An observed increase in air temperature can lead to significant changes in the phenology of plants and, consequently, changes in agricultural production. The aim of the study was to evaluate the spatial differentiation of thermal resources in Poland and their variability during a period of changing thermal conditions in Europe. Since the variability of thermal conditions is of paramount importance for perennial crops, the study focused on apple, plum, and cherry orchard regions in Poland. The analysis was conducted for the period of 1951–2010 using air temperature daily data. Thermal resources have been defined using the growing degree days (GDD) index calculated independently for the whole year and during in frost-free season for three air temperature thresholds: 0, 5, and 10 °C, which determine the non-winter period, growing season, and the period of full plant growth, respectively. In addition, due to the high significance for perennials in particular, the incidence and intensity of frost during flowering were calculated. In this study, a detailed analysis of the spatial differentiation of thermal resources was first performed, followed by an evaluation of long-term variability and associated change patterns. The obtained results confirmed an increase in thermal resources in Poland as a consequence of the lengthening of the growing season. However, the frequency and intensity of spring frost, especially during flowering or even during ripening of plants, remain a threat to harvests in both the eastern and western parts of the country.

Nearly every study dealing with temperature extremes underscores the lack of a universal and broadly used method of identifying such events. The most popular are relative methods, which are based on the empirical distribution of temperature at each location (i.e., percentiles). The aim of this study was to evaluate the effects of the various percentile-based methods of defining hot days on the analysis of their frequency of occurrence, trends, and geographic patterns in summer in Europe. The basis for the research consists of daily maximum (TX) and minimum (TN) values of air temperature for 1961–2017 for Europe obtained from the E-OBS database. A hot day occurs when air temperature exceeds the 90th percentile–based threshold. These thresholds are determined using the following: (I) various temperature metrics (TX and TN), (II) various baseline periods (1961–1990, 1971–2000, 1981–2010), and (III) different timeframes within the year that the percentile is calculated for (summer season, separate summer months, and each calendar day). Our results indicate that the use of different variants of the percentile-based definition leads to differences in the geographic patterns of frequencies of and trends in summer hot days in Europe. The differences are especially substantial within the results obtained using various temperature metrics and baseline periods, and they are relatively small when different timeframes within the year that the percentile is calculated for are considered. On the example of the case study, we also show how the use of different research approaches may affect the intensity and spatial extent of an extreme temperature event.

Extremely hot or warm weather over the course of the year may have significant impacts on many aspects of human life, the economy, and the natural environment. Until now a thorough assessment of changes of extreme heat or warm events in Europe was hindered by the number of metrics employed, time periods examined, and most studies being conducted in the summer season only. Here, we employ the Extremity Index (EI) to investigate long-term trends in extremely hot or warm days in Europe over the course of the year, with a special focus on their frequency, spatial extent, and intensity. An extreme temperature event (ETE) is defined as a day with an unusually high temperature for a given location and season, even if such a temperature would not be considered extremely high in an absolute sense. The research is conducted in five spatial domains that together cover a large portion of Europe. The period of the most recent 70 years is considered. In all examined regions, mainly significant increasing trends since 1950 are evident for seasonal EI; therefore, also for ETE frequency, intensity, and spatial range. Yet, every region is characterized by its own event pattern, and trends across the continent strongly vary geographically and seasonally. Our study highlights that examined trends of temperature extremes are accelerating and in the last 40 years the rate of change has been even more than three times greater than in the entire study period. The greatest changes were noted for the summer season in Central Europe and Eastern Europe for the most recent 40-year period.

The aim of the study is to estimate the trend in summer maximum air temperature extremes in Poland during the period 1951–2015 by demonstrating the changes in the magnitude of temperature anomalies, temperature “surplus”, as well as the area influenced by extreme temperature occurrence. To express the latter two variables, daily maps of maximum air temperature were created to calculate the total area affected by temperature extremes. To combine the effect of spatial extent and temperature anomaly, an Extremity Index was introduced. The results confirmed an increase in summer maximum air temperature of about 0.4 °C per 10 years, evidenced also in the increase of summer extremeness. Positive anomalies have dominated since the 1990s, with the largest anomalies occurring during the summers of 1992, 1994, 2010 and finally 2015, the most exceptional summer during the analyzed period.

The research study aimed at assessing multiannual variability of heat wave occurrence in the lowland part of Germany between 1966 and 2015 and determining the role of atmospheric circulation in their occurrence. The analysis was conducted with the use of two independent datasets, that is, the dataset of Germany’s National Meteorological Service, Deutscher Wetterdienst, and American meteorological reanalysis database of the National Centre for Environmental Prediction/National Centre for Atmospheric Research. This article defines a hot day as a day with maximum temperature of >30 °C, and a heat wave as a sequence of at least three such days. The observed warming translated into an increase in a number of hot days and, consequently, an increase in the frequency of heat wave occurrence. In the analysed 50-year period, the smallest number of heat waves was observed between 1976 and 1985, and the largest number between 2006 and 2015 in the lowland part of Germany. The occurrence of heat waves in lowland Germany was related to anticyclonic circulation.