A wildfire isn't a big campfire. It's a weather phenomenon that moves at the pace of the wind, runs up slopes, and throws its embers hundreds of metres ahead of itself.
Three ingredients, then three accelerators
Every fire needs fuel, heat and oxygen. A wildfire adds three factors that decide its speed and violence:
- Fuel: how much, how dry, how continuous. Dry grass, pine needles, scrub, bark, and resinous pines that burn like torches.
- Weather: wind pushes the flames and brings oxygen; heat and dry air dry everything out. In southern France, firefighters talk of the rule of three 30s: over 30 °C, under 30% humidity, over 30 km/h of wind, and the risk becomes extreme.
- Terrain: a fire climbs a slope much faster than it comes down, because the flames preheat the vegetation above them. A common rule of thumb says its speed doubles with every ten degrees of slope.
The three levels of fire
- Ground fire smoulders in humus and peat, with no visible flame, sometimes for weeks, and breaks out further away.
- Surface fire runs through grass, needles and scrub. The most common.
- Crown fire jumps from tree to tree through the tops. The fastest and most violent, and it can't be stopped from the ground.
Anatomy of a wildfire
- The head, moving with the wind: the most dangerous part.
- The flanks, on the sides, calmer.
- The rear, creeping slowly against the wind.
- Spotting: embers and burning bark, carried by the wind and the smoke columns, start new fires far ahead. In strong wind, hundreds of metres, sometimes several kilometres.
A grass fire driven by a strong wind can move at more than 10 km/h (6 mph), faster than a person running over rough ground. A very large fire can create its own thunderstorm cloud, a pyrocumulonimbus, which throws lightning and unpredictable winds.
Where they come from
In France, about nine fires in ten are caused by people: carelessness (cigarette ends, barbecues, work, garden bonfires), accidents (power lines, sparks from machinery and trains), and arson. Lightning starts a small share, often in mountain areas.
What they amount to
- France burns on average a few tens of thousands of hectares a year; more than 70,000 hectares in 2022, including about 30,000 in Gironde, far from the Mediterranean areas where they were expected.
- Australia lost about 20 million hectares in the 2019-2020 summer.
- Canada broke its record in 2023, with more than 15 million hectares burned.
A warmer climate lengthens the fire season, and fires are moving north and uphill.
Forests and fire
Some forests need fire: some pines only release their seeds in the heat of a fire, and natural fire clears the undergrowth. The problem comes from the modern combination: dense, unmanaged forests, houses built in the middle of them, longer droughts.
What really protects
- Fuel clearance: in France's at-risk areas, the law requires owners to clear vegetation 50 metres around buildings, and 10 metres either side of access roads. A house surrounded by a cleared space is defensible; a house buried in pines isn't.
- Fuel breaks: firebreaks, vineyards, fields, tracks, which break the continuity of vegetation.
- Access tracks for emergency services, with water points.
Smoke
Wildfire smoke contains fine particles that go deep into the lungs. It travels hundreds or even thousands of kilometres. On smoky days, an FFP2 or N95 filtering mask protects far better than a surgical mask, see Masques, filtres et tenues de protection.
What firefighters do: Fighting wildfires. What to do yourself when fire comes: Wildfire, defending yourself and getting out.