Infographics. When can we talk about a heatwave? We take stock in the departments of Lorraine and Franche-Comté.

Up to 36° in Besançon, 35° in Nancy and Metz, and 34° in Vesoul... The intense heat continues to hit France this Tuesday, August 12. The second heatwave of the summer promises to be longer and more intense. This episode is linked to a rise of hot air from North Africa.

But at what stage are we talking about a heatwave and what are the reasons for it? We take stock of the situation in the departments of Lorraine and Franche-Comté.

Thresholds set at the level of each department

A heatwave is a period of intense heat, both day and night , lasting at least three consecutive days . Public Health France and Météo France have set alert thresholds for each department “to prevent an impact on mortality,” according to a regional epidemiological bulletin from Public Health France in Bourgogne-Franche-Comté.

In Meurthe-et-Moselle, for example, the temperature must exceed 34 degrees during the day and not fall below 18 at night.

• Lorraine : Meurthe-et-Moselle (18°C - 34°C), Meuse (18°C - 34°C), Moselle (19°C - 34°C) and Vosges (18°C - 34°C).
• Franche-Comté : Doubs (19°C - 33°C), Jura (20°C - 34°C), Haute-Saône (18°C - 34°C) and Territoire de Belfort (18°C - 33°C).

The combination of these minimum and maximum temperatures corresponds to the biometeorological index (BMI) . This index makes it possible to determine the different heatwave alert thresholds.

To determine local minimum and maximum temperatures, Météo France uses measurements taken at reference weather stations from June 1st in each department. Particular attention should be paid to high nighttime temperatures, as the body has difficulty recovering.

Finally, other factors that could worsen this health situation are also monitored: air humidity, air pollution, and even population gatherings. These factors are taken into account qualitatively when deciding to issue an alert. When the alert is issued, actions are implemented and a description of the health situation is provided at the departmental level (e.g., number of deaths, visits to emergency rooms, use of SOS doctors associations, etc.).

An alert system every summer since 2004

These thresholds and alert levels have been set every summer since 2004. This system follows the 2003 heatwave, which resulted in an estimated 14,800 deaths (+60%) and affected all age groups. The first signs of a health impact were noted by emergency physicians and funeral directors, as well as by the deaths of workers.

Since 2004, the heatwave plan has aimed to address these difficulties . "It is based on prevention actions targeting vulnerable people and the general public, and on management measures aimed at institutions and professionals in the health and social sectors," explains the High Council for Public Health in a 2019 report on the heatwave and health alert system.