About 90% of wildfires are started by human activity, typically igniting fine fuels such as vegetation, grass, pine needles, litter and twigs. Recent weather has made ignition far easier, with drought in western Europe turning vegetation into tinder.
Fuel moisture and abundance
Fine fuel moisture code values climbed above the 96th percentile this month, while France and Spain's eighth-wettest winters on record left an abundance of fine fuel ready to burn. Once established, fires can spread into heavier fuels including shrubs, roots and deeper litter if conditions are dry enough. These materials burn hotter and for longer, can smoulder long after flames appear to have extinguished and generate towering columns of superheated air capable of carrying embers far beyond the blaze.
Heatwaves and winds
This year's prolonged dry spell and three record-breaking heatwaves have allowed heavier fuels to dry substantially. Winds have also made fires harder to predict. France's largest blaze, in Gironde, initially spread westwards towards the coast on an easterly breeze before winds abruptly turned westerly on Friday, driving the fire back towards Bordeaux. Fires of this scale can also generate their own strong, gusty winds, creating unpredictable behaviour that complicates firefighting efforts.
Thunderstorms and risk ahead
Showers and thunderstorms near the largest fires in France and Spain brought little rainfall on Saturday and may have worsened conditions. Rain evaporating beneath a storm cloud cools the air, making it denser, so that it rapidly sinks to the ground before spreading in all directions. These downdrafts can fuel sudden, erratic fire behaviour. Cloud-to-ground lightning without accompanying rainfall can also ignite new fires.
With increasingly high pressure expected to delay any meaningful rain until the end of the week, while increasing the likelihood of a fourth summer heatwave across much of Europe, the threat of further large wildfires is likely to persist into August.
Flooding in South Asia
Elsewhere, heavy rainfall and glacial melt in Afghanistan, Pakistan and India over the past week and a half have triggered severe flooding, with at least 180 people reported dead. Monsoon rain, colliding with out-of-season western disturbances, has brought up to 250mm of rain in 24 hours – almost half the average annual rainfall of London, UK – causing rivers to burst their banks and triggering landslides.
