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Researchers at Imperial College and fire services warn that a vapour pressure deficit\u2011driven ‘firewave’ \u2014 simultaneous wildfires across urban green spaces after roughly ten consecutive very dry days \u2014 is making UK cities more vulnerable and calls for VPD\u2011based early warnings and targeted operational responses.<\/p>\n<\/div>\n
Cities across the UK are confronting a new and unsettling hazard as hotter, drier summers make urban green spaces far more combustible. Researchers at Imperial College London have coined the term \u201cfirewave\u201d to describe the simultaneous outbreak of multiple wildfires across a city during extended dry spells \u2014 a phenomenon brought into sharp relief this week as crews tackled separate heath and gorse fires in London and a large gorse blaze on Arthur\u2019s Seat in Edinburgh. The Scottish Fire and Rescue Service confirmed a major incident on Arthur\u2019s Seat that mobilised multiple appliances and specialist resources as crews worked overnight to contain the blaze. According to reporting by national broadcasters, the recent outbreaks underline how quickly long\u2011dormant fuels in parks and heaths can become a threat to urban populations.<\/p>\n
That danger is not just anecdotal: a peer\u2011reviewed study from Imperial College examined London Fire Brigade incident records and Met Office weather data for 2009\u20132022 and identified vapour pressure deficit (VPD) \u2014 a measure of how much moisture the atmosphere can draw from the land \u2014 as a principal driver of urban wildfire risk. The paper finds that VPD explains more of the variability in wildfire occurrence than temperature or relative humidity and quantifies five distinct \u201cfirewave\u201d events over the 2009\u20132022 period. The authors recommend using VPD as an operational early\u2011warning metric to signal when city vegetation has become unusually desiccated and primed for rapid fire spread.<\/p>\n
The researchers and London\u2019s fire commanders have identified a practical tipping point: after roughly ten consecutive very dry days, vegetation across wide areas can pass a moisture threshold that makes it dramatically more flammable. \u201cVegetation doesn\u2019t just become a bit more flammable,\u201d Professor Guillermo Rein of Imperial College told the BBC; \u201cit becomes much more flammable. Once the moisture content of the vegetation drops below a certain threshold, even a small spark can lead to a fast\u2011spreading fire.\u201d That scientific framing helps explain why multiple small ignitions \u2014 most often human in origin, whether accidental or deliberate \u2014 can cascade into city\u2011wide emergencies.<\/p>\n
London\u2019s experience in July 2022 offers a stark illustration of the stakes. The London Fire Brigade describes 19 July 2022 as its busiest day since the Second World War, dealing with more than 1,100 incidents and thousands of calls as heat and drought sent fires racing through parks and open spaces and, in some cases, into residential areas. One large blaze in Wennington destroyed dozens of buildings and led to mass evacuations, prompting the brigade to declare a major incident and later to review operational learning and community support needs. Brigade commanders say the day exposed how simultaneous fires can overwhelm resources and put lives and property at acute risk.<\/p>\n
In response to that changing threat landscape, the London Fire Brigade has invested in new tactics and kit tailored to wildland\u2011urban incidents. The brigade\u2019s operational guidance explains how drones now provide aerial optical and thermal imagery, loudspeaker capability and targeted reconnaissance that improves commanders\u2019 situational awareness and helps direct scarce resources. Complementing those aerial tools, pilot wildfire response vehicles \u2014 off\u2011road 4x4s equipped with roughly 475\u2013500 litres of water, pump\u2011while\u2011driving systems, misting lances and specialist beaters \u2014 have been deployed to reach terrain that conventional appliances cannot and to create firebreaks or dampen fuels close to homes. A brigade spokesperson says these assets have become central to a \u201cmore proactive approach\u201d to safeguarding London\u2019s green spaces and neighbouring communities.<\/p>\n
The policy implications run beyond equipment. Imperial\u2019s study and accompanying commentary argue that existing public\u2011health style heatwave warnings do not capture the distinct and rapidly escalating fire risk that arises in urban landscapes. Researchers propose adopting the \u201cfirewave\u201d concept in early\u2011warning systems so that messaging and operational readiness are aligned with the meteorological conditions that actually drive urban ignitions. The Met Office has been asked for comment on whether its heatwave definitions and public alerts should incorporate VPD or a similar measure of ecological dryness.<\/p>\n
There are limits and nuances to the picture. The vast majority of urban fires are still started by people, and so behavioural interventions \u2014 public awareness campaigns, targeted restrictions in high\u2011risk periods, and local stewardship of vulnerable open spaces \u2014 remain vital. The Imperial team also highlights adaptation measures for green space management to reduce fuel continuity, and London\u2019s post\u20112022 planning has emphasised training, strategic stationing of specialist vehicles, and community liaison as part of resilience building. At the same time, many northern European cities lack a historical institutional memory of dealing with wildfires, which increases the risk of being ill prepared as climate patterns shift.<\/p>\n
As summer heat and drought become more common, the argument from scientists and fire services is straightforward: better metrics, clearer public messaging and targeted operational investment can reduce the likelihood that a handful of sparks become a city\u2011wide emergency. The evidence from peer\u2011reviewed research and from front\u2011line services suggests that recognising and preparing for \u201cfirewaves\u201d \u2014 rather than treating urban heat purely as a comfort or health issue \u2014 will be an increasingly necessary part of urban climate resilience.<\/p>\n