Is your city crystal or liquid? No, this isn’t a Facebook quiz. It’s a serious scientific question about urban design, and whether a city’s layout can affect how hot it gets at night compared to its rural surroundings — a phenomenon known as the urban heat island effect.
If you live in a grid-like city, such as New York or Chicago, whose design at the nanoscale looks like atoms in a crystal, you are more likely to be hotter at night than if you live in a chaotically arranged city, London or Boston, for example, which resemble the disordered atoms of liquid or glass.
The geometric arrangement of cities — which the researchers call “texture” — can make a big difference, according to a new study. In fact, a city’s texture is the single biggest influence on its heat island effect, with “crystalline” cities showing a much greater buildup of heat than their “liquid” or “glass-like” counterparts, the scientists said.
“Grid-like cities have smaller cluster sizes, thus their buildings occupy relatively greater portions of those areas, even though — for the entire city — grid-like cities might have the same density as non-grid like cities,” said Roland Pellenq, a senior research scientist at the Massachusetts Institute of Technology and one of the study authors. “Because the local density for grid-like cities is higher than for non-grid like cities, they essentially trap more heat during the day and are more efficient at releasing it at night.” The study appears in the journal Physical Review Letters.
The urban heat island effect occurs when a city’s building materials, such as concrete and asphalt, absorb heat during the day and radiate it back at night to a much greater extent than areas covered in vegetation. It’s one reason why it always feels hotter in the city at night than in the nearby country. The effect, in fact, can add as much as 10 degrees F to evening temperatures in cities such as Phoenix, according to the researchers.
As climate change continues to drive temperatures higher, resulting in more and prolonged dangerous heat waves, people living in cities will increasingly carry the burden produced by urban heat island effects, including an upsurge in heat-related illness, air pollution, and energy use as locals crank up air conditioners to stay cool. A better understanding of what exacerbates the heat island effect could help city planners and other officials cope with these growing problems, the researchers said.
“For new cities, or even neighborhoods, our findings can be used…in designing block layouts that would help optimize temperature,” Pellenq said. “If a city is located in a hot climate, and the city wants to minimize temperature and reduce energy consumption in buildings, the grid layout should be avoided. If, however, a city is located in a cold climate, then perhaps some local clusters of buildings on a grid-like structure should be utilized with large green areas in the midst of neighborhoods to offset the negative impacts of [the urban heat island effect] during summer time.”
He stressed, however, that city planning is a complex process that involves many factors affected by layout—for example, traffic efficiency. “Human comfort and well-being, transportation, safety, [and] energy consumption, all must be taken into consideration,” he said. “An equation for a perfect city design doesn’t exist. For many, it’s a trial and error procedure fostered naturally in some ways, but artificially through other [ways] by urbanization. The challenge urban planners face is how to optimize all those parameters. Our study only sheds information on one of them.”
The team used mathematical models developed to analyze atomic structures in materials — ”tools of classical statistical physics” — to calculate how a city’s design influences the heat island effect, Pellenq said. They adapted formulas initially created to analyze how individual atoms in a material are affected by forces from the other atoms, then reduced these relationships to simpler statistical descriptions of the distances of buildings to each other.
They then applied them to building patterns extracted from satellite images of 47 cities in the United States and other countries, ultimately ending with a single index number for each city ranging between zero (total disorder) and one (perfect crystalline structure). They factored in temperature data for each city, both within the city, and outside of it, to determine the difference. The variation in the heating effect seems to result from the way buildings radiate heat that can be reabsorbed by other buildings directly facing them, the researchers said.
“There is a very distinctive structural difference between Chicago, or New York City or Washington, D.C.,” Pellenq said. “As our data shows, D.C. has a definitely less organized structure, which translates to a lower urban heat island effect. The hottest cities at nighttime would be Chicago and New York City, while the coolest would be Dallas or Seattle.”
The researchers evaluated every U.S. state individually and found, for example, that in Florida alone, urban heat island effects cause an estimated $400 million in excess costs for air conditioning. “The $400 million amount is the average annual air conditioning cost just for the residential sector, assuming an urban heat island effect of 2 degrees C across all cities in Florida,” Pellenq said. “In reality, this figure will be much higher, because the commercial sector makes a great contribution to cooling energy loads in dense urban areas.”
Pellenq said the idea for the study evolved from their interest in examining the nanoscale structure of various construction materials, rather than any personal nighttime experiences in hot cities, although “we have traveled to New York City during summertime and were not particularly fond of the excessive heat,” he said.
Although the research “started off as a fun, short summer experiment,” the scientists realized after testing concrete in the lab “that roughness patterns on the surface of tested materials resemble the view that we have right outside of our building across the river, the skyline of Boston,” Pellenq said. “We began to ponder whether the complex network of buildings and roads, known to us as cities, looks similar to a nanoscale surface of a rough material.”
After establishing a link between nanoscale physics and urban planning, they began thinking about larger issues, among them “addressing the question of the relevance and importance to our society,” Pellenq said. “Why should we care that cities are similar to crystals or liquids? [Because] we all have the common goal of changing the world to a better place.”
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.