Order Makes Cities Easy to Navigate—It May Also Make Them Hotter

Physics and statistics can describe how building patterns relate to cities’ tendency to hold heat

heat island
How a city is arranged can influence whether it heats up in comparison to surrounding areas courtesy the researchers/MIT News

The structured grid of New York streets help tourists find their way around, but the order may actually have an unwanted side effect: It makes the city hotter.

new study  in Physical Review Letters suggests that the orderly buildings exacerbate the sweltering effects of the urban heat island. Conversely, the layout of more disordered cities like Boston​ actually releases the warmth that could have been trapped by buildings and pavements.

The findings could help city planners build urban environments that will better suit our changing climate and a future of more frequent and intense heatwaves, David L. Chandler writes in a release for MIT News.

Urban areas can be up to 5.4 degrees Fahrenheit warmer on average than the surrounding, more rural areas, according to the U.S. Environmental Protection Agency. On the best summer days, that can mean that things are just a tad bit more toasty. On the worst summer days, heat waves become dangerous. Heat kills more people every year in the U.S. that any other weather-related event, according to a 2014 report from Climate Central. Individual days in individual cities can spike up to 27 degrees Fahrenheit. More heat means more air pollution and greater energy use. And climate change will only exacerbate these effects.

So understanding what factors contribute to the heat island effect is important. For decades, researchers have known that urban building materials are great at absorbing heat during the day and radiating it at night, Chandler explains. The research team, led by senior research scientist Roland Pellenq of MIT and National Center for Scientific Research calls their measurement of city patterns the "texture" of the city.

The team borrowed a concept from classical statistical physics that describes how the atoms of a given material are affected by the forces of other atoms. But instead of atomic arrangement, they applied the idea to the spacing between buildings‚ using this to describe the order of cities on a scale that ranges from the chaos of liquids to ordered crystal structures.

They used their framework to analyze satellite images of 47 U.S. cities and came up with a single number to describe each city's structure. The cities studied ranged in order from 0.5 — a more "liquid-like" structure— to 0.9, a "crystalline" one. For example, Los Angeles is a liquid-like city while Chicago is crystal-like. 

The researchers also collected temperature data from inside and outside the cities to determine the effect of each city on heat. They then compared that data to their crystalline rating scale and found that this texture number corresponded strongly with the intensity of the heat island effect.

The world is increasingly urbanized—between new cities and expansion of existing ones. The model could help city planners tailor the arrangement of buildings to particular regions, either minimizing heat island effects or taking advantage of them. "If you’re planning a new section of Phoenix," says Pellenq in the press release, "you don’t want to build on a grid, since it’s already a very hot place. But somewhere in Canada, a mayor may say no, we’ll choose to use the grid, to keep the city warmer."

Already more than half the global population lives in cities, according to the United Nations. And heat island effects can even reach those living thousands of miles away, if weather patterns align, Joseph Stromberg reported for Smithsonian.com in 2013. Research like this latest study could help us make smarter decisions about the future.

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