Using Reflective Surfaces to Address Urban Heat Pollution

Photo credit: Dustin Phillips, CC BY-NC-ND 2.0

As summer enters full swing throughout the United States—bringing with it record-setting heat waves, threats of power outage, and cases of heat-related illness—scientists, engineers, and urban planners are considering urban surface alternatives that might reduce the effects of heat pollution in cities.

Many cities around the world record average temperatures between 2 and 4°C warmer than neighboring rural areas. This phenomenon, known as the Urban Heat Island effect, occurs because typical urban surfaces (like pavement) absorb more heat than natural surfaces (like grass) which often characterize rural regions.

For the individuals who live in urban centers, stakes are high (and climbing higher). From 2004 to 2018, the Centers for Disease Control and Prevention (CDC) recorded an annual average of 702 heat-related deaths in the United States. As global warming increases the likelihood of extreme weather events like heat waves, the need for effective remediation against heat pollution grows. Otherwise, we could see cities throughout the world become unlivable for part or all of the year.

In a paper recently published by the journal Nature Communications, researchers from the University of Pittsburgh’s Swanson School of Engineering model the effects of reflective surface applications on conventional urban surfaces in a neighborhood. Coauthors Sushobhan Sen and Lev Khazanovich used Computational Fluid Dynamics to model air currents in a prototypical neighborhood as it was subjected to alternate spatial distributions of reflective surfaces. Their research yields promising insights that can be used to address urban heat pollution.

The team found that widespread application of reflective surfaces reduced air temperatures throughout the simulated neighborhood by up to 1.9°C, but this came with a significant investment. Alternate models revealed that temperature reductions as great as 1.1°C could be generated by adding reflective materials to 50% of existing surfaces, rather than 100%.

In cases where researchers located reflective surfaces upstream from conventional surfaces, cooler air currents penetrated the downstream homes in addition to upstream homes. Because reflective surfaces are cooler than conventional surfaces, dominant wind cools as it passes through high reflectance areas and continues to travel downstream, cooling the rest of the city at a reduced cost.
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Sen notes that strategic placement of reflective surfaces is key. The effectiveness of high reflectance surfaces declines when they are placed downstream of or parallel to the dominant wind direction—less mixing of air restricts cooling to the part of the neighborhood with reflective surfaces. In order to maximize the effectiveness of minimal resources, engineers and urban developers must evaluate both spatial distribution of reflective surfaces and dominant wind streams throughout neighborhoods. In doing so, they enable a substantial decline in city temperatures with half the material investment required of total surface application.
 
Click here to read Sen and Khazanovich’s work in the journal, Nature Communications. To learn more about heat-related deaths in the United States, click here.