Why Cities Run Hot

THE URBAN HEAT ISLAND: WHY CITIES STAY WARM A. On a summer evening, a person driving from the countryside into the centre of a large city may notice the temperature rising by several degrees along the way. This pattern, known as the urban heat island effect, occurs in cities across Canada, from Montreal to Vancouver. It is caused mainly by the materials that make up the urban landscape. Asphalt, concrete and dark rooftops soak up heat from the sun all day and give it back slowly once the sun goes down, so city neighbourhoods often cool down much later than nearby farmland or forest. On the hottest nights, the difference can be enough to decide whether people sleep comfortably. Researchers often measure the effect by comparing readings taken at the same moment in a downtown core and at a rural weather station outside the city. B. Other features of city life add to the problem. Tall buildings that stand close together can trap warm air and block breezes that would otherwise carry heat away. Cars, air conditioners and industrial equipment all release waste heat directly into the streets. At the same time, cities usually have fewer trees and less open soil than rural areas. Plants cool their surroundings by releasing water through their leaves, a process similar to sweating, and when vegetation is replaced by pavement, this natural cooling is lost. Large parking lots and flat commercial roofs, which cover a surprising share of many suburbs, add further surfaces that soak up heat. C. The consequences are more than a matter of comfort. During heat waves, the extra warmth can be dangerous, especially for older adults, young children and people with chronic illnesses. Those who live in older apartment buildings without air conditioning are at particular risk, because indoor temperatures can remain high throughout the night, giving the body no chance to recover. Higher temperatures also increase demand for electricity and can worsen air quality, since heat speeds up the chemical reactions that form smog. Hospitals and paramedics often see a rise in heat-related calls when hot days follow one another for several days in a row. D. Many Canadian municipalities are now looking for ways to cool their streets. Planting trees along sidewalks and in parking lots provides shade and restores some natural cooling. Light-coloured or reflective roofing materials can bounce sunlight away instead of absorbing it, and green roofs covered with plants can help insulate the buildings beneath them. Some cities have opened cooling centres in libraries and community halls where residents can rest during extreme heat, and others have added splash pads and drinking fountains in parks. Planners note that these measures work best when they focus first on the neighbourhoods that have the least shade and the most vulnerable residents. Some programs also offer residents free trees to plant in their own yards.