The Problem With ‘Cool Pavements’: They Make People Hot

SAM BLOCH OCT 3, 2019


A tool to help solve the problem of urban heat islands could have an unwelcome side effect, new research in L.A. finds.


Updated: October 07, 2019

About two months ago, Ariane Middel walked the empty streets of Sun Valley, a suburban neighborhood in Los Angeles’ San Fernando Valley. The roads there had recently been coated with an asphalt mixture called CoolSeal, which lowers air temperatures by reflecting the energy from sunlight, rather than storing it and converting it into heat.



페인트 사용 "쿨로드" 실제로 주변 공기를 덥게 만드나? 


실효성 의문 제시


    몇 년 전, 우리는 로스엔젤레스가 어떻게 어떤 아스팔트 도로를 쿨실(CoolSeal)이 만든 가볍고 페인트 같은 재료를 칠하고 있었는지에 대한 기사를 공유했다. 그들의 희망은 그것이 그들의 도시에서 가장 더운 지역의 열섬 효과를 감소시킬 것이라는 것이었다. 최근의 한 연구는 이 코팅이 실제로 시가 희망하는 효과를 내지 못할 수도 있다는 것을 발견했다.




미국기상학회가 발간할 보고서 사본에 근거한 시티랩에 따르면, 쿨센스 제품이 코팅된 길을 걷는 사람은 실제로 일반적인 아스팔트 포장도로보다 7도 더 따뜻한 온도를 느낄 수 있다고 한다.


아스팔트가 태양열을 흡수하는 반면, 코팅은 즉시 그것을 반사시켜 공기 온도를 높이지만 포장 자체는 더 차가워지는 것으로 조사되었다.


이 연구는 로스엔젤레스의 샌 페르난도 계곡에서 매우 흔하고 밤에 온도가 내려가지 않은 덥고 화창한 날에 완료되었다는 점에 유의해야 한다. 이는 전반적으로 시원한 포장도로가 주변 온도를 낮출 수 있다는 것을 의미할 수 있지만, 여전히 한낮의 사람들에게는 도움이 되지 않을 수도 있다는 것을 의미한다.


우리가 이전에 추측했듯이, 눈부심도 햇빛을 반사하는 밝은 도로와 함께 증가한다. 이 프로젝트의 연구원 중 한 명인 아리안 미델은 시티랩에 이 코팅이 영향을 받는 지역에 최대 10%의 햇빛을 더해주며 이 중 대부분은 출근 시간대에 이른 저녁 시간대에 있다고 말했다.


밝은 색상과 반사 지붕 재료와 코팅은 현재 포장 코팅보다 훨씬 더 인기가 있는데, 이는 건물의 내부 이득과 지붕에서 일어나야 하는 적은 양의 인간 활동 때문에 그렇다. 하지만, 이 가벼운 지붕 위에서 작업해야 하는 계약자들과 기계들에 대해 궁금하게 만든다: 그것들은 눈부심 증가 외에도 더 높은 온도에 노출되어 있을까?


황기철 콘페이퍼 에디터 큐레이터

Ki Chul Hwang, conpaper editor, curator


edited by kcontents




For hours, Middel and a team of researchers dragged an elaborate heat sensor, mounted to a garden cart, down the streets and sidewalks to grab meteorological data. A climate scientist at Arizona State University in Tempe—where air temperatures soar into triple digits—Middel didn’t feel all that hot. But after crunching the data, she discovered the reflected sunlight hadn’t disappeared: She had probably absorbed it.


According to her sensors, on a hot, dry day, a person could feel more than 7 degrees warmer on a “cool pavement,” as the reflective roads are called, as opposed to a normal blacktop.


That discovery is significant. Los Angeles, like many other cities, is trying to cool down, and city officials have been counting on cool pavements as part of the solution. But if they can make people feel that much warmer, they may actually undermine the city’s ambitious plans. Summer days won’t feel better for pedestrians. They could, in fact, feel worse.


***





Scientists have been trying to lower urban temperatures with cool pavements for decades, since the identification of urban heat islands. Cities are hotter than rural areas, because they’re covered in dark, impermeable surfaces that have low albedo, or solar reflectance—meaning they absorb heat from sunlight. They get hot, and warm the air above. Light-colored surfaces, however, reflect more of that sunlight back into space, and keep cities cooler.


In the 1990s, scientists at Lawrence Berkeley National Laboratory calculated that if all pavements and roofs in downtown Los Angeles increased albedo by 25 and 35 percent, respectively, air temperatures would drop by nearly 3 degrees. If that happened throughout the city, and was combined with other efforts, the cooling effects could be even greater. In the years since, more studies have emerged, with varying predictions. They all agree: If done right, cool pavements can help cities move the needle on ending heat islands.


While reflective coatings are now fairly common on roofs, they haven’t taken off on roads. That’s due to concerns about wear and tear, and potential glare, says David Sailor, who directs urban climate research at Arizona State University. (He wasn’t involved with Middel’s study.) More often, he says, cool pavements are found in parking lots of buildings seeking LEED credits.


Los Angeles, however, has taken up the idea with zeal. Since 2015, the city has covered around 50 city blocks in various reflective coatings and seals. In April, Mayor Eric Garcetti announced a plan to cover 250 lane-miles of city roads by 2028.


Although city officials have observed cool pavements’ lower surface temperatures for years, they had not measured air temperatures and human comfort effects. In July, Middel and V. Kelly Turner, a UCLA urban planner, studied unshaded streets in Pacoima and Sun Valley, taking readings on reflective pavements and traditional asphalt. They measured air temperature, wind speed, humidity, and radiation for 10 hours, from morning to sundown.




The results were troubling. On a typical Los Angeles summer day, with a high of 88 degrees, they found the reflective roads could make people feel much hotter. Just before noon, the mean radiant temperature—a calculation of the amount of heat emitted by surrounding surfaces—was more than 7 degrees higher on the coated pavement. That temperature, which is another way of measuring human comfort, decreased as the day went on, but in the afternoon, the roads still felt more than 3 degrees warmer.


The warmer feeling, Middel says, is almost entirely attributable to solar radiation reflected off the roads. Normally, asphalt sucks it in, and it dissipates slowly into the air. These roads, however, reflected it back at a rate of 130 watts per square meter—akin to adding 10 percent more direct sunlight. That reflection was visible as glare, which was “really big” in the early evening, she says, just as people were getting home from work.


“That’s a negative effect on the human body, there’s no doubt about it,” says Larry Kalkstein, a bioclimatologist.

Middel and Turner stress that this is a small sample. They can’t draw conclusions about how cool pavements might affect human comfort in other circumstances, like on cloudy or windy days, or during Los Angeles’ wetter winter season, or even on streets built differently, with taller buildings or more trees. And, they point out, they didn’t study how the roads behaved at night, when they’re expected to have greater cooling impacts.


Nevertheless, Los Angeles has no shortage of dry, sunny days. And the city lacks shade. The streets Middel and Turner studied, in other words, are fairly typical. For those reasons, the measurements, which Turner describes as preliminary findings, are concerning to other scientists.




“That’s a negative effect on the human body, there’s no doubt about it,” says Larry Kalkstein, a bioclimatologist at the University of Miami who was not involved in Middel and Turner’s research. The extra heat implied by their findings, he says, would makes people sweat more, and force them to drink more water. For the most vulnerable populations, like the elderly, homeless, and obese, it could contribute to respiratory failure, heart attack, heat strokes, and under extreme conditions, death.


***


Pedestrians walk past a digital thermometer reading 113 degrees Fahrenheit in the Canoga Park section of Los Angeles in August 2015. (Richard Vogel/AP)




The results, which were provided exclusively to CityLab in advance of their publication by the American Meteorological Society later this month, are likely the first real-world, empirical assessments of human comfort—or discomfort—caused by cool pavements in any American city. They’re not a complete surprise. Modeling studies have predicted that reflected energy could be absorbed by pedestrians, and in hot, dry cities, make people feel warmer. One such study, conducted for El Monte, an L.A. suburb, suggested pedestrians could avoid those heat effects if they were protected by shade trees, and more than 16 feet away from the road. George Ban-Weiss, one of the co-authors and an environmental engineer at USC, is also measuring L.A.’s cool pavements, separate from Middel and Turner.


Despite the implications for pedestrians, city officials are continuing on with the program. Cool pavements are an anchor of the mayor’s plan to reduce the urban heat island by 3 degrees by 2035. Supporters are excited about the potential cooling benefits. Moreover, L.A. is the archetypal American road city, blanketed in thousands and thousands of miles of asphalt roads. They’re a symbolically important place to make an impact on urban heat.  


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https://www.citylab.com/environment/2019/10/cool-pavement-materials-coating-urban-heat-island-research/599221


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