Effects of tree seasonal characteristics on thermal-visual perception and thermal comfort

To understand effects of tree seasonal characteristics on pedestrian thermal-visual perception and explore their influence on outdoor thermal comfort, four typical deciduous shade trees, local to Xi'an China, were measured for annual crown characteristics. Meteorological variables and pedestrian thermal and visual perceptions were simultaneously collected. Universal Thermal Climate Index (UTCI) and visual conditions were used to stratify our results. We found that: (1) Neutral UTCI (NUTCI) under trees decreased in order of G. biloba, P. orientalis, Y. denudata and M. azedarach. (2) The ability of trees to reduce UTCI was related to leaflet area, crown color (degree of red and green [a*] and brightness of the color [L*]) and diffuse non-interception (DIFN). The thermal sensation vote (TSV) reduction was related to crown vertical projected area (Acrown) and L*. (3) The ability of trees to reduce the illumination intensity was related to Acrown, DIFN and L*, and the sun sensation vote (SSV) reduction was related to L*. (4) NUTCI in the environment with high illumination was lower than that with low illumination. When UTCI was 1°C−37 °C, respondents who reported ‘slightly bright to too bright’ primarily perceived a neutral thermal sensation, followed by these who felt ‘neutral’ and ‘slightly dark to too dark’. Respondents who reported high aesthetic assessment scores tended to vote warm TSV in winter. Our results could provide selection baselines for urban shade trees in optimizing overall pedestrian comfort and promoting urban livability.