When heat waves hit, people start looking for anything that might lower the temperature. One solution is right beneath our feet: Pavement. Think about how hot the soles of your shoes can get when you’re walking on dark pavement or asphalt. A hot street isn’t just hot to touch — it also raises the surrounding air temperature.
Research shows that building lighter-colored, more reflective roads has the potential to lower air temperatures by more than 2.5 degrees Fahrenheit and, in the process, reduce the frequency of heat waves by 41 percent across U.S. cities. But reflective surfaces have to be used strategically — the wrong placement can actually heat up nearby buildings instead of cooling things down. As researchers in MIT’s Concrete Sustainability Hub, we have been modeling these surfaces and determining the right balance for lowering the heat and helping cities reduce their greenhouse gas emissions. Here’s how reflective pavement works and what cities need to think about.
Why surfaces heat up
All surfaces, depending on the amount of radiation they absorb or reflect, can affect air temperatures in cities. In urban areas, about 40 percent of the land is paved, and that pavement absorbs solar radiation. The absorbed heat in the pavement mass is released gradually, warming the surrounding environment. This can exacerbate urban heat islands and worsen the effects of heat waves. It’s part of the reason cities are regularly a few degrees warmer in summer than nearby rural areas and leafy suburbs.
To estimate a pavement’s reflectivity, we use a measure called albedo. Albedo refers to the proportion of light reflected by a surface. The lower a surface’s albedo, the more light it absorbs and, consequentially, the more heat it traps.