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Five ways to keep cities cool during a heat wave

Along with the warmer and wetter winters in Britain and Europe, our summers are getting hotter with more frequent periods of extremely hot weather. And warm weather not only affects agriculture, infrastructure and nature, it also affects us.

Heat can affect human health, especially for younger and older people and people with pre-existing conditions. It reduces our ability to concentrate and learn. Heat waves have been linked to increased aggressive behavior. The consequences are both direct in terms of productivity loss and indirect in terms of longer-term economic effects.

I was recently involved in the Cooltowns project, working with researchers from Belgium, France and the Netherlands. Our aim was to show how cities can develop heat resilience strategies by using local interventions to increase thermal comfort in priority areas.

We started developing a method, captured in our Urban Heat Atlas, to identify where heat stress was most likely to occur. We then mapped vulnerability to heat by identifying priority locations for those most at risk.

We found that transport hubs, where people gather to wait for buses or trains, schoolyards and market squares fall into this category, as do walking and cycling routes from residential areas to amenities and workplaces.

Thermal comfort refers to how individuals feel under different conditions and we used mobile weather stations to calculate the physiologically equivalent temperature. This is a measure that takes into account humidity, incident radiation (the amount of solar energy hitting the ground), wind speed and air temperature.

Prior to our study, there was hardly any information about the effectiveness of different interventions at physiologically equivalent temperatures and no information at all from Northern Europe. We took measurements and identified five of the most scalable solutions that reduce the impact of heat waves:

1. Shadow

It will come as no surprise that people are cooler in the shade than in full sun. Planting trees has many benefits and is therefore likely to be the first choice for landscape architects and urban planners, although shade intensity varies by species. Planting trees along roadsides can be a challenge due to underground utilities such as pipes and cables. They require maintenance, especially as trees establish their root systems.

Thermal image of dark purple areas of cool shade under trees and orange-pink areas exposed to sunlight away from trees
Infrared image showing the effect of shade on soil temperature.
Cool Towns team, Province of East Flanders

Trees and other vegetation, including grass, appeared to have a cooling effect that extended beyond the immediate area. Where planting trees is not possible, textile shade structures can be used. These can easily be removed in winter to allow the sun to penetrate.

2. Water features

Fountains and water playgrounds have a cooling effect when people can have direct contact with the water and when water falls to the ground, it then cools through evaporation. However, although the water itself, as with rivers, is cooler than the surrounding area, the effect is localized even though it is greater downwind.

city ​​street with trees, green leaves and shadow, water fountains spouting up from the pavement, children in swimming shorts playing in the water, buildings in the background
These water fountains on Kasteelplein in the center of Breda provide cooling on hot days in this stony part of the city.
Municipality of BredaCC BY-ND

3. Special paving

Permeable or vegetated paving consists of a matrix of interlocking recycled plastic grids or concrete paving. This provides a solid surface, while grass or other low plants can also grow in the holes and is cooler than asphalt or solid paving. This also allows water to infiltrate into the soil, which in turn reduces the risk of surface water flooding. Low-growing plants are used so as not to cause problems for wheelchair or stroller users.

4. Green walls

Green walls come in many forms and are effective at insulating buildings, reducing the need for internal temperature control. They look attractive and benefit biodiversity. Like green roofs, these reduce the urban heat island effect. However, the influence on thermal comfort can only be felt nearby.

The Green on the central square of Antwerp, a green wall by Bold Architects in Belgium.
Greentexx from SIOEN

5. Better city planning

Together, the total of all parks, private gardens, green roofs, street trees and water bodies (collectively known as green-blue infrastructure) reduces the urban heat island effect of building materials that absorb and later release heat. This cools down cities, especially at night.

Heat-resilient strategies are becoming increasingly common as decision makers respond to the impacts of the changing climate and achieving net-zero targets. Landscape architects and urban planners can be encouraged to incorporate small-scale thermal comfort interventions that together mitigate the urban heat island effect.

Barriers to implementing new development and modernization programs are not limited to initial expenditures. Challenges include ongoing maintenance costs, cold weather comfort, public safety and acceptability, as well as access for routine and emergency services, all of which must be taken into account. The Cool Towns project roadmap provides practical advice, with examples, for developing city-wide heat resilience strategies.


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