Fire Weather Explained: The Science Behind Europe's Worsening Wildfire Seasons

Wildfires need three things: fuel, ignition and weather that lets fire spread faster than people can stop it. Fuel changes over decades. Ignition is close to constant, since most fires in Europe start with human activity. Weather is the variable that changes week to week, and it is where the climate signal shows up most clearly.

Vapour pressure deficit: the number that matters most

Vapour pressure deficit, or VPD, measures how strongly the atmosphere pulls moisture out of vegetation. It rises roughly exponentially with temperature. A three-degree increase in afternoon temperature does not make plants three per cent drier; depending on humidity, it can double the drying rate. This is why modest warming produces disproportionate increases in fire danger, and why VPD has replaced simple temperature in most modern fire-danger indices.

Fire danger does not rise in step with the thermometer. It rises with the atmosphere's thirst, and thirst compounds.

Heat domes and the missing night

A heat dome is a persistent high-pressure ridge that traps warm air beneath sinking, compressing air. Two consequences matter for fire. First, the ridge suppresses cloud and rainfall for days or weeks. Second, it keeps overnight minimum temperatures high, which prevents the humidity recovery that normally lets fires lie down after dark. Crews rely on those hours to build containment lines. When the night stops cooling, containment stops being achievable and suppression becomes damage limitation.

Live and dead fuel moisture

• Dead fuel — leaf litter, fallen branches, cured grass — responds to humidity within hours. It governs how easily a fire starts and how fast it runs.
• Live fuel — shrubs and tree canopy — responds over weeks to months. It governs whether a surface fire climbs into the crown.
• When live fuel moisture drops below roughly 80 per cent of dry weight in Mediterranean shrubland, crown fire becomes plausible on almost any windy afternoon.
• Spring rainfall deficits are therefore a better predictor of a severe August than August temperatures alone.

Wind, terrain and the plume

Wind supplies oxygen and tilts flames toward unburnt fuel; slope does the same thing through convective preheating, which is why fire runs uphill several times faster than across flat ground. The most dangerous behaviour, though, is a plume-dominated fire — one whose own convection column becomes strong enough to generate erratic surface winds and, in extreme cases, a pyrocumulonimbus cloud with its own lightning. Plume-dominated fires are effectively unfightable at the head and account for a disproportionate share of fatalities.

What the science says about attribution

Attribution studies consistently find that warming has increased the frequency of extreme fire weather days in the Mediterranean basin, while being far more cautious about any individual fire. That distinction is worth holding onto. Climate change does not light fires. It lengthens the window during which a fire, once lit, cannot be stopped — and that window is what the current European season has been demonstrating.