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Planting trees in the downstream direction of the wind to avoid low winds can help keep an open area cooler, a study has suggested.
The study, Effects of different tree layouts on outdoor thermal comfort of green space in summer Shanghai, was published in scientific journal ScienceDirect December 22, 2022.
The researchers were Dayi Lai of School of Design, Shanghai Jiao Tong University; Yiqing Liu, School of the Built Environment, University of Reading, the United Kingdom and Bingqin Yu of School of Environmental Science and Engineering, Shanghai Jiao Tong University.
The team looked into the impact of tree layouts on the microclimate of generic green areas to look at how to create a thermally comfortable urban green space. The framework of trees will influence the microclimate when designing and building a green space, the report said.
Microclimatic parameters such as air temperature, humidity, thermal radiation and wind speed in a green space control the heat transfer between a human body and the surrounding environment. Thus, it largely determines the thermal comfort level, the paper said.
The research used a validated numerical model and compared the microclimate and thermal comfort of a generic green space with 17 different layouts on a typical summer day in Shanghai, China. The city sees a hot and humid summer and the monthly average daily maximum air temperature in July and August can reach 32 degrees Celsius.
Photos of the portable weather station used to measure the air temperature, relative humidity, global radiation, global temperature and wind speed of the studied site for the four measuring locations: (1) on pavement, (2) on grass, (3) under a single tree, and (4) under grove. Photo: ScienceDirect
The shade of the trees significantly reduced the temperatures by providing a significant reduction in shortwave solar radiation, the study said. Evenly distributing the trees on the site only increased the fraction of shaded area to 14.7 per cent from 11 per cent, it found.
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The major influence of tree layouts was on wind distribution. Tree shade reduced air temperature, while the low wind “cavity” increased the air temperature, the study found. However, the air temperature differences within the simulated sites for all layouts were less than 1°C.
Thus, it is unnecessary to scatter the trees for better shading since the increase in the shaded area by scattering was limited and clusters of trees can still be used to create an aesthetic appearance, the research suggested.