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Street smarts required in heat mitigation

May 5, 2020

ASU researchers investigate how reflective coatings on city streets affect pedestrians when the heat is on

One day last July, Ariane Middel and two other Arizona State University researchers headed west on Interstate 10. Squeezed inside their van were MaRTy 1 and MaRTy 2, mobile biometeorological instrument platforms that can tell you exactly what you feel in the summer heat. All five were destined for Los Angeles. 

The researchers and their colleagues were headed to L.A. to start investigating how solar reflective coatings on select city streets affected radiant heat and, in turn, pedestrians’ comfort on a typical summer day.

The Los Angeles Bureau of Street Surfaces has pioneered the use of solar reflective coatings in a quest to cool city streets. 

The idea is, if you coat a street with a lighter color than traditional pavement black, it will actually lower the surrounding temperatures.

But Middel and her collaborators now wanted to see what effect reflective coating had on pedestrians.

“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, assistant professor in the ASU School of Arts, Media and Engineering and a senior sustainability scientist in the Julie Ann Wrigley Global Institute of Sustainability. “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.”

Thanks to remote-sensing satellites, decades of data exist on the Earth’s land surface temperature; that is, how hot a single point on the Earth’s surface would feel to the touch. But that data should not be confused with near-surface ambient and radiant temperature, the heat that humans and animals “experience,” said Middel, lead author of the study and director of ASU’s SHaDE Lab, which stands for Sensable Heatscapes and Digital Environments.

Ariane Middel

Ariane Middel with a MaRTy unit on the Tempe campus. Photo by Ken Fagan/ASU Now 

The researchers’ study is the first to measure the thermal performance of solar reflective coatings using instruments that sense meteorological variables relevant to a pedestrian’s experience: radiant heat, ambient temperature, wind and humidity.

The researchers focused on two variables, surface temperature and radiant temperature over highly reflective surfaces. They took MaRTy 1 and 2 on hourly strolls through a Los Angeles neighborhood to measure a pedestrian’s heat exposure over regular asphalt roads, reflective coated roads and sidewalks next to the roads.

MaRTy, which stands for mean radiant temperature, looks like a weather station in a wagon. The station measures the total radiation that hits the body, including sunlight and the heat emitted from surfaces like asphalt.

The study showed 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.

“So, if you’re a pedestrian walking over the surface, you get hit by the shortwave radiation reflected back at you,” Middel said.

The study also found that the coating didn’t have a big impact on air temperature, only half a degree in the afternoon and 0.1 degrees Celsius at night.  

The upshot, said V. Kelly Turner, assistant professor of urban planning at UCLA and the study’s co-author, is that to cool off cities, urban climatologists and city planners need to focus on different solutions or combinations of solutions depending on a desired goal. 

“The solutions are context dependent and depend on what you want to achieve,” Turner explained.

A solution that addresses surface temperature is not necessarily suited to the reduction of building energy use. For example, if you want cooler surface temperatures on a playground because children are running across its surface, a reflective coating would be best. But if you want to reduce the thermal load on people, planting trees or providing shade would be more effective.

But what happens if you combine trees with cool pavement? Does the cool pavement lose its ability to reduce surface temperature? Or perhaps the cool pavement is costly to maintain when the trees drop their leaves?  

“So, reflective coating is not a panacea,” Turner said. “It’s one tool.”

It should also be noted that temperature is a multifaceted measurement of heat. Surface temperature, ambient temperature and mean radiant temperature are distinct from one another and require distinct solutions when it comes to mitigating heat.

“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 researchers report their findings of the Los Angeles study in, "Solar reflective pavements — A policy panacea to heat mitigation?” which was published on April 8, 2020 in the journal Environmental Research Letters. Co-authors on the paper include Florian Schneider and Yujia Zhang of ASU, and Matthew Stiller of Kent State University.

Top illustration by Alex Cabrera/ASU Now

Science writer , Media Relations and Strategic Communications

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Shadow hunting: ASU urban climatologist helps us keep our cool

ASU prof maps campus to find the shadiest route.
Shade can make you feel up to 18 degrees cooler, ASU urban climatologist says.
May 30, 2017

Ariane Middel builds mobile weather station to map the most comfortable route through campus on a hot summer day

In an Arizona summer, the best parking spot is not the one by the door. It’s the one a quarter-mile away under a tree.

Ariane Middel, Arizona State University urban climatologist and hunter of shadows, knows this and has created a method to show us a cool way through the shade.

Middel, an assistant research professorMiddel is also a senior sustainability scientist in the Julie Ann Wrigley Global Institute of Sustainability and on the honors faculty at Barrett, The Honors College at Arizona State University. The School of Geographical Sciences and Urban Planning is an academic unit of the College of Liberal Arts and Sciences. in the School of Geographical Sciences and Urban Planning and a faculty affiliate of the Urban Climate Research Center, has developed a tool that will someday show pedestrians the shadiest — and hence coolest — route to their destinations. It also will tell planners and architects where they should throw shade.

She and a team of computer scientists used high-resolution Google Earth images to generate 180-degree “fish-eye” views to calculate whether a specific location would be in the sun or shade during a certain time of day. It’s a big-data approach that seizes upon Middel’s research.

Two years ago, she led a study to investigate thermal comfort at the university’s student union in Tempe. She and her team found that shade was the most important factor for comfort. Shade can make you feel up to 18 degrees cooler, while air temperature might only vary by 5 degrees. That got her interested in studying shade further.

“Keeping cool and staying comfortable is really difficult here in the desert in summer,” Middel said. “Out of all the variables that determine how comfortable you feel, shade is the most important factor, more important than air temperature, relative humidity, even the clothing that you’re wearing. Shade is the most important factor. That led me to look into the impact of shade on your thermal comfort when you’re outdoors.”

Video by Ken Fagan/ASU Now

The issue with shade is that it “travels” during the day, depending on the position of the sun. If you look at a north-south urban canyon, you will have shade from the building to the east in the morning, almost no shade at noon, and shade to the west in the afternoon. Middel wanted to look at the distribution of shade on campus and also assess the implications for thermal comfort.

To do it, Middel built a mobile weather station. (She calls it a mean radiant temperature cart. It looks somewhat like WALL-E.) Using the cart, Middel generated fish-eye views for the Tempe campus (one view every 5 yards) and then calculated thermal comfort for two times of day, 9 a.m. and 5 p.m., for a hot summer day last August.

“To make sure that the outputs from the model that we develop make sense, we have to ‘validate’ the outputs using real-world measurements,” she said. “I built a mobile platform that can measure thermal comfort (using three net radiometers that, simply put, measure the direct sunlight and the heat from surfaces that hit the human body from all sides).”

The next step was using the thermal comfort maps to find the most comfortable route from the Brickyard building on Mill Avenue to the Memorial Union. The algorithm suggested a more shaded route — longer than a direct route, but with much more shade.

“What we found measuring thermal comfort using the mean radiant temperature cart last summer was that thermal comfort varies a lot on campus,” Middel said. “It varies much more than air temperature.

“Mean radiant temperature, or thermal comfort, can be up to 60 degrees Celsius in the sun, but then it’s only 30 degrees Celsius in the shade. This is how you would feel this temperature as opposed to the air temperature you get from the weather report. There’s a huge variation due to trees, shade from buildings, shade from overhangs, different surface materials — all of this can be measured using this cart.”

Knowing the thermal comfort conditions at a very fine scale can be very helpful to architects and planners. It can be used in urban planning to assess whether an area has enough shade or maybe needs more shade.

University landscape architect Byron Sampson constantly hears pleas for more shade on campus. A tool like Middel is developing would be very useful to him, he said.

“If the interface will let the user know the shadiest route, we can learn where interventions need to take place and predict pedestrian movement through the university during different seasons,” Sampson said. “The paths do change and evolve over time.  As we look at the form of the built and open spaces in terms of development, we should be able to influence the architectural form to increase the shaded routes in areas where trees cannot be used to do the same.”

ASU urban climatologist Ariane Middel

This and top photo: ASU urban climatologist Ariane Middel uses her mean radiant temperature cart to measure thermal comfort of different spots on campus. Photos by Ken Fagan/ASU Now

As the Phoenix metro area becomes more urban and densities increase, shade will become the asset an area is judged by, Sampson said.

“If you look at the older neighborhoods with large trees (hint: more shade), the property values seem to be higher and stable,” he said. “This could also be a means to have interventions in poorer areas to increase the comfort level and significantly reduce the heat island in the Valley.”

Development on the tool continues. Middel has been using an existing thermal comfort model to translate the fish-eye information into thermal comfort. The model isn’t suited for large areas, because it only works for one fish-eye photo at a time.

“We are currently developing our own outdoor thermal comfort model,” she said. “Once working, (it) will be able to calculate thermal comfort at street level for any city where Google Street View data and basic weather station data are available.”

As Shakespeare said in “A Merchant of Venice,” “Some there be that shadows kiss; Such have but a shadow’s bliss.”


Scott Seckel

Reporter , ASU News