Urban Trees: Strategies for Reducing Urban Heat Island in Cities

by Veronica Westendorff, PLA, ASLA, SITES AP

Heat Island Effect Diagram
Parks, open land, and bodies of water can create cooler areas within a city because they do not absorb the sun’s energy the same way buildings and paved surfaces do. / image: U.S. Environmental Protection Agency

It doesn’t take a scientist to know. In the middle of summer, walking down the streets of almost any city, there is a notable wave of heat rising off the sidewalks where old trees have deteriorated or been removed and either no replacements or new, young trees which barely cast a shadow across the surface of the walkway are in their place. In contrast, sidewalks and streets lined with mature trees offer respite for pedestrians and cyclists. We cross the street to stand in the shade of a building or under the cooling canopy of the trees around us.

While this change in temperature, referred to as Urban Heat Island (UHI), is noticeable during the day, the real impact of UHI is felt at night, when the sun has set and the impervious surfaces around us hold and slowly release the heat of the day (Norton et al., 2015). This heat begins to compound, and the following day begins at a higher temperature, increasing the overall heat in these areas. Differences in temperature may vary by as much as 22 degrees Fahrenheit (Urban ReLeaf, 2016) and are markedly higher in urban areas with more impervious surfaces and less green space. Land cover type plays a large role in moderating these effects. Impervious areas and sealed soil act almost the same in the creation of UHI, while greened areas that include shrub cover and areas with trees and urban forests lessen the effects of UHI (Norton et al., 2015).

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