# Spatio-Temporal Analysis of Surface Urban Heat Island and Canopy Layer Heat Island in Beijing
## 4 Discussion
### Analysis of Changes in Intensity and Center of SUHI and CLHI  
Firstly, as shown in Figures 5 and 6, from a seasonal perspective, the intensity of SUHI and CLHI showed obvious seasonal variations, and we noticed that the intensity of nighttime SUHI and CLHI was stronger in the cold season and weaker in the hot season, while during the daytime was obvious in the hot season and weakened in the cold season,  
Figure 8: The distribution map of the annual average area and direction of (**a**) SUHI and (**b**) CLHI at the times of four crossings from 2003 to 2020. Figure 7: The broken line graph of annual mean intensity, longitude and latitude of (**a**) SUHI and (**b**) CLHI at the times of four border crossings from 2003 to 2020.  
which is consistent with the results of existing studies [30]. It has been shown that NDVI and albedo are the main drivers of heat islands; however, the factors that primarily influence heat islands vary by land use type [31; 32]. For example, higher evapotranspiration values associated with vegetation and water bodies result in lower heat island intensities [33]. As shown in Figure 9, During the cool season, NDVI values may be relatively low due to lower temperatures and slower vegetation growth. This indicates a decrease in vegetation cover and an increase in surface exposure as in Figure 10d, which may lead to an increase in surface albedo, as exposed to surfaces which usually have a higher albedo. On the contrary, during the hot season, vegetation growth is vigorous, NDVI values are high, vegetation cover increases, and surface albedo decreases, as in Figure 10b. Albedo is the ability of the surface to reflect solar radiation. Higher albedo means that the surface reflects more solar radiation and therefore the surface temperature may be lower. During the day, solar radiation is the main source of surface temperature [34]. Since urban areas usually have higher building densities and less vegetation cover, they absorb and store more solar radiation, resulting in higher intensities of SUHI and CLHI during the day. In the cool season, more solar radiation is reflected back into space due to the higher albedo of surface cover (e.g., bare soil, snow and ice, etc.) [35; 36; 37], resulting in less heat being absorbed by the surface as in Figure 10d, but at night, less heat is also released from these areas, and thus the nighttime heat island effect may be stronger. During the hot season, areas of lower surface albedo (e.g., vegetated areas) absorb more solar radiation and release heat during the day, leading to a stronger daytime heat island effect. Figure 9: Monthly time series of NDVI in Beijing from (a) 2003 to (b) 2020. Figure 10: Spring, summer, autumn and winter distribution of (**a**,\(c\),\(e\),_g_) NDVI and (**b**,\(d\),\(f\),_h_) albedo. One year (e.g., 2020) was selected to perform the correlation analysis between the monthly average NDVI and albedo distributions in June and July and the surface temperature, the relationship between the surface temperature and NDVI and albedo was depicted by scatter plots, and the results are shown in Figure 11a-d. Combining the scatter plots of surface temperature distribution with NDVI and albedo and their correlation coefficients for June and July 2020, it can be seen that the surface temperature distribution is negatively correlated with NDVI and albedo, which echoed the prior study [38], with correlation coefficients of Figure 11a,b being around 0.8, and those of Figure 11c,d being around 0.6, indicating that the that the heat island effect is attenuated in regions with high vegetation cover and high albedo. NDVI measures the growth and density of vegetation, thus indirectly reflecting the green space coverage in the city. Green spaces have higher transpiration and lower surface temperatures, which can help mitigate the SUHI and CLHI effects. At night, green spaces have lower temperatures due to less heat being absorbed and stored during the day, which may result in the center of SUHI and CLHI being concentrated in less vegetated areas [39].