The impact of urban morphology on thermal environment under extreme heat: A case study in Nanjing, China

The diverse urban morphology leads to different microclimates within urban areas. However, little is known about the relationship between different urban morphology types and their impact during extreme heat events, particularly in humid subtropical climates. To address this gap, we analyzed six types of blocks in Nanjing, China, distinguished by three building heights and two street orientations, to understand their differential impacts on the thermal environment under extreme heat conditions. Meteorological observations were carried out through a combination of on-site measurements and numerical modeling, with the analysis being based on the simulated data. The results showed that: (1) Blocks with a Northeast to Southwest (NE-SW) orientation offer more effective cooling compared to a North to South (N-S) orientation. (2) The Physiological Equivalent Temperature (PET) categorized as ‘Very Hot’ (above 41 °C) was consistently lower on N-S oriented blocks than on NE-SW oriented blocks throughout the day, with this difference diminishing as building height increased. (3) The worst thermal comfort conditions were observed on NE-SW oriented blocks with low-rise buildings, where the ‘Very Hot’ level accounted for 63 % of the time between 10:00 and 19:00. In contrast, the proportion dropped to 42.7 % for middle-rise N-S oriented blocks, indicating better thermal comfort. Our findings provide new insights for urban planners and policymakers, suggesting that thoughtful urban morphology planning can contribute to the development of healthier and more environmentally friendly cities.