The best way to keep cities cool is to include as many green surfaces as possible. Trees provide shade, cooling the areas around them. Green facades stay cooler than bare ones. In addition, water evaporates from all types of greenery, creating natural cooling. Maintaining a high proportion of green spaces in urban areas keeps a city cooler, makes it healthier and more attractive, and allows more room for biodiversity.
The goal is to keep cities as comfortable as possible during hot periods. Keeping average urban air temperatures low can best be achieved by maintaining a high proportion of greenery in the city.
- On average, plants reflect more solar radiation than pavement does;
- Plants create shade, keeping the surfaces they cover cooler and preventing too much heat from being stored in substrates or facades; and
- Plants extract heat from their immediate surroundings through evaporation.
In general, having 10% more greenery (e.g. 40% instead of 30%) leads to an approximately 0.5°C reduction in air temperature. Of course, this may not a very noticeable difference on a hot day (a difference between, say, 34.5 en 35°C), but on average it does make both daytime and nighttime temperatures slightly cooler. Kluck et al, 2020
In periods of extreme heat, this is not enough. The other way to keep a city liveable is by creating sufficient green spaces. In the shade of a tree, the perceived temperature is 3 to 19°C lower than in the sun. Creating shade is therefore the most effective means of reducing perceived temperature. Trees are the most effective means of realising cool urban spaces and providing relief to a city’s residents on hot days, because they give shade and cool the air through evaporation. Kluck et al, 2020
Evaporation is a highly effective means of cooling; it requires energy and therefore draws heat from the surroundings. Features that create cooling through evaporation are water elements such as ponds, ditches or fountains and green elements such as parks, trees, lawns or green roofs. Evaporation is the most effective means of lowering ambient air temperatures. Its cooling effect is strongest during the day, because that is also when plants transpire. Because less heat is stored in surfaces during daylight hours, the effect also continues at night. Increased evaporation has an equal cooling effect on both air temperature and perceived temperature. However, shade plays a much larger role than evaporation in lowering the perceived temperature; therefore, in relative terms, evaporation has little influence on perceived temperature. Green elements need to have enough water to be able to evaporate. If vegetation dries out or if it gets too hot, evaporation stops, which can even lead to a slight increase in air temperature. This must be considered when planning urban green spaces.
Just as green facades remain cooler than bare ones, grass can reduce the surface temperature of substrates. Grass-covered ground can be as much as 24°C cooler than concrete pavement. In addition, grass reduces the perceived temperature by up to 3°C. The smaller a lawn is, however, the less efficiently it cools: a one-metre-wide strip, for example, has a local cooling effect of only 0.6°C. Kluck et al, 2020
At the city level, increasing evaporation by creating as many green and blue surfaces as possible is the most effective way to keep air temperatures cooler. The more vegetation and water there is, the more of the sun’s energy is captured by evaporation and the less energy remains to heat up the city.
Having a clear ambition is imperative in realising more green spaces. It can give greenery a more prominent position in urban design projects, comparable to that of the spatial requirements for roads and car parks, or for cables and pipes.
Additional advantages of increasing the amount of urban greenery
A higher proportion of green in the city helps to limit drought, subsidence and siltation by increasing the total permeable surface area and thus allowing groundwater to be replenished.
A greater proportion of green allows for more biodiversity in cities. Of course, the actual level of biodiversity is dependent on the arrangement and design of the greenery.
Reducing urban temperatures by adding greenery can reduce the summertime need for mechanical cooling devices such as airconditioning.
Urban greenery is essential for the well-being of city residents.
Finally, research shows that real estate prices in green neighbourhoods are higher.
Source: Kluck et al, 2020