On hot days, it can feel hotter in some Pennsylvania cities, according to a new analysis by Climate Central, a non-profit science communications group.
That’s because of the urban heat-island index – additional heat absorbed and released by buildings, roads, and pavement. The study showed that Philadelphia, Pittsburgh, Harrisburg and Wilkes-Barre all felt at least 8 degrees hotter, with some regions of the cities feeling up to 13 degrees hotter.
The heat effect can be worse in neighborhoods that are low-income and have a higher population of people of color, the group says.
Climate Central says its estimates are more accurate than in the past. Previously, the group estimated the heat-island index using land-cover type and population density at the census tract level.
That meant some land types – industrial, office buildings and abandoned spaces – could be combined, and the population density could be less, said Jen Brady, the lead analyst behind the study.
In this study, the estimates are at the block level, where the land-cover type is more uniform and the population density is higher. Each block within a census tract has between 600-3,000 people. That means the study is averaging over a smaller area, which increases the accuracy of the estimates, Brady said.
Within a city like Philadelphia, “you may see land-cover varying from dense high rise, to heavy industrial areas to low-rise residential areas to green spaces,” and each of those have different levels of additional heat, Brady said.
The population density amplifies that because of things like traffic-related emissions and cooling systems blowing out hot air.
The urban heat island index calculates how many additional degrees that area would experience compared to an area outside the city without concrete and asphalt structures. Brady said the index shows “the effects of climate change – extreme heat events – are made worse by cities.”
In big cities like Philadelphia and Pittsburgh, more than half the population feels more than 8 degrees of additional heat. In Wilkes-Barre, that’s close to 75%. In all, that’s over a million people.
But the burden of heat is not shared equally within the cities, Brady said.
In a joint analysis between Climate Central and Columbia University, researchers found that neighborhoods that were traditionally redlined tended to be hotter than non-redlined areas in 150 out of 179 U.S. cities.
Redlining was a racially biased practice set by a U.S. governmental agency called Home Owners’ Loan Corporation in the 1930s. The practice gave grades, ranging from A-D, to neighborhoods based on “security” of loans or investments made in the area.
Brady said that has had a lasting impact.
“What do you do if a building or a lot is abandoned, how do you rebuild that? Do you build a park there or leave it as an abandoned lot of pavement?” she said. “If more money was put into maintaining neighborhoods and that effort wasn’t put into other neighborhoods, that’s going to lead to this disparity.”
The 1968 Fair Housing Act outlawed the redlining policy. But the study looked at neighborhoods set in the 1930s and found, for example, that in Philadelphia, the difference in the mean temperature between formerly ‘grade A’ and ‘grade D’ neighborhoods, today, is close to 12 degrees.
Brady said it all comes down to the built environment of the area. An environment with large stone buildings, abandoned structures, and pavements absorbs a lot of heat that is often released back at night.
“So when the city should be cooling off, you have all this absorbed heat that’s sitting in all this material that’s going back into the environment,” she said.
But, there are solutions at every level, Brady said.
Policies like Philadelphia’s Cool Roof Code, which requires roofs on newly constructed buildings and additions to existing buildings to use highly reflective paints, can help, Brady said.
Last year, the City of Philadelphia created the Commercial Corridor tree planting and care program. In partnership with community groups, the city has planted over 240 trees in nine corridors in priority regions.
In 2023, the city got a $12 million grant from the USDA Forest Service to implement the Philly Tree Plan, a 10-year plan to plant and preserve urban tree canopy in the city with priority given to regions that are experiencing the most heat.
This story is produced in partnership with StateImpact Pennsylvania, a collaboration among WESA, The Allegheny Front, WITF and WHYY.
