The urban heat island (UHI) effect occurs when urban areas are hotter than surrounding areas. This phenomenon is caused by a high concentration of dark, impervious surfaces such as roofs, walls, and roads combined with a lack of trees and green space. Tall buildings that block or slow air movement, known as urban canyons, contribute to the formation of heat islands, along with waste heat released by vehicles and air conditioning.

CRRC UHI illo FINAL CRRC Edit 2021 10 07 1
This illustration describes the factors that contribute to urban heat islands (UHI), as well as factors that help mitigate UHI. Urban heat islands occur when the temperature in urban environments is higher than surrounding areas. High surface temperatures lead to elevated air temperatures, especially at night. Heat islands increase heat-related discomfort, illness, and death. They also cause greater air conditioner use, which increases energy costs and air pollution. Urban heat has a disproportionate impact on disadvantaged communities (HSU et al., 2021; Hoffman et al., 2020; and Wilson, 2020). Image credit: Cool Roof Rating Council

Heat islands are dangerous for our communities because they increase the risk of heat-related illness and death. They also make working outside uncomfortable or even dangerous and can decrease productivity and school performance. Heat islands also increase peak power demand because air conditioning systems must work harder to keep spaces cool, which makes power interruptions more likely and increases air pollution.

Studies show that low-income communities and communities of color are impacted more severely by UHIs than other neighborhoods. This is because less investment has been made in trees, parks, and infrastructure in these areas. Check out the below video by The Verge for an example of temperature disparity between neighborhoods in New York City.

Cool Roofs and Solar-Reflective Walls as Urban Heat Island Mitigation Strategies

Cool roofs and solar-reflective walls have an important role in combating UHIs. Due to their ability to reflect heat back into the atmosphere, installing reflective materials on buildings decreases the building’s surface temperature, as well as temperatures inside the building. This means that, compared to buildings with conventional roofs and walls, less air conditioning is needed to keep occupants comfortable and buildings without air conditioning stay cooler. Cool roofs and solar-reflective walls also contribute to increased solar reflectance (albedo) of a community, which can help decrease outdoor air temperatures.

Urban Heat Island Research

Below are several recent research publications on urban heat islands, mitigation strategies, and heat disparity among communities.

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