Regulation of outdoor thermal comfort by trees in Hong Kong

Urbanization is transforming human society in many ways. Besides all the obvious benefits, it also brings negative impacts such as the well-documented urban heat island (UHI) effect and the magnified human heat stress. One way to reduce human heat stress is to increase vegetation density in urban areas, because they can provide evatranspiration and shading benefits. However, given the diversity of tree species and their morphological properties, it is important to understand rationally how different trees regulate thermal comfort. In this study, we investigated the impact of various trees on urban micrometeorological conditions in both open space and high density settings, and how they regulate outdoor thermal comfort. The study shows that trees planted in high density settings are more effective in improving pedestrians' thermal comfort than those in open spaces. The study further shows that trees with a large crown, short trunk, and dense canopy are the most efficient in reducing mean radiant temperature (Tmrt). Therefore we recommend five specific ways to facilitate the integration of tree planting into urban design. In a broader sense, our studies suggest that urban trees should be planted strategically to improve human thermal comfort as an integral part of all modern urban developments.