Determinants of urban–rural land surface temperature differences – A landscape scale perspective

• Urban–rural surface temperature difference (ΔLSTUHI) averages at 4.2 K.
• Small urban areas (<1 km2) show ΔLSTUHI of more than 8 K.
• The urban–rural NDVI difference (ΔNDVI) is the major driver of ΔLSTUHI.
• The surrounding landscape is of crucial importance for quantitative UHI assessments.

Urban areas are known to have a local climate different from that of surrounding rural landscapes and the temperature difference constitutes an urban heat island (UHI). Despite being described as ‘heating islands’, cities are not isolated from their environment. But only few studies respect the UHI as a relative measure and consider both the drivers of the temperature level of the urban area and of the surrounding landscape in their UHI assessments. A series of 124 Landsat satellite images spanning four decades were used to derive land surface temperatures (LST) of 77 urban areas (between 20 and 1600 ha) and their surroundings in the Province of South Tyrol, northern Italy. These data were used to calculate urban–rural LST differences (ΔLSTUHI) and NDVI differences (ΔNDVI) for quantifying the urban heat island effect. ΔLSTUHI were recorded between 1.0 K and 8.1 K, with a mean ΔLSTUHI of 4.2 K. The results show pronounced UHI effects already for small cities of less than 1 km2, but suggest that the urban size is of only minor importance for the ΔLSTUHI magnitude in case of urban areas smaller than about 10 km2. Urban–rural land surface temperature difference (ΔLSTUHI) was identified to be primarily driven by ΔNDVI, together with solar irradiance and land use. Threshold values for the occurrence of ΔLSTUHI extremes are provided. Implications for landscape and urban planning are presented that underpin the crucial importance of considering the surrounding landscape for quantitative assessments of UHI intensity.