Factors affecting urban climate at different times of the day in China: A case study in Yibin, a riverside mountain city

Urban heat islands and global warming degrade urban climate substantially and negatively affect residents’ lifestyle. Experts have explored strategies to mitigate their effects. They found that vegetation and water surfaces had a significant cooling effect, which varied with the time of the day. Thermal performances of the two elements echoed the strategies of nature-based solutions in cooling. Meteorology stations in different counties of one city are sources of big databases providing regionally long-term meteorological data, available for climate studies. The present study used this data to investigate the variation in cooling effects in Yibin, a riverside mountain city located in China. The cooling effects of vegetation and water surface (indicated by river width) were significantly affected by the time of the day. Increasing vegetation cover reduced air temperature by a maximum of 2.2 °C (R² = 0.615, at 4:00 h). A similar cooling effect was observed with river (R² = 0.5149, at 14:00 h). Nevertheless, the cooling effects fluctuated during the day. Vegetation had significant thermal effects in the late afternoon and early evening (4:00–7:00), which is the coolest time during the day. Conversely, waterbodies showed limited effect on weather conditions in the evening. They were thermally adjustable in the afternoon (13:00–16:00, daily warmest time). This phenomenon is attributed to hourly weather changes. Overall, R² values of vegetation-temperature models decreased, while those of water-temperature models increased with temperature. These findings represent an advance in our understanding of urban climate. Also, it is important to find that the two key cooling factors (vegetation and river width) behave differently at different times of the day. To achieve full day cooling, it is imperative that future planning should incorporate proper landscaping in their design works. This finding is helpful for future generations in terms of climate change adaptation.