IELTS Reading: Matching information

On a calm, clear summer night, a thermometer in the centre of a large city can read several degrees higher than one in the fields a short drive away. This ‘urban heat island’ is not a recent discovery. In the early nineteenth century Luke Howard, an amateur meteorologist better remembered today for giving the clouds their names, compared temperatures recorded in London with those from the countryside around it and found that the city was, on average, the warmer of the two, especially at night. Two centuries later, with more than half of the world’s population living in towns and cities, what Howard noted as a curiosity has become a pressing practical concern. Heat islands raise the demand for electricity, worsen air pollution and, during heatwaves, cost lives.

The most important cause lies in what cities are made of. Asphalt, concrete, brick and dark roofing absorb most of the sunlight that falls on them: a fresh asphalt road may reflect as little as five per cent of it. Grass, crops and soil reflect rather more, and plants use much of the remainder to evaporate water rather than to warm the ground. By day, the absorbed energy is conducted into the body of roads, walls and roofs and stored there, which is why a pavement can be too hot to touch by late afternoon. After sunset, when the open countryside cools rapidly, these materials go on releasing their stored heat into the air for hours. The gap between urban and rural temperatures is therefore usually greatest not in the heat of the afternoon but during the night.

The shape of a city adds to the problem. In a street lined with tall buildings, sunlight that would otherwise be reflected back towards the sky strikes wall after wall and is largely absorbed, and at night the buildings block much of the open sky, so heat that would escape upwards is trapped between them. Tall buildings also slow the wind that might carry warm air away. Replacing fields and woodland with hard surfaces removes a natural cooling system as well: plants give off water vapour through their leaves, a process that draws heat from the air. Finally, cities produce heat of their own, from vehicles, industry and, increasingly, air conditioners, which cool the inside of buildings by pumping heat out into the street. The hotter the streets become, the more air conditioning is used, which makes the streets hotter still.

Measuring a heat island is less simple than it sounds. The traditional method compares a weather station in the city with one in the surrounding countryside, but the answer depends heavily on exactly where the two stations happen to stand: one beside a car park and another in a sheltered valley will exaggerate the difference. Researchers therefore also carry out ‘traverses’, driving or cycling across a city with instruments mounted on a vehicle, which reveal how temperature varies from one street to the next. Satellites offer the widest view, but they record the temperature of surfaces such as roofs and roads, not of the air, and in full sun a dark roof can be tens of degrees hotter than the air just above it. ‘Satellite images are superb for finding hot spots,’ says Dr Anika Rowe, an urban climatologist, ‘but they show where surfaces are hot, which is not the same as where people are hot.’

The consequences for health are serious. The European heatwave of 2003 is estimated to have caused tens of thousands of deaths, many of them in large cities. What makes urban heat so dangerous is not so much the afternoon peak as the absence of relief at night: a body that cannot cool down during sleep begins the next day already under strain. Older people are most at risk, particularly those who live alone, along with people with heart or lung conditions and those who work outdoors. Nor is the heat shared equally. In many cities, poorer districts have fewer trees and more tarmac than wealthier ones and can be several degrees hotter on the same afternoon, so that heat adds to disadvantages that already exist.

Cities have three broad ways of fighting back. They can make surfaces more reflective, most simply by painting roofs white; they can add vegetation, which shades the ground and cools the air through evaporation; and they can bring water into streets and squares. The most successful programmes combine all three. One mid-sized city in southern Europe began by mapping surface temperatures block by block and comparing the map with data showing where elderly residents lived alone. In the twelve neighbourhoods where heat and vulnerability overlapped, it offered building owners free roof coatings, replaced the asphalt of school playgrounds with grass and permeable paving, and planted more than 4,000 street trees. Drinking fountains and shaded benches followed. After three summers, the city reported that afternoon air temperatures in the treated streets had fallen by about 1.5 degrees compared with similar streets elsewhere.

None of these measures is without drawbacks. Reflective surfaces send sunlight back upwards, and at street level some of it strikes pedestrians; one study of a reflective pavement found that although the air above it was slightly cooler, people walking across it at midday absorbed more heat. In cities with cold winters, white roofs can also raise heating bills by reflecting away warmth that would have been welcome. Trees have limitations of their own. A sapling may take twenty years or more to provide meaningful shade, and it needs water during the very droughts when cooling matters most. In narrow, busy streets, a dense canopy can trap vehicle exhaust close to the ground. Fountains and misting systems, meanwhile, consume water that dry cities can ill afford. There is no single cure, only a set of partial remedies that must be matched carefully to each street.