How to build a heat-resilient city

This story was co-published with Gizmodo. It is also part of Record High, a Grist series examining extreme heat and its impact on how — and where — we live.

Cities are hot. When you cover the ground with asphalt and concrete, jam millions of cars together on congested streets, and erect thousands of buildings that leak their own heat, you create what experts call an “urban heat island.” Daytime temperatures in these places can be as much as 7 degrees Fahrenheit higher than surrounding rural areas, and things don’t get much cooler at night.

As climate change fuels a succession of historic heat waves, the urban heat island effect in many American cities is pushing the limits of human survivability. That’s the case in desert cities like Phoenix, where temperatures crested 110 degrees F for 30 straight days this summer, and also in cooler climes like Chicago, which has seen a series of scalding triple-digit weeks over the past few months. 

Dealing with this type of heat requires more than isolated interventions — reflective roofs here or mist machines there. Rather, the crisis of the past summer has shown that most American urban centers will need to consider a revamp from the ground up.

Drawing on feedback from climate experts, architects, and urban planners, as well as successful technologies pioneered by warm-weather communities around the world, Grist set out to design a city built from scratch to handle extreme heat, all while reducing carbon emissions. The buildings and streets in this cool metropolis incorporate basic design principles such as shade and foliage, but they also include bespoke architectural solutions such as wind-trapping towers and special absorbent polymers. The finished product shows how much work is needed to adapt to the extremes of climate change, but it also shows how much more humane and people-oriented our cities can be.

City Centers

With their tall towers of steel, abundant concrete, and frequent lack of green space, dense downtown areas are known for scorching temperatures that last day and night. But there are many design elements that can help cool these cityscapes, from shaded sidewalks and bus stops to reflective glass. Even the positioning of tall buildings, like those on a street like Manhattan’s 5th Avenue, can create what is known as an “urban canyon,” blocking the sun from reaching the street during the morning and the afternoon. This keeps the pavement cool for most of the day, and reduces the risk of heat stroke and overexposure.

“You can’t generalize and say that density is bad,” said Sara Meerow, an associate professor of urban planning at Arizona State University who studies heat risks. “If you plan your density well, you can build in ways that are not going to increase heat risks.”

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1. SHADED STRUCTURES: Waiting 20 minutes for the bus in triple-digit weather isn’t just unpleasant — it can be dangerous. Bus stops, train stations, and other outdoor transit facilities are some of the biggest heat pinch points in the urban environment. The easiest way to address this risk is to install shade structures. But urban planners told Grist communities need to make sure these are big enough to fit more than a person or two if they hope to increase ridership: Earlier this year, Los Angeles debuted a prototype called La Sombrita, which was designed to provide shade to people at bus stops in places where the city couldn’t build full shelters. But the structure was so skinny that it couldn’t block out the sun for more than one person at a time.  

2. TREES: It might seem simplistic, but planting trees and other vegetation is one of the most effective heat-mitigation strategies available. During the summer time, the area below a tree receives just 10 to 30 percent of the sun’s energy. The transpiration of water through their leaves also has a cooling effect, and combined with shade, it can lower temperatures by 2 to 9 degrees Fahrenheit. What’s more, trees come with other benefits, like improving air quality and reducing runoff. Unfortunately, there is a long history of low-income and minority neighborhoods — communities that were historically redlined and received fewer government resources — lacking greencover. A 2021 analysis found that neighborhoods with majority-people of color had on average 33 percent fewer trees compared to majority-white neighborhoods. To reverse this discrimination, many cities are targeting their tree planting campaigns toward these neighborhoods.

3. MISTERS: Even with shade structures available, bus stops and public plazas can still be overwhelmingly hot. An easy way to reduce the danger in these waiting areas, and provide passersby with a little refreshment as well, is to install misting machines or fountains in the places where the heat is most intense. These misters use a fraction as much water as the average home, so they’re feasible even in desert climes: The city of Phoenix, for instance, installed a mister at a bus shelter in the Uptown neighborhood, an area that doesn’t have many trees. The creators of the popular pilot picked a west-facing bus stop, helping to combat its long afternoon sun exposure.

4. COOL PAVEMENT: Ever walked across a parking lot and felt the heat burn through your shoes? Pavements heat up when they absorb energy from the sun. So-called “cool pavements,” however, are made with materials that reflect more energy than they absorb and porous materials that allow for evaporative cooling. There are a lot of options on the market: Traditional concrete pavements can be modified by using reflective binders instead of asphalt ones. Others use resin from trees as binders. Porous asphalt, rubberized asphalt, and bricks made from clay also increase reflectivity and porosity. And vegetative pavements consist of intricate lattice structures made of plastic, metal, or concrete with space for grass to grow. These pavements are also often painted in light colors that reflect more heat.

But cool pavements have one major downside. When the sun is at its highest, heat reflected off its surface can actually be absorbed by the people and structures nearby. “During the midday hours, from about 11 to 1, the cooler pavement does increase the radiant heat burden,” said V. Kelly Turner, an associate professor of Urban Planning and Geography at the University of California, Los Angeles. To counter that, cities should target them for areas that are highly trafficked in the morning and evening hours, she said.

5. URBAN CANYON EFFECT: While tall buildings are typically associated with exacerbating the urban heat island effect, they can also provide heat relief by providing shade when positioned correctly. High-rise buildings block sunlight and create canyons or passages that are cooler than surrounding areas during the day. In the Middle East, traditional construction practices include aligning buildings with sunlight and wind direction in order to provide shade and increase airflow, which reduces temperatures. These arterial roads, called “sikkak,” are narrow alleyways within blocks and increase walkability in neighborhoods, improve pedestrian safety, and have lower temperatures. They’re commonly found in markets and historic centers in Middle Eastern cities. Urban planners in Abu Dhabi employed sikkak to increase connectivity within larger blocks. 

6. REFLECTIVE GLASS: Reflective glass can also prevent buildings from absorbing too much heat as the sun shines through their windows, though engineers warn these installations need to be done carefully. If pointed to the ground, the reflected sunlight and its heat can make conditions worse. The windows on London’s famous Walkie Talkie skyscraper, for example, used to melt metal and plastic at street level until the designers fitted it with a set of louvered sunshades known as a “brise soleil.”

7. GREEN WALLS: Skyscrapers themselves absorb a lot of sun, and keeping them cool can be challenging. One way is to deck out the facade of a building with ivy or another variety of plant: Not only do the leaves and foliage block the sun before it can heat up the surface of the building, they also cool down the surrounding air when they release water through a process known as evapotranspiration. Foliage-covered structures have become prevalent in Middle Eastern cities like Tel Aviv.

8. AIR FLOW: To increase air flow, some designers of supertall buildings in Chinese coastal cities have opened up the base floors of their towers, creating openings that allow cooling sea breezes to pass through. This effect is even more powerful when combined with trees that blunt the sun’s heat. “If you fill that [urban] canyon… with street trees, or you rip out the first floor of a building, then where the people actually are walking around, they’re not going to be exposed to the same level of heat,” said Evan Mallen, a researcher at the Georgia Institute of Technology who studies the relationship between heat and building design.

Residential Areas

But most American space is not high-rise. More than half of the U.S. population lives in what they identify as more of a suburban area, peppered with single-family homes and low-rise office buildings in layouts that rely on car travel. Many of the solutions that apply to dense cities, such as shade and trees, can be applied here, but these communities also have different challenges — and solutions. For one, more than half of all the energy used in single-family homes comes from heating and air conditioning. Overall, home cooling accounts for 6 percent of total electricity consumption in the U.S. Building smarter, more heat-resilient homes, particularly with sustainable or natural materials, can lower temperatures and energy bills, and reduce the associated carbon emissions that come with AC use. 

“That is the future,” said Turner. “We are a ways off from it, but we need to be coming up with ways to make scalable building homes with [natural] materials. As a general rule of thumb, the more unnatural the material is, the more it becomes like hot pavement.”

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1. WINDOW FILMS AND AWNINGS: During the summer, more than three-fourths of the sunlight that falls on windows becomes heat. Shade structures like awnings or overhangs and reflective coatings can help keep the heat out. Since these structures also reflect sunlight during winter months, they’re best used in regions of the country with high temperatures year-round. For regions that are warm primarily during the summer months, interior shades such as blinds, shutters, and curtains may be best. To straddle both climes, researchers are also developing thin adhesive films that reflect heat when temperatures are high and remain transparent at lower temperatures. While these smart windows are available on the market, they can be cost prohibitive for most homeowners.

2. COOLING TOWERS: Wind catchers, tall chimney-like towers attached to the sides of homes and buildings, are great passive cooling systems and make use of pressure differences within a building to increase ventilation. These “Barjeel” towers are a common sight in the United Arab Emirates and other Gulf countries. Air entering the building is cooled down through wet cloths; warmer air inside the structure rises and escapes through towers. The wind catchers are typically four-sided, although cylindrical, hexahedral, and tetrahedral towers also exist. A variation of this idea is the solar chimney, which has been around for centuries. A chimney structure made with heat-absorbing materials such as glass or metals is used to heat a specific section of air within a building. As the hot air rises, it creates a natural vertical ventilation flow that circulates cool air.

3. REFLECTIVE SURFACES: Painting roofs, walls, and pavements white is a cheap and effective way to reduce temperatures. From the Cycladic Islands of Greece to cities in Rajasthan, India, those who live in Mediterranean and tropical climates have long harnessed the ability of white paint to reflect heat and keep communities cool. A growing number of cities already have “cool roof” requirements in their building codes. The products available on the market range from a straightforward pot of white paint to a special goo that rolls out on city streets and can reduce surface temperatures anywhere from 8 to 20 degrees Fahrenheit. “The availability of cool roofs is ubiquitous across almost all roof types,” said Kurt Shickman, the director of extreme heat initiatives at Adrienne Arsht-Rockefeller Foundation Resilience Center. “From a market perspective, it’s a no brainer… More recently, bringing that cool paint technology down to streets and parking lots, that’s something that a lot of cities have been experimenting with.”

4. COVERED PLAYGROUNDS: Playgrounds, with their metal slides, swings, and even the dark asphalt on the ground, are some of the hottest parts of a city. Researchers have found that temperatures of playground equipment can get as high as 189 degrees — hot enough to burn a kid’s skin in less than three seconds. A key culprit is the material used to design them: Rubber play surfaces, metal ladders, handlebars, and railings, artificial turf, and other synthetic materials all retain heat, have a higher conductivity to skin, and cause higher air temperatures. 

So what’s the solution? Shade. Whether natural or built, shade can dramatically lower temperatures and create a much safer play environment for children. A shaded playground, whether that’s from a large elm tree or tarp or some other structure, can be 30 to 40 degrees cooler. In Jackson, Mississippi, two moms designed canopies that cover 75 percent of a local playground. The shade sails also have an added benefit: protection from ultraviolet rays. “It’s like wearing sunscreen without having to wear it,” one of the moms told a local TV station.

5. CONSTRUCTION MATERIALS: Designing homes with materials that have insulating properties is one way to reduce the need for air conditioning — and the costs and carbon emissions that come along with it. Materials such as stone, concrete, clay, and mud have an ability to absorb and retain heat as opposed to conducting it through during the day, which keeps inside temperatures low. When these materials are shaped into blocks with air pockets, they’re particularly good at providing thermal insulation. Air is a poor conductor of heat, and air cavities in building materials can prevent heat penetration. These methods stand in stark contrast to the common building typologies in the United States, where more than 90 percent of new homes are made out of wood.

Researchers are also increasingly experimenting with so-called phase change materials, substances that melt at specific temperatures, thereby absorbing heat from the surrounding area and cooling it down. Paraffinic waxes and different types of salt hydrates are some common types of phase change materials. When injected into walls, floors, and roofs, they’ve been found to lower temperatures by up to 7 degrees Fahrenheit. One study in Casablanca, Morocco, found that when the roofs of homes contained a copolymer made of ethylene and paraffin, temperatures indoors declined by 2.7 degrees. While there are a few companies offering construction materials infused with phase change materials on the market, it hasn’t yet found widespread commercial success, and the Department of Energy has several studies underway to assess the effectiveness of the technology.

Commercial Zones

Outside of skyscraper-filled downtowns and leafy suburban neighborhoods, most American cities also contain expanses of factories, shopping, warehouses, and industrial sites — areas that can’t be ignored in the planning for heat-proof cityscapes. Big-box stores and strip malls may blast AC on the inside, but they tend to be barren of trees, congested with heat-emitting cars, and surrounded by asphalt parking lots. The same is true for manufacturing areas, where power plants and factories also leak heat into surrounding streets.

Solving this thermal buildup isn’t as simple as installing a mister or increasing airflow — urban designers need to plan from the ground up with reflective materials and ample shade to reduce the urban heat island effect and make these commercial spaces safe for daily use as temperatures climb. 

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1. WASTE HEAT CAPTURE: In addition to creating a large buffer around industrial facilities, companies can also cut down on waste heat by investing in heat capture technology. A heat exchanger at a big factory can suck up leaking heat and cycle it back into the facility, which also cuts down on energy demand. This capture can make a building more energy efficient by capturing the 20 to 50 percent of energy that gets wasted as heat. One estimate from the Environmental Protection Agency suggests that catching the usable waste heat in the U.S. could generate 7.6 gigawatts of power, enough juice for millions of homes.

2. BUFFER ZONES: Factories run huge generators at all hours of the day, pouring heat onto surrounding streets. Power plants combust natural gas in the heat of summer to power air conditioners across the city. Semi-trucks idle on big trucking routes and depots, burning diesel as they stand still. All of this industrial activity generates what researchers call “waste heat.” Experts say the best way to mitigate this heat is through good urban design. If a city concentrates factories in one neighborhood, it should place a protective buffer around those factories, separating them from residential areas with forests or green space. The same is true for peak-load power plants, which in cities like New York often sit mere feet away from large apartment complexes.

3. PUBLIC TRANSIT:The way we design transportation systems is key to combating high temperatures. A car-choked thoroughfare produces much more waste heat than a tram or bus lane. “Land use and urban development patterns matter,” said Meerow. “We need to be making sure that we’re conserving open spaces…and promoting alternative forms of transit does kind of have a heat benefit.” In addition, these systems need to have reliable service. If buses and trams arrive on time and at short intervals, people can plan their trips so they don’t have to wait for 20 minutes in the heat.

4. SOLAR PANELS ON PARKING: Parking lots can get extremely hot, from the vast dark pavement to the metal cars that soak up heat. But there’s a win-win option available: Cover these lots with solar panels that double as shade structures. Not only do the panels stop cars, pavement, and people from overheating, but the solar energy generated can power the nearby stores, giving businesses a discount on their monthly energy bills. Such setups can be found from the Lincoln Financial Field stadium, home to the Philadelphia Eagles, to France, which passed a law last year requiring solar panels in all large parking lots.

5. GREEN ROOFS AND WALLS: While reflective paint is a quick and easy way to reduce rooftop heat absorption, many cities have also experimented with putting foliage on rooftops and along walls of large-footprint buildings. Even a thin layer of leaves — such as ivy or creepers — can block the intensity of the sun’s rays, which also reduces the need to crank up the air conditioning on the inside. The foliage on these roofs can range from low rows of shrubs to trees, depending on how much water and maintenance a building owner can provide. 

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