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Pyrocumulonimbus Fire Clouds Intensify Europe’s Wildfires
What’s the News?
Severe wildfires in France and Spain have generated pyrocumulonimbus clouds, commonly called fire clouds or pyroCbs. A major blaze near Saumos in France’s Gironde region produced what was reported as the country’s first documented pyroCb event.
The fire cloud generated strong winds and lightning, making the wildfire more unpredictable and capable of igniting new fires. The Gironde blaze burned more than 42,000 hectares, while large-scale evacuations were undertaken across affected parts of France and Spain.
Pyrocumulonimbus Cloud
A pyrocumulonimbus is a thunderstorm cloud generated by intense heat from a wildfire or another major heat source.
- Common names: Fire cloud or fire-generated thunderstorm.
- Abbreviation: PyroCb.
- WMO terminology: Cumulonimbus flammagenitus.
- It can rise to approximately 15 kilometres and may carry smoke and aerosols into the upper troposphere or lower stratosphere.
- Unlike an ordinary thunderstorm, its initial upward convection is driven mainly by the extreme heat of a fire.
How Does It Form?
- An intense wildfire heats the surrounding air.
- Hot air, smoke, ash and water vapour rise rapidly through convection.
- The rising air expands and cools.
- Water vapour condenses on smoke and ash particles, forming a pyrocumulus cloud.
- If the plume continues rising into an unstable and sufficiently moist atmosphere, it develops into a pyrocumulonimbus thunderstorm.
The required conditions generally include:
- Extreme fire intensity.
- Hot and dry conditions near the surface.
- Cooler and relatively moist air at higher levels.
- Atmospheric instability that supports rapid vertical movement.
Why Are Fire Clouds Dangerous?
- Lightning: PyroCbs can produce lightning that ignites new fires away from the original blaze.
- Strong downdrafts: Descending air can generate sudden and violent surface winds.
- Changing fire direction: Fire-generated winds can rapidly alter the speed and direction of a wildfire.
- Ember transport: Powerful updrafts may carry burning material over long distances.
- Reduced visibility: Dense smoke and ash complicate aerial and ground firefighting.
- Stratospheric pollution: Smoke injected high into the atmosphere can travel across continents and affect air quality and atmospheric processes.
Europe’s Wildfire Crisis
The Gironde wildfire near Bordeaux:
- Burned more than 420 square kilometres, equivalent to over 42,000 hectares.
- Forced approximately 220,000 people to evacuate within the region.
- Destroyed or damaged more than 240 houses.
- Produced repeated fire-cloud formations and fire-generated lightning.
Spain also experienced widespread fires, with more than 170,000 hectares reportedly burned during the year and its largest active blaze affecting around 50,000 hectares in Ávila.
Climate Change Link
Climate change does not directly ignite every wildfire, as many fires originate from human activity or natural causes such as lightning. However, warmer conditions can:
- Increase the frequency and severity of heatwaves.
- Dry vegetation and soil more rapidly.
- Extend the duration of fire seasons.
- Create larger quantities of combustible material.
- Increase the probability of extreme fire behaviour and pyroCb development.
A wet winter can also promote vegetation growth; subsequent drought and extreme summer heat can convert this vegetation into highly flammable fine fuel.
Additional Info
- Pyrocumulus and Pyrocumulonimbus
- Pyrocumulus: Fire-generated cumulus cloud that may not produce a thunderstorm.
- Pyrocumulonimbus: More powerful vertical development accompanied by thunderstorm activity, lightning and dangerous winds.
- Wildfire Monitoring
- Satellites detect heat signatures, smoke plumes and rapid cloud development.
- Lightning-detection instruments can help identify pyroCb intensification.
- NASA’s Fire Information for Resource Management System—FIRMS provides satellite-derived information on active fires.