Why sizzling cities are mapping hot spots street by street

The city of Reno, Nevada, is breaking records in ways it doesn’t like: A 2024 analysis of 241 cities showed that Reno has heated up faster than any other city in the United States.

While the country as a whole warmed by 2.6 degrees Fahrenheit on average between 1970 and 2023, Reno saw an increase of 7.6 degrees. A heat wave that hit Reno in July 2024 made for the hottest month in the Biggest Little City’s documented history.

Reno’s heat is not evenly spread. In the summer, neighborhoods near the airport feel oven-like compared to the more affluent, cottonwood-shaded homes near the Truckee River. The sidewalks of pavement-heavy shopping centers and casinos in the city center offer little relief; those in the green-space-rich neighborhoods of South Reno are kinder. The city’s main thermometer, located at the airport, doesn’t tell you anything about those differences.

And neither, it turns out, do temperature-mapping satellites — their resolution is far too coarse.

Which is why I found myself attaching a snorkel-like sensor to my car window on a clear and toasty August day, along with 75 other volunteers with their own sensors and cars. We split up along 20 routes and cruised through Reno in the morning, afternoon or evening. My partner and I drove a slow evening loop in the Galena neighborhood, where large, lush properties — the median home price is over $1 million — are flanked by patches of sagebrush. Even at 7 p.m., it was 92 degrees.

By the time we finished the loop, dusk had set in and my car thermometer had dipped to 83 degrees. But as we headed to the volunteer station in the city center, the gauge climbed back up to 89. Such gradients are common in cities, and commonly remarked upon, but they’re rarely precisely measured and mapped.

Today, locating the hottest parts of cities with precision is critical for guiding efforts to contend with heat’s dangerous effects. As climate change brings more intense, frequent and longer-lasting heat waves, heat-related illnesses and deaths also climb. High-resolution maps can alert officials to spots facing the greatest risks, so they can plan. It’s especially important when heat risk overlaps with poverty, where communities may have less access to air conditioning and fewer ways to stay cool.

Maps pieced together by the sensors “will help us be able to target, down to the street level, where we can plant more trees to help people better endure the hotter days of summer,” says Brian Beffort, sustainability manager for Washoe County, home to the Reno-Sparks metro area. The maps will also guide where to focus efforts to weatherize buildings so they require less energy to cool.

Campaigns to record temperatures across city neighborhoods and create better heat maps are on the rise. Reno is one of more than 80 US communities that since 2017 have completed a heat mapping project with the aid of citizen scientists, efforts overseen by the National Oceanic and Atmospheric Administration. NOAA has also supported a few international mapping efforts in cities such as Nairobi, Kenya, and Salvador, Brazil.

Local officials are using the data to plan how to adapt to, and fend off, rising urban temperatures. Some have begun to plant trees, install reflective materials and take other measures to cool the hot spots.

Hot spots within heat islands

Cities tend to be warmer than the open spaces around them, a well-known phenomenon called the heat island effect. Roads, parking lots and roofs tend to absorb more heat than soil and vegetation. City buildings add surfaces that hold on to heat while reducing circulation of cooling breezes. Human activities, from powering cars and appliances to simply existing (think body heat), give off what’s called anthropogenic heat.

Climate change is amplifying this effect. While the average global air temperature has increased about 2 degrees Fahrenheit, “overheating in cities is really five to six times higher,” says Mat Santamouris, an architectural scientist at the University of New South Wales in Australia. In Sydney, where he lives, the annual number of days with an average daily 24-hour temperature exceeding 65 degrees F — a threshold for thermal comfort without needing cooling — has gone up more than threefold since 1990, an increase 10 times that of the surrounding natural areas. During heat waves, the heat island effect is often magnified, with temperature differences between urban and undeveloped areas growing even wider.

The heat island effect isn’t uniform within a city, and heat maps can line up with class and racial disparities. Affluent areas tend to have more trees, parkland and water features, while poorer communities likely have more heat-absorbing paved surfaces and less tree canopy. That translates into a greater risk for some people as temperatures inch up.

Excessive heat is among the most dangerous of weather-related hazards. During heat waves, heat-related illnesses can increase up to 11 percent for every 1 degree Celsius (1.8 degrees Fahrenheit) increase in the surrounding air, researchers find. One analysis of temperature and health data collected in summer months in California reported that a 10-degree F temperature increase was associated with a 2.3 percent increase in deaths. Heat waves trigger more emergency department visits, with young children and those over 65 facing the highest risk. There’s also an uptick in hospitalizations from kidney problems, cardiovascular disease, diabetes and electrolyte imbalances.

Perhaps surprisingly, city officials working to better prepare for heat waves have not known where to expect the most extreme urban heat. Relying on temperature data from satellites provides a relatively crude map: Climate research has historically relied on remote sensing made up of pixels that correspond to 30-by-30-meter squares — and while that’s useful for depicting larger weather and climate patterns, it misses a lot. Community-led heat-mapping projects fill those gaps by logging temperature and humidity every second at precise locations, five feet above the road, as volunteers drive their assigned routes.

“A satellite can’t tell you what you feel like when you’re walking down the street,” says Max Cawley, director of climate research and engagement at the Museum of Life and Science in Durham, North Carolina, who helped coordinate a volunteer heat mapping campaign in that area.

Heat mapping volunteer campaigns have revealed disparities as high as 20 degrees between parts of the same city, says Joey Williams, a program manager at the climate adaptation consulting firm CAPA Strategies, which works with city data in partnership with NOAA. In a 2019 study analyzing both satellite and car-mounted sensor data taken from campaigns in three mid-Atlantic cities, he and coauthors found stark within-city microclimates, with temperatures varying by over 15 degrees F. In a midsize or large city on a 90-degree-plus day, it’s typical to see differences of at least 10 degrees, he says.

Back in Reno, the measures I helped to take last summer revealed a dramatic maximum temperature difference of 23 degrees F. Part of that gap was elevation-based — some neighborhoods sit a few hundred feet higher. Still, southern Reno neighborhoods with ample greenspace were about 10 degrees cooler on the afternoon of the mapping day compared with more central hot spots near highway and industrial areas. Some less-vegetated suburbs in the adjacent city of Sparks were also running hot.

Targeted cooling

In many communities, heat campaign data are already being used to target spots most in need of cooling resources.

After a mapping campaign in Toledo, Ohio, officials found that the city’s hot spots often overlapped with census tracts identified by the Biden administration’s Climate and Economic Justice Screening Tool as lacking in commercial investment and overburdened by pollution. (In January 2025, the Trump administration removed the mapping tool and references to the project from government websites.) Some parts of the city were 11 degrees hotter than others at midafternoon on the summer mapping day. City sustainability coordinator Beatrice Miringu says that the data helped to secure a $6.1 million grant to plant more than 10,700 trees, funded by the 2022 Inflation Reduction Act.

In the Raleigh-Durham area of North Carolina, Cawley says that heat-map data from the area’s 2021 campaign are helping officials to identify streets to spray with reflective titanium dioxide, a covering that helps to bring down street-level temperatures. They are also displayed at the Museum of Life and Science and incorporated into public school physics and history curricula.

Neighborhood organizations can also use the data to inform heat-safety outreach by, for example, calling or texting people when temperatures climb to dangerous levels.

“Heat is a hyper-local impact. It imperils people really differently on a neighborhood-by-neighborhood, and sometimes block-by-block, level,” says Cawley. “People who are impacted ought to have a pretty big say in how this is mitigated and dealt with.” Some neighborhoods, for example, might need immediate help paying for increased air conditioning use; after all, trees need years to grow into a shady canopy.

Often, the most effective way to expand canopy is to maintain the health of existing trees rather than only planting new ones — though such maintenance efforts are more expensive and get less press, says Erica Smith Fichman, city forester for Philadelphia’s Parks and Recreation Department, where officials use heat mapping data to design the city’s forestry plans.

Data from Reno and elsewhere will also help to improve local climate models and so better predict warming impacts down to the street level, says John Mejia, a climatologist with Nevada’s Desert Research Institute. Mejia led a similar mapping effort in Houston and advised another in Las Vegas. He has built an urban climate model of Las Vegas, which he can use to “play God” and, for example, add more trees to the simulation to observe their effect on temperatures. “We can use the heat map data to contrast the model output versus the observations,” he adds, thus improving the model.

Designing for overheated cities

As more and more cities roll out heat-mitigation efforts, their impact on the heat island effect will be closely followed. Research suggests that smartly designed plans should bring relief, says Santamouris, coauthor of a 2023 review on urban overheating in the Annual Review of Environment and Resources.

Greenery alone can reduce peak temperatures by up to 2 degrees F, and while trees obviously provide shade, they also release water vapor, which can cool the air in drier climates, he says. Roofs and pavements covered in light-colored, reflective material so they absorb less heat can decrease temperatures by up to 3 degrees. Added together, cool surfaces and vegetation can reduce peak temperatures by up to about 5 degrees during the day and even more at night, Santamouris says.

Innovative surfaces can provide even more benefit. Santamouris’s team, for example, is working on new, “super cool” materials that can potentially reduce heat islands by 9 degrees.

Appropriate heat treatments depend on a region’s climate, says James Voogt, an urban climatologist at Western University in Ontario, Canada, who coauthored a paper on cities and climate change in the 2020 Annual Review of Environment and Resources. In hot and humid areas, he says, increasing ventilation to carry heat away is crucial. In Southeast Asia, for example, it makes sense to design building developments with gaps to allow cool breezes to snake through structures. Where the air is dry, such as in the US Southwest, adding shade is more important.

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The researchers, local officials and nonprofit leaders who worked on the heat mapping campaigns in various cities are continuing to collaborate, says Tom Albright, deputy state climatologist and a geographer at the University of Nevada, Reno, who helped organize the Reno campaign. “There might be scientific questions that we can address really effectively only when we have the benefit of looking across multiple communities.”

The exercise was also an eye-opener for some of the heat-mapping volunteers — revealing how much the experience of heat varies from neighborhood to neighborhood, for better or for worse. In Toledo, says Miringu, she received comments from participants who had never been to certain parts of the city before recording temperatures there. “It was quite a realization. If you’re driving in an area with no canopy, and then you get into an area with canopy, there’s that sudden shift in how you feel,” she says. “It makes that information no longer a data point, but an experience.”

And though the challenge of surmounting global climate change may often feel impossible, efforts to cool cities are a way for communities to try to cope, at least in their small patch of the Earth. When it comes to urban heat islands, “I’m quite a bit more optimistic that we’re not going to see that accelerate,” says Albright. “I think we have a lot more ability to control it locally.”