Why is it so hot? If you live in a city, the answer might surprise you.

Yahoo

26-07-2025

PHILADELPHIA ‒ It was another brutally hot day in a summer that has seen more than its share of them. Temperatures were in the high 80s, but with the oppressive humidity, heat indices reached into the low 100-degree range.
And in Hunting Park, a working-class neighborhood in North Philadelphia, it felt even hotter. Heat radiated in waves off concrete sidewalks and asphalt streets. Garage owners left their bay doors open and residents sat on the front stoops and porches of their row houses, hoping to get a little relief from even more sweltering temperatures inside.
"There's a lot of asphalt and cement," said Jemile Tellez Lieberman, who works in research, health equity and community engagement for Esperanza, a neighborhood nonprofit.
All that pavement, along with a constant flow of cars, densely packed housing, and a lack of greenery, means Hunting Park is warmer than leafier pockets of the city ‒ an urban heat island where temperatures can be as much as 7 degrees higher than the rest of the city during the day.
There are pockets like this all over Philadelphia, and in cities all over the country.
One solution, say a growing number of researchers and urban advocates, is trees.
"If you plant a tree, this will benefit your neighborhood in 5, 10, 50 years," said Lauren Marshall, director of landscape restoration for the Arbor Day Foundation, a nonprofit dedicated to planting trees.
Dangerous heat, disproportionate impact
Adding trees doesn't just make a neighborhood look better. It makes it healthier, too, Marshall said.
"Heat kills more people than any other natural phenomenon in the United States," she said. "So trees will be more important than ever, especially in urban areas."
Visualizing the issue: Yes, it's hotter in the city. See how the urban heat island effect works.
In 2023, more Americans died from heat-related causes than in any year since 1999, when tracking began. More than 2,300 people died from heat that year, the study, from the American Medical Association study found. But those researchers believe that was likely an undercount.
Children, the elderly and people with preexisting health conditions are the most vulnerable to high temperatures. And low-income communities are more likely to swelter in heat islands, research by the U.S. Environmental Protection Agency and others has shown.
A study by Climate Central found that 41 million Americans live in census tracts across 44 cities with an urban heat island index of 8 degrees or higher. New York (9.5 degrees), San Francisco (8.8), Chicago and Miami (8.3) and Seattle (8.2) are among the U.S. cities with the highest urban heat island indices.
The effect can differ among cities. Some, including Albuquerque, Indianapolis, Las Vegas, Philadelphia, Sacramento, San Antonio and Tulsa, have higher temperatures concentrated within an urban core.
Other cities including Atlanta, Denver, Baltimore, Boston, Chicago and Milwaukee, have more diffused areas of heat intensity, with smaller differences in temperature between their urban core and their outlying areas.
How trees can help people, and how people can help trees
Like a lot of urban neighborhoods across the United States, Hunting Park faces a wide range of challenges, from poverty to underfunded schools. So prioritizing tree-planting can be a tough sell, Esperanza's Tellez Lieberman said.
Esperanza, which supports the Latino community in Hunting Park, has found that engagement is key, she said. "We let (residents and business owners) take the lead in discussions about what trees they want and where to plant them."
It also takes a lot of trees to make a difference.
A 2019 study found that a neighborhood has to be at least 40% covered by trees ‒ counting pavement and buildings ‒ to substantially limit heat. That amount of tree coverage can reduce temperatures by seven to nine degrees, according to the University of Wisconsin researchers.
Esperanza works with the Arbor Day Foundation, Pennsylvania Horticultural Society, Tree Philly and the city's Department of Parks and Recreation to plant trees every the spring and fall. The trees are provided at no cost to residents, and they are planted in places where they can grow and flourish.
"Trees need maintenance," Tellez Lieberman noted. "They can tangle with wires or their roots can upend pipes, and it can be cost-prohibitive to get one removed, so we work with people to pair the right tree with the right person for support and maintenance."
The Arbor Day Foundation uses data, including GIS (geographic information systems) mapping, its GIS director Derrick Frese explained, to help determine which areas should be prioritized because they have too little greenery, denser populations and higher temperatures.
They're beginning to examine the benefits of trees in a more data-focused way as well, Frese said. "Now we can capture a ton of information, like how much water runoff is captured, how many trees are planted, how many people are impacted."
But the benefits can go beyond a bit of shade and a little green in a gray cityscape, said Marshall of the Arbor Day Foundation.
"What I love about trees is that they are part of the solution to so many problems," she said. "Improving air quality, mitigating water runoff. They can even bring in some food, like fruit, draw in more wildlife, and we've even heard it brings more people to walk around."
This article originally appeared on USA TODAY: Why is it so hot? Cities swelter when trees disappear.
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