French Firefighters Alert Public to 'Rule of Three 30' During Heatwave

French firefighters are warning of extreme wildfire risks as a heatwave triggers the "rule of three 30" across the country.

This meteorological threshold is critical because the simultaneous occurrence of these three conditions allows wildfires to ignite and spread with dangerous speed. When these markers are met, authorities issue maximal vigilance alerts to protect forested regions and residential areas.

The rule of three 30 defines a high-danger state when three specific thresholds are exceeded at once. First, the temperature must rise above 30 °C ^[1]. Second, wind speeds must exceed 30 km/h ^[1]. Finally, relative humidity must drop below 30 % ^[1].

These conditions have been observed across much of France during the current heatwave, particularly in southern regions ^[2]. Vigilance has also increased in forested areas such as the Orne and Sarthe, where patrols and anti-fire barriers are being utilized ^[3]. The risk is not limited to France, as the same rule has been referenced to indicate high risk levels in Belgium ^[4].

The danger comes from how these factors interact. High temperatures dry out vegetation, low humidity removes moisture from the air and fuel, and strong winds push flames rapidly across the landscape ^[2]. This combination creates a volatile environment where small sparks can quickly evolve into uncontrollable blazes.

The urgency of these warnings follows a destructive start to the season. Reports indicate that 8,700 hectares have already burned in France since the season began ^[5]. Firefighters continue to monitor weather data closely to anticipate where the next major ignition may occur.