Aim of the study:
As a result of the increase in average air temperature associated with the present-day climate change, heatwaves are forecast to occur with increasing frequency. What is more, they will be longer, more intense, and potentially very dangerous for human health and life. The present work reports the day-to-day variation in air temperature and the variation of thermally characteristic days in Siedlce from 2001 to 2023.

Material and methods:
Day-to-day values of average and maximum air temperatures were obtained from the Siedlce Meteorological Station of the Institute of Meteorology and Water Management – National Research Institute (IMGW-PIB). The day-to-day variation in air temperature was determined across six ranges, and their frequencies in each month were calculated. Additionally, based on the maximum temperature, the number of thermally characteristic days (from cold to hot) was identified, and sequences of hot and very hot days were determined. The direction of trends in the number of hot and very hot days was also established.

Results and conclusions:
In the Siedlce region, days marked by significant day-to-day temperature changes occurred on average 4.8 days per year, and they were the most frequent in January (5.5 days/year) and least frequent in October (3.7 days/year). Significant strain on the body due to day-to-day variation in temperature, ranging from 6 to 9°C, was recorded on about 0.5 days per year, on average. In 2001–2023, there were 39 hot days and 9 very hot days per year, on average. When comparing the number of hot and very hot days between the periods 2001–2012 and 2013–2023, an increase of about 6 hot days and 3 very hot days was observed. The trend coefficients for these changes were also positive, amounting to 0.43 and 0.24 days, respectively. In each month of the warm season (April to September), sequences of hot days occurred, while very hot days did not occur only in April and May. The most frequent were sequences of hot days that were 4 to 6 days long.