Keeping Cities Cool

The use of pervious concrete has become an attractive way to improve drainage in the same areas that used to constrain it.

Before all the stoplights, parking lots and high rises, trees provided shade and, along with other vegetation, released water into the atmosphere that evaporated and cooled the ground and surrounding air. Surfaces that were once permeable and moist are now impermeable and dry. According to the U.S. Environmental Protection Agency (EPA), the sun can heat up dry, exposed urban surfaces such as pavements to temperatures 50–90°F hotter than the air. Meanwhile, shaded and moist surfaces remain close to air temperatures. In downtown areas, some North American cities are up to 9°F warmer than surrounding suburban and rural areas.

Moreover, the urban heat island effect doesn’t let up at night. The slow release of heat from urban infrastructure keeps the heat on even after sunset. In fact, temperatures after sunset can be as much as 22°F warmer in urban areas than the surrounding area. Why is it important? The EPA estimates that if current heat island trends continue, cities will be 10°F warmer by 2050. As temperatures rise, cooling demands and electrical consumption increase proportionately. EPA-supported research indicates that 5 to 10 percent of current electricity demand is used to compensate for the heat island effect alone. The increased demand adds pressure to the electrical grid during peak periods. Additionally, elevated temperatures from urban heat islands can affect a community’s environment. Due to the increased energy usage, there are greater emissions of air pollutants and greenhouse gas emissions.

This Is How

It is well known that lighter surfaces tend to reflect solar light and heat while dark surfaces tend to absorb light and heat. A measurable parameter of solar reflectance is albedo, which is simply the percentage of solar energy or short-wave radiation (typically visible light reflected by a surface). A higher albedo signifies greater ability to reflect light away; thus, greater albedo reduces the amount of solar energy absorbed by a structure and keeps it cooler.

Concrete pavements provide excellent reflectance, increasing albedo and effectively contributing to lowering urban temperatures.