Impacts of sub-daily rapid temperature drops on winter outdoor thermal comfort under climate change: A case study in Hong Kong

As climate change progresses, extreme heat and cold events are well documented. However, the sub-daily rapid temperature drop (RTD), characterized by a fast ambient temperature decrease within hours, remains understudied especially in subtropical regions where local residents are less adapted to cold, despite its significant impacts on human thermal comfort and health. This study examines wintertime RTD events in Hong Kong, inspecting the extreme condition defined by a threshold of 9 °C temperature decrease within 6 h on a single day, identified by the sliding window method. Using global climatic dataset and WRF model coupled ENVI-met simulations across three specific decades (2010–2019, 2040–2049, and 2090–2099), we found both increasing intensity (9.8 to 10.2 °C mean temperature drop magnitudes) and frequency (8.2 to 9.4 mean occurrence days) of RTD events across six representative urban districts during winters (November–March) under SSP2–4.5 climate scenario. Physiological Equivalent Temperature (PET) evaluations revealed that during RTD periods, (1) thermal comfort spans up to six distinct classes (from “Hot” to “Cool”), with elderly residents showing attenuated thermal responses (smaller ∆PET) that may mask their greater physiological vulnerability and lead to insufficient protective behaviors; (2) unshaded areas and sparsely vegetated spaces experience PET variations up to twice those of shaded and densely vegetated areas. These findings underscore the importance of incorporating RTD considerations into climate-resilient urban design, particularly emphasizing age-sensitive planning and strategic vegetation deployment in subtropical cities. • Developed a sliding-window statistical approach to detect rapid temperature drop (RTD) events. • Revealed intensifying RTD trends (magnitude 9.8 to 10.2 °C, frequency 8.2 to 9.4 days) by 2100 under SSP2–4.5. • Quantified RTD-induced PET swings spanning up to six comfort classes (e.g., “Hot” to “Cool”). • Elderly show reduced thermal sensitivity to RTDs despite higher physiological risks. • Unshaded and sparsely vegetated spaces offer reduced thermal buffering in response to RTDs.