Scientists Just Discovered A New Key Reason Why Cities Get So Hot

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Sunset in London during the summer time.

Scientists have long been aware of the “urban heat island” effect — the tendency of large cities to get 1.8 to 5.4°F warmer than surrounding areas in the day, and 22°F warmer at night. The assumption has been that this was because all the concrete and other structures stored up heat, and the lack of vegetation reduced evaporation.

But a new study published last week in Nature says that’s not entirely right: convection, or the movement of air through the city, plays a big role in the daytime. The lack of vegetation and the presence of urban structures still matters, but because of their effect on air movement rather than on evaporation: “The ‘rougher’ surfaces of the vegetation triggers turbulence, and turbulence removes heat from the surface to the atmosphere,” said Lei Zhao, a doctoral student at the Yale School of Forestry and Environmental Studies, and the study’s lead author. “But where there is a smoother surface, there is less convection and the heat will be trapped in the surface.”

Interestingly, this effect on convection actually played much different roles depending on whether a city is found in wetter or drier climates. In dry areas like the American southwest, vegetation is shorter and scrubbier, which makes the rural areas far less adept at dissipating heat. As a result, urban areas actually become comparably more efficient at releasing heat back into the atmosphere, making the cities about 1.5°F cooler. But in wetter climates, urbanization reduces the efficiency of convection by as much as 58 percent.

The study examined 65 different cities in the U.S. and Canada, using satellite data of land surface temperatures and vegetation cover. The researchers then applied various climate models to the data to tease out the contributions of different variables — such as radiation, convection, evaporation, heat storage, and human-generated heat — to the heat island effect.

“One of the major barriers to mitigating the effects of UHI [urban heat island effect] is the lack of quantitative attribution of the contributing factors,” continued Zhao.

One unfortunate take-away from the study is that managing convection effects will be largely impossible, since it would require massive alterations to the height and structural design of buildings in cities throughout the country. An alternative possibility the researchers brought up was changing “albedo” — the amount of solar radiation reflected back out to space from various city surfaces like rooftops and roads — to reduce the absorption of heat in cities. Studies have shown that while both white roofs and “green” roofs — ones planted with vegetation or gardens — have various advantages, the white roofs tend to to the best job of reflecting heat, which also comes with energy savings from the reduced need for cooling. It’s an approach to climate adaptation that multiple American cities are already taking on of their own accord.

Another concern is the danger of future heat waves, and how those will interact with the convection problems in some cities. “There is a synergistic relationship between climate conditions and the urban heat island,” said Xuhui Lee, the Sara Shallenberger Brown Professor of Meteorology at the Yale School of Forestry and Environmental Studies, and another one of the study’s authors. “This relationship suggests that the urban heat island will exacerbate heat wave stress on human health in wet climates where temperature effects are already compounded by high humidity.”

“This is a huge concern from a public health perspective.”

A recent study by Climate Central showed that U.S. summers are already warmer than in the 1970s, and by the end of this century Boston could be seeing the same kinds of summers that Miami sees now. Another recent granular study of the risks climate change poses throughout the American economy warned that heat waves could have significant impacts on both human safety and worker productivity by 2100, especially in the eastern half of the country.

About 3.5 billion people, or more than half the global population, already live in cities according to the World Health Organization. And by 2050, the group projects that portion will rise to 70 percent of the people on Earth living in urban areas.


This post has been edited for clarity.

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