Academic Reading — All-Types Test 3

Reading passage

The Overheated City A Anyone who has crossed a sunlit car park in high summer knows that a built-up area can feel markedly hotter than the open countryside beyond it. This everyday sensation has a name. Climatologists call it the urban heat island, a phenomenon in which towns and cities register air temperatures several degrees above those of the fields, woods and water that surround them. On a still night the difference can reach seven or eight degrees Celsius, and it is at night, rather than at midday, that the contrast is usually sharpest. The reasons lie less in the sun overhead than in the fabric of the city itself. Early studies of the effect, conducted more than a century ago in the fog-bound capitals of northern Europe, first established that the warmth was a property of the settlement and not merely of the weather passing over it. Since then, observations from hundreds of cities across every inhabited continent have confirmed that the pattern is remarkably consistent, whatever the local climate. B The chief culprit is the material from which streets and buildings are made. Brick, asphalt and concrete are dense and dark, and they behave very differently from soil and vegetation. Throughout the day these surfaces absorb solar radiation and store it as heat deep within their mass. When the sun sets, the stored energy is released slowly back into the air, so that the built environment continues to radiate warmth for hours after a meadow has cooled. A green field, by contrast, loses much of its incoming energy to evaporation, because the moisture held in soil and leaves is drawn upward and carried away as vapour. Cities, having stripped away that moisture and sealed the ground beneath an impermeable crust, lose this natural cooling mechanism almost entirely. C Shape matters as much as substance. A city is not a flat plate but a landscape of canyons, its narrow streets flanked by tall walls that trap radiation through repeated reflection. Heat that might otherwise escape upward bounces from façade to façade and is gradually reabsorbed. The same walls obstruct the wind, and without a breeze to carry warm air away, temperatures climb still higher. To this must be added the heat that the city generates for itself: engines, factories, air-conditioning units and the bodies of millions of people all pour warmth into the surrounding air. On a busy weekday this manufactured heat is far from trivial, and in the densest districts it may rival the contribution of the sun. D The consequences reach well beyond discomfort. During prolonged hot spells the heat island can turn a city into a genuine hazard, for elevated night-time temperatures deny residents the cool respite that ordinarily allows the body to recover. The elderly, the very young and those already unwell are the most exposed, and mortality figures climb accordingly. Higher temperatures also drive up the demand for electricity as air-conditioners labour through the night, and the power stations that answer that demand emit yet more heat and pollution, tightening a vicious circle. Warm, stagnant urban air, moreover, hastens the chemical reactions that produce ground-level ozone, so the heat island quietly degrades the quality of the air that citizens breathe. E Fortunately, the same understanding that explains the problem also points towards its remedy. Planting trees along streets and preserving parks restores the cooling power of evaporation while casting welcome shade. Painting roofs in pale colours, or covering them with living vegetation, reflects sunlight that a dark surface would have absorbed. Some municipalities now surface their pavements with lighter, more reflective materials, and encourage architecture that admits the passage of air. None of these measures is a complete cure, and each carries a cost, but together they can shave several degrees from a summer night. As the world's population gathers ever more densely into cities, and as the global climate warms, the modest science of keeping the streets cool is likely to become a matter of increasing urgency. What was once a curiosity noted by a handful of meteorologists has become a central concern of those who design the places where most of humanity now lives.