Researchers from Wageningen University used the warm days in August to map out the urban climate in two Dutch cities. The research team drove two cargo bicycles with measurement equipment during various times of a 24 hours’ day through Rotterdam and Arnhem. The results may indicate to which extent heat stress may become a problem for the inhabitants.
Future projections of climate change show that frequency of heat waves will increase substantially in the next decades. Particularly in cities heat stress may become a serious problem due to the so-called Urban Heat Island effect (UHI), the phenomenon where the average temperature in the city is higher than in the surrounding area.
The measurements on 6 August in Rotterdam showed that during day time the city centre was two degrees warmer on average than Zestienhoven airport, which is located outside the city. A striking observation was that the city park De Twee Heuvelen was 2.4 degrees cooler than Zestienhoven. This means that the differences in the afternoon in the city can rise to 4.4 degrees centigrade. During the late evening (22-24 hours), the city centre was more than 5 degrees warmer than Zestienhoven. The route near the national Green Heart (Doenkade) turned out to be even cooler (2 degrees C) than Zestienhoven. The difference in temperature between the city and countryside consequently amounted to more than 7 degrees during nocturnal hours.
In the late afternoon the felt air temperature – the air temperature perceived by the human body – was 28 degrees C at Zestienhoven, the temperature at the city centre of Rotterdam (in the sun and out of the wind) would feel more than 6 degrees higher – so well above 30ฐ C. Surprisingly, similar effects were measured in the much smaller city of Arnhem.
For the measurements, days with maximum temperatures above 25ฐ C were necessary. With the two cargo bikes with measurement equipment, the researchers cycled along two previously determined routes through a number of characteristic urban districts, such as an industrial area, an older residential area, a city park and the harbour area. The researchers plan to take more measurements later this year and in 2010.
For technical reasons, the researchers use cargo bikes to transport the measurement apparatus. With a cargo bike it is easy to manoeuvre through the narrow streets in the city, while the instruments remain horizontal. The cargo bikes are equipped with a thermometer that registers the temperature, a humidity meter, a sensor for wind direction and wind speed, sensors that measure the amount of sunlight and sensors for the exchange of heat radiation. The measurements were conducted every second. In addition, the route was photographed at fixed intervals from 50 cm above the ground with a fisheye lens pointed upwards. This can be used to determine the percentage of the sky that is "covered" with buildings or greenery as seen from street level. This coverage largely determines the strength of the urban heat island effect. The felt temperature is determined by the air temperature combined with radiation, humidity and wind. The instruments are powered by a solar panel mounted on the baggage carrier.
With the measurement data, the researchers will first determine the strength of the UHI in Rotterdam and the factors that affect the local differences within the city. The data provide an indication of the extent to which heat stress is, or will become, a problem. Furthermore, the data will be used to further develop simulation models for urban climate. The results will evolve into design rules or tools that can be used by urban developers and designers to mitigate the effect of climate change on heat stress.
The measurements were carried out by staff and students of the Meteorology and Air Quality Group, and the Earth System Science and Climate Change Group of Wageningen University and Research Centre. The measurements in Rotterdam are part of the Heat stress in the city of Rotterdam project, that is partly financed by the special climate research programme ‘Knowledge for Climate’ and the municipality of Rotterdam. The municipality of Rotterdam is lead partner in this project. Other consortium partners are: TNO, Deltares and Waterwatch.
The measurements in Arnhem are part of the EU Future Cities project, in which the municipalities of Tiel, Arnhem, Nijmegen, Hastings, the West-Flemish region, the German water boards, and the French Rouen-Seine-Am้nagement are collaborating. The municipality of Arnhem is lead partner of the Dutch consortium in this project.
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