Chicago is not known for its blistering temperatures like, say, Phoenix, which is experiencing a relentless stretch of 100-degree days this month. But a recent report analyzing temperature disparities within cities found that Chicago was among the cities with the largest population living in urban heat islands, above San Antonio, San Diego and Phoenix. 

Urban heat islands are pockets of a city that absorb and retain heat more than surrounding areas due to dense concentrations of pavement, buildings and other urban features and limited natural land cover to reduce temperatures.

Five cities—New York, Houston, Los Angeles, Dallas and Chicago—are home to the highest percentage of the population living in areas with temperatures elevated by at least eight degrees Fahrenheit, according to the analysis by Climate Central of the urban heat island effect in 44 U.S. cities. A dozen of the cities studied, including Chicago, have a proportion of their population living in areas with an urban heat island index of 12 degrees, meaning they are that much hotter than the cities’ baseline temperatures.

Twelve degrees “is quite high, especially when you’re thinking of this as additional heat on top of what with climate change is already becoming quite extreme temperatures,” said Jennifer Brady, lead analyst at Climate Central.

Low-income communities in cities tend to be located in areas with less tree canopy and more heat-absorbing infrastructure, making them more likely to be in urban heat islands than wealthier communities.  They are also more prone to health risks related to heat, like heat strokes, as they are less likely to have central air conditioning and have more limited access to affordable healthcare. The risks are exacerbated by heatwaves, which are occurring across the country this month and are increasing due to climate change.

Nine cities have over one million people living in urban heat islands, totaling 40 million people across the U.S., according to the climate research nonprofit. 

Brady said she was surprised that these extra hot areas were dispersed across the cities. 

“I was expecting that there was a big concentration of urban heat islands in the city cores, but in most cities, there’s a fairly sustained urban heat island index throughout the city and even around the city’s boundaries,” said Brady.

The analysis found that the parts of Chicago with the highest elevated temperatures were in the downtown area, Northwest Side and Southwest Side based on the land cover zones and population density.

Chicago also ranked third for the largest population, almost 12 percent, living in parts of the city with temperatures at least 10 degrees higher than the city’s base temperature, following New York with 41 percent and San Francisco with 30 percent, Climate Central found.

According to Brady, cities like Phoenix and San Antonio may have fewer people living in urban heat islands, even with limited natural landscape, partially because of sprawl and lower population density. A factor that may expose more people in some cities than others to higher temperatures is more concentrated heat-absorbing infrastructure, like roads and buildings in downtown spaces, which is more common in older cities. 

“There’s a lot of contributions that sort of infrastructure makes to raising the urban heat islands, so that’s why you see some of those higher numbers in Chicago, New York and Philadelphia,” said Brady. 

The City of Chicago, known for its frigid winters, has recently taken more interest in studying heat disparities. In April, it announced its partnership with the National Oceanic and Atmospheric Administration to map urban heat islands. The city is one of 18 that will be collaborating with NOAA this year to collect data they said is meant to inform city decisions addressing the disproportionate health impacts of extreme heat on vulnerable communities.

“Heat waves are increasing in frequency, duration and magnitude,” Allison Arwady, commissioner of the Chicago Department of Public Health, said in a June announcement of the heat mapping project. “There are actions we can take in partnership with the community to inform both the focus of our efforts and solutions we can take together.”

Close to 500 Chicagoans signed up to help the city determine the hottest pockets of the city. This Friday, the city plans to deploy a group of them to drive around the city with sensors throughout the day, gathering temperature data. The city plans to release the map of the data collected in September. 

Brian Daly, senior planner at the Chicago Metropolitan Agency for Planning, said the findings could help inform where to target heat education in the short term and implement natural solutions to reduce heat exposure in specific neighborhoods in the long term.

“We will be experiencing a range of temperatures that are not what we are used to and not what our infrastructure was designed for,” said Daly.

Air conditioning in Chicago is not universal, and summers in Chicago are more humid than in other cities, which can be more dangerous and put people at higher risk of heat-related illness. 

Chicago Metropolitan Agency for Planning is currently developing its own heat vulnerability index based on population sensitivity to heat, exposure to heat and capacity to adapt to the heat as part of its effort to target climate resilience measures, said Daly.

Brady from Climate Central said she sees research in urban heat islands as an opportunity for communities to come up with hyper-local solutions, like more trees and green roofs, to bring some of these numbers down. 

“There are very localized solutions that can be put in place to help mitigate to some extent,” said Brady. “This has a little bit of hope in that way because it doesn’t have to be this big global thing.”