City ovens Cities, especially in summer, are hotter than the surrounding countryside. They are urban heat islands. Why? Because bricks, concrete and asphalt absorb more heat than vegetation - and fewer trees means less shade. The effect is strongest in high-rise areas, where the heat-absorbing surface area of buildings is vastly greater than that of the natural landscape they replaced. Add to this the heat thrown out of buildings by air conditioning systems, and generated by cars and factories, and the result can be sidewalks hot enough to fry an egg. The difference is usually even greater at night, as buildings hold onto their heat far longer than fields or woodland. The urban heat island effect is not new - British meteorologist Luke Howard wrote about it back in the 18205 - though it is intensifying as cities grow. Typically, cities can be a couple of degrees warmer during the day and up to 6°C warmer at night, but in the desert metropolis of Phoenix, Arizona, the different can be 10 °C or more. Why is this extra city heat a problem? For a start, it is expensive. One study calculated that the US spends a billion dollars each year on air conditioning to combat urban heat islands - though some of that may be recouped by the reduced need for winter heating. The heat also aggravates pollution by generating toxic chemicals such as ozone. What's more, it kills people. Around a thousand people die of the heat in the US each year, mostly city-dwellers. Couple that with the gradual rise in average temperatures caused by climate change (no, the urban heat island effect does not explain global warming), and we have big trouble ahead - especially at night, a vital period for cooling overheated bodies. Europe saw a glimpse of the future in August 2003, when its worst heatwave in 600 years saw temperatures in Paris exceeding 1.0°C during the day and barely falling below 30°C at night. Some 20,000 people died. Heat islands affect the local weather too. Strong thermal updrafts caused by the extra city heat can generate rain-bearing clouds and even thunderstorms, though usually downwind of, rather than over, the cities themselves. One NASA study found 28 per cent extra rainfall in summer at up to 60 kilometres downwind of cities such as Dallas and Atlanta. So how can we cool things down? Planting more trees and grass, and excavating more lakes, will help provide shade and dissipate heat, says Qihao Weng of Indiana State University, who is exploring ways of mitigating urban heat in Indianapolis. For instance, in Miami summer electricity bills are around 10 per cent lower in areas where more than a fifth of the land is covered in trees. If more planting isn't possible, however, then there's always the option of painting buildings white to increase their reflectivity. |
