## 3 Results and Discussion
### Response of Urban Heat Island Effect to Land Use/Land Cover Change
#### 3.3.2 Dynamic Response of UHI Evolution to LULC Transition  
When masking the spatiotemporal variation maps of NLST levels (Figure 3) over the corresponding LULC transition ones (Figure 5) during the observed sub-periods, we found that the thermal characteristic variation did not always correspond to the LULC transition as expected [15; 25; 29]. To further investigate the dynamic response of the UHI effect to LULC change, the land transition types and their contributions to enhanced NLST levels were counted. Figure 7 illustrates the area proportion of different land transition types for the inward transfer types of high temperature (level V) and sub-high temperature (level IV). In general, the large amount of construction land transferred from other LULC types significantly raised the NLST level and deteriorated the thermal environment [20; 26; 30; 44]. However, the main LULC transitions contributing to thermal deterioration varied in different sub-periods. During 1987-1996 (Figure 7a), the three main contributors of raising NLST level I to level IV and level V were the land transitions from water bodies to farmland (23.4% and 34.9%), from water bodies to construction land (21.0% and 15.9%) and from farmland to construction land (16.1% and 6.4%). The land transitions from water bodies to construction land and from farmland to construction land contributed 38.3% and 56.1% to thermal variation from level II to level V and from level III to level V,  
Figure 7: Area proportion of different LULC transitions for the inward transfer types of high temperature (level V) and sub-high temperature (level IV) during 1987–1996 **(a)**, 1996–2007 **(b)**, 2007–2016 **(c)** and 1987–2016 **(d)**. In the legend, Fa., Gr., Wa. and Co. represent LULC types, namely farmland, greening land, water body and construction land, respectively. The symbol “-” indicates the transition process between different LULC types. For example, Gr.-Fa. represents the conversion from greening land to farmland. respectively. The conversion of farmland to construction land was the main driving force in a slightly deteriorating thermal environment. For example, it contributed 57.4% to pushing level III to level IV. During 1996-2007 (Figure 7b), the land transition from water bodies to construction land was the major contributor to the outward transfer of NLST level I to level IV and level V, with contributions of 77.2% and 54.6%, respectively. In addition, the transition from farmland to construction land contributed more than 60% to the inward of UHI levels (levels IV and V) from non-UHI levels (levels II and III). Overall, the inward transfer of construction land from the other LULC types was mainly responsible for the thermal deterioration in this period. Previous studies confirmed that the UHI effect is context sensitive [14; 42; 46]. Rapid urbanization had significantly changed the context characteristics of the study area during this sub-period, which warmed the whole area but weakened the temperature difference among different LST levels. During 2007-2016, the mutual transitions among different NLST levels were not frequent (shown in Figure 3 and Table 4) excluding the one from level III to IV, due to relatively stable LULC conditions (Figure 7c). The expansion of construction land slowed obviously in our study area, as urban expansion of Wuhan in this sub-period had sprawled outside the third ring-road [28]. The inward transfer of level IV and level V from other NLST levels was attributed to the land transition from farmland to construction land near the third ring-road. During 1987-2016 (Figure 7d), on the whole, the land transitions from water bodies and farmland to construction land were responsible for the majority of thermal deterioration. Specifically, the transition of water bodies to construction land was the main driving force in rapidly upgrading NLST level I, leading to 55.8% and 58.6% of level I being converted into level IV and level V, respectively.