Changes in thermal conditions in the northern part of the Mazovia (central Poland) between the late 19th and early 21st centuries

The paper examines changes in thermal conditions in Płońsk (central Poland) between the end of the 19th century and the beginning of the 21st century. The study uses two air temperature datasets: newly found and digitised dataset from a period 1881–1895 and a dataset from a nearby Institute of Meteorology and Water Management – National Research Institute (IMGW-PIB) weather station from a period 1951–2010. No statistically significant changes in mean daily air temperature (Tmean) were found between 1881–1895 and 1951–1980. The increase in Tmean of ca. 1.5°C was found between 1881–1895 and 1981–2010. As Płońsk is a small town this increase can be considered as unbiased by the Urban Heat Island intensification. The largest shift in the cumulative empirical distribution function of Tmean between both studied time periods was noted for the summer in the interval of values from 17.6 to 20.0°C and for the winter in the interval of values from 0.1 to 2.5°C The longest spells of days with a statistically significant warming were identified in mid-January (increase of at least 5.0°C between 13 and 16 January), late March – early April and in the first half of August (the longest spell of ten consecutive days). A significant reduction in a duration of the thermal winter (a reduction of 25 days) and extending of the summer (an increase of 17 days) were detected as well as the shift in the timing of transitional seasons.