This is the time of day when it is statistically at its coolest in weeks like these. And then it becomes clear: The city has a problem with the heat. Because while in peripheral areas such as Röttgen, Ückesdorf or the south of Ippendorf, cool air from the foothills and from the side valleys of the Kottenforst refreshes heated areas, the heat tends to accumulate in Neu-Tannenbusch, Duisdorf, Beuel, Bad Godesberg and in the city center, like in a furnace.

In many city districts - above all the city center of Bonn - the minimum night temperature on summer nights with stable high pressure is more than seven degrees higher than in the undeveloped countryside. Even open spaces such as those around the Annaberger Hof in the Kottenforst, which also heat up like an oven during the day, provide important reserves of cool air for the city at night.

The climate maps for day and night in Bonn were recently created as part of the ZURES research project. The acronym stands for Future-Oriented Vulnerability and Risk Analysis to Promote Resilience of Cities and Urban Infrastructures. Funded by the German Federal Ministry of Education and Research, the University of Stuttgart, the Technical University of Dortmund and the UN University Bonn have been working since 2016 in the project cities of Bonn and Ludwigsburg to gain insights into adaptation to rising temperatures and climate change.

In Bonn, this is important right now for several reasons. "On the one hand, Ennert and Venusberg form a certain natural basin, which restricts the supply of cool air," explains Helbig. On the other hand, more living space is urgently needed in the city, which continues to grow. There are calls for increased densification and filling in the gaps in housing development. At the same time, all property owners and residents must prepare for rising temperatures. A local climate forecast by the University of Duisburg-Essen from 2011 expects an increase in the number of days with temperatures above 30 degrees Celsius (86° F) from an average of 7.8 to 20.9 per year when comparing the decades 1991 to 2001 and 2051-2061. A climate forecast by the Cologne-Bonn Region e. V. expects 18 of these warmer days per year as climate change progresses.

The number of summer days above 25 degrees Celsius (77° F) is even expected to nearly double from 37.1 to 70.3 days, according to the Essen-based calculations. And the nights will also get warmer. The number of so-called tropical nights (anything above 25° C/77°F) will statistically increase tenfold from 0.1 to 1.0. These are important parameters, because the climate increasingly causes health problems for people during the runs of several consecutive hot days in combination with tropical nights. Then, the heat load is not only extreme during the day, but even at night one has no chance to recover due to lack of sleep.

What does it mean for urban land use planning that hot days are becoming more frequent, especially in densely populated areas? Where do green spaces need to be preserved, and cool air corridors preserved or created? Where is it necessary to remove sealed surfaces? Where do children up to preschool age or elderly people live who are susceptible to heat stress? And where, on the other hand, can one still build or expand without major negative effects?

ZURES is intended to provide answers to these questions. For the two maps, the city area was flown over with laser sensors and measured three-dimensionally. Green and street areas and other data were included in the calculation. In the end, a model was created on the computers of the Geo-Net company in Dresden and Hanover that shows heat traps and cool-air pockets in a grid of ten by ten meters. In addition to temperature, variables such as humidity, wind and radiation intensity were taken into account. In Germany, comparable models have so far only been available for the capital city of Berlin.

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

Cooling, on the other hand, comes primarily from the hillsides. Anyone who has built at the foot of the Ennert is doing well in terms of heat. In the west, ventilation is provided almost exclusively by the valleys of Katzenlochbach in the north and Godesberger Bach in the south. Helbig believes, "If you were to put a complete building block in there, it certainly wouldn't be a good idea." On the other hand, there are construction areas in the flat north of the city, such as the projected Rosenfeld construction area, which will probably make relatively little difference to the flow of air in the city.

In the map with the daily maximum readings at 2 p.m., the data basis is different. Here, the perceived deviation from the statistical mean temperature in Bonn of 13.7 degrees Celsius was mapped to the greatest midday heat in a stable high-pressure situation without clouds. In the peripheral development to the surrounding countryside, the perceived temperature rises to 30° C and higher on such days. In densely built-up core zones and on the major roads, however, the readings can easily rise to a perceived 43° C and more. In this case, we are already talking about extreme heat stress.

"Here, it will be important to create spaces where it is still possible to reside," says Löffler. Shading and greening are important aspects. Water surfaces could also contribute to air cooling with their evaporation effect. After all, open spaces alone do not provide a cool breeze. During the day, it can be just as unbearable in fields and open meadows as it is in the city. In parts of the Rheinaue, temperatures of around 40 degrees Celsius (104° F) are also felt on such days.

In the next step, the data obtained will be used to create a detailed map, from which even laypersons will then be able to gain insights into local weak points. For all its accuracy, the map says nothing about buildings that protrude or materials used. Wood, for example, conducts heat differently than concrete or metal. Helbig's conclusion: "The map shows where smaller-scale assessments are required.”

(Orig. text: Martin Wein / Translation: Carol Kloeppel)