The heat goes on\u2014changes in indices of hot extremes in Poland

Abstract

On the basis of temperature observations at 60 meteorological stations in Poland, changes in the indices associated with the presence of extremely high air temperatures were examined. Indices associated with heat waves, such as the number of hot days (Tmax ≥30 °C) in the summer months (June, July, August) and beyond the summer months (May, September), the number of extremely hot days (Tmax ≥35 °C), the duration of the longest hot spell in the year, as well as the number of tropical nights (Tmin ≤20 °C) were calculated. Spatial distribution of change rate in days per decade for the studied indices as well as the significance level of the observed trends is illustrated. Also current values of the studied indices (for 1991–2013) are examined and compared with the reference period, 1961–1990. For eight of 11 analysed indexes, increase has been detected in last decades with the help of the Mann-Kendall test at a significance level ≥0.05 or better, for a large group of stations. Statistically significant increases of the number of hot days in summer, the number of tropical nights in a year, and duration of the longest hot spell in summer were found for more than half of the stations. Distinct changes in the duration of heat waves were also noted. In 1961–1990, the longest hot spell lasted for 10 days while in 1991–2013, there were many hot spells longer than that, while the longest hot spell recorded in this period lasted for 17 days. Beyond summer, changes in the number of hot days were smaller. In May, a statistically significant increase was recorded for only three stations, while in September the downward trend was dominating and for eight stations it was statistically significant.