Anyone that’s ever been out walking on a hot summer day has probably experienced an uncomfortable phenomenon: sometimes, the heat radiated from the pavement below is just as hot as that coming from the sunlight above. In a quest to cool city streets, the Los Angeles Bureau of Street Surfaces has pioneered the use of solar reflective coatings with the idea that coating streets with a lighter color will lower the surrounding temperatures. It’s an interesting theory, and one that has attracted the attention of researcher and urban climatologist Ariane Middel.
“If you’re in a hot, dry, and sunny climate like Phoenix or L.A., the mean radiant temperature has the biggest impact on how a person experiences the heat,” explains Middel, who is also a senior sustainability scientist in the Julie Ann Wrigley Global Institute of Sustainability and the director of Arizona State University’s SHaDE lab. “The mean radiant temperature is essentially the heat that hits the human body. It includes the radiation from the sun, so if you are standing in direct sunlight you will feel much hotter than in the shade.”
Last July, Middel traveled to LA to investigate how solar reflective coatings on select city streets affected radiant heat and, in turn, pedestrians’ comfort on a typical summer day. She was accompanied by two researchers and MaRTy 1 and MaRTy 2, mobile biometeorological instrument platforms that can tell you exactly what you feel in the summer heat. The researchers took the instruments on hourly strolls through an LA neighborhood to measure a pedestrian’s heat exposure over regular asphalt roads, reflective coated roads, and sidewalks next to the roads.
They found that the surface temperature of the coated asphalt road was up to 6 degrees Celsius cooler than the regular road in the afternoon. However, the radiant heat over coated asphalt was 4 degrees Celsius higher than non-coated areas, basically negating any heat-limiting factor. The study also showed that coating didn’t have a big impact on air temperature, only half a degree in the afternoon and 0.1 degrees Celsius at night.
“We need more of these experiments,” Middel said. “There have been a lot of large-scale modeling studies on this. So, we don’t know in real life if we get the same effects. The urban environment is so complex, and models have to always simplify. So, we don’t know what really happens on the ground unless we measure, and there haven’t been these types of measurements in the past.”
The full result of their study was published in the Journal of Environmental Research Letters, in a paper titled "Solar reflective pavements — A policy panacea to heat mitigation?”