At this time, it is statistically the coolest during weeks like these. And then it becomes clear: The city has a problem with the heat. While in peripheral areas such as Röttgen, Ückesdorf or the south of Ippendorf, cool air from the hillsides and the side valleys of the “Kottenforst” forested area refresh the hot parts of the city, heat builds up like a furnace in Neu-Tannenbusch, Duisdorf, Beuel, Bad Godesberg and in the city center.

In many parts of the city - especially the city center of Bonn - the minimum nighttime temperature on summer nights with stable high pressure conditions is more than seven degrees higher than on undeveloped land. Even open spaces, such as those around the “Kottenforst”, which also heat up like an oven during the day, supply important cold air reserves for the city at night.

The climate charts for day and night in Bonn have recently emerged as part of the research project ZURES. The abbreviation stands for “Future-oriented Vulnerability and Risk Analysis” and the project aims to develop resilience of cities and urban infrastructures when it comes to heat stress. Supported by the Federal Ministry of Research, since 2016 the University of Stuttgart, Technical University of Dortmund and the UN University Bonn have been working on the subject of adaptation to rising temperatures and climate change in the project cities of Bonn and Ludwigsburg.

In Bonn this is important for several reasons. Helbig explains that the higher elevation of Ennert on one side of the Rhine and Venusberg on the other side, contribute to a bowl-like area which restricts the flow of cool air. Meanwhile, the city, which is forecast to continue to grow, urgently needs more living space. Demands for densification and building in areas that are still vacant are being considered. At the same time, property owners and residents have to adjust to rising temperatures. A local climate forecast by the University of Duisburg-Essen from the year 2011 anticipates a rise in hot days of more than 30 degrees from an average of 7.8 to 20.9 per year in the decades from 1991 to 2001 and 2051-2061.

The number of summer days with more than 25 degrees is expected to almost double from 37.1 to 70.3 days. And the nights are getting warmer too. The number of tropical nights increases tenfold statistically from 0.1 to 1.0. These are important parameters, because the warmer weather both day and night means an increase in health problems. Not only is the heat a challenge during the day but it becomes difficult to recover at night with a lack of sleep.

What does it mean for urban land use planning and where do you need to maintain green areas, preserve or create cold air corridors? Where is it necessary to unseal surfaces? Where do lots of children or older people live who are prone to heat stress? And where can we still build without major negative effects? These are some of the questions ZURES is trying to answer.

One key finding: The climate modelers have completely overestimated the Rhine as a climate factor. "We would have expected a certain current effect that cools the banks of the Rhine." On the night map, however, the built-up areas on the river are deep red. In fact, there is often a wind on the shore, but not a cool one. "As a large body of water, the Rhine heats up during the day. At night it partially releases the heat back into the air," explains Helbig.

Cooling comes mainly from the hillsides. Those living at the foot of the Ennert are in a good place when it comes to cooling off. In the west, the valleys of the “Katzenlochbach” stream in the north and the “Godesbergerbach” stream in the south provide ventilation. But the flat areas in the north, such as the Rosenfeld development project will be unlikely to change anything regarding air flow in the city.

Midday high temperatures vary from 30 or more degrees Celsius in the outskirts of the city, but in densely built-up core areas and on major roads, it can feel like up to 43 degrees Celsius or more.

Shading and green areas are an important aspect. Even water surfaces could contribute to the air cooling with their evaporation effect. Because open spaces alone do not make for a cool breeze. On fields and open meadows, it can be as unbearable during the day as in the city. In parts of the Rheinaue, temperatures of about 40 degrees are also felt on such days.

Orig. text: Martin Wein

Translation: ck