London’s urban centre has the joint-fourth most extreme urban heat island (UHI) “hot spot” of six major cities around the world, with temperatures 4.5°C hotter than rural surroundings, according to new research by global sustainable development consultancy, Arup.

The report, Urban Heat Snapshot, analyses AI and satellite images from space using Arup’s urban heat modelling tool, UHeat. Mapping the most extreme “hot spots” in a 150km2 sample of the urban centres from London, Cairo, Los Angeles, Madrid, Mumbai and New York, the snapshot is among the first international UHI international comparisons using both day and night-time datasets.

UHeat draws on an advanced climate model by the University of Reading in the UK and demonstrates how advanced digital tools can bring academic models to real-word scenarios and examine the causes of the UHI effect. It can then rapidly model solutions to show how the strategic deployment of nature and other interventions can help cities reduce the impact of hot spots. 

In the majority of cities, the hottest spots had less than 6% vegetation cover, while the coolest spots had over 70%. These were found almost entirely in parks, away from residential and commercial areas, and contributed to massive temperature swings within the cities surveyed. The Kilburn and South Hampstead area in London, with 38% vegetation cover, experienced heat over 7°C hotter than Regent’s Park with 89% vegetation cover, just a short distance away. This was reflected globally, with Madrid’s built up downtown experiencing heat almost 8°C hotter than nearby El Retiro Park.

The authors are calling on global city leaders, urban designers and planners to better understand how their designs can mitigate hot spots, particularly for the most vulnerable communities. 

Worryingly, three of the cities studied, including London, experienced the worst UHI “hot spots” during the evening or night-time. Urban heat is a particular problem at night, due to materials like cement absorbing heat in the day then slowly releasing it when the sun goes down. This causes stress and health issues and acutely impacts vulnerable citizens – including children and the elderly.

Arup’s Urban Heat Snapshot is one of the few international comparisons of the UHI effect on air temperatures during both day and night-time. This method is much closer to people’s actual experience than cruder satellite analysis of land surface temperatures, which can be skewed by the dramatic highs of heat-absorbing materials on roofs and roads.

Extreme temperatures are proving lethal, with a recent study by the European Health Institute estimating that extreme heat killed more than 61,000 people in Europe alone in 2022.* It is increasingly understood that poorer neighbourhoods are also more at risk to heat exposure, due to factors such as a lack of trees and a lack of air conditioning.**

The report authors have outlined five key recommendations for cities to focus on, including implementing nature-based solutions and establishing cool islands, as well as offering global examples of solutions in action. 

* Ballester, J., Quijal-Zamorano, M., Méndez Turrubiates, R.F. et al. Heat-related mortality in Europe during the summer of 2022. Nat Med 29, 1857–1866 (2023). 

** A study by Yale School of Forestry and Environmental Studies (T Chakraborty) of 25 cities around the world found that in most (72%) of cases, poorer neighbourhoods experience elevated heat exposure due to factors such as lower vegetation density. 

Modelling urban heat has been the reserve of the few, mainly those in academia. UHeat bridges this gap, drawing on the urban climate science and models developed by academia and combining them with increasing amount of city data available through remote sensing. It gives those on the ground shaping cities the tools they need to rapidly understand the impacts of their design on urban heat.

Dr Ting Sun

Survey contributor, UCL, Institute for Risk and Disaster Reduction