Introduction to Urban Heat Islands
As cities continue to grow, so does the problem of urban heat islands (UHIs). UHIs are created when there is a lack of vegetation and an abundance of pavement and buildings, which absorb and radiate more heat than natural surfaces. This can make cities up to 10°F-15°F hotter than their surroundings.1 Tree and vegetation can mitigate this by providing shading and cooling through evapotranspiration.
Trees provide shade that helps cool the air and reduce the heat absorbed by pavement and buildings. They also evaporate water from their leaves, which cools the atmosphere through evapotranspiration. In addition, trees help reduce air pollution and improve mental health and well-being.
It is vital to plant the right tree in the right place to maximize its cooling effect. The best trees for mitigating UHIs are those with large leaves that provide dense shade. Fast-growing trees are also beneficial as they quickly offer cooling benefits.
The Causes of Urban Heat Islands
A heat island is a metropolitan area that is significantly warmer than its surrounding rural areas. The leading cause of heat islands is the replacement of natural landscapes (such as trees and vegetation) with built environments (such as asphalt and concrete). These built surfaces have a low albedo, meaning they absorb more heat from the sun than they reflect. Conversely, trees and vegetation have high albedo, reflecting more sunlight than they absorb.
Another cause of heat islands is the “heat island effect.”2 This occurs when warm air near the ground is trapped by taller buildings, causing the air to become even more generous. The island effect is most pronounced during calm weather conditions when there is little wind to mix the air around.
Figure 1 (Source: https://www.epa.gov/heatislands/learn-about-heat-islands)
(Figure 1) During the day, surface temperatures are more variable than atmospheric air temperatures, but at night, they are often similar. Because water does not absorb the sun’s energy in the same way that structures and paved surfaces do, it maintains a relatively constant temperature day and night. This is demonstrated by the dips and spikes in surface temperatures over the …
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