• To trigger the heat wave alert, which currently concerns 23 departments in France, the average temperatures over three days must exceed a certain level and drop below another at night.

  • But depending on whether you live in Lille or Marseille, these thresholds are not the same.

  • 20 Minutes

    explains how they were established.

" It's hot !

More and more hot”, sang Passi in a classic of the summer of 1997. A time when the heat waves still seemed exceptional to us and when we predicted a great future for CD-ROMs.

That said, the definition of "too hot", and the triggering of the heat wave alert - which currently concerns 23 departments in France - does not have the same threshold everywhere.

Thus, depending on whether you live in Lille or Marseille, daytime and nighttime temperatures exceeding a certain level for at least three consecutive days do not have the same values.

In Marseille, for example, temperatures must exceed 35 degrees during the day and not drop below 24 at night.

In Lille, the heat wave alert threshold values ​​are set at 33 for the day and 18 at night.

That is a significant difference of five degrees, you will have done the calculation.

This is how a department like Pyrénées-Atlantiques, with no less than 20 degrees at night and more than 34 during the day, is on heatwave orange vigilance, while Bouches-du-Rhône, where these values ​​will be reached, is not.

So, would the inhabitants of the south resist biologically better to the heat?

This is a question which in reality has not been studied, explained to 20 Minutes Etienne Patin, genetics researcher, and which would require dissecting the genome of many individuals to identify genetic mutations.

But how are these thresholds established?

They were set by the Health Watch Institute in the wake of the traumatic and deadly heat wave of summer 2003, during which an excess mortality of 14,800 deaths was observed.

That is about a 60% increase compared to that expected, notes the authors of a study entitled

Heat wave alert system and health: principles, foundations and evaluation

, published in March 2012 by the health authorities.

Distinguish between "true alerts" and "false alerts"

They detail the methodology used to deploy this alert system, "based on the monitoring of forecasts of biometeorological indicators and on a system of departmental alert thresholds".

In search of the “best possible compromise between good specificity and good sensitivity”, the authorities tested different indicators “for previously defined levels of excess mortality” (50% in Paris, Lyon, Marseille and Lille and 100% for the others) .

Among the value indicators tested, we find the average minimum and maximum temperatures, an indicator mixing the two, another taking the average temperature of the dew point, or even a thermohygrometric index (a function between temperature and relative humidity).

These indicators were calculated for 14 large pilot French cities,

All these tests were intended to distinguish between “true alerts”, “false alerts” (days when high temperatures are not associated with high excess mortality) and “missed alerts”.

The index finally adopted was the combination of minimum and maximum temperatures averaged over three days.

For each pilot city, this system was tested retrospectively over the years 1973-2003.

"It showed a good ability to detect major heat waves for which excess mortality was actually observed (especially in 2003, 1983 and 1976)", continues the study.

This was later extended nationwide.

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