Spatiotemporal Patterns of the Evolution of the Urban Heat Island Effect and Population Heat Exposure Risks in Xi’an, One of China’s Megacities, from 2003 to 2023

Under the dual pressure of rapid urbanization and global warming, the urban heat island (UHI) effect has been intensifying, accompanied by a continuous increase in heat exposure. As a typical example of rapid urbanization in China, Xi’an is facing severe challenges. However, previous research on diurnal variations in long-term UHI effects and heat risks is insufficient. So, this study utilized the temperature level threshold method and the heat exposure risk assessment model to investigate the spatiotemporal evolution characteristics of diurnal variations in the UHI and population heat exposure risks in Xi’an from 2003 to 2023. The results indicate that (1) over the past two decades, both the summer UHI intensity and the population heat exposure risks in Xi’an exhibited an overall intensifying trend, (2) spatial expansion followed a radial diffusion pattern centered on the urban core, with heat risk levels decreasing outward, (3) the nighttime expansion of high-level UHI zones and risk areas was slightly less than during the daytime, and (4) changes in the thermal environment often preceded population aggregation, indicating a lag effect in the evolution of heat exposure risks. This study deepened the understanding of the UHI and heat exposure for governments and planners and can help propose scientific UHI mitigation measures.