After a summer of record temperatures and heatwaves, new analysis from Ordnance Survey has found the City of London has the most heat-retentive urban environment in Great Britain, with the density of its buildings and hard surfaces combining with almost no green or blue space to release the heat they absorb. More than eight in ten British cities are already classed as highly vulnerable to this phenomenon, known as urban heat islands, and the City of Westminster and the wider London area are both projected to reach severe heat vulnerability between present day and 2070 to 2100 as summer temperatures continue to rise.
Urban heat islands form where dense buildings, roads and hard surfaces absorb solar heat during the day, keeping cities significantly warmer than surrounding countryside. Unlike rural areas, which cool rapidly after sunset, cities retain the same heat overnight. Tall buildings and narrow streets reduce wind speeds that would otherwise carry heat away, while a lack of trees and water sources eliminate the natural cooling effects of shading and evaporation. Where cities lack sufficient green and blue space to moderate temperatures, this effect intensifies throughout both day and night.
Drawing on its detailed mapping of Britain’s built and natural environment, OS analysed data including building density and height, hard surface coverage, tree canopy, open vegetation and water across every city in England, Scotland and Wales. OS combined this with satellite readings on observed land surface temperatures compared to their surrounding natural environments from 4 Earth Intelligence, and Met Office climate modelling air temperature projections to produce a Heat Vulnerability Index tracking how each city’s heat exposure is projected to evolve through to the end of the century, and how effective current sources of natural cooling are in releasing extreme retained heat.
London: Britain’s most heat-retentive urban environment
The City of London and City of Westminster, the two densely built inner-city areas at the heart of Greater London, rank among the most heat-retentive urban environments in Great Britain. The City of London records the highest built-environment score and the least effective natural cooling sources of any city in Britain, with its high concentration of buildings, roads and made surfaces converging with limited green spaces, tree cover and water which could carry extreme heat out of the city. Westminster follows closely, ranking second on both measures, ranks third for overall heat vulnerability today and second in both future time periods, according to the OS Heat Vulnerability Index.
Heatwaves alone have contributed to an estimated total of 2,877 heat-associated deaths in May and June this year, and the impacts of urban heat islands in London are already being felt. Research published in The Lancet Planetary Health in 2025 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, with a social cost estimated at up to £987 million. A separate study commissioned in support of Heat Ready London, the Mayor of London’s heat resilience initiative, estimated the total economic cost of the 2022 London heatwaves at £1.5bn, spanning productivity losses, health impacts, education disruption, energy demand, transport and emergency services.
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[Figure 1. The OS Heat Vulnerability Index, mapped to identify which of London’s postcodes are most vulnerable to retaining severe heat in near-term climate modelling].
The changing geography of urban heat
Nationally, OS analysis finds that 83% of Britain’s cities (59 cities) are already highly vulnerable to retaining extreme urban heat today. By 2040, rising summer temperatures are projected to push every city moderately exposed to retaining extreme heat into the high vulnerability category, and by the turn of the century 63% (45 cities) are set to cross into severe heat vulnerability, the most serious level in the Index as rising temperatures reshape the geography of urban heat vulnerability across Britain. The consequences extend far beyond uncomfortable summer days. Urban heat can increase risks to health, particularly for older and vulnerable people, while putting additional pressure on homes, workplaces, transport, energy infrastructure and public services.