Paving the way to a cooler future
An ASU engineer explores ways to reduce the urban heat island effect
This article originally appeared in Chain Reaction, an ASU science magazine and website
Do you ever walk across a parking lot on a summer night and feel heat radiating upward? This happens because the pavement soaks in the sun’s heat during the day. It holds onto that heat and releases it well into the night.
Cities contain a lot of pavement and other man-made materials that hold onto heat. As a result, they can be as much as 20 degrees Fahrenheit hotter than the surrounding countryside. This temperature difference is called the urban heat island effect.
High temperatures cause a spike in energy use as people try to keep their houses cool. This increases the air pollution and greenhouse gases released from power plants, which are harmful to the environment. The heat island effect also causes more heat-related health problems. And of course, extreme heat is just plain uncomfortable.
In the heat of the night
Heat islands develop as cities and towns grow, replacing plants and natural landscapes with roads, buildings and pavement. Built-up areas do not hold water as well as the natural landscape, causing them to collect more heat and hold onto it longer. These man-made materials are major contributors to the heat island, particularly roads.
“If you look at the surfaces in an urban city like Phoenix, sidewalks, parking lots, roadways and highways are about 40 to 45 percent of the overall landscape cover,” says ASU associate engineering professor Kamil Kaloush.
“That’s why we should think about roadways when we talk about the urban heat island effect.”
Kaloush is a faculty member in the School of Sustainable Engineering and the Built Environment, one of ASU’s Ira A. Fulton Schools of Engineering.
He’s also director of ASU’s National Center of Excellence for SMART Innovations, which looks for engineering solutions to environmental problems. (SMART stands for Sustainable Materials and Renewable Technologies.)
Researchers at the center study alternative pavement materials and designs. They want to keep paved surfaces cooler during the day and reduce the amount of heat they release in the evening.
“When I think about the urban heat island, I think about a nighttime phenomenon,” says Kaloush. “During the day most materials are hot. But what we’re concerned with here in the Southwest region is nighttime mitigation. When people go to their homes and get active with outdoor activities, you really want to have a cool environment as much as possible.”
Density makes the difference
Kaloush has found that some types of pavement absorb more heat and hold onto it longer than others. Two basic properties cause these differences: a material’s density and its reflectivity.
Density is how tightly a material is packed into a space. It is measured as mass per unit of volume. For instance, a rock is denser than a crumpled piece of paper of the same size.
When it comes to pavement, density is determined by the amount of space between each piece of rock, or aggregate, that the pavement is made of.
“Usually dense materials conduct heat at a faster rate,” says Kaloush. “Depending on how thick and how large a material is, it can retain heat much longer than a porous material.”
A porous material is the opposite of a dense one. It has more space between its components, which allows heat to escape faster.
Think of it this way: If you’re trying to make your way through a forest, but the trees are tightly packed, it will take you much longer to get out than if the trees were spread out. The same principle applies to materials. If there’s not a lot of space to move, the heat will stay put.
“With dirt or gravel, it might be hot during the day but at nighttime it cools down because it is loose material with air pockets,” says Kaloush. “So anytime you have a large, dense mass, it will have the tendency to conduct and keep that heat in the material, slowly re-emitting it into the atmosphere.”
Reflectivity is how much light a surface reflects. It also plays an important role in a material’s temperature. Materials that reflect more stay cooler. White or light-colored surfaces reflect more light than dark ones.
Highway hot spots
So if we know what contributes to the heat island effect, why aren’t all of our roads made of light-colored, porous materials?
“You have to be careful,” cautions Kaloush. “You really have to think about the project and the issues at hand and select the parameters that are going to get you the best performance for the daytime or nighttime. There’s not really one solution that fits all.”
Roads need to be durable against the wear and tear of traffic. Denser materials, while hotter, hold up better under lots of use. Simply making materials lighter in color presents challenges as well. The light would be reflected, lowering the temperature of the pavement, but in some cases it could be reflected up toward people walking on it. Kaloush compared it to getting sunburned at a ski resort.
How and where materials are used also makes a difference in how much heat they release in the evening. Kaloush discovered this when he saw a nighttime satellite image of Phoenix, which revealed the hottest surfaces of the city.
“I was surprised to see that you have a good footprint of the road network standing out as a hot surface at night,” Kaloush said. “So I was very interested to look into this further. More importantly, I was looking at different highways and I was seeing some are cooler than others. So I was really curious to know what makes that section of a highway cooler than another section.”
He found that highways that run lower in the ground than others tend to be hotter. The sound walls that surround them to trap traffic noise also trap the sun’s heat. They release it into the atmosphere later than highways at ground level or without sound walls.
Pavement materials key to solution
The amount of traffic on a highway also plays a role in its temperature. Cars and trucks on the road can increase air temperature from engine heat and emissions. However, they actually make the pavement surface cooler.
“In the beginning, I thought the traffic would only contribute to the heat, with the heat of engines and such—but I was wrong,” says Kaloush. “The movement of the cars actually helps cool the surface of the roadways. So if you have more traffic it cools down much faster, compared to a parking lot with no traffic.”
The materials used to make the pavement, where they are located, and how we use them all affect the urban heat island effect.
“Roadways are a big part of the urban landscape, and we can design them in different ways that actually work for our benefit,” says Kaloush. “Having all roadways white or black is not going to be the solution for all. We need to think through and evaluate different designs that are best suited and effective for what you are trying to achieve overall.”
Written by Pete Zrioka