# Characteristics of Urban Heat Island in China and Its Influences on Building Energy Consumption
## 3 Results and Discussion  
### Spatial and Temporal Variation of UHII in Main Chinese Cities  
The annual mean UHII is shown in Figure 2.  
The annual mean UHII ranges from 0.03 to 3.1 \({}^{\circ}\)C; 52.3% of cities have an annual mean UHII between 0.5 and 1 \({}^{\circ}\)C. The relationship between annual mean UHII and daily maximum UHII or median daily UHII in different cities was analyzed and the results are shown in Figure 3. It is found that the annual mean UHII has positive correlations with the maximum of daily mean UHII (Figure 3a) and median of daily mean UHII (Figure 3b). With R\({}^{2}\) = 0.97, the median of daily mean UHII can represent well the annual mean UHII. The maximum daily mean UHII has a weaker correlation with annual mean UHII (R\({}^{2}\) = 0.26) as indicated by more scattered data points in Figure 3a compared to Figure 3b. It suggests that large-scale background weather, such as extreme heat waves [52], can affect the daily mean UHII occasionally, leading to extreme values of daily UHII. The deviation of the maximum daily mean UHII from the annual mean UHII is large, which suggests that we should consider extreme weather conditions in urban planning and policymaking to avoid dramatic damage.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Building Climate Zones** & **Heating/Cooling** & **COP Values** \\ \hline SCR & For heating \(COP_{h}\) & 1.6 \\ \cline{2-3} CR & For cooling \(COP_{c}\) & 2.8 \\ \hline HSCWR & For heating \(COP_{h}\) & 1.8 \\ \cline{2-3} HSWWR & For cooling \(COP_{c}\) & 2.8 \\ \hline \hline \end{tabular}
\end{table}
Table 2: _COP_ of a split air conditioner used in a residential building for heating and cooling, which were provided according to standards [50].  
The characteristics of temporal variation of UHII are shown in Figure 4. Figure 4a presents the typical variation of synthetic daily UHII profiles based on the whole year data for SCR cities (Equation (2)), i.e., UHII\({}_{\rm{SD}}\). A \"U\" shape forms for most cities. UHII\({}_{SD}\) begins to decrease around 6 am after sunrise because the rural air temperature increases rapidly because of its relatively low thermal capacity. During 10:00-14:00, the UHIISD reaches the lowest values and begins to increase after around 16:00 when the solar radiation decreases dramatically [53]. As for the seasonal mean UHII (Figure 4b), the values are lowest in summer and highest in winter. The relatively low value of solar radiation intensity and high anthropogenic heat emissions due to heating demand in buildings contribute to the high UHII in winter [52]. The magnitude of UHII during spring has a value similar to that of the solar radiation intensity.  
Figure 3: (**a**) The relationship between annual mean UHII and daily mean maximum UHII. (**b**) The relationship between annual mean UHII and median daily UHII.  
Figure 2: Annual mean UHII for 61 prefecture-level cities and 136 county-level cities in 2019.  
during autumn. It should be mentioned that the temporal variation of UHII in other cities presents a similar trend as in typical SCR cities shown in Figure 4. For simplicity, the data are not shown here.