# Satellite Remote Sensing of Surface Urban Heat Islands: Progress, Challenges, and Perspectives
## Key Research Findings
### Relationship between Surface and Air UHIs  
Since air UHIs are commonly impeded by lacking sufficient spatial detail for urban land use planning [25; 26; 27; 28], predicting the air temperature in both urban and rural areas indirectly from satellite observations has gained considerable interest [25; 32; 188; 189; 190; 191]. Finding a simple and general relationship between SUHIs and air UHIs is not an easy task and the published relationships remain empirical [29]. Generally, the largest air UHI intensity is observed at night, while the SUHI peaks during daytime [62]. Air UHIs are often weak during the late morning and throughout the daytime and become more pronounced after sunset, due to the slow heat release from urban structures.  
Surface and air UHI differences and the timing of their peaks depend on the urban and rural surface properties (e.g., moisture, aerodynamic roughness, albedo, emissivity, and thermal admittance), season, geography, and prevailing weather conditions [29; 192]. [PERSON] et al. [20] carried out a campaign in Leipzig, Germany, providing LST from airborne flights and air temperature (T\({}_{\text{air}}\)) from weather stations and a mobile traverse. A higher SUHI magnitude was detected in the morning than in the evening, whereas the diurnal trend was opposite for the canopy layer heat island (CLHI) [20]. [PERSON] et al. [193] examined 48 cloudless, anti-cyclonic nights in Birmingham, UK, from 2002 to 2007 by using MODIS LST and T\({}_{\text{air}}\) from ground stations. The nighttime SUHI-CLHI comparison revealed a linear relationship, whose slope was partially explained by the built-up area fraction inside the satellite pixels [193]. For the city of Milan, Italy, surface UHIs, retrieved from 403 MODIS summer images available four times a day from 2007 to 2010, highlighted how the SUHI has a diurnal cycle, with the highest intensity in the early afternoon. This is halved during the night. By contrast, the CLHI is absent during the daytime and emerges after sunset showing features similar to the nighttime SUHI [25]. The same daytime/nighttime behavior was found by computing SUHI from MODIS and CLHI from meteorological stations across 300 urban areas in the United States [124]. Surface and air UHIs retrieved from a year of MODIS data over Beijing, China, showed agreement during the night regardless of the season, but an evident difference during the daytime, with the SUHI\(-\)CLHI intensity difference being small and negative during the cold months while being large and positive during the warm months [189]. [PERSON] et al. [194], using T\({}_{\text{air}}\) from a meteorological network and MODIS LST during the summer months in 2013 in Birmingham, UK, demonstrated that SUHI is clearly linked to land use, whereas CLHI is more influenced by advective processes that cause different spatial patterns. Surface and air UHIs exhibited exponential decay trends moving away from the urban area of Wuhan, China, with different magnitudes. This was more evident in the summer than in the winter [195]. In the region of Hangzhou, China, the UHIs computed by T\({}_{\text{air}}\) from weather stations and LST from Landsat images were not comparable, which demonstrates the importance of the choice of dataset, acquisition time, and weather conditions for SUHI\(-\)CLHI comparison [196].