Street Grids May Make Cities Hotter

In 1995, an unbearable heatwave in Chicago, with temperatures reaching above 100 degrees, killed an estimated 739 people. The extreme heat was in part a consequence of the urban heat island effect, which makes downtown areas ostensibly warmer than their surrounding suburban and rural areas. The difference is even starker at night: even as the temperature cools, the release of heat absorbed during the day by asphalt and densely packed buildings can make the downtown area some 20 degrees warmer in some cities.

By now, scientists know that the density of buildings, the absorption of light by those buildings, and the relative lack of vegetation in cities are major contributors to the urban heat island effect. It’s why cities like Chicago are hoping to find relief through green roofs and reflective construction materials, or through planting more trees and banning cars. In a more radical move, Los Angeles even began painting their roads white as part of Mayor Eric Garcetti’s effort to bring down the city’s temperature by just under 2 degrees over the next 20 years.

Yet Roland Pellenq, a senior research scientist at MIT’s Concrete Sustainability Hub, wondered how much the actual layout of a city area contributes to the intensity of the urban heat island effect. To find the answer, Pellenq and his team turned to physics.

In a paper published last month in the journal Physical Review Letters, he and his team concluded that the “texture” of a city makes a big difference. Grids may be making cities much warmer than nearby suburbs and countryside.

It all began while Pellenq was on his coffee break, looking at Boston’s skyline from across the Charles River. To him, the buildings resembled water molecules piled up on a surface. So, he thought, “why don’t we calculate the relative position of [the buildings] and see if there is a pattern of order that emerges?” Pellenq told CityLab. Indeed, the fingerprints of cities like Boston and Los Angeles mirror the disorderly atomic structure of liquids and glass, while the likes of Chicago and New York City, with their streets and avenues perpendicular to one another, exhibit a more orderly configuration found in crystals.

Pellenq’s team calculated the temperature difference between urban areas and their suburban or rural counterparts for 14 U.S. cities, using years’ worth of data from the National Oceanic Atmospheric Administration. To find the correlation between texture and temperature, they first quantified the “local order” of each city’s urban layout, using Google Maps to analyze how buildings are organized within a three-mile radius of a weather station.

Then, adapting formulas that physicists use to measure atomic interaction in condensed materials, they assigned each city a score between 0 and 1, with 0 indicating a liquid-like layout in which buildings are loosely packed and organized randomly, and 1 signifying a perfectly ordered structure. (Most cities fell between 0.5 and 0.9.)

The researchers concluded that the more tightly packed that cities are, the more intense the urban heat island effect is—no surprise there. “The heat problem is really a short-range interaction between buildings,” said Pellenq. “Basically, one surface of a building is radiating heat to the next [building].” That energy tends to accumulate within so-called heat canyons created between those buildings.

“The heat problem is really a short-range interaction between buildings,” said Pellenq.

More surprising, though, they found that cities with more rigid grid-like street patterns (that is, a higher local order) tended to display a higher temperature difference between their urban and rural areas. This has to do with air flow, said Pellenq. In disorganized cities, the air tends to flow uniformly with little or no interruption. But the perpendicular streets of Chicago and the like often trap heat by disrupting that airflow.

As existing cities expand and new ones crop up, particularly throughout Asia and Africa, Pellenq said this finding should be incorporated into planning. Designers of urban districts in colder regions may want to consider a grid-like pattern to retain heat, while those in warmer regions might consider introducing some form of disorder into the layout.

Forethought like that can make a big difference over time in terms of financial spending and climate change. Consider that roughly nine out of 10 households in the U.S. have air conditioners, which are turned on even before the peak of summer. And as the world heats up, countries like China and India are increasingly mirroring America’s addiction to artificial cooling. One study estimated that the world is poised to add 700 million AC units by 2030, which translates to massive energy costs and consumption—not to mention a big boost in carbon emissions. But beyond that, Pellenq noted, the design of a city can mean life or death as climate change fuels more deadly heatwaves in the future.

The urban heat island effect is complex, of course, and there’s more research needed to round out Pellenq’s analysis. He hopes to study the air flow in cities and how that factors into his model, and wants to expand his research to beyond that three-mile radius. But as cities are already demonstrating, architecture, materials science, and natural landscaping each has its own role in curbing the urban heating island effect. So everything has to be considered.

“It’s not one [factor] or the other,” Pellenq said. “It’s probably one and the other.”

Learn more at CityLab