Why the City Is (Usually) Hotter than the Countryside

The smoothness of the landscape and the local climate—not the materials of the concrete jungle—govern the urban heat island effect, a new study finds

summer day
In cities, where the urban heat island effect can raise the local temperature several degrees higher than nearby rural areas, summer is a time to cool off wherever you can. © Tannen Maury/epa/Corbis

There’s a good reason why city dwellers flee to the countryside to cool off in the heat of summer: Rural areas are usually not as hot. Because of the “urban heat island” effect, cities of a million or more people can be 1–3°C (1.8–5.4°F) warmer on average—and as much as 12°C (22°F) warmer in the evening—than the surrounding area, according to the U.S. Environmental Protection Agency.

There are plenty of reasons why cities are hotter. All those people, their buildings and the machinery inside create heat. Air conditioning, for example, can raise temperatures by more than 1°C, Arizona State University researchers reported last month. Buildings and other structures can store more heat during the day than plants; at night, they emit some of that heat, contributing to warmer temperatures when it’s dark out. Cities might also have less reflectiveness, which would let them soak up more of the Sun’s heat.

But most researchers have considered the biggest contributor to the urban heat island effect to be the reduction in evaporation that occurs when plants are replaced by concrete. That evaporation, the thinking goes, absorbs energy and keeps the countryside cooler.

According to a new study, though, that difference in evaporation isn’t the big factor behind the heat island effect. Local climate may matter more. Lei Zhao, of Nanjing University of Information Science and Technology in China, and colleagues published these findings today in Nature.

Zhao and colleagues began with a question: Would similar cities located in different climates experience the same increase in temperature from the urban heat island effect? From NASA satellite data of 65 U.S. cities, they could see some trends: At midnight, bigger cities tended to experience a greater difference in temperature, compared with the surrounding rural area, than smaller cities. At mid-day, though, wetter cities experienced a greater heat island effect.

The researchers then created a computer model that let them evaluate the importance of various factors that might influence the urban heat island effect. These included features such as evaporation, heat created by humans and their structures and heat stored by cities. Also included were differences in the land and in convection—the transfer of heat from the city or rural region to the atmosphere.

From the model, the researchers could see that these last two factors were important, and they interacted with the local climate. In humid areas, such as those on the U.S. East Coast, “convection is less efficient at dissipating heat from urban land than from rural land,” Zhao and colleagues note, and these cities are usually about 3 degrees warmer than the nearby countryside. The dense vegetation of the rural area is aerodynamically rougher than the city, which increases the efficiency of convection, letting more heat move from the land into the atmosphere.

In dry regions, where that rural vegetation is lacking—think Las Vegas—the opposite actually occurs. “On average, the urban land is about 20% more efficient in removing heat from the surface by convection than is the rural land,” the researchers write. And in a few of these cities, the convection difference is great enough that they don’t experience the urban heat island effect.

At night, though, no matter the climate, the release of heat stored during the day drives the heat island effect. That heat gets trapped in the shallow layer of atmosphere near the surface, and to dissipate, it has to move more horizontally to escape. For bigger cities, the heat will have to move farther, so the center of a big city will tend to be much warmer than the middle of a small city, the researchers note.

The urban heat island effect is more than just an annoyance for city-dwellers—it can also exacerbate the health problems associated with heat stress, adding to already uncomfortable conditions, especially in dry years in normally humid areas, the researchers say. But this new research gives some insight into which measures might help to alleviate some of that heat.

Reducing the heat from our air conditioners and other machinery might seem like a simple solution, but according to the computer model, that wouldn’t really help much because anthropogenic heat isn’t a huge factor. Tackling the big contributors—convection efficiency and heat storage—however, isn’t a practical solution “because it would require fundamental changes to the urban morphology,” Zhao and colleagues note.

What would help, though, is increasing the reflectiveness of the city, they say. That would decrease the amount of heat the city absorbs during the day and even help indirectly at night, by decreasing the amount of heat available to be released after the sun goes down. Plus, it’s easy—flat city roofs can be painted white or another reflective color.

White roofs also have additional benefits, such as reducing energy use because not as much air conditioning is needed to balance out the heat absorbed by buildings. And lowering energy use also means lowering carbon emissions—a perk that extends beyond the dynamics of one urban heat island.

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