Decadal Trends in Urban Heat and Cool Islands: Insights from a Long-Term Sensor Network in Bragança, Portugal

In 2011, a network of 23 sensors was installed in Bragança, a small city in the North of Portugal (Koppen Geiger - Csb). It has been continuously running ever since, recording air temperature across a wide range of local climate zones (LCZs), extending beyond the city limits. The collected data was used to evaluate seasonal and spatial patterns, temporal trends, and climatic anomalies, while changes over time were also assessed. Results indicated a higher intensity of the UHI in the more urbanized Local Climate Zones (LCZs), with significantly elevated intensities during the summer (up to 4^oC). The Urban Cool Island Effect (UCI) was also notable, occurring with greater intensity during the early hours on summer days. Despite some consistency across measurements for each LCZ, microclimate effects generated variability within single classifications.  Trend analysis indicates a gradual increase of up to 2°C in air temperature over the studied period, attributable to the combined effect of urbanization and global climate change. The year 2022 stood out as the warmest in the analyzed series. The assessment of climatic anomalies shows that as urbanization intensifies, the UHI exacerbates extreme climatic events, directly impacting the urban quality of life. This underscores the urgent need for action to mitigate the effects of urbanization on climate change. Finally, some recommendations for local urban climate planning are presented, and some benefits of long-term urban meteorological monitoring in small to middle-sized cities are discussed.