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An urban heat island (UHI) is an urban area that is significantly warmer than its surroundings (suburbs or rural areas). The phenomenon has been understood since the early 19th century, with studies conducted by scientists on the urban climate of London.

Urban areas have a higher concentration of concrete and paved surfaces compared to rural regions, where vegetation like grass, fields, or forests predominates. This vegetation plays an important role in cooling the air, while materials like asphalt and concrete absorb heat, leading to a rise in temperatures. Within cities, buildings and narrow streets hinder airflow, trapping heat. Human activities such as heating buildings and vehicle emissions further contribute to this heat emission.

Collectively, these factors create what is called the “urban heat island effect”, whose impact is mostly felt at night when temperatures in urban areas can be up to 5-10°C higher than surrounding areas. This phenomenon stems from the gradual release of stored heat from structures such as buildings and roads.

The prevalence and severity of UHI have dramatically risen due to global urbanization and climate change. Studies show the impact of the UHI effect in cities like New York where temperatures rise by up to 8.0°C, while London or Paris regularly experience temperatures up to 5-6°C higher compared to the surrounding areas. China also faces severe UHI effects, with Beijing’s outskirts experiencing a 3.3°C rise from 1961 to 2000, and Shanghai’s reaching 7.4°C.

Illustration: climate.copernicus.eu

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Health and social impacts of Urban Heat Islands

A study from January 2024 by the Swiss Federal Institute of Technology in Lausanne, covering 85 cities, highlights that UHI effects are not merely about comfort for city inhabitants but pose significant health risks. These include heightened risks of respiratory and cardiovascular issues, leading to reduced life expectancy. Heat exposure also correlates with non-fatal health issues like heat strokes, dehydration, decreased labor productivity, and learning.Other studies show that built environment characteristics, such as green spaces, urban layout, city size, and spectral reflectance, not only create temperature gaps between urban and rural areas but also contribute to temperature variations within cities. These disparities can lead to the uneven distribution of heat-related health burdens across different sociodemographic groups. Very often, poor communities are located in the most densely built areas, where the UHI effect affects them disproportionately. This tends to increase social inequalities, especially those related to the exposure to the effects of climate change.

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Cities’ Strategy: Planning and Mitigation

In response to the UHI challenge, municipalities, urban planners, and solution providers have started to design strategies to mitigate its effects.

• One obvious and efficient response is to increase the surface covered by vegetation and water in urban areas, such as parks, artificial ponds, planting tree lines on existing streets and increasing the surface of so-called “green roofs”. Some cities, like Geneva, have started uncovering existing streams which used to flow underground.
• The use of alternative building materials, such as timber, has also been experimented as potential response to the UHI effect.
• Changing the color of building roofs to lighter tones so as to reflect the sunlight.
• Planning the orientation of new buildings in such a way that wind paths are created to allow for the airflow from the cooler places, like rivers or parks, into the city streets.

However, knowledge through measurement and modelization remains the centerpiece of any efficient UHI mitigation strategy. Urban planners need to know how and in which conditions temperatures vary within urban environment. They need to assess where changes to the urban landscape would have the most impact in combating UHI. And for this, they need accurate data.Solutions such as satellite imagery analysis, remote temperature sensors (IoT), 3D visualization and data modelization tools have recently been developed by a number of solution providers. The idea behind any strategy is to combine these solutions to build an accurate picture of a city “Urban Heat Map” in order to rationalize and facilitate planning and mitigate the effects of Urban Heat Islands in the coming years.

Illustration: Latitudo 40 – Urban Heat Map of the city of Genoa

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Example: Grenoble

To illustrate one of such strategies, Grenoble-Alpes-Metropole, a UTA member, has called on solution providers to build an integrated “socio-technical” system enabling to visualize and model the impact of a construction project on Urban Heat Islands (UHI), Daytime Thermal Comfort (DTC), and CO2 capture data through fertilization and vegetation. The project, launched through the European initiative, Climaborough, aims to raise the awareness of citizens to the UHI problem, guide the urban planners in taking decisions concerning new building, greening, or redevelopment projects, and support the private sector in their current and future initiatives. The project thus aims to include the entire community into its strategy towards mitigating the consequences UHI in particular and the effects of climate change more broadly. )

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Conclusion

Mitigating the urban heat islands effect will be a long process, which will necessarily be part of a wider smart city strategy to help cities to combat and, where needed adapt, to climate change. Yet, in some regions, the urban heat Island effect may be the most visible and severe effect of climate change. By highlighting the problem of UHI and taking steps towards tackling it, cities may raise awareness to climate change challenges and mobilize both the public and private sectors to take action, fostering a collective effort towards climate resilience and sustainable urban development.

Join us in the mission to combat the urban heat island effect and build climate-resilient cities. Reach out to Urban Technology Alliance today to explore how we can support your efforts tackling this important issue and driving sustainable urban development. Together, we can make a difference. Contact us now to get started.