The expected lower heating of the ground can be detected by measuring sensors and a high-resolution infrared camera.
Light-coloured surfaces reflect the sun's incoming shortwave radiation more strongly than dark surfaces, which can be quantified with the help of the albedo expression. The albedo is a number between 0 and 1 and represents the fraction of sunlight diffusely reflected by the surface. This means the greater the albedo of a surface is, the more shortwave radiation is reflected into the atmosphere, and the surface heats less quickly and strongly. Therefore, bright surfaces have a greater albedo than dark surfaces.
Due to the high density of buildings and the general high degree of sealing in cities, temperatures are often higher than in the surrounding areas, which can also be seen on our heat maps. The temperature difference between the city and its surrounding area is particularly noticeable during the summer months and on hot days and nights.
There are various measures to counteract the urban heat island effect, for example, urban green spaces or green roofs. There are also so-called "cool pavements" approaches, whereby materials with a high albedo value are deliberately used as surfaces.
As part of a pilot project in collaboration with the Swiss Rhine ports, meteoblue implemented such a Cool Pavement strategy. The aim is to investigate the differences in surface and air temperature between a normal (dark) asphalt road and a light road to conclude the effect of the Cool Pavement strategy.
On 7 September 2023, a road section of approximately 800 m2 in Kleinhüningen (Basel) was painted white. A few weeks before, various measurement instruments have been installed to on the edge of the road to measure air temperatures before and after the road painting.
In addition, a high-resolution infrared camera was used to take before- and after-images of the street's temperature, in collaboration with the Atmospheric Sciences Groupe of the University of Basel. The comparative images of the infrared camera show the street's surface temperature and very impressively visualize the albedo effect between the dark asphalt road and the white-painted road.
On both recording days, the air temperature at the measurement time was about 21° Celsius. Comparing the average surface temperature of the white-painted road to the previous black asphalt surface, the surface temperature decreased by 8.5°C.
As the image shows, it is very clear what an enormous effect the surface texture has on the temperature balance. The change in surface temperature also changes the air temperature to a certain degree. This change will be analysed later in the project once a sufficient time series of temperature measurement data is available.
However, even though the white road heats up much less quickly and strongly than the dark asphalt road, some difficulties may occur. For instance, the road section does not remain permanently white due to vehicle pollution and the general deposition of dirt particles. The discolouration towards a darker surface also reduces the albedo and, therefore, the cooling effect. In addition, a surface with a high albedo leads to an increased perceived temperature, meaning that when you walk across the street, it seems warmer than it is. Finally, the sunburn risk increases, and other road users may feel disturbed or blinded by the bright surface.
Cool Pavement approaches already exist worldwide and are being applied again and again. In road construction, asphalt roads (low albedo) are increasingly being rehabilitated with a surface layer of concrete (high albedo). Mixed pavements with a light grain are also being used. Particularly in the United States, road sections are often sprayed with highly reflective surfaces as pilot projects.
We will keep you informed about future evaluations of this project.
