3.1. Land Surface Temperature
and Figure 6
display the distribution of estimated LST, based on 30 × 30 m grid cells. A similar pattern is observed during all four seasons: areas with higher LSTs tend to be concentrated in the northern part of the river, where the built-up area situated. Low LSTs are mainly concentrated in the western and eastern parts of Paris, where the green area coverage is quite high. The areas holding higher LSTs are located in the city center, extended to the west in spring and winter and the east in summer and autumn.
The proportions of high-LST (above average) area in Paris are 56.51% (20.2 °C in average), 58.13% (27.85 °C in average), 54.01% (18.27 °C in average) and 47.13% (4.93 °C in average) in spring, summer, autumn and winter, respectively. There is a slight downward trend from spring to winter.
Compared to Paris, the mean values of LST in Geneva display a different patterns for each season. Areas with higher mean LST are located within the city center in Spring and Summer, close to the lake. The high LST areas are observed in the southwest of the city in autumn, and on the west side of the city in winter. In general, mean LST is higher in Spring in Geneva while in Paris, the higher mean LST is observed during summer. From Spring to winter, it can be observed that the highest and the minimum LST all decrease. The proportion of high LST (above average) area in Geneva are 34.93% (17.58 °C in average), 33.42% (26.86 °C in average), 40.14% (20.42 °C in average) and 52.68% (5.9 °C in average) in spring, summer, autumn and winter, respectively. There is a slightly upward trend from spring to winter, which is opposite to Paris.
3.4. Grid-Based Results
As shown in Figure 11
and Figure 12
, the dark red area represents higher LST. The light red area represents the lower LST. Circles with larger radius represent areas with greater impervious surface density. In Paris, areas with greater impervious surface density are located along River Seine, associated with higher LST. The light red areas are generally covered by smaller radius circles, indicating smaller density of impervious surfaces. The bar chart describes the coverage of each density group. The group with higher density of impervious surfaces has a greater proportion of coverage. The mean LST of each density group increased significantly (p
< 0.01) along the four seasons. Among the four seasons, the largest difference between the highest mean LST and the lowest mean LST was observed in summer, reaching 2.9 °C, around 0.7 °C per 20 ha/km2
increase of impervious surface density. The smallest difference is observed in winter, equaling 0.8 °C, corresponding to around 0.2 °C per 20 ha/km2
increase of impervious surface density. The largest error bar is observed in the density group of 20–40 ha/km2
, with only 1.2% coverage in the whole study area. More than 70% of the grids are located within the density group of 80–100 ha/km2
, showing the smallest error bar.
In Geneva, areas with greater impervious surface density are not always associated with a high LST. The distribution of impervious surfaces is similar to that of Paris. The higher impervious surface densities are located around Lake Geneva, where the pattern of LST changed with season. The city center, connected with Lake Geneva, has a higher impervious surface cover and a relatively higher LST. A low LST is observed in the southwestern part of Geneva in spring, where around one third of the area showed a higher impervious surface density. The east bank of Lake Geneva, where the LST is lower, showed higher impervious surfaces densities in summer, autumn and winter. The group with a smaller density of impervious surfaces covered a larger proportion, which is opposite of the results for Paris. The mean LST of each density group increases significantly (p < 0.01 in spring, summer, p < 0.05 in autumn and winter) with the LST for all seasons. The largest difference between the highest mean LST and the lowest mean LST is observed in spring, equally 5.1 °C, which corresponds to around 1.3 °C per 20 ha/km2 increase of impervious surfaces density. The smallest difference is observed in winter, equally 0.6 °C. The temperature differences in spring and summer in Geneva are greater than those found in Paris, while the LST differences in autumn and winter are similar to the ones of Paris.
As shown in Figure 13
, most of the areas with greater green area density are situated in the west and east of Paris. Few are located in the northeastern part of the city, where LSTs are very low. Generally, areas with a larger green area density are associated with lower LST values. The largest coverage of green area is observed in the density group of 0–20%, which is near 70% (Figure 13
e), while the second largest coverage is observed in the density group of 80–100%. The mean LST of each density group decreases significantly with LST (p
< 0.01 and p
< 0.05). Similar to the pattern of impervious surfaces, the largest difference between the highest mean LST and the lowest mean LST is observed in summer, equally 3.3 °C, which corresponds to around 0.8 °C per 20 ha/km2
decrease of green area density. The smallest difference (0.8 °C) is observed in winter. The differences in LST in summer and autumn measured for green area were larger than those for impervious surfaces during the same season. The LST differences in spring and winter were very similar. The largest two error bars are observed in the density group of 40–60 ha/km2
and 60–80 ha/km2
, with coverage less than 5% in total.
Contrary to the distribution of impervious surfaces in Geneva, greater green area densities are situated close to the edge of the city (Figure 14
) where impervious surface densities are smaller. Generally, most greater densities of green area are correlated with lower LST, except for winter. The western part of Geneva, where LSTs are very high, is almost fully covered by larger densities of green area. The group with a larger density of green area has a larger proportion of low LST (Figure 14
e). The mean LST of each density group went up firstly, and then went down in spring and summer. There is no general rule for the pattern of LST in autumn and winter. The overall LST increased by 0.9 °C in autumn, and 0.02 °C in winter. An unusual value, similar to Paris, is also observed in the density group of 40–60 ha/km2
in winter. The largest error bar is observed in the density group of 0–20 ha/km2
, with around 10% coverage.
The overall patterns of green area with LST in Geneva are different from Paris. Comparing impervious surfaces and green area maps, the area covered by smaller dots in green area maps (Figure 14
a–d) is covered by larger dots as shown in impervious surfaces maps (Figure 12
a–d). LST decreases with green area density, while it increases in autumn.