Air or surface temperatures in urban environment over the case study of Toulouse, France: Confusion and synergy

Extreme urban heat leads to health issues and even deaths. Therefore, urban stakeholders need to adapt cities to reduce these health impacts. The locations exposed to heat within the city, as well as the cooler areas need to be identified. The way to diagnose them from quantitative observations is not an easy task, as ambient air temperature, which is of primary importance for health impacts, is rarely observed within cities. The use of Land Surface Temperature (LST) from satellite sensors is often used as a proxy of air temperature. However, both parameters cannot be conflated and their comparison is necessary in order to highlight their differences and similarities to avoid such confusion and misleading interpretation for urban heat-related studies. In this paper, we aim to analyze the differences in the spatial variability and intensities of both parameters, at day and night during summer and winter as well as their corresponding air temperature Urban Heat Islands (AUHI) and Surface Urban Heat Island (SUHI). The case study is the one-million inhabitants metropole of Toulouse, France. While air temperature and LST are on average similar in wintertime, large differences occur during summer. During summer nights, air temperature and LST exhibit a similar spatial distribution but they do not exhibit the same range. We confirm that LST should not be used as a quantitative proxy for air temperature during summer daytime or nighttime and provide some reflections on how to use the LST in order to be complementary to the air temperature.