Quantifying the effect of rain events on outdoor thermal comfort in a high-density city, Hong Kong.

Rainfall events often cause a modification to atmospheric conditions. The impact of this phenomenon on human thermal comfort has however been less well studied. Therefore, this paper quantifies the effect of rainfall events on human thermal comfort in a hot-humid subtropical city, Hong Kong. Firstly, rainfall events were categorized based on time of occurrence, i.e., morning (on or before 11:00 LST), afternoon (12:00-15:00, LST), early evening (16:00-18:00), and all-day events. Thereafter, human thermal comfort on typical non-rainy (sunny) days and rainy days was estimated and compared by using the radiation-driven physiological equivalent temperature (PET) and non-radiation-driven temperature-humidity index (THI) and compared. Results revealed variable and stable hourly patterns of PET and THI thermal classification, respectively under different rainfall event category. The insensitivity of THI values could be due to the retained strong contribution of both input parameters (air temperature and relative humidity) on both rainy and non-rainy (sunny) days. An understanding of the mechanism of thermal changes before, during, and after rainfall events based on statistical analysis suggests a strong interplay between moisture content and air temperature as determinants of thermal comfort in the hot-humid city and not necessarily the radiation parameter. This finding suggests that while PET clearly shows the impact of rain-event; it is principally due to the strong contribution of the lowered radiant temperature in its calculation while in reality, the critical determinants of thermal comfort in such period in a hot-humid subtropical environment like Hong Kong are the moisture content and ambient temperature. Finding from the study could enhance occupational health and safety management of outdoor workplaces.