check on vulnerable people.
But the risks increase as the episode continues.
Heatwave promotes dehydration, heat stroke, worsening of cardiovascular or respiratory diseases and can be particularly dangerous for the elderly, infants, pregnant women or people with chronic illnesses.
This is also why health authorities closely monitor these episodes: it is not only the temperature that counts, but its duration and the body’s inability to recover when the nights remain stifling.
In summary, a heat peak can be intense but brief, a heat wave is long-term, while a heatwave corresponds to a prolonged episode presenting a proven health risk. A nuance that changes everything when it comes to protecting your health.