Fire weather: understanding wildfire risk

The three factors that make the danger

Vegetation dryness first: after weeks without rain, grass and brush become fuel ready to ignite at the slightest spark. Air relative humidity next: below 30%, fine vegetation dries out in hours. Wind finally: it feeds oxygen, lays flames forward and throws embers sometimes hundreds of metres ahead. Wind is what turns an ignition into a disaster.

The most feared combination fits in three numbers: above 30 °C, below 30% humidity, above 30 km/h of wind, the “30/30/30 rule”.

Official fire danger ratings

Most fire-prone countries publish a daily fire danger rating: Météo-France's “météo des forêts” map (green to red, per department, since 2023), the US National Fire Danger Rating System, Canada's Fire Weather Index (FWI), which has become the international reference formula. At the highest levels, access to some forests is legally restricted.

kanari shows an estimated weather risk on each French department page, computed continuously from the same ingredients (temperature, humidity, wind, recent rain). It gives the real-time trend without replacing the official maps, which carry legal force.

What the risk changes in practice

On high-risk days, mundane actions become dangerous: outdoor work with spark-generating tools, barbecues, cigarette butts, parking on dry grass (a catalytic converter can ignite it). Firefighters pre-position resources and water bombers fly armed patrols: attacking a nascent fire within 10 minutes multiplies the chances of stopping it before the first hectare.

The right reflex: check the day's risk in the morning, avoid any flame or spark, and keep an eye on the real-time map when the wind picks up.

Frequently asked questions

Also worth reading

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