Occurrence of Heatwaves in Selected Regions of Poland and Greece and the Characteristics of their Biometeorological Conditions

This work aimed to characterise the occurrence of heatwaves and the related bioclimatic conditions in two climate-distinct regions of Europe. This study was based on data of maximum daily air temperature (T_max), as well as air temperature and humidity at 12:00 UTC, from two meteorological stations in Poznań (Poland) and Thessaloniki (Greece), spanning the summer seasons (the period from May to September) from 1966 to 2022. A hot day was defined as a day with T_max >90th percentile of the station climatology. Heatwaves were defined as sequences of at least three hot days based on the identified hot days. This study revealed a significant variation in thermal conditions over the examined period in both stations. Both stations showed a statistically significant increase in average T_max, although changes were more intense in Poznań (0.41°C/10 years). Like T_max, the 21st-century seasons also stood out in terms of the number of hot days compared to the analysed multi-decade period. This study showed a statistically significant increase in analysed days at both stations, with changes being more intense in Thessaloniki (3.5 days/10 years). The study revealed a significant variation in biometeorological conditions during heatwaves at both stations. Heatwaves in Poznań were characterised by less burdensome conditions compared to those in Thessaloniki.