Wetenschap - 29 november 2011

More greenery, less heat

On a normal day in Wageningen, it is 2.4 degrees warmer in the city than outside. Both in summer and in winter. But no sweat: more greenery makes it cooler.


Foto: .

The difference in temperatures between city and countryside can be as much as more than five degrees on a warm summer's day. Meteorologists call this the heat island effect: the city is like a heated island on a cooler countryside. Researcher Gert-Jan Steeneveld (Meteorology and Air Quality) calculated this effect for more than twenty Dutch cities, with help from colleagues and students.
That cities can retain heat is a known phenomenon. We spend long summer evenings outside on warm terraces. Until lately, this heat has not been considered as a problem in the Netherlands. 'Because of the mild climate, people thought that there wouldn't be any problem,' says Steeneveld. But climate changes and a series of hot summers have changed that. Heat stress and what to do about it are 'hot' issues.
Steenveld is the first to chart the island effect for Dutch cities. He had to use data from amateur meteorologists because there are hardly any official weather stations in the cities. The Dutch weather institute KNMI does not take any measurements in cities. Amateurs do, mostly with expensive professional equipment over a long period of time. Amateur data is therefore a godsend.
Heat stress
The Wageningen researcher calculated the maximum daily island effect by getting the difference between the temperature in a city and the temperature at the nearest KNMI station. He arrived at 2.4 degrees for Wageningen, a moderate heat island effect in the Netherlands. But differences among cities can be relatively big. In Rotterdam, for example, it is 2.8 degrees, while it is less than half of this in Groningen. This is due to the location near to the sea and stronger winds in the north. Steeneveld also calculated the heat stress which the island effect can cause. As a whole, the heat stress is not that bad. The (empirical) threshold is seldom exceeded. And yet, a third of the cities are affected by heat stress on seven days in a year. In a city such as Rotterdam, this number can even be doubled.
Luckily, there is a simple remedy against heat: plant more greenery. Steeneveld stresses that there is a direct correlation between greenery and the heat island effect. 'One percent more green coverage can lower this effect by 0.06 degree.' It is important that city designers know about this. Steeneveld: 'They have two elements at their disposal: water and greenery. We have also researched into the effects of water, but these are less straightforward.' Cities retain warmth partly because of their buildings. Tall buildings located close to one another do not allow much heat to dissipate directly. Cities with a high population density are therefore more affected by the heat island effect.  The reverse is also true: tall buildings make a city cooler in the mornings.