Cities are hot and polluted places, much hotter than the rural areas immediately around them. If you thought you were imagining it, it’s true: the countryside is indeed cooler than the city.
This happens due to the phenomenon known as the urban heat island. This term refers to the fact that cities accumulate heat and have higher temperatures compared with the surrounding rural areas. London, for example, records temperatures six degrees Celsius above what is recorded in the surrounding areas. What can urban planners do? And why do cities register higher temperatures than the bordering zones?
1
One of the major culprits for the rise in city temperatures are dark building materials, which absorb heat during the day and release it at night, contributing to a constant heat cycle. A good example of this is road asphalt, which is a great absorber of heat.
To tackle this problem, there are cities, such as Los Angeles, that are painting roads white to reflect light and reduce the amount of heat absorbed. It is an expensive pilot project (about $40,000 per mile, or 1.6 km) but it is estimated that painting only one third of the city’s roads could lower the overall temperature by 1°C.
2
Another way to reduce the temperature is to plant vegetation on the rooftops of buildings. For example, in a pilot project in Chicago, it was found that roofs with vegetation were 7°C to 8°C cooler than roofs without vegetation. (France approved in 2015 a law requiring new buildings to have either vegetation on the roofs or to install solar panels).
This concept was taken to the extreme at Bosco Verticale, in Milan, a building with hundreds of trees planted on its balconies.
3
Careful urban planning can also reduce temperature. For example, the city of Masdar, in the United Arab Emirates, was built to be used by people and not cars. Thus, buildings can be much closer to one another, creating shade and lowering the exterior temperature on the streets. On the other hand, ventilation towers 45 meters tall pull cooler air from above the city down to street level, creating a ventilation system that keeps the city up to 15°C cooler than the surrounding desert.
Although this kind of idea is not easy to implement in existing cities, architects can create corridors in cities that promote air circulation and cleanse them of pollutants. An example is the city of Stuttgart, Germany, which banned the construction of tall buildings precisely to create ventilation corridors.
