Climax, heat wave and heat wave correspond to precise climatological criteria, vigilance during heat waves also taking into account the possible health effects, explains Météo-France.
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What do heat peaks and waves mean?
“A heat spike is a brief episode of 24 to 48 hours in which temperatures are higher than normal for the season,” explains Météo-France on its website.
“A heat wave is an episode of temperatures well above the seasonal norm for several consecutive days,” continues France’s Public Service for Meteorology and Climatology.
The definition of a heat wave varies from region to region depending on temperatures and how long they last, as populations do not have the same adaptability.
At the national level, “a heat wave is recognized when the national thermal indicator (average of daily measurements of average air temperature in 30 weather stations) reaches or exceeds 25.3°C and remains high for at least three days,” adds Météo-France.
And a heat wave?
A heat wave is “an episode of high temperature, day and night, over a prolonged period (at least three days)”, explains Météo-France.
Meteorologists “have defined threshold values that vary from department to department. In Toulouse, for example, Météo-France speaks of a heat wave when the maximum temperature is over 36°C and the minimum temperature is over 21°C over the three days and three nights.”
Temperatures can be lower in northern France.
Heatwave vigilance can be yellow, orange or red, the latter threshold corresponding to both an extraordinary meteorological event and a health alert justifying maximum mobilization, with the risk of excess mortality.
Multiplication of episodes
These hot spells usually affect mainland France between early July and mid-August, but can also occur outside of this period.
46 heat waves have been recorded in France since 1947, the last one dated August 2022, before the current wave began on August 17.
These waves “have been significantly more numerous in recent decades.” In the past 38 years, their numbers have been four times higher than in the previous 38 years. The number of days with heat waves has increased ninefold,” states Météo-France.
This “is part of the global warming logic that implies that such episodes could occur more frequently or later during the summer season,” explains Météo-France. By 2050, their frequency is expected to double.