What is an urban heat island—and how can design prevent it?

Climate change is fueling extreme weather around the globe. During the past month, there have been catastrophic floods in Vermont and South Korea, raging wildfires across Canada (there are 881 active wildfires in the country as of today), and massive heat waves scorching the Western and Southwestern United States as well as parts of Europe. Inevitably, extreme heat and rising temperatures have become a staggering reality. In 2022, there were reports of heat so extreme it shattered 7,000 records across the United States, and heat waves in Europe caused widespread droughts and more than 61,000 deaths.

An important aspect missing from these reports, however, is that the world’s cities are compounding the problem. Cities are constructed with elements—from heat-absorbing materials to fewer trees for shade and greenery—that make them far hotter than nearby rural areas, resulting in what meteorologists call the “urban heat island effect.” But what is an urban heat island; why do urban heat islands form; what can architects to do prevent them; and how can technology help?

When you think of the world’s biggest cities, you probably picture towering steel skyscrapers, concrete car parks, and crowds window-shopping on sidewalks. Together, these are the perfect ingredients to create an urban heat island.

This is because during the day, dark surfaces such as asphalt act as “heat sponges” by absorbing far more solar radiation than lighter surfaces while tall buildings block wind and provide multiple opportunities to reflect sunlight. At night, all this stored heat is slowly released into the air, adding to the additional waste heat from the thousands of air conditioners that city dwellers need to stay safe through hot weather.

“All this means is that our cities are not really cooling down as much at night as they should be,” says Dr. Ariane Middel, assistant professor at Arizona State University (ASU). For years, she’s researched and simulated urban heat as part of her climate research group The SHaDe Lab. The majority of her work is currently based in Phoenix—a city that could soon become uninhabitable due to climate change exacerbated by the urban heat island effect.

“This is a problem that people in city governments have become increasingly aware of,” says David Hondula, assistant professor at ASU and director of the Office of Heat Response and Mitigation for the city of Phoenix—the first publicly funded office of its kind in the United States. “What hasn’t been clear is who’s responsible for tackling the challenges in our cities related to heat.”

With his team, Hondula has been specifically tasked to fight extreme heat in the city before it’s too late. Recent projects include the city’s “Cool Pavement” pilot program, conducted in partnership with ASU, which revealed that reflective asphalt coatings reduced average surface temperatures 10.5 to 12 degrees Fahrenheit compared to traditional asphalt at noon and during the afternoon hours.

Fighting extreme heat is something that every other city government would be smart to replicate—and the sooner, the better: By 2050, the urban population is projected to more than double its current size, and nearly seven out of 10 people in the world will live in cities. That means that as climate change creates increasingly extreme heat waves, many more communities will be at risk.

The good news is, cities are rising to the challenge of finding cross-disciplinary solutions.

September 2022 marked the first gathering of chief heat officers from cities around the globe, organized by the Atlantic Council’s Adrienne Arsht-Rockefeller Foundation Resilience Center, which spearheaded the global Extreme Heat Resilience Alliance. At the event, the all-female group of policy makers shared the challenges of adapting to the harsh realities of climate change; the event marked the release of the nonprofit’s report Hot Cities, Chilled Economies: Impacts of Extreme Heat on Global Cities, which examines the impact of global warming in 12 major urban areas. While some cities are further ahead with mapping and analysis, they share a focus on expanding cooling infrastructure, especially micro-parks and tree canopies.

It’s not just city governments that can make a difference; architects and designers are increasingly being tasked to mitigate urban heat islands. Tasks can range from optimizing a building’s orientation to something as simple as adding more water features to city parks or painting rooftops lighter colors so they reflect heat.

Passive design is a key component of heat mitigation. Encouraging natural ventilation within buildings, for instance, can minimize the use of air conditioning and keep the heat out on sunny days. And it’s not just about what’s happening inside buildings: Encouraging natural ventilation between buildings makes a big impact. “I don’t think many people believe our urban environment is the best it could be,” says Hajime Aoyagi, design department supervisor at Nikken Sekkei, a Japanese firm that ranks as the second largest architectural practice in the world.

He explains that for the Nikken Sekkei team, it’s crucial to plan projects for heat mitigation from the outset. Designing buildings made of sustainable construction materials such as wood can help alleviate the heat island effect, as wood has a much lower thermal capacity than concrete. Making sure buildings are positioned to avoid excessive buildup of heat is also highly effective.

“Since we are a firm that is often involved in large-scale development and tend to work on projects at multiple locations in Tokyo simultaneously, we can design from a broader perspective,” Aoyagi says. “We were involved in the development of the Yaesu Exit of Tokyo Station and intentionally kept a low roof on the central concourse to create a route for the wind from Tokyo Bay to inland areas.”

For Dr. Middel, creating shade is the most important factor architects should consider when designing cities today. “When you look at how people experience heat, it’s not just about air temperature,” she says. “You also need to look at the radiant temperature, which is how we measure the heat load on the human body. That’s strongly driven by the availability of shade. If you’re in the shade, you feel much more comfortable than if you’re in the sun, no matter where that shade is coming from. That means architects should use buildings, trees, or even photovoltaic canopies to create shade wherever possible.”

Not doing so is a risky strategy. As more building certifications like LEED and directives like London’s thermal comfort guidelines begin to reward heat-reduction measures, not taking a city’s heat into consideration can result in costly retrofitting years down the line.

In the short term, it could even lead to urban designs being ignored altogether. “We had a project with a very popular park that was redesigned to have this path leading into it,” Dr. Middel says. “That meant neighborhood residents wouldn’t have to take their car to the park. Sounds great, right? Turns out, nobody would use that path. It wasn’t performing in the way the city’s architects had expected, and when we measured the heat conditions with our metrological instruments, we knew why. There was no shade whatsoever, and it was miserable to walk on. Fixing this would be a time-consuming and expensive process.”

Microclimate-analysis technology can help take the guesswork out of designing a project to mitigate the heat-island effect. “Technology can help you run scenarios, such as where to plant trees so they’ll have the maximum effect on heat,” Dr. Middel says. “You no longer need to plant them on-site, then see what happens. You can simulate it all on a computer first.”

Using tools such as Autodesk Forma, for example, architects and urban designers can create simulations and easily test mitigation strategies during the planning and design phases. Thermal comfort maps can be created for a specific date and time so anyone can see how a site will handle extreme temperatures. Heat maps can be generated to show exactly when outdoor spaces like public parks and squares will be most and least usable during the day. Together, these types of technologies can ensure cities are developed to be cooler, as quickly and efficiently as possible.

“The most important thing is to act now,” Hondula says. ”Heat is not an isolated problem for anyone. We need to be taking steps today because the city of 30 or 40 years in the future is largely going to be shaped by the decisions we make over the next five to 10 years. With widespread deployment of cooling technologies like trees and reflective surfaces, some modeling suggests we could end up with a city in the future that is cooler than we have today, even with continued global-scale warming. That is a very encouraging sign.”

Note: Currently the Autodesk Forma microclimate analysis is not able to support the effects of various ground types or building materials, instead assuming that the temperature of these surfaces is equal to air temperature. These scenarios are created using the 95th-percentile warmest climate results based on weather data for the previous eight years.

This article has been updated. It was originally published in October 2022.