Forest fires, heatwaves, thunderstorms... How does Météo France choose alert levels?

Marseille, Narbonne, and the departments of Gard and Hérault in southern France are facing devastating blazes this summer. In the Aude department, the fire that broke out in Ribaute has already caused the death of one person and covered more than 15,000 hectares as of Wednesday, August 6. The day before, the department had been placed on red alert for forest fires.

With the intense summer heat, weather warnings are multiplying across France. Thunderstorms, heatwaves, fires... Météo France uses a four-level alert scale to warn of the severity of these phenomena: green, yellow, orange, and red.

Météo France's vigilance map, which covers today's and tomorrow's dates, is compiled by the institute's forecasters. These experts exchange data with regional stations to determine the risk level department by department.

Certain exceptional circumstances may influence this decision. A major holiday weekend or the organization of large-scale outdoor events warrants increased vigilance.

Created two years ago, following a summer of 2022 marked by unprecedented fires, the forest fire alert is not a traditional alert. It is based on the analysis of other weather predictions such as drought, winds, or extreme temperatures, and primarily targets Mediterranean areas, which are more prone to these episodes.

When a red alert is issued, it means that the risk of fire is very high, well above seasonal averages, and that fires can start suddenly and spread very quickly.

Given that 90% of fires are caused by human activity according to Météo France, prefects can issue orders to limit risky behavior . Barbecues, fireworks, or certain agricultural and forestry work generating sparks can therefore be prohibited. Access to forest areas can also be temporarily restricted to prevent any fires from starting.

In the event of a fire, the public is encouraged to take shelter in a building with cleared areas while waiting for emergency services to arrive. However, it is strongly recommended not to take refuge in a car, which is highly vulnerable to flames.

In place since 2004 following the 2003 heatwave , the weather alert is in place from June 1 to September 15. It takes into account the duration of the episode: orange for a heatwave of at least three days and three nights, and red in the event of extreme, prolonged temperatures, covering a large geographical area and likely to have a significant health impact.

Alert thresholds vary by department, depending on their climate and the impact of previous episodes. To assess a red alert, Météo France cross-references weather data with the biometeorological index (BMI) established by Public Health France, which measures the effect of weather on human health.

In the event of an alert, the instructions are clear : avoid going out during the hottest hours, hydrate regularly, even if you don't feel thirsty, and stay in cool places.

The red storm alert is triggered when a storm of exceptional violence is expected, representing a major threat to people, property and infrastructure.

Unlike other alerts, there are no strict numerical thresholds for activating it. It relies on analysis by forecasters who assess the expected intensity of gusts, precipitation, and hail, as well as the area's capacity to cope with them. Saturated ground or dense urbanization can increase the alert level.

If the red alert is activated, Météo France recommends taking shelter in a solid building capable of withstanding the storm, and above all avoiding driving, which could become trapped in the event of a flash flood.