Greenery in the neighborhood as a measure against heat
The summers of 2018, 2019 and 2020 saw one heat record broken after another, and these years featured long periods of drought. Building intensified the heat.

October 9, 2020
The summers of 2018, 2019 and 2020 saw one heat record broken after another, and these years featured long periods of drought. Building amplifies heat. Extreme heat in built-up areas can have many undesirable consequences, ranging from increased mortality, through declining labor productivity to deteriorating water quality and infrastructure problems.

Greening of public spaces is increasingly used as a heat mitigation measure. With this trend, the demand for scientific information on cooling by greenery increases: how can cooling effects in urban areas be maximized and maintained during droughts? There is a particular need for concrete guidelines for the construction and design of “climate greenery,” aimed at mitigating the effects of climate change in the built environment.

In the project “Effective greenery for climate adaptation in the city” existing scientific knowledge is translated into practical guidelines for greening urban areas and elaborated into templates for application in common Dutch neighborhood types. While the templates focus on maximum cooling, where possible they will also be tailored to practical aspects such as maintenance costs and consequences for underground infrastructure.

Research in Wageningen's Tarthorst neighborhood
The templates will be tested in practice: are they easy to apply in a real redevelopment task? The municipality of Wageningen has put forward the possibility for such testing in practice. The municipality is considering redesigning the public space of a number of neighborhoods and, in doing so, wants to be able to absorb increasing heat as well as possible through green space provision. The Tarthorst and the Roghorst in Wageningen are typical examples of cauliflower neighborhoods. This neighborhood type is one of the common standard situations in urban areas studied in the project.

In the spring, the existing greenery of the Tarthorst was mapped by students of the Hogeschool van Hall-Larenstein. They calculated the current heat situation and possible effects of greening using a kind of weather model for neighborhoods and cities. In addition, Wageningen Plant Research and Wageningen Environmental Research took measurements of the cooling effect of various trees in the neighborhood on a summer day with plenty of sunshine. The latter is done with special instruments, which in addition to air temperature and humidity also measure wind speed and effects of solar radiation (“heat trackers”). The cooling effect can thus be expressed as a reduction in the wind chill, or temperature as people experience it.
Up to almost 10 degrees cooler during the day
The results of these research activities overwhelmingly confirm that shade by trees contributes greatly to cooling during the day. The differences in wind chill temperatures between places in the shade and in the sun amounted to up to almost 10°C. This meant the difference between a situation with moderate or severe heat stress, in the sun, or one with hardly any heat stress, in the shade. Even greater differences are possible, but were not observed on the measuring day in the Tarthorst. The air temperature hardly plays a role in this type of cooling. It is about the same in the shade as in the sun. The much-heard claim that a tree acts like an air conditioner is therefore not true. Heat images showed that tree leaves were generally as warm or slightly warmer than the air. Heat flows from hot to cold, so the air is still heated, or at least not cooled. But of course, thanks to their transpiration, trees do remain significantly cooler than stony surfaces such as walls and roofs of a house or black asphalt. In this way, a tree contributes less to the warming of its surroundings than stone houses, streets or paved plazas. Also, objects shaded by a tree will heat up less. This not only directly contributes to lowering the wind chill on site, but also reduces the heating of the city at night.