Why Cities Run a Fever

On a summer night, anyone who has travelled from the middle of a large city to its outskirts will have noticed a distinct change in the air. In the centre, the pavements seem to radiate stored warmth long after sunset; a few kilometres out, among fields or trees, the temperature drops noticeably. This difference, which can reach several degrees, is known as the urban heat island effect, and it is becoming a major concern for planners as both cities and summer heatwaves grow. The causes are numerous, but the most significant is the nature of the materials with which cities are built. Asphalt, concrete and dark roofing absorb a great deal of sunlight during the day and release it slowly after dark, so that urban areas have little opportunity to cool down. Compounding this is the scarcity of vegetation. Plants and soil do not merely provide shade; they also cool the air through evaporation, an effect comparable to the way sweat works on human skin. Remove them, and the cooling disappears with them. Architecture adds further complications. Tall buildings packed closely together form what are often called urban canyons, which trap heat and obstruct the breezes that might otherwise carry it away. Meanwhile, the activities of the city itself, including traffic, industry and the air-conditioning units that expel hot air into the street, generate additional warmth. The result is a vicious circle: the hotter the city becomes, the more energy people use to stay cool, and the more heat their machines produce. Fortunately, the remedies are better understood than the problem once was. Planting trees along streets is among the most effective, since a mature tree can lower the temperature of its surroundings substantially. Pale or reflective roofs, sometimes called "cool roofs", return a larger share of sunlight to the sky, while green roofs and walls add insulation and shade. Some cities have gone further. Medellín, in Colombia, planted green corridors along roads and waterways, and officials there have reported that average temperatures in the treated areas fell by roughly two degrees. It would be a mistake, though, to regard such measures as purely technical. Poorer neighbourhoods typically contain fewer trees and more pavement, which means that the burden of excessive heat is rarely shared equally. Not until planners address this imbalance will cooler cities become genuinely healthier cities for all of their residents.