Cooling the Heat: Designing Cities for Climate Resilience
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Our cities are hotter than surrounding rural areas. Photo: ARUP
Urban heat poses a growing challenge for cities around the world. When we discuss this issue, we're often referring to the Urban Heat Island (UHI) effect. Simply put, it’s a phenomenon that describes the warmer temperatures our urban areas experience compared to their rural surroundings. But what exactly drives UHI, and why should we be concerned?
Impact on Human Health and Urban Functionality
Heat, often referred to as the “silent killer,” has a profound impact on human health and well-being. Excessive heat can lead to a variety of health problems, including heat exhaustion, heatstroke, and even cognitive decline. Sleep quality is compromised, and productivity dips. Vulnerable populations - such as outdoor workers, the elderly, and low-income communities - are at greater risk.
As designers, we need to ask how the built environment contributes to unsafe living conditions in cities and what we can do to reduce urban heat. It's also important to understand who is most affected by these heat impacts and how we can address their needs. We therefore need to recognize the factors driving the Urban Heat Island effect, how we can predict it, and how we can design urban areas with these challenges in mind. Equally, we need to find ways to mitigate and retrofit existing urban areas to make them more resilient to heat.
Understanding the Urban Heat Island Effect
The UHI effect is primarily caused by the way cities are designed, how they evolve over time, and the materials they are made from. Cities, with their dense concentrations of concrete and asphalt, trap and amplify solar radiation and convert it to latent heat which increases local air temperatures. This is a stark contrast to the cooler rural areas surrounding them. This temperature disparity is exacerbated by various factors, which include:
Urban Form and Design: The layout of the city, building height and density can significantly impact how air flows through the urban environment. For example, tall, closely packed buildings can create canyons that trap heat due to limited access to the sky, which combined with reduced wind speeds during a heatwave and reduced ventilation, prevents heat from escaping.
Anthropogenic Heat: The heat generated by humans, such as that released by buildings through their cooling systems into the surrounding urban environment.
Material Choices: The albedo of building materials plays a crucial role: dark materials absorb solar energy, while lighter ones can reflect it. Additionally, thermal mass influences how heat is stored and subsequently released back into the urban environment - contributing to the UHI effect at night.
Blue and Green Infrastructure: Vegetation has a cooling effect since it absorbs solar radiation and releases moisture into the air through evapotranspiration. The availability of blue infrastructure, such as rivers and lakes, as well as man-made water features, also has a cooling effect.
Climate Context: The geographical location of a city plays a significant role in UHI formation.
A 2021 study by the Intergovernmental Panel on Climate Change (IPCC) showed that urban form and design had the greatest heating impact, with vegetation tending to have the largest cooling impact. Therefore, as designers, we are often looking at how these two factors can be more effectively integrated with one another.
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UHeat: Measuring and Mitigating Urban Heat
Addressing the UHI effect requires data-driven insights. But how do we measure heat across vast urban areas? Traditional methods, such as satellite imagery capture land surface temperatures, but these often fail to reflect the temperatures felt by humans, which are more accurately represented by air temperatures.
To tackle this challenge, Arup developed the UHeat tool in collaboration with our partners at University College London and University of Reading. It uses detailed data inputs—ranging from land cover to population density—to model urban heat effects and evaluate mitigation strategies. To predict air temperatures, it combines these inputs with the physics-based Surface Urban Energy and Water Balance Scheme (SUEWS) model. By comparing current urban conditions with rural environments, UHeat provides valuable insights into UHI intensity and helps us understand the ideal scenario for a given climate—one without the UHI effects of buildings or roads.
Through UHeat, cities can test mitigation scenarios, such as greening, white roofs, or changes in building density, as well as understand the effects of climate change. Our aim at Arup is to provide city authorities, urban planners, developers, and any relevant stakeholder with vital information on urban heating, to improve city planning, public health and resilient infrastructure.
Using Uheat for insights and solutions. Photo: ARUP
Arup Urban Heat Snapshots
As part of Arup’s Urban Heat Snapshots campaign, the UHeat tool has been used on several cities across the world including Los Angeles and New York in the Americas. These snapshots identified hotspots and cool spots across each city, revealing that proximity to green spaces and vegetation significantly influences urban temperature, with poorer neighbourhoods often experiencing the highest heat levels due to lack of vegetation. In the case of New York City, the maximum temperature difference between the hottest and coldest spots was predicted to be 4.5 ℃ / 8 ℉.
Urban heat snapshot of New York City. Photo: ARUP
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A Case Study: Building for Climate Resilience in Toronto
Motivated by our work with Evergreen, a Canadian non-profit, we understood the benefits of retrofitting schoolgrounds for flood resilience and UHI mitigation and saw an opportunity for site selection. To address this, Arup developed a standardized site-selection framework for Toronto, combining data from sources like Statistics Canada, the City of Toronto, and the Toronto and Region Conservation Authority. This framework incorporates three key factors: social vulnerability (such as income, age, and homelessness), flood risk (using data from the TRCA), and UHI (using UHeat data to identify area of higher temperature). By combining these factors, we generated a vulnerability score that pinpointed four key areas in Toronto that present as opportunities for intervention and addressing community-wide needs.
This approach allows us to tailor solutions for different neighborhoods in Toronto: the east and west are driven by flood and social factors, while the city center’s vulnerability stems from UHI and social issues. Downtown, which faces all three factors, requires the most focused attention.
By using this kind of evidence-based framework, Arup can help future clients prioritize investments and create healthier public spaces. Over time, this framework could be expanded to other areas and incorporate additional factors like pedestrian connectivity or urban ecology.
Key factors contributing to vulnerability throughout the City of Toronto. Photo: ARUP
Conclusion: Investing in the Future of Urban Heat Mitigation
The future of our cities lies in our ability to design and retrofit spaces that are resilient to heat. Using advanced tools like UHeat, city authorities, developers, and planners can make data-driven decisions that create healthier urban environments. By investing in green infrastructure and other climate-resilient strategies, we can ensure that cities are prepared to cope with the growing threat of urban heat while improving the quality of life for their inhabitants.
Through these efforts, we can help transform cities from heat traps into cooler, more sustainable, and equitable places to live.
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Dr. Tariq Saeed is an Urban Heat and Microclimate Specialist at Arup in Toronto. As the Americas Environmental & Building Physics Skills Leader, he is responsible for driving forward innovative and strategic initiatives to deliver advanced solutions for Arup’s clients.
Email: Tariq.Saeed@arup.com
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