Cities show characteristic heat island effects

10/23/2013 - Cities heat up stronger than the rural regions surrounding them – and if climate change continues, this will become a risk for the inhabitants. On the basis of satellite data, researchers have now more comprehensively than ever before investigated this so-called heat island effect for thousands of cities in Europe. This effect can be noticed in everyday life: If you ride your bicycle from the green surroundings into a city on a hot day, you will often notice a temperature change. The larger a city is, the stronger is the effect, previous studies assumed. Now scientists could for the first time show that the urban heat island effect is in fact increasing with the size of the city – yet only up to a certain threshold value. The analysis revealed that even large cities are getting hotter than their surrounding by only two to three degrees on the average.
Cities show characteristic heat island effects
Tempera­ture differences of up to 4-6 degrees are not unusual during summers in large cities. (Photo: Thinkstock)

“This is an important finding in view of the trend towards urbanisation in many parts of the world – even in the case of larger megacities, the heat island effect is expected to be limited,” says Diego Rybski from the Potsdam Institute for Climate Impact Research. The individual measurements of cities however disperse around this typical value. Tempera­ture differences of up to 4-6 degrees are not unusual during summers in large cities. This effect however almost disappears in winter.

The detailed analysis of individual cities also shows that cities in warm climate zones can have a reverse, thus cooling, effect. It still needs to be investigated precisely why this is the case. On this basis, planning strategies could then be developed.

“The heat island effects is most relevant when it is already warm as for instance during a constantly hot summer,” says co-author and head to the research group, Jürgen Kropp. The cities are even warmer then, which can have health consequences espe­cially for older people. An increased occurrence in heat waves is to be feared in view of climate change. “In combination with an aging society, health problems are expected to increase,” says Kropp. “Knowledge about the urban heat island effect can thus help to take prevention measures.”

Article: Zhou, B., Rybski, D., Kropp, J. (2013): On the statistics of urban heat island intensity. Geophysical Research Letters (Early View, online) DOI: 10.1002/2013GL057320

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