UK Cities Most Vulnerable to Extreme Heat: New Index Reveals Risks 2026

Extreme heat is becoming a critical concern for UK cities, and a new heat vulnerability index reveals which urban areas are most at risk. According to research by Ordnance Survey (OS), the Met Office, and 4 Earth Intelligence, cities like Portsmouth, Leicester, and Exeter are set to experience the most significant warming by the end of this century. This analysis highlights how urban heat islands—where concrete and asphalt absorb solar radiation—can make cities up to 10°C hotter than surrounding rural areas.

Understanding Urban Heat Islands and Vulnerability

Urban heat islands (UHIs) occur when natural surfaces like soil and vegetation are replaced by buildings, roads, and other infrastructure. These materials store heat during the day and release it at night, preventing cities from cooling down. The research shows that areas with the least tree cover, green space, and waterways are most susceptible to extreme heat.

The heat vulnerability index developed by OS and its partners assesses 71 cities across England, Scotland, and Wales. It combines factors like population density, surface materials, and green space to rank each city’s current and future heat risk.

Top Vulnerable Cities in Great Britain

Portsmouth, with a population of 208,000, tops the list as the most vulnerable city to extreme heat both today and in the future. Other cities facing high risk include Leicester, Exeter, Bristol, and Nottingham. These cities have high levels of impermeable surfaces and limited green infrastructure, exacerbating heat retention.

By 2040, the index suggests all 71 cities will reach high heat vulnerability, and by the end of the century, nearly two-thirds will face extreme heat conditions. This summer was provisionally the UK’s hottest on record, underscoring the urgency of addressing urban heat risks.

Why Some Cities Are More at Risk

The primary factors driving vulnerability include the proportion of built-up area, lack of vegetation, and population density. Cities with extensive concrete and asphalt surfaces, like Portsmouth, retain more heat. Additionally, coastal cities may experience higher humidity, making heat feel more oppressive.

Green spaces and waterways play a crucial role in cooling urban environments. Trees provide shade and release moisture through evapotranspiration, lowering surrounding temperatures. Cities that invest in green roofs, parks, and reflective materials can mitigate the urban heat island effect.

Impacts on Public Health and Infrastructure

Extreme heat poses serious health risks, including heat exhaustion and heatstroke, particularly for vulnerable populations like the elderly and those with pre-existing conditions. It also strains energy systems as demand for cooling increases, and can damage roads, railways, and buildings.

Adaptation strategies are essential. The research emphasizes the need for urban planning that incorporates climate-resilient design. Planting more trees, creating green corridors, and using cool pavements are actionable steps cities can take to reduce heat vulnerability.

Key Takeaways

• Portsmouth is the most heat-vulnerable city in Great Britain.
• All 71 cities analyzed will face high heat risk by 2040.
• Urban heat islands can make cities up to 10°C hotter than rural areas.
• Green spaces and waterways are critical for cooling.
• Immediate adaptation measures are needed to protect public health.

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