Thermal seasons in Warsaw during the period 1961-2013

This study determines the approximate duration of the thermal seasons in Warsaw, and investigates their long-term variability. The measurement data come from the Ursynów meteorological station at the Warsaw University of Life Sciences (WULS) (λ_E 21°02’52’’, φ_N 52°09’38’’, 102.5 m a.s.l.), situated in the south of Warsaw, and comprise mean diurnal air temperatures for the years 1961-2013. In order to identify thermal seasons, the method of five-day rolling averages was applied, which revealed a substantial year-by-year variability. The achieved results were close to those found by other methods for stations situated within the same climatic region. Over the analysed period, winters and springs have become shorter, whereas the other two seasons have become longer. Nevertheless, the positive trend was only statistically significant for thermal summers. The increasing length of the thermal summer in Warsaw Ursynów may be due to the influence of urban heat islands in built-up areas.