As the UK reaches the end of the hottest summer on record, new research from the Ordnance Survey (OS) has revealed which cities are most vulnerable to extreme heat.
And it is bad news for the people of Portsmouth, with the city coming out at the top of the list.
Experts say that the city, which is home to 208,000 people, is particularly at risk due to its warm summer weather, hard concrete surfaces, and lack of green space.
This creates an effect known as an ‘urban heat island’, which can make poorly cooled cities like Portsmouth up to 10°C (18°F) hotter than the surrounding countryside.
By 2040, human–caused climate change is expected to drive London‘s City of Westminster into second place for the most heat–vulnerable city.
Plymouth, meanwhile, will jump up the risk table from fifth place today to become the third most vulnerable city by 2040 and onwards to the turn of the century.
Nick Bolton, CEO of the OS, says: ‘These findings matter because cities are where millions of people live, work and spend their everyday lives.
‘Extreme and sustained heat can affect people’s health and wellbeing, how comfortable our homes and workplaces are, and put additional pressure on transport, energy and emergency services.’
As the UK reaches the end of the hottest summer on record, new research from the Ordnance Survey (OS) has revealed which cities are most vulnerable to extreme heat
Portsmouth tops the ranking of the UK’s most heat–vulnerable cities every year from now until the turn of the century
In the countryside, temperatures tend to fall quickly after sunset as heat escapes back out into the atmosphere.
Meanwhile, trees, plants and bodies of water offer natural cooling effects via shade and evaporation.
Cities, on the other hand, are filled with concrete, asphalt, and buildings that hold onto more of the heat they have absorbed during the day.
Tall buildings and narrow streets block the winds that would otherwise carry away hot air, while a lack of green spaces reduces natural cooling effects.
This means that urban areas can be significantly hotter than nearby rural regions, and are hit much harder by extreme heat.
However, this heat island effect doesn’t affect every city the same way, and even small differences in urban planning can have a massive impact on heat risk.
The OS combined its own maps with satellite temperature measurements, provided by the company 4 Earth Intelligence, and Met Office climate modelling, to see how this effect will affect different cities over time.
They used this to create a Heat Risk Index, ranking each of the 71 cities granted by royal charter across England, Scotland and Wales.
The dense urban cores of the City of Westminster and City of London rank as the country’s most heat retentive areas, although not the most vulnerable to extreme heat
Today 83 per cent of Britain’s cities are considered ‘highly vulnerable’ to urban heat, with the remaining 12 cities ranked as ‘moderately vulnerable’.
By 2040, rising summer temperatures will push every city into the high vulnerability category and, by 2100, 63 per cent of cities will cross into ‘severe’ vulnerability, the highest ranking on the scale.
Mr Bolton says: ‘What the Index shows is that the severity of urban heat islands is not uniform, and the cities most at risk of overheating during a heatwave are places where the built environment has grown at the expense of the natural one.
‘They show where exposure is greatest as temperatures continue to rise, which of a city’s existing green and blue spaces need protecting, and where different interventions could have the highest impact in protecting communities.’
The City of London and the City of Westminster ranked as the most heat–retentive areas of the UK, with the least effective natural cooling of any cities.
Tall buildings, limited green spaces and a high concentration of roads combine to trap more heat and prevent these dense urban areas from cooling, with deadly consequences.
Recent research found that approximately 399 of the 785 heat–related deaths in Greater London during the summer of 2018 were attributable specifically to the urban heat island effect rather than background temperatures alone.
Meanwhile, a study commissioned by the Mayor of London estimated that the 2022 London heatwaves cost the city £1.5billion, including productivity losses, health impacts, education disruption, energy demand, transport and emergency services.
Cities like Milton Keynes, St Asaph and Durham have much better natural cooling capacity and are, therefore, less vulnerable to urban heating
Looking to the future, Leicester is the city expected to see the biggest rise in the UK’s heat vulnerability rankings.
Today, the city of 388,300 sits at number 10 on the Heat Vulnerability Index, but is expected to leap to fourth place by the end of the century.
Jonathan Hendry, CEO of 4 Earth Intelligence, says: ‘Cities are on the front lines of climate change, and while the challenge is global, its impacts are often hyper–local.
‘Exposure can vary street by street within a single city, and the areas consistently subjected to higher temperatures than their surroundings are the ones that overheat first during a heatwave.’
Across the UK, heatwaves are already having a deadly impact, with urban areas feeling the worst of the effects.
New figures, provided by the Office for National Statistics, show that more than 40,000 people have died in England due to extreme heat between 1988 and 2025.
Those deaths were especially concentrated in London, where 7,675 extra people died on the hottest days over the last four decades.
However, these figures do not include this year’s May and June heatwaves, in which the UK Health Security Agency estimates that 2,877 died
