## 3 Results and Discussion  
### Spatiotemporal Variation of Urban Thermal Characteristics  
#### 3.1.1 Spatial Distribution of Different Surface Temperature Levels  
Figure 2 drafts the spatial distribution maps of different NLST levels. Lower temperatures (levels I and II) had significant spatial correlation with large urban water bodies such as the Yangtze River, Han River, East Lake and Shahu Lake, expressing a cooling effect [32]. However, the distribution patterns of different temperature levels varied over time. Figure 2a,b share similar patterns in 1987 and 1996 with higher temperatures (levels IV and V, defined as urban heat island in this study) were mainly distributed along the main roads and concentrated in urban industrial areas in the city core. Compared with the surrounding scattered lower temperatures, the concentrated higher temperature area presented an obvious \"heat island\" effect. Figure 2c,d indicate that higher temperatures sprawled throughout the study area in 2007 and 2016. Areas covered by a higher temperature were scattered rather than concentrated in the study area, which expanded the spatial extent rather than the intensity of the heat island effect. Figure 2: Spatial distribution maps of normalized land surface temperature (NLST) levels in 1987 (**a**), 1996 (**b**), 2007 (**c**) and 2016 (**d**). Between 1987 and 1996, the most dominant NLST level conversion was from medium temperature (level III) to sub-high temperature (level IV). The converted area was 67.7 km\({}^{2}\), accounting for 47.8% of the outward transfer of level III (the transfer from level III to other NLST levels, the same below) and 67.5% of the inward transfer of level IV (the transfer from other NLST levels to level IV, the same below). It was mainly detected in the newly expanded construction areas in Qingshan, Jianghan and Zhuankou districts (Figure 3a). Between 1996 and 2007, the most common transitions were from high temperature (level V) to sub-high temperature (level IV) (86.6 km\({}^{2}\)) and from sub-high temperature (level IV) to medium temperature (level III) (66.9 km\({}^{2}\)). They were mainly distributed along the dense streets and traffic road in the inner city (Figure 3b). It should be noted that the decreased LST levels did not mean an improvement in the thermal environment. Actually, the majority of the study area during this sub-period was involved in warmer temperatures due to rapid urbanization, which lowered the temperature difference between NLST levels and changed the area proportion of different LST levels. Between 2007 and 2016, the main LST transition was from medium temperature (level III) to sub-high temperature (level IV), with the converted area of 79.2 km\({}^{2}\). It contributed 76.3% to the inward transfer of level IV and 73.6% to the outward transfer of level III. The next largest transition was that from sub-high temperature (level IV) to medium temperature (level III) (56.9 km\({}^{2}\)). The interconversions between level III and level IV were scattered in the densely developed areas (Figure 3c). Overall, between 1987 and 2016 (Figure 3d), 186.7 km\({}^{2}\) of sub-high temperature (level IV) was transferred from other LST levels. Among them, 89.4 km\({}^{2}\) and 73.5 km\({}^{2}\) were from medium temperature (level III) and high temperature (level V), respectively. The transition from level III to level IV was the main contributor to both the inward transfer of level IV (47.9%) and the outward transfer of level III (70.7%).