Effects of urban trees on local outdoor microclimate: Synthesizing field measurements by numerical modelling

In this study, we investigated the effects of trees on the local urban microclimate and human thermal comfort under different local weather conditions, in a small urban area in Assen, the Netherlands. In both summer and winter, continuous air temperature and relative humidity measurements were conducted at five selected sites having obviously different environmental characteristics in tree cover. Measurements demonstrated that in summer the microclimatic conditions at each observation site showed significant differences. The cooling effects of trees on clear and hot days were two times higher than on cloudy and cold days. In winter, air temperature was slightly reduced by the evergreen trees, and weather conditions did not cause a notable change on performance of trees on the microclimate. ENVI-met, a three-dimensional microclimate model was used to simulate the spatial distribution of temperature and humidity. After selecting representative days, we simulated the study site as it currently is and for a situation without trees. Spatial differences of trees’ effects were found to vary strongly with weather conditions. Furthermore, human thermal comfort is indicated by the Predicted Mean Vote model. During the hottest hours, trees improved the thermal comfort level via reducing ‘very hot’ and ‘hot’ thermal perception by about 16 % on clear days and 11 % on cloudy days. Generally, our findings demonstrate that urban microclimate and human thermal comfort convincingly varies in close geographical proximity. Both are strongly affected by the presence of local trees. Weather conditions play an important role on the trees’ performance on the summer-time microclimate.