Researchers investigate Würzburg: Green inner cities noticeably reduce heat stress

Plenty of green spaces and trees contribute significantly to more comfortable temperatures in cities, especially in densely populated areas. This is now confirmed by a study by the Technical University of Munich, which had previously investigated this in Würzburg. However, the proportion of flora must be significant for this.

According to a study, a green infrastructure with many trees can lower the air temperature in cities and thus reduce heat stress for people. However, around 40 percent green space is necessary to ensure cooler temperatures, reported the Technical University (TU) Munich on the results of the empirical study lasting several years. The researchers called for green spaces to be strategically planned in order to reduce thermal stress even in densely built-up urban quarters.

According to the researchers, the average air temperature at the inner-city locations examined in Würzburg was 1.3 degrees Celsius higher in summer and five degrees higher in winter than in suburban locations. The differences were particularly influenced by the number of buildings, explained Stephan Pauleit from the Technical University of Munich.

A total of 97 hot days with more than 30 degrees were counted within three years at a market square where there is not a single tree. On the other hand, there were no extreme heat stress days at the suburban locations during the study period. “Our study showed that about 40 percent of green space in the built environment, including lawns, green roofs and green walls, could cut extreme summer heat stress in half without increasing winter cold stress,” said the study Researcher Mohammad Rahman.

In heterogeneous urban ecosystems, trees in particular would have multiple biophysical functions. With their treetops, they reduce the entry of short-wave radiation onto the ground by up to 90 percent, which is particularly the case in summer. Trees also cool their immediate surroundings by one to eight degrees, increasing relative humidity. However, trees could also have negative effects. For example, they could prevent the vertical and horizontal mixing of the air in narrow street canyons, so that polluted air at pedestrian level is not diluted and discharged.

Grass growth, on the other hand, reduces thermal radiation due to higher reflection compared to the built environment. However, it allows higher wind speeds to reduce the summer heat load and higher solar radiation, which at the same time minimizes the winter cold load.

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