How Reflective Paint Can Combat The Urban Heat Island Effect

Published: Friday, July 24, 2020 - 12:44pm
Updated: Friday, July 24, 2020 - 12:45pm
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MARK BRODIE: Phoenix has started a pilot program aimed at mitigating the urban heat island by applying cool pavement to a number of streets and a park. City officials say the cool pavement has the potential to reverse increasingly warmer nighttime temperatures. Reflective paint has also been tried in different cities on other surfaces, including roofs and sidewalks. The idea is the paint will make those surfaces cooler and make them more comfortable for people. But new research shows it might not always be that effective at the second part of that equation. My next guest was part of a team studying this during a project in California's San Fernando Valley. Urban climate researcher Ariane Middel is an assistant professor in ASU's School of Arts, Media and Engineering. She joins me. And Ariane, why is reflective paint seen as something that could reduce the impact of urban heat islands?

ARIANE MIDDEL: Reflective pavement works in a way that a highly effective coating is applied onto a surface, say, an asphalt road, which usually has a very low albedo or reflectivity. And as you apply this reflective coating, what happens is that more of the energy that comes in from the sun and hits that surface is actually reflected back into the atmosphere and not stored in the surface as heat.

BRODIE: So what types of colors and what types of surfaces are we talking? Like how different is that type of surface than what we typically think of as a road?

MIDDEL: If you look at a typical road, say, a road, an asphalt road that is fairly new, that could have albedo or reflectivity of about .08, which means 8% of the energy that hits the surface is being reflected back. If you look at a surface such as a concrete sidewalk, on the other hand, that's a little bit lighter if you look at it — if you compare it visually, it has a lighter color. So those surfaces usually have an albedo of about .2, .3. So 20 to 30% of the energy is reflected back into the atmosphere. So those cool pavements or those coatings are typically applied to asphalt roads to increase the albedo of .1, two,.2 or .3.

BRODIE: Do we know how much that can reduce the temperature? Like, do we know how much of an impact this can have?

MIDDEL: Typically, if you look at a very hot summer day, like the one that we looked at in the study, temperatures were in the range of, say, about 9 degrees Fahrenheit in the afternoon. You can get a surface temperature cooling as much as 10 degrees Fahrenheit from this highly reflective paint.

BRODIE: Now you created a device called MaRTy to measure two streets with heat reflective paint in the San Fernando Valley in California. How did that work? What you find?

MIDDEL: We have this device called MaRTy, and it's essentially a garden card that has a bunch of metrological sensors attached to it. And one specialty of MaRTy is that MaRTy doesn't just measure air temperature, which you would get from your weather forecast or from the weather station at the airport. But MaRTy actually measures the amount of radiation that hits a human body. So MaRTy can measure how much radiation is actually reflected back from this cool pavement at a person as the person is walking over the road or over the sidewalk or any other given surface.

BRODIE: And what did those readings tell you? Was there anything surprising in what you found?

MIDDEL: We found that the cool pavement was actually doing its job. So in terms of reducing the surface temperature, we found a reduction of about 7 to 10 degrees Fahrenheit in the afternoons. If you were to touch the asphalt road that has been coated with the highly reflective coating, it would be 7 to 10 degrees Fahrenheit to the touch cooler than if you were to touch the regular asphalt road. But at the same time, MaRTy measured that because there is an increase in the reflection of the energy back into the atmosphere, and this reflection actually hit a person that walks over this, over this road. So the increase in energy on this person or the heat load on this person is up to 7 degrees Fahrenheit or 4 degrees Celsius. So if you walk over this highly reflective paint, you would feel an in — slight increase in temperature.

BRODIE: Which sounds like it kind of defeats the purpose of having that reflective paint there in the first place.

MIDDEL: Yeah, I think, I think with reflective paint, it's, it works really, really well and has been tested very well on cool roofs, for example. So a lot of efforts have been conducted to, to place this coating on top of roofs where you don't have pedestrians. And it's working really well there because the roof, again, it reflects the energy back into the atmosphere. But as you are applying these measures to the ground and to the street level where you have pedestrians, you really have to think about pedestrians as well.

BRODIE: So in a city like Phoenix, where it is very sunny and there are lots and lots of days that are above 100 degrees Fahrenheit for the high temperature, what to you is the best way to maybe use this type of heat mitigation so that the temperatures can be reduced, but also you're not, as we've talked about, reflecting additional heat onto pedestrians or other people?

MIDDEL: Roofs are, in fact, the perfect location to apply reflective paint. As you move to the ground where people are, it has to be a combination of different interventions. You could envision having a mix of planting trees, putting shade structures, maybe having the reflective paint in areas where you're really worried about surface temperatures and. about touching certain areas. If you think about your pool deck. That's an area where you don't want to burn your feet. So that's a perfect place to apply the highly reflective paint because you're, you're more concerned about the surface temperature itself than the radiation that would hit a person's body. So it really depends on the location and on the context of the location. If it's, you know, what strategy you would like to apply.

BRODIE: Yeah, really sounds like context and what, what you're trying to achieve matters here. As you reference with the pool deck, you want to make sure your feet don't get burned. And if the tradeoff is that the rest of your body is a little warmer, well, that's the tradeoff you have to make. Whereas on a street or a sidewalk, the calculation might be a little different.

MIDDEL: If you think about bus stops, too, there are, oftentimes you have these metal benches on bus stops. So that would be a perfect place to apply cool paint because that would help people to sit down without burning themselves. But then if you think about a place such as the center of the road, that, in areas where you have a lot of pedestrians that are walking next to the road, may not be the best area. But we honestly don't know. I think we need much more testing to really know what's happening on the ground.

BRODIE: All right. That is Ariane Middel, an urban climate researcher, an assistant professor in the School of Arts, Media and Engineering at ASU. Ariane, thanks for your time. I appreciate it.

MIDDEL: Thank you so much for having me.

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