Exploration of applicability of UTCI and thermally comfortable sun and wind conditions outdoors in a subtropical city of Hong Kong

This study conducted simultaneous physical measurements and questionnaire surveys to study the outdoor thermal comfort in the subtropical, high-density Hong Kong. With an innovative analysis method, the study reveals that mean radiant temperature (T_mrt), sun and wind desirability, and wind speed (v) were the top three factors influencing the surveyed thermal sensation vote (TSV), which was not well estimated by the Universal Thermal Climate Index (UTCI); and that UTCI underestimated the effects of v and T_mrt at higher air temperatures (T_a) but overestimated the impacts of T_a and relative humidity on TSV of subjects in Hong Kong. Accordingly, a combined sun and wind condition index (SWI) was defined to explore the combined effects of T_mrt and v on thermal comfort. Within the meteorological parameter ranges of 12.0 ≤ T_mrt ≤ 64.0 ℃, 0 < v ≤ 4.0 m/s, and 12.0 ≤ T_a ≤ 36.0 ℃, over 50% of the subjects voted comfortable within the SWI range of −0.1 to 0.2; and the SWI range of −0.3 to (−0.1) and that of 0.1 0.2 were more comfortable at T_a > 26.0 ℃ and T_a ≤ 26.0 ℃, respectively. A v-T_mrt diagram was thus constructed to indicate the comfort zones of sun and wind combinations, which can be used to estimate the comfort levels of outdoor space in the design stage.