Exploring the Long-Term Trends of Outdoor Thermal Comfort Under Urban Heat Island and Climate Change Effects in Taiwan’s Cities

Outdoor thermal comfort is a crucial indicator for assessing urban livability. With the intensification of climate change and the urban heat island effect (UHI), thermal comfort in Taiwanese cities has gradually deteriorated. This study analyzes changes in outdoor thermal comfort in two major cities—highly urbanized Taipei and rapidly urbanizing Taichung—using the Universal Thermal Climate Index (UTCI) from 1990 to 2023. Hourly climate data (temperature, humidity, solar radiation, and wind speed) from both city centers and suburban coastal areas were collected and used to calculate hourly UTCI values. Additionally, annual overheating hours and their severity were examined. The results show that, although Taipei is located in a cooler climate zone, its annual overheating hours and severity have exceeded those of Taichung over the past 34 years, along with a higher growth rate. Particularly the growth rate, Taipei’s annual growth rate is approximately 1.6 to 1.8 times that of Taichung. Analysis of the urban and suburban data reveals that Taipei’s urban expansion has already impacted the surrounding suburban areas, accelerating the deterioration of thermal comfort, with a discomfort growth rate approximately twice that of the city center. In contrast, Taichung’s suburban areas have not yet experienced significant urban development, resulting in a growth rate of thermal discomfort that is only one-third to one-half of that in the city center. Overall, the findings highlight that the impact of the UHI effect on thermal comfort far outweighs that of climate change. As urbanization continues, the negative effects of UHI will become increasingly significant.