A Spatiotemporal Analysis of Extreme Minimum Temperatures in Portugal (1980–2024) Using Severity Heatmaps

Rising nighttime temperatures, particularly 'tropical nights' where minimum temperatures (Tmin) exceed 20°C, pose increasing public health risks by limiting nocturnal cooling. A comprehensive assessment of extreme Tmin trends across mainland Portugal was conducted for the 44-year period from 1980 to 2024, with a focus on changes in both the frequency and intensity of nighttime heat. High-resolution ERA5-Land reanalysis data (1,012 grid-points) were employed to compare two 22-year subperiods (1980–2002 and 2002–2024), with extreme Tmin events defined using the 99th (Q99 = 21.75°C) and 99.9th (Q99.9 = 24.41°C) percentiles. Two key metrics were calculated: the Frequency of Extreme Tmin (FET), representing event occurrence, and the Tmin Anomaly Percentage (TAP), representing event intensity. These metrics were integrated into severity heatmaps, which classify each grid-point into four categories based on whether the most recent period shows an increase relative to the early period: (i) concurrent increase in both FET and TAP, (ii) increase in FET only, (iii) increase in TAP only, and (iv) no significant increase in either metric. This classification facilitates the identification of areas experiencing the most pronounced intensification of nighttime heat. The results reveal a substantial increase in nighttime heat, particularly across southern Portugal, where exceedances of the Q99 threshold have increased by up to 45%. Additionally, TAP values indicate that extreme Tmin events have become, on average, 1.8°C hotter in the recent period (2002–2024). These findings provide relevant insights for public health and urban adaptation planning, while the severity heatmaps present a transferable tool for risk assessment applicable to other Southern European regions experiencing similar climate pressures.