VPT Practice Test 2

The Urban Heat Island

¹On a summer afternoon, a downtown street can feel noticeably hotter than a field just a few miles outside the city. Scientists call this difference the urban heat island effect. The term describes a city, or part of one, that stays warmer than the less developed land around it. The effect has no single cause; instead, several features of city life combine to trap and produce heat.

²The most important cause is the material that covers a city. Asphalt roads, dark rooftops, and concrete sidewalks absorb much of the sunlight that strikes them and store that energy as heat. Grass and soil, by contrast, hold water, and as that water evaporates, it carries heat away from the ground. Because cities replace so much vegetation with pavement, they lose much of this natural cooling.

³Buildings add to the problem in other ways. Tall structures packed closely together block breezes and trap warm air between them. Air conditioners, vehicles, and factories also release waste heat directly into the streets. Interestingly, the difference between city and countryside is often greatest after sunset, when pavement slowly releases the heat it stored during the day while open fields cool quickly.

⁴The consequences reach beyond discomfort. Higher temperatures raise the demand for electricity, which increases energy costs and can strain power systems during heat waves. Hot nights also give people's bodies less chance to recover from daytime heat, a particular danger for older adults and for people without air conditioning.

⁵Cities are not powerless, however. Planting trees provides shade and restores some of the cooling that vegetation offers. Roofs coated with light-colored, reflective materials absorb less sunlight than dark ones. Some cities are also testing paving materials designed to stay cooler. None of these steps can make a city as cool as a forest, but together they can make summer streets safer and more livable.