Studying urban heat islands (UHIs) in Southern Europe is crucial, as they amplify heat risks under climate change. UHIs and their temporal variability at seven urban–rural pair locations in Spain were analysed from 1970 to 2023. The UHI was defined as the air temperature difference between each urban site and its neighbouring rural sites, and trends were analysed using the non-parametric Mann–Kendall test with Sen’s slope estimator. Based on daily minimum air temperature data, results indicated a mean UHI intensity ranging from −0.15 °C in Alicante to 2.28 °C in A Coruña. The UHI annual trend was significant, increasing in Valladolid (0.023 °C/year) and Alicante (0.009 °C/year) and decreasing in Santander (-0.015 °C/year). Seasonal analysis showed statistically significant trends in Valladolid, particularly in spring and summer (0.029 °C/year). In Alicante, an increase of around 0.012 °C/year was observed in spring and summer, while Madrid showed a trend of 0.012 °C/year in winter. However, a warming effect at the rural site was identified in Barcelona (−0.028 ºC/year in autumn) and in Santander −0.025 °C/year in spring and summer), corresponding to negative UHI trends. The influence of synoptic patterns on UHI yielded values between 3 and 4 °C in A Coruña and Madrid for anticyclonic southeasterly, anticyclonic southerly, and southeasterly air flows. Lower intensities were found in Barcelona (2.5 °C) and were associated with hybrid anticyclonic westerly flows. UHI intensities below 2 °C were obtained at the other locations, with the lowest values being linked to hybrid cyclonic westerly and cyclonic north-westerly flows.
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