Spatiotemporal evolution and influencing factors of surface urban heat island footprint across different-sized cities

With the development of urbanization, the urban heat island (UHI) effect has gradually increased. The UHI footprint (FP) is an important indicator of the UHI effect and can quantitatively characterize its spatial pattern, it is essential to investigate the state of UHI FP for mitigating UHI problem. Therefore, the logistic model was applied in this study to assess the spatiotemporal characteristics and evolutionary trends of the surface UHI (SUHI) FP of three cities of different sizes (Beijing, Shijiazhuang and Cangzhou) from 2001 to 2020. The results indicated that the SUHI FP area tended to increase as the city size increased. The average areas of daytime SUHI FPs in Beijing, Shijiazhuang and Cangzhou were 2192.69 km2, 832.32 km2 and 222.83 km2, respectively. During the daytime, the fractal dimension index (FRAC) increased with city size and time, and the footprint expansion intensity index (FEII) increased with decreasing city size. However, for the FEII of the normalized SUHI FP, the values for super huge city were generally negative. And there was a significant positive correlation between the SUHI FP and built-up area (BA), population (POP). The study is required to provide comprehensive references for sustainable urban planning and development.