Exploring urban heat islands with a simple thermodynamic model

The urban heat island (UHI), where urban areas experience higher near-surface temperatures than surrounding rural areas, has long been recognized as a serious issue in urban climatology due to global warming and rapid urbanization. This study investigates the key mechanisms of the UHI through a simple theoretical thermodynamic model. Using a simple day-night model based on the surface energy balance (SEB), we demonstrate that the UHI primarily results from two mechanisms: reduced diurnal temperature range (DTR) due to larger heat capacity of urban materials and increased mean temperature due to lower urban albedo. These mechanisms explain why the UHI intensity is stronger at night than during the day. The UHI intensity obtained from the theoretical model shows a qualitatively similar diurnal variation to that found in observations, supporting the applicability of the theoretical model for understanding the UHI. An analysis of temporal dynamics of UHIs in a megacity (Seoul) and a major city (Suwon) in South Korea shows that the long-term changes in the UHI in both cities are significantly correlated with those in the urban-rural difference in DTR, highlighting the role of urban heat storage in the UHI. In particular, this study emphasizes that well-known UHI mechanisms can be explained in a simple and intuitive manner through a time-integrated theoretical framework, underscoring the academic value of simplified models in interpreting complex urban climate processes. Moreover, this approach demonstrates broader applicability beyond UHI research, suggesting that such models can serve as practical tools in diverse climatic and environmental contexts.