Trends and regional European patterns in “atmospheric potential” for the urban heat island effect in the ERA-5 reanalysis.

The canyon layer urban heat island effect (UHI) is the best known example of how urban surfaces affect the microclimate. Lots of studies have focused on the how the UHI magnitude is determined by the urban morphology such as e.g. green vegetation fraction and sky-view factor. Although these aspects play a crucial role, the UHI needs first of all an atmospheric state that promotes UHI formation, e.g. high solar insolation, clear skies, low synoptic wind speed, and stably stratified nights. Given the ongoing climate change the frequency of occurrence of these favorable conditions may change. This study explores trends in this so called “atmospheric potential” for the UHI for summer periods from 1950 till 2025 using ERA5 re-analysis data. The atmospheric potential is expressed in a ratio that contains solar radiation, diurnal temperature range (DTR), and wind speed as observed in the countryside. For Europe we find clearly spatially different patterns of changing UHI potential. We find that the atmospheric potential for the UHI has increased by 0.6 K, apart from the U.K. the Alps, Norway and Sicily (decrease of 0.1-0.4 K). Further analysis reveals that of the three governing terms, the trends in DTR and surface solar radiation are the dominant drivers for the increase in UHI potential. Apart from a band from the U.K. to Bulgaria, the wind speed term reduces the UHI potential across Europe, although only by ~0.1K. We conclude that apart from urbanization effects, also the atmospheric potential for UHI is subject to a substantial long-term trend.