In periods of excessive heat, urban daytime temperatures can be uncomfortably high. This is especially true in places where there is no shade from trees, or where there is a lot of radiant heat from pavement and buildings that have absorbed heat from the sun. Differences in perceived temperatures from one area to another in the city can therefore be as high as 22°C Klok et al., 2019. It is important to offer cool, shady spaces throughout the city to keep it liveable during hot periods. This respite from heat can be provided by trees, awnings, arcades, and cool buildings such as thick-walled public buildings and churches. A place feels cool if its perceived temperature remains below 35°C on hot days.
Creating shade is the most effective way to lower the local perceived temperature, because it prevents the body from warming up due to solar radiation. This can be achieved in many ways. Trees are most effective, as they not only provide shade but also contribute to cooling through evaporation. If there is little space for trees, good alternatives are awnings, pergolas, parasols, arcades, and narrow streets with high buildings, as can often be found in Mediterranean countries. Some shade provisions have the advantage that they can be removed in the evening, allowing the heat stored in the ground and buildings to escape at night and cool the city down faster.
Smart planning in the planting of trees can aid outdoor cooling, as can placing benches on the north side of buildings or in other shady locations. Trees cool most efficiently when they cast shade on both sidewalks and building facades, as these surfaces then radiate less heat.
Creating cool spaces
Cool oases are especially important during heatwaves. At such times, a grid of strategically placed cool spaces can keep a city liveable. Regularly spaced areas of shade and coolness along walking routes from homes to amenities such as shops and public transport, or in crowded areas, keep public spaces pleasant and accessible to the elderly and other vulnerable populations.
Cool spaces need to provide shade but should also be attractively arranged so people will want to spend time there. Optimally, they should provide a combination of shade (from trees, buildings, awnings or canvases), seating areas, greenery and water. Visitors to cool spaces say that a green atmosphere, greenery and benches make a location attractive. A cool space should have a minimum size for purposes of usability, ecological quality and attractiveness as a place to rest and spend time. The recommended minimum size for cool spaces is 200 m2 Kluck et al, 2020.
The elderly are the least mobile and most vulnerable population group, and a walking time of 5 minutes between two cool places seems to be the maximum for them. This amounts to a walking distance of up to 300 metres between two cool spaces. Nuijten, 2008.
Useful guidelines for a cool space are therefore:
- It should be a pleasant place to spend time, with shaded seating and attractive greenery, and it should exude a sense of calm, cosiness and safety.
- Any surface that remains below 35°C PET (perceived effective temperature) during an average hot summer day can potentially be considered part of a cool space. An area of 200 m2 is large enough to provide heat relief, but a larger area is needed to create a pleasant space to spend a longer amount of time.
- Swimming areas and fountains (both decorative and accessible) in an already cool space provide extra cooling and are highly appreciated.
- Klok, L., Rood, N., Kluck, J. & Kleerekoper, L., Assessment of thermally comfortable urban spaces in Amsterdam during hot summer days. International Journal of Biometeorology, 63(2), 129-141, 2019
- Kluck, J., Klok, L., Solcerová, A., Kleerekoper, L., Wilschut, L., Jacobs, C., & Loeve, R. De hittebestendige stad, Een koele kijk op de inrichting van de buitenruimte, Hogeschool van Amsterdam, 2020
- Nuijten, D., Dwingend vergroenen: Onderzoek naar de toepassing van het richtgetal van 75 m2 groen per woning uit de Nota Ruimte en de relatie met de kwaliteit van het groen in de stad. Afstudeerscriptie Sociaal-Ruimtelijke Analyse, SAL-80436, Wageningen Universiteit, 2008