An observational analysis of the urban heat Island in the eastern Mediterranean coast: The case of Thessaloniki, Greece

Urban areas are increasingly affected by rising temperatures and frequent heatwaves (HWs), mainly due to climate change and rapid urbanization. Since the Mediterranean region has been recognized as a climate change hotspot, Thessaloniki, a coastal city in Northern Greece, offers a representative case for exploring the Urban Heat Island (UHI) effect and variability. Based on the hourly surface air temperature data retrieved from a network of stations within five strategically chosen Local Climate Zones (LCZs) during the period 2018–2022, our analysis evaluates the UHI intensity (UHII) and its spatial patterns across areas characterized by different urban morphology. The highest monthly UHII was estimated at 2.1 °C in the urban core. The results revealed a clear UHI spatial gradient across the LCZs, with daily UHII ranging from − 2.3 °C to 2.4 °C during daytime and from 0 °C to 2.8 °C at night, with peak intensities observed in summer. Spatiotemporal variations strongly corresponded to each station’s LCZ, highlighting the influence of the urban form on the UHI patterns. Comparison of the UHI under HW and non-HW conditions revealed a notable intensification during HW events. Heatwaves were found to amplify the nocturnal UHI intensity by 0.7 to 1.0 °C, most pronounced in dense urban areas, while daytime fluctuations remained minimal. Statistical tests were conducted to ensure robustness and significance of the results. Finally, our study compares the findings with prior research to track the UHI evolution in Thessaloniki and draws parallels with other Mediterranean coastal cities. In synthesis, this work contributes to linking specific urban features to the UHII in a site representative of the Southern European coastal climate. The findings provide guidance for urban planning to improve thermal comfort for urban residents in a climate change hotspot where heatwaves are projected to increase in frequency, intensity and duration.