Science - October 2, 2018

More wind in the city than outside it

Text:
Roelof Kleis

It can be windier in a city than in the surrounding countryside, discovered PhD candidate Arjan Droste. He calls this surprising phenomenon the wind island effect.

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The name is a nod to the already known heat island effect, through which a city is often hotter than the countryside. But does something similar happen for wind? That is completely contrary to intuition, admits Droste, who works in the Meteorology and Air Quality chair group. ‘That can’t be right, was my first thought when I saw the results of my model calculations. The average wind speed was higher in the city than in the countryside. Whereas the wind can blow where it wants in the country, with no buildings in its way.’ But it turned out to be right. And so what initially looked like an error provided material for an interesting paper in the latest edition of Environmental Research Letters.

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Little is known about wind in cities. According to Droste, this is mainly because it is so difficult to measure wind there. ‘Because of the buildings, nearly every location has its own wind speed. There is a lot of variation. And the wind speed varies at different heights as well.’ Droste took a different approach to the problem. ‘I made a straightforward set of comparisons which describe the atmospheric boundary layer, the lowest layer of the atmosphere, where our weather takes shape. I zoomed in on the way the wind speed changes over time, both in the countryside and in the city.’ And the unexpected result was the wind island effect. ‘In the early morning it is not as windy in the city, but then it changes and the wind picks up in the afternoon.’

Phenomenon
Droste reckons this phenomenon has to do with the interaction between the atmospheric boundary layer and the troposphere above it. ‘At the border between the two layers, the air mingles and that speeds up the wind. That acceleration above the city is different to what happens in the countryside because the atmospheric boundary layer is broader above the city.’ The effect is small but Droste believes it is significant for models that calculate urban air quality, for example.