Nocturnal influencing patterns on outdoor thermal environmental parameters along an urban road in summer: A perspective of visual index

Urban surface compositions played a key role in affecting outdoor thermal environments in most cities in middle and low latitude in summer. It is essential to reveal crucial influencing patterns of urban surface compositions on outdoor thermal environment at night, when people do outdoor activities. In this study, vertical urban compositions and outdoor thermal environment were observed at three nights. Vertical urban compositions were represented with four visual indices, categorized into "Sky" and "Land"; "Sky" included sky visual index (SVI), while "Land" included construction visual index (CVI), road and pavement (imperious surface) visual index (R&PVI) and green view index (GVI). "Sky" and "Land" accounted for 27.15%-90.18% explanatory power in total at the three nights. "Sky" accounted for low explanatory power (0-27.13%), "Land" contributed highly (3.49%-73.24%), and their synergies were low (-6.62%-22.28%). In most time, SVI and R&PVI contributed low independent and joint effects, while CVI and GVI contributed highly. Contributions of the four visual indices to the outdoor thermal environment increased after sunset, probably due to heat release from "Land" in the energy process at ground surface. Besides, the influencing pattern was affected by the weather condition, such as precip-itation, wind velocity and accumulated net radiation during daytime.