Understanding winter outdoor thermal comfort in subtropical regions: Evidence from Hong Kong's cold weather events

Outdoor thermal comfort (OTC) is essential for public health. While the negative impacts on OTC during hot periods have been extensively studied, particularly in subtropical regions, there is a lack of evidence regarding cold conditions in these regions. Existing literature indicates that residents in regions with mild winters are less adaptable to cold snaps compared to those living in regions with cold winters. Overlooking the effect of cold days in warmer regions may lead to an underestimation of their impacts on thermal comfort and health, especially for vulnerable populations. Taking Hong Kong as a case study, this study assesses the cold weather events and their impacts on OTC in subtropical regions. Cold warning days, as defined by daily minimum air temperature at or below 12℃ and 7℃ respectively by Hong Kong Observatory, were used to identify cold weather events. Results show that during 2010-2019, Hong Kong experienced 98 cold days including 11 very cold days in downtown Nam Shan Estate, and 143 cold days including 16 very cold days in the new town of Tin Shui Wai. Physiological Equivalent Temperature (PET) evaluations by ENVI-met modelling reveal that the percentage of cold discomfort in spatial patterns exceeded 50% on cold warning days for both downtown and new towns. Compared to typical winter days, cold warning days resulted in 14.3–45.5% increase in the duration of thermal cold discomfort. Outdoor open spaces with less vegetation tend to be the most vulnerable areas throughout the day, identified as ‘cold spots’ in both downtown and new towns, potentially posing health risks to the elderly who are the primary users of urban outdoor spaces. Findings of this study underscore the critical need to identify and address the potential health risks associated with cold snaps in subtropical regions, particularly for vulnerable populations.