Safety. What is the "three-30 rule," a marker of increased fire risk?

Three departments in the south of France - Vaucluse, Var and Bouches-du-Rhône - were placed on Tuesday at "very high" red risk of forest fires by Météo France's forest weather service .

This means that "weather conditions make the risk of the outbreak and spread of forest and vegetation fires very high compared to summer norms."

Several fires are also raging this Tuesday. A fire broke out in the town of Pennes-Mirabeau, bordering Marseille, and is spreading "very quickly." Near Narbonne, the fire that started on Monday was still raging Tuesday afternoon , with firefighters anticipating a "difficult afternoon" due to the weather conditions.

The risk of forest fires is "very high" this Tuesday because several factors that increase susceptibility to fires are present. According to the National Center for Forest Property, three factors are essential for assessing susceptibility to fire: wind speed, air temperature, and atmospheric humidity.

These weather conditions, which contribute to increasing risks, are summed up by firefighters as the "rule of three 30s." "30°C or more, 30% humidity or less, and a wind of 30km/h or more. When these three factors are combined, the area becomes "severe risk" and is put on red alert. The longer these conditions last, the greater the risk," explains Fabien Didier, who heads the Aisne Departmental Fire and Rescue Service (Sdis) in an article published on the departmental council's website .

"The more active and powerful the wind, the faster the fire will spread, the hotter it is, the faster the fire will start and spread, and the less humidity there is in the air, the drier the environment, the more the fire can spread," Bruno Ménard, general secretary of the French Volunteer Firefighters Union, explained to Actu.fr.

In Narbonne, the weather forecast showed 27°C this Tuesday afternoon, leading emergency services to fear difficult conditions. Especially since "the wind will continue to blow strongly with gusts of up to 75 km/h and the humidity in the air will be 20%, which is an aggravating factor," Colonel Christophe Magny, director of the Aude Sdis, explained to Le Parisien .

In Marseille, too, weather conditions were particularly unfavorable, with a strong mistral wind blowing and 28°C in the air. Along the Mediterranean coast, "vegetation is becoming dry," which could "fuel fires," notes Météo-France in its bulletin published Monday.

According to the meteorological agency, winds are expected to decrease on Wednesday, and "high-risk areas should therefore decrease significantly." Météo-France, however, warns that the risk of fires breaking out will remain high "given the dryness of the vegetation, which will have become even more pronounced during this windy period."