Strategic city planning can mitigate 'urban heat island' effect, make cities cooler in summer: study

People eat lunch on a hot day in Toronto on Thursday, June 23, 2022. THE CANADIAN PRESS/Nathan Denette

New research from Penn State University found specific urban factors can reduce the "urban heat island" (UHI) effect, which is the tendency of cities to trap heat.

The peer-reviewed study, published in the journal Buildings, suggests trees can cool air temperatures, mean radiant temperature and "predicted mean vote index," which is what researchers use to evaluate thermal comfort levels. 

Additionally, higher building-height-to-street-width ratios, where taller buildings provide shade to narrower streets, and pavement that is better at reflecting sunlight contribute to lower temperatures and greater comfort levels.

According to Guangqing Chi, professor of rural sociology, demography, and public health sciences at the College of Agricultural Sciences, the findings can help encourage cities to plan more effectively, particularly for low-income communities, people of colour and the elderly, who are disproportionately affected by the UHI effect.

“Global warming makes some human habitats unbearably hot, but more so for socially and historically disadvantaged communities,” Chi explained in a news release published on Friday. “This interdisciplinary project provides an effective, equitable urban design solution for enhancing resilience against extreme heating.”

The study, which is a collaboration between College of Agricultural Sciences and the College of the Arts and Architecture’s Hamer Center for Community Design in the Stuckeman School, focused on Philadelphia.

According to the researchers, the city experiences high rates of both poverty and extreme weather with 22.8 per cent of people in the city living below the poverty level. The city also has experienced extreme weather events, such as its snowiest winter, two warmest summers, wettest day, and two wettest years on record since 2010.

"Heat island hotspots and mortality rates tend to be greater in urban blocks with a socially disadvantaged population,” Farzad Hashemi, a Hamer Center researcher, said in the news release. “This is due to the combination of physical factors, such as impervious surfaces and lack of vegetation, and social factors, such as vulnerability to heat-related health effects.”

For this study, researchers used the social vulnerability index (SVI) as well as data about tree coverage to identify two Philadelphia neighbourhoods—one with the lowest SVI and high tree coverage, and another with the highest SVI and the lowest tree coverage.

Researchers analyzed several characteristics of the neighbourhoods including construction materials, land cover, how well pavement reflects sunlight and heat generated by human activities, among others. Researchers also ran 24-hour simulations for six scenarios—winter, spring, summer, fall, extreme hot and extreme cold—in each neighborhood to see how different factors affected each other.

Researchers found the trees’ cooling effect is limited to their immediate surroundings, while areas without trees had significantly higher mean radiant temperatures. Additionally, the effect of trees on air temperature decreased as distance from areas with many trees increased.

Chi said these findings suggest ways urban climate knowledge can contribute to better city planning and design.

“For example, using taller and denser buildings alongside pavement surfaces with higher albedo could be considered to support more comfortable thermal conditions, particularly in urban blocks with less vegetation coverage,” Chi said. “Overall, the study highlights the importance of considering urban morphology and vegetation coverage in the design of sustainable and livable urban environments.”

Hashemi said the study can form the basis for more in-depth research in the future.

“Our study focused on Philadelphia, which is in the U.S. Northeast region,” he said. “Expanding the data to incorporate more cities from other climate zones, including hot and dry or extremely cold, could give us a better understanding of the effects of urban properties on varying climates and social vulnerability indexes.”

 

Reporting for this story was paid for through The Afghan Journalists in Residence Project funded by Meta.