托福听力

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Listen to part of a lecture in an environmental science class. Today we're going to talk about urban heat islands, a phenomenon where cities are significantly warmer than surrounding rural areas. This happens because concrete, asphalt, and buildings absorb and re-emit heat from the sun more than natural landscapes like trees and soil. For example, on a hot summer day, a city can be up to 7 degrees Fahrenheit warmer than nearby countryside. Now, one interesting solution that some cities are adopting is green roofs. These are rooftops covered with vegetation and soil. They help in two main ways: first, plants provide shade and release moisture through a process called transpiration, which cools the air. Second, the soil layer acts as an insulator, reducing heat absorption. A study in Chicago showed that green roofs can lower rooftop temperatures by up to 40 degrees Fahrenheit on a hot day. However, green roofs are not a one-size-fits-all solution. They require significant upfront costs and regular maintenance, such as watering and weeding. Also, in very dry climates, the water needed for the plants can be a drawback. So while promising, they are just one tool in our fight against urban heat.