This simple solution can provide relief from the heat in cities

(CNN) — Summers are becoming increasingly dangerous, especially in cities, where temperatures soar due to heat dissipation effects caused by tall buildings, concrete and asphalt. But there may be a simpler and potentially cheaper way to reduce urban heat: retroreflectors.

Applying retroreflective materials to buildings reduces their surface temperature by 19.9 degrees Celsius (36 degrees Fahrenheit) and air temperature by about 2.7 degrees Celsius (5 degrees Fahrenheit), according to a study published Monday in the journal Nature Cities.

Elie Bou-Zeid, co-author of the study and professor of civil and environmental engineering at Princeton University, says reducing the temperature of buildings is “very, very important” for pedestrians, “because those walls emit a lot of heat at you. “

“Cooling those surfaces will make you feel a lot more comfortable,” he said. “It’s almost like living in the shadows.”

Cities tend to be much hotter than their surrounding suburbs and rural areas because of the way they are built. Tall buildings, dark roofs, asphalt and concrete absorb the sun’s rays and reflect their energy into the environment as heat: this is the so-called urban heat island effect.

Urban designers have begun to implement simple solutions to counter the phenomenon of urban heat, such as painting streets white, planting more trees, and building green roofs. But researchers say the use of retroreflective materials would have a more powerful cooling effect.

Retroreflectors take advantage of a simple concept: three cubic corner-shaped mirrors reflect light in the direction from which it comes. This can also be achieved with a bowl shaped mirror.

These are used in traffic signs and bicycle reflectors, for example, to reflect headlights into the eyes of drivers. These are also used in running clothing, which have reflective stripes. Cats also have similar natural retroreflectors in their eyes, which is why they appear to glow when light hits them.

According to Bou-Zeid, retroreflective materials could be developed as sheets or coatings to be installed on city surfaces. Since the material itself is relatively inexpensive, it could be a low-cost solution to an increasingly dangerous phenomenon as temperatures continue to rise due to planet-warming pollution.

Smaller-scale versions of this idea have already been tested in the Netherlands and Italy. But no large-scale examples exist yet, because of how difficult it is to retrofit existing buildings.

“In general, the construction sector is a little slow to adopt new technologies,” Bou-Zeid tells CNN. “We hoped to at least demonstrate with this work that retroreflective materials have great cooling potential. And the next step would be to develop this technology, which already exists, but in buildings.”

Some US cities have taken steps to combat extreme heat. The streets of Los Angeles were painted grayish-white to reflect sunlight and keep the air temperature cooler, just as a light shirt will keep you cooler than a dark one under the hot summer sun. . But researchers say this solution is less effective than retroreflective materials, which reflect sunlight back toward the sun “and not toward other objects on the ground,” Bou-Zeid explains.

But this has a downside: It would make winters even colder – reflecting sunlight at a time when it is beneficial – which could lead to new public health problems. According to Bou-Zeid, it would be possible to design reflective materials that would “turn off” when necessary.

“When it’s very hot in the summer, they turn white and reflect a lot towards the sky,” explains Bou-Zeid. “When it’s very cold in winter, they become dark, they don’t reflect much and they absorb mostly solar energy.”

To help mitigate the urban impact of the climate crisis, cities should implement as many measures as possible to adapt to and mitigate these changes, says Xinji Huang, lead author of the study and a doctoral researcher at Princeton.

“It should be noted that retroreflective surfacing, or any other single heat optimization strategy, cannot be a one-size-fits-all solution for different urban environments around the world,” Huang told CNN. Instead, “to deal with the increasing stress of global urban heat requires a multi-technology cooling portfolio with cooling technologies tailored to local conditions.”