Academic Reading — All-Types Test 18

Reading passage

Why Cities Run Hot: The Urban Heat Island A On a still summer night, the difference in temperature between the centre of a large city and the open countryside a few kilometres away can be startling. Readings taken in the built-up core are often several degrees higher than those from surrounding fields, and on occasion the gap has exceeded ten degrees Celsius. This persistent warmth is known as the urban heat island, a term first applied in the nineteenth century by observers who noticed that towns seemed to trap heat long after the sun had set. The effect is not a matter of a single hot afternoon; it is a durable feature of the climate that a city creates around and above itself, most pronounced once darkness falls and the countryside cools quickly while the city lags behind. Calm, cloudless conditions sharpen the contrast further, whereas a brisk wind stirs the warm and cool air together and blurs the boundary between town and field. B The chief cause lies in the materials from which cities are built. Concrete, brick, asphalt and stone absorb solar radiation during the day and store it as heat, releasing it slowly through the evening and night. Natural surfaces behave very differently. Soil and vegetation lose much of the energy they receive through evaporation, because water drawn up by plants and held in the ground turns to vapour and carries heat away with it. A city, having replaced meadow and woodland with hard, dry surfaces, loses this cooling mechanism almost entirely. The dark colour of many urban materials compounds the problem, since dark surfaces reflect little sunlight and absorb a great deal, whereas paler surfaces bounce more of it back to the sky. C Geometry matters as much as material. The narrow spaces between tall buildings, sometimes called urban canyons, trap heat in two ways. During the day the vertical walls intercept sunlight that would otherwise strike the ground and be partly reflected upward and away; instead the radiation bounces from wall to wall and is progressively absorbed. At night the same walls block the open view of the cold sky that a flat field enjoys, so the heat stored in the fabric of the buildings cannot escape upward as freely. The result is that a densely built district holds its warmth far more stubbornly than an open one, and the taller and closer the buildings, the stronger the trapping tends to be. D Human activity adds a further layer of warmth. Vehicles, factories, air conditioners and the countless machines of daily life all discharge heat directly into the surrounding air, a contribution sometimes described as anthropogenic, meaning generated by people rather than by nature. Air conditioning is a particular irony: as residents cool their interiors, the units expel the extracted heat outdoors, raising the outside temperature and prompting yet more cooling. On the hottest days this feedback can become significant. The waste heat from human activity is modest compared with the sun, but in a dense city centre packed with engines and appliances it is far from negligible, and it is at its most influential precisely when the weather is already uncomfortable. E The consequences reach well beyond discomfort. Elevated night-time temperatures deny residents the cool respite that normally allows the body to recover, and prolonged heat is associated with a rise in illness and, during severe episodes, in deaths, particularly among the elderly and the unwell. Higher temperatures also drive up demand for electricity to run cooling systems, straining power supplies at the very moment they are most needed. The burden, moreover, is rarely shared evenly; poorer districts often have fewer trees and less open ground than wealthier ones, and so endure the sharpest heat. Yet the phenomenon is not beyond human influence. Planting trees, laying out parks, fitting reflective or pale roofing, and introducing green walls and rooftop gardens can all lower local temperatures appreciably. Because the causes are so directly the work of human hands, the remedies lie there too, and a number of cities have begun to treat the cooling of their streets as a serious matter of public health.