How we can cool our cities
Southern Europe is currently battling a heat wave. Jan Carmeliet explains how cities can tackle the summer heat. It is a balancing act that calls for finely tuned measures.
More and more people are becoming aware of the increasing risk of being exposed to severe heat waves during the summer months. City inhabitants even more than others. The urban heat island effect makes cities more vulnerable to heat extremes than their rural surroundings. This is because impervious surfaces in cities made of asphalt and concrete heat up more during the day than vegetation. Cities are also subject to more heat-producing human activities such as traffic and industry. Night-time cooling, in contrast, is lower in cities, as the heat radiation to the sky is blocked by buildings.
People often ask what can be done to reduce extreme heat in cities. Unfortunately, there is no magic solution, as urban climates are extremely complex. However, there are some ways of combating it. Planting trees is often suggested as being the best way of mitigating urban heat, as trees provide shade and transpirative cooling for pedestrians. Wind flow in ventilation corridors such as main streets running in the main wind directions is often also used as a heat mitigation measure. Employing light colours that reflect sunlight and evaporative cooling from wet porous pavements are also cooling measures. Shading and ventilation corridors can be extremely effective, but attention must be paid to their details and possible trade-offs.
Trees are not the last resort
Trees cast shadows and indeed cause localised cooling. However, this very much depends on their species, size and age. The most effective are mature, large deciduous trees with high leaf density and extensive root systems, which enable them to draw and evaporate a lot of water from the soil. When viewed on a large scale, however, trees contribute to heating, albeit of a lesser magnitude, because they block the flow of air and limit the removal of heat. Consequently, large trees that block the airflow should be avoided in ventilation corridors to maximise their cooling potential.
“Ventilation corridors may turn into hot air corridors that are unsuited as walkways for pedestrians.”Jan Carmeliet
Street corridors without trees may heat up substantially during the day, however, when unshaded pavements and walls absorb and store solar heat. This may turn ventilation corridors into hot air corridors that are unsuited as walkways for pedestrians. Since pedestrians need cool paths, urban planners need to have a sound understanding of both the positive and negative effects of trees.
Adaptation strategies are necessary
If we are to tackle periods of extreme heat, we need to start combining the mitigation strategies outlined above with heat adaptation strategies. These are part of a city’s comprehensive heat response planning and involve forecasting and monitoring heat waves and teaching people how to behave during periods of extreme heat. Infrastructure such as cooling centres is also needed. This takes the form of air-conditioned, quiet rooms where people can take a few hours' break from the stress of the heat. When developing mitigation measures such as cool walkways, urban planners need to consider the use of open spaces and access to cool spaces.
We also need to consider general climate zones. Trees in hot, humid climates are less effective for cooling purposes, as they have a lower rate of transpiration on account of the high relative humidity of the surrounding air, leading to less transpirative cooling. While cross ventilation (often recommended for urban dwellers) may be an adequate way of cooling hot and humid buildings, as it enhances skin transpiration, it is not effective in hot-dry climates.
Finding local hot spots
The right tools based on scientific data are needed to develop prevention or adaptation measures. Comprehensive all-physics models that have been validated by climate sensing data and correctly represent the complexity of the urban climate against the background of climatic conditions are needed to optimise the design of urban heat mitigation measures. These models take into account a variety of physical and meteorological phenomena as well as environmental, social and economic processes.
My team has developed models with a resolution of ten centimetres that can be used for local urban climate simulations for a summer that includes heat waves. However, these models require a huge set of input data, such as climate boundary conditions of temperature and wind speed, the geometry and material properties of building components, street composition and vegetation. Such local urban climate models allow us to assess the neighbourhood’s current thermal comfort, find local hotspots that exhibit extreme heat stress and put forward effective heat mitigation scenarios.
These results need to be communicated to stakeholders and inhabitants in a readily understandable, graphical and interactive manner. This in combination with the long-term mitigation of climate change by reducing the flow of heat-trapping greenhouse gases into the atmosphere will enable us to reduce air and surface temperatures and improve the thermal comfort of our cities.