In firefighter jargon, this is called the “rule of three 30s.”
When the humidity in the air is less than 30%, the temperature is above 30°C and the wind speed exceeds 30 km/h on average, this forms a veritable "powder keg" that can transform a simple sparks into a massive fire and complicates the task of firefighters once the fire starts.
“All these conditions are checked off on the real-time observation maps and the very unfavorable forecast”, warns the Infoclimat site this Wednesday morning.
Several fires have already been underway since the beginning of the week.
In Gironde, 6,000 hectares have burned in less than 24 hours since Tuesday, and the worst is undoubtedly to come.
To assess the #fire risk, #firefighters rely on the so-called "three 30" rule: #temperatures > 30°C, humidity < 30%, wind > 30 km / h.
Unfortunately, all these conditions are checked off on the observation cards.
in real time @infoclimat, and the very unfavorable forecast... pic.twitter.com/zORkRirGc4
– Asso Infoclimat (@infoclimat) August 9, 2022
The origin of this “three 30s” rule dates back to 2018, when operating techniques evolved.
"We conducted a feedback on the previous 20 years, which allowed us to highlight the most unfavorable weather parameters for forest fires", describes Commander Xavier Wiik, technical adviser to the National Federation of firefighters of France (FNSPF).
“A temperature above 30 degrees, coupled with humidity below 30% and a wind of at least 30 km/h are the worst conditions, both for a fire to start and for it to intensify” , completes the manager.
“Risk Anticipation”
In concrete terms, this rule serves “above all for risk anticipation”, adds Xavier Wiik.
In each department, the firefighters have forecasts provided by Météo France and comprising numerous parameters.
Depending on the climatic conditions expected at a particular location, the system and the chain of command can be reinforced there.
Read alsoFires and forest fires: “We are facing an explosive weather cocktail”
“Despite the sometimes heavy rains, we do not find the usual soil moisture and groundwater reserves.
We are facing an explosive weather-Molotov cocktail,” warned Grégory Allione, president of the FNSPF, in early July.
This very unfavorable context will continue on Thursday and Friday over a large part of the country.
"We have such a low humidity rate, which will drop below 20%, and temperatures well above 30°C", describes Sébastien Brana, administrator of the Infoclimat site.
As for the wind, "we will be there punctually in the Center and the North-West on Thursday and Friday", he describes.
The Alpes-Maritimes, for their part, will remain “rather spared because we have a high humidity level”, adds Xavier Wiik, chief of operations of the Departmental Fire and Rescue Service in the Mediterranean territory.