The research paper ‘The physical and psychological impact of street greenery on thermal comfort’ is published in press in the special issue ‘Green urban infrastructure and Climate adaptation’ of Landscape and Urban Planning. In the study Wiebke collaborated with colleagues Bert Heusinkveld, Sanda Lenzholzer and Bert van Hove. The study is part of the Climate-proof cities programme which finished by the end of last year.
The paper presents results of micrometeorological measurements and surveys with pedestrians in nine streets in the Rivierenwijk in the city of Utrecht. The streets were characterized by similar spatial characteristics but various amounts of street greenery (street trees and green front gardens) in order to investigate the impact of street vegetation on thermal comfort. They focussed on warm summer days.
The results show that large tree crowns have a significant physical impact on thermal comfort by lowering mean radiant temperature. The psychological impact on thermal comfort (perceived thermal comfort) tended to be related to the amount of street greenery. However, tey could not identify statistically significant differences. Hence, they suggest that an enhanced ratio of street greenery in the visual field experienced by pedestrians leads to a better thermal perception.
Abstract: This study focuses on the benefits of street greenery for creating thermally comfortable streetscapes in moderate climates. It reports on investigations on the impact of street greenery on outdoor thermal comfort from a physical and psychological perspective. For this purpose, we examined nine streets with comparable geometric configurations, but varying amount of street greenery (street trees, front gardens) in the city of Utrecht, the Netherlands. Mobile micrometeorological measurements including air temperature (Ta), solar and thermal radiation were performed, enabling the calculation of mean radiant temperature (Tmrt). Additionally, semi-structured interviews with pedestrians about their momentary and long-term perceived thermal comfort and their esthetical appreciation of the green street design were conducted. Measurements showed a clear impact (p = 0.0001) of street greenery on thermal comfort through tree shading: 10% tree crown cover within a street canyon lowered street averaged Tmrt about 1 K. In contrast, our results did not show an influence of street greenery on street averaged Ta. Interview results indicated that momentary perceived thermal comfort tended to be related to the amount of street greenery. However, the results were not statistically significant. Related to long-term perceived thermal comfort respondents were hardly consciously aware of influences by street greenery. Yet, people significantly (p < 0.001) valued the presence of street greenery in esthetic terms. In conclusion, street greenery forms a convenient adaptive strategy to create thermally comfortable and attractive living environments. Our results clearly indicate that both physical and psychological aspects of thermal comfort have to be considered in urban design processes.