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The wind was analyzed on the basis of speed and direction data from three observation times at 41 meteorological stations in Poland from 1966–2018. The average annual and seasonal wind speeds and the incidence of atmospheric calms were calculated. Wind frequencies from different directions were determined using a 16-sector wind rose. Particular attention was paid to winds exceeding 8 ms−1. The seasonal and annual trends of strong winds were counted. The average wind speed was 3.6 ms−1 and ranged from 1.4 ms−1 in Zakopane to 12 ms−1 on Śniezka. On the daily scale, the highest wind speed appeared at noon. In the annual cycle, the average wind speed at noon was the highest in spring and in the other times in winter, the lowest was always in summer. Windless weather occurred most often in the morning (12.3% of observations), less often in the evening (8.3%) and the least often at noon (3.0%). The long-term average of the highest annual wind speed oscillated between 8.3 ms−1 in Tarnow and 42.5 ms−1 on Śniezka. Strong winds were most often recorded from the west, west southwest and west northwest directions. In southern Poland it was also the southern direction. The decreasing trends in annual and seasonal series of wind speed were observed in Poland as well as in other European countries. A number of authors attribute this trend to an increase in surface roughness.

The book presents the results of climate research throughout Poland in the pre-instrumental period (using proxy data), instrumental period (using mainly statistical methods, based on data from weather stations and grid data) and projected changes (using regional climate models). A total of 1100 years are covered, i.e. the period from about 1000 to 2100. The majority of examined climate elements, meteorological phenomena and indices show statistically significant changes at least in certain areas of Poland and at certain seasons of the year. Moreover, many elements demonstrate significant year-to-year variability and temporal fluctuations. Changes of particular climate elements are interrelated. The primary causative factors are both anthropogenic changes (greenhouse gas emissions resulting in increased greenhouse effect and global warming, local sources of air pollution) and natural changes: (1) circulation factors: changes in the intensity and location of atmospheric activity centres, changes in the frequency of advection from a specific sector, and the frequency of cyclonic and anticyclonic systems over Poland and (2) radiation factors (changes in values of global solar radiation, sunshine duration and cloudiness). These changes, especially visible after the 1980s, affect the trends of most climatic elements, meteorological phenomena and indices. The effects of these changes, both positive and negative, are evident in people’s daily lives (e.g. decrease in bioclimatic cold stress, increase in bioclimatic heat stress, changes in conditions for recreation and sport) and economy (e.g. improvement of thermal agricultural conditions, changes in energy demand for heating buildings, air conditioning and refrigeration). A better understanding of the relationships of trends of the different climatic elements should be the aim of further research into the climate of Poland.

The study covers the entire area of Poland. Part II of the book presents the results of the study of climate change before instrumental measurements in Poland in the last millennium. More than 50 proxy series including documentary evidences, dendrochronological records and varved sediment records were analysed. On this basis, reconstructions of: summer and winter air temperature, winter severity, precipitation (liquid and solid) and droughts were performed. Part III analysed the long-term changes and variability of 36 climate characteristics for 14 climate elements, indices, meteorological phenomena and weather types using data from 79 weather stations. Most of the climate elements studied were analysed for the base period 1951–2018 (68 years). For some elements and indices shorter study periods were considered. In addition, climate variability was analysed for 10 long measuring series up to 239 years. Variability of circulation indices over a period of 147 years (1873–2019) was investigated as well. Uniform research methods common to all elements and indices were used: (1) coefficient of variability (%), (2) absolute trend (unit/10y); (3) relative trend (%/10y). Part IV of the book deals with projected changes in temperature, precipitation and thermal indices related to the agriculture and energy sectors. In this study, 8 regional climate models from the EURO-CORDEX experiment were used for 2 representative concentration pathways: (1) corresponding to radiative forcing value +4.5 W.m−2 in 2100 (RCP4.5) and (2) corresponding to radiative forcing value +8.5 W.m−2 in 2100 (RCP8.5) relative to pre-industrial values. Two future time horizons were carried out for each concentration pathway: (1) near future: 2021–2050 and (2) far future: 2071–2100 with reference to the period of 1971–2000. Future projections were created for: 8 characteristics of temperature, 10 characteristics of precipitation, 5 characteristics for agriculture indices and 4 for energy demands indices.

Analyses of homogeneity of climate series and correction of heterogeneous series were undertaken in order to avoid making incorrect conclusions about climate change in Poland. The climate series homogenisation procedure was typically carried out in several stages: (1) collecting information on the history of meteorological stations (metadata), especially on significant changes in their location and measurements methods; (2) selecting the test for homogeneity control, calculating values of the test, selecting series with test values exceeding the specified critical level; (3) correction of non-homogeneous series; (4) recheck of the revised series. The correction of the climatic series analysed for the purposes of this book was carried out using various methods, specific to the climate element analysed. The most common was the relative and absolute Alexandersson test (Standard Normal Homogeneity Test, SNHT). The most common reasons for breaking the homogeneity of the climatic series were a significant change of station location and a change of the measuring instrument. However, in many cases, the cause of the series’ heterogeneity is not clear.

The aim of the study is to analyze the occurrence of hot days and heat waves in Poland, their intra-annual distribution, and their long-term variability, and to present the circulation factors favoring the appearance of extensive waves in the country. Hot days were days with Tmax not lower than the threshold value defined by the 95th percentile of summer Tmax in the period 1961–1990. Atmospheric circulation was described using sea level pressure, geopotential of 700 and 500 hPa level, and horizontal and vertical wind on these levels. A statistically significant increase in the number of hot days in the entire study period and a significant acceleration in growth after 1980 were shown. In the entire analyzed period, only 11 waves were found covering at least 25% of the country area and lasting no less than a week. Among them, only one occurred before 1990, and more than half were observed in the last decade. Four circulation patterns favoring the extensive heat waves were distinguished differing the location of main baric center location. Spatial and temporal distribution of vertical velocity anomalies allows distinguishing clear phases of strengthening, stabilization, and weakening of anticyclone accompanying the occurrence of a heat wave.

The aim of the paper was to present the temporal and spatial variability of precipitation in the Łodź province in the period 1961–2015. Daily precipitation totals from 20 precipitation stations in the province were used. It was shown that the mean yearly precipitation sum averaged across the province was 582 mm, and at particular stations ranged from 515 to 648. The totals were observed to be highest in the southern part of the province and they dropped to the north, where water deficits happened most often. Summer totals were more than double the winter ones. Spring and autumn precipitation totals were similar, with a slight advantage to spring totals. Daily sums most often fall in the range 1–10 mm. Daily totals exceeding 50 mm were extremely rare, and appeared once every few years, mainly in summer. At the moment, there are no statistically significant changes in the structure and level of precipitation, but there is a slight tendency towards increasing winter totals and decreasing summer totals.

This work gives an overview on how the projected changes in the extremes in Poland might impact human health and economy. For that purpose, statisti-cally corrected data from 7 regional climate models were used. A significant increase of extreme hot events (i.e. heat waves, tropical nights) is projected for Central and Southern Poland for the end of the 21st century which might seri-ously affect a society living in large urban areas. Less extreme cold events im-prove thermal comfort in winter. The negative impact of the warming will affect energy systems with higher demand for electricity in summer and agriculture: an earlier beginning of the growing season and flower blooming will enhance the risk of frost damages in spring, whereas excessive heat will reduce yields in summer. Polish tourism should benefit from higher thermal comfort (except for hot July and August in the far future and warming in the winter season bring-ing snow cover depletion in the near future)., Ovaj rad daje pregled kako projicirane promjene ekstrema u Poljskoj mogu utjecati na ljudsko zdravlje i njezino gospodarstvo. U tu svrhu korišteni su statistički korigirani podaci iz 7 regionalnih klimatskih modela. Za srednju i južnu Poljsku predviđa se za kraj 21. stoljeća značajan porast ekstremnih vrućih događaja (tj. toplinskih valova, tropskih noći), što bi moglo ozbiljno utjecati na društvo koje živi u velikim urbanim područjima. Manje ekstremni hladni događaji poboljšavaju toplinsku ugodnost zimi. Negativni utjecaj zagrijavanja utjecat će na energetske sustave s većom potražnjom električne energije ljeti, a u poljoprivredi: raniji početak vegetacijske sezone i cvjetanje cvijeća povećat će rizik od oštećenja od mraza u rano proljeće, dok će pretjerana vrućina smanjiti prinose ljeti. Poljskom turizmu trebala bi pogodovati veća toplinska ugodnost (osim vrućeg srpnja i kolovoza u dalekoj budućnosti te zagrijavanja tijekom zimske sezone u bliskoj budućnosti, što će dovesti do smanjenja snježnog pokrivača).

Snow accumulation and associated synoptic conditions in the Russian Arctic are analysed based on snow depth data from 1950 to 2013 from the All-Russian Research Institute of Hydrometeorological Information—World Data Centre data set. The mean duration of snow coverage in the Russian Arctic is approximately eight to nine months. While the period of snowmelt is usually very short (one or two months), snow accumulates during most of the cold season (October–May). Snow accumulation is associated with negative anomalies of sea level pressure and positive anomalies of air temperature, which means increased cyclonic activity over the analysed region. The cyclones differ in intensity and localization, depending on the area of snowfall. In the western part of the Russian Arctic the cyclones and air masses that bring snowfall may originate from the North Atlantic, while in the eastern part they originate from the Bering Sea, Okhotsk Sea or the North Pacific. The cyclones that bring snowfall may also form locally along the zonal border between two different air masses: the very cold, polar, continental air originating from the Siberian High and the Arctic air from the north, which is often warmer and always more humid than the continental air. Keywords: Polar climate; snow cover; air circulation patterns. (Published: 30 March 2016) To access the supplementary material for this article, please see the supplementary files in the column to the right (under Article Tools). Citation: Polar Research 2016, 35 , 25916, http://dx.doi.org/10.3402/polar.v35.25916

Various indices of hot weather frequency and intensity were analysed in the area of Poland in the period between 1951 and 2006. An increase of majority of them was shown in the whole year and all summer months but September, when significant decrease in all indices was apparent. The correlation of selected hot weather indices and precipitation totals in a month of hot weather event and the preceding months were also calculated to check if prolonged dry weather can constitute a forcing factor for hot event creation. Because significant correlations appear mainly in the cases when precipitation is for the same month as the hot weather index, it seems that in Poland the presence of high pressure systems is a more important factor of hot event creation than dry weather.

Gridded data sets of observations are frequently used for the evaluation of climate extremes in climate models. However, it is necessary to understand how the behaviour of extremes is affected in such data sets. The aim of the paper is to analyse how the smoothing effect is related to the number of stations used for calculating the values in each gridcell. A very dense network of stations with highly correlated records of temperature and precipitation was used. The set of grid points was established and for each grid point all data from stations located in the circles with a radius of 75 km in the case of temperature and of 50 km in the case of precipitation were used. Two distributions were compared. The first one was built of the data averaged from all selected stations. The second one was built of all the data from the same stations. Apart from high correlation of averaged records, the effect of reducing extremes is strong. In the case of temperature in winter, the areal averaging causes the overestimation of temperature in the left tail of distribution for the percentiles below the tenth. In summer, the strongest effect is in the right tail and the temperature in underestimated. In the case of precipitation, the higher the number of stations averaged the lower the number of dry days. At the same time, the highest daily totals are underestimated in the highest 5 percentiles (equal and higher than the 95th percentile). All the effects increase with the increasing number of averaged records.

In Łodź, the total cloud cover has significantly decreased in the second half of the 20th century. This change is opposite to that of the low-level cloud cover in the warm part of the year. But the most important are variations in frequencies of different cloud types. The stratiform clouds, St and Ns, became less frequent, whereas the convective ones, Cu and Cb, became more frequent. The rise in Ac and high-level clouds and the fall in As clouds were also observed. These changes are consistent with other observations in central and eastern Europe. Because different cloud types are created in different conditions, their variability is a valuable tool for assessment of changes in processes taking place in the troposphere. Such reorganization in cloud types distribution indicates significant changes in the vertical structure of the troposphere: fall of the frequency of temperature inversions in the lower troposphere and stronger lapse rate. Copyright © 2007 Royal Meteorological Society

Primary goal of presented study is to classify the most frequent patterns of the upper tropospheric jet stream over Europe. Wind fields were grouped into separated classes with the help of the correlation-based Lund’s technique. The treatment of vector fields with Lund’s method was achieved by replacement linear Pearson coefficient with vector correlation coefficient. The outstanding features of the upper-level circulation and ground-based weather associated with each jet type were analysed. Finally, basic statistics of jet stream patterns (frequency, duration time, day-to-day changes of jet structure) as well as their trends were estimated. The analysis was conducted on the basis of mean daily wind components at 200 hPa level, air temperature at 850 hPa, sea-level pressure, vertical velocity and geopotential at 500 hPa level. Data set was extracted from the NCEP/NCAR Reanalysis. The warm half-year in the period 1950–2001 was taken into consideration. The first 15 most frequent jet types, including 60.8% of the sample, were selected. Three jet stream types (C, E and I) are associated with distinct temperature changes in western Europe. Another three types (B, F and O) cause significant thermal advection in eastern and central Europe. Seasonal differences in frequency and duration time of jet stream patterns are also observed. Meridional types (A, C and D) dominate in spring, while in summer, patterns with intensified zonal flow prevail (B, E and J). At last, it is worth noticing that the majority of selected jet types pronounce an increase in day-to-day changes of wind field, which may indicate slight enhancement of circulation dynamics in the upper troposphere.

Data from two automatic stations in Łodź (one urban and one rural) for the period 1997–2002 are analyzed to reveal urban–rural contrasts of such parameters as air temperature, relative humidity, water vapour pressure and wind speed. Under favourable weather conditions the highest temperature differences between the urban and rural station exceeds 8 °C. Relative humidity is lower in the town, sometimes by more than 40%. Water vapour pressure differences can be either positive (up to 5 hPa) or negative (up to −4 hPa). Wind speed at the urban station is on average lower by about 34% in night and 39% during daytime. Regression analysis shows that for rural winds lower than 1.13 m s−1 urban winds can be stronger than rural speeds. Attention has also been paid to singularities in the course of the analyzed parameters over 24 hour periods. It is shown that the typical course of the urban heat island intensity under favourable conditions is similar in all season. Four stages of this course have been distinguished. Wind speed differences also seem to change in a typical way. Case studies show that humidity contrasts, unlike temperature, can evolve in different ways under fine weather conditions. Types of relative humidity evolution are proposed.

Mean monthly air temperatures in Cracow for the period 1826–1990 were used. The temperatures of the coldest (Tlow) the warmest (Thigh) periods of the year, and mean annual temperatures (Ta), were calculated. The persistence of temperatures was examined using the correlation coefficients between monthly temperatures in succeeding months. A significant temperature persistence was found for a cold season (December-March) and also for the summer (July-August). Long-term fluctuations were analysed by means of 20-year and 30-year running mean values and the positive and negative squared deviations of moving 20-year subperiods. Two warm stages were found. One at the beginning of the present century, the second at the end of the period analysed. The first warm phase was connected with a decrease in the degree of continentality. Warm phases were also linked with a decrease in the temperature variability. The spectrum and the moving spectrum in 40-year subperiods of monthly and annual temperature records were calculated. An 8-year period was found in Tlow, Ta, and in the monthly series of the cold season, and 4- and 5-year periods were found in Thigh and in the monthly series of the warm season. By comparing the temperature in Cracow with that at the neighbouring stations of Wieliczka, Bochnia, and Tarnow, it was found that the difference in annual means increased from −0.3°C at the end of the previous century to 1.0°C in the last two decades. About 0.5°C of this increase occurred in the last 40-year period of abrupt expansion and industrialization of the city of Cracow.

The correlation coefficients between air temperature, precipitation, and geopotential heights of the 500 hPa level for six stations in Europe, located along the 20°E meridian from Scandinavia to Greece, for the period of 1951–1985 are calculated. The fields of correlation coefficients for each station and season are described. It is observed that the correlation coefficients of temperatures are, as a rule, significantly greater than for precipitation. How meridional and zonal circulation determine temperatures and precipitation has been examined.

Seasonality in the interannual variability of atmospheric moisture over Europe is investigated using precipitable water (PW) from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data set for 1979-2004. Over Europe the summer PW and its interannual variability (expressed by standard deviations) are essentially larger than those of the winter season. The largest seasonal differences are found over eastern Europe and European Russia, where the summer PW climatology and magnitudes of its interannual variability exceed respective winter characteristics by a factor of 2.5-3.8. The first and second empirical orthogonal function (EOF) modes of winter PW over Europe are associated, respectively, with the North Atlantic Oscillation (NAO) and the East Atlantic teleconnection pattern. During summer the leading EOFs of PW are not linked to the known regional teleconnection patterns. Our analysis revealed that EOF-1 of summer PW is associated with sea level pressure (SLP) pattern characterized by two action centres of opposite polarity over northwestern Siberia and over a broad region including southern Europe, the Mediterranean Sea and part of northern Africa. The EOF-2 of summer PW is associated with cyclonic/anticyclonic SLP anomalies over Scandinavia and southwestern Europe. It is shown that PW and precipitation variability are positively coupled during the cold season but not for the warm season. Instead, during the warm season we found a significant link between regional PW and air temperature variability, indicating an important role of local heating in variability of summer PW over Europe.

This section describes long-term observed climatic changes in atmospheric parameters. The focus is on surface climate conditions, but changes in atmospheric circulation are discussed as they often are behind climatic variability seen on regional and local scales. For a summary introduction on mean atmospheric states and conditions in the Baltic Sea Basin see Annex 1.2 with sections on the general atmospheric circulation (A.1.2.1), surface air temperature (A.1.2.2), precipitation (A.1.2.3), clouds (A.1.2.4), and global radiation (A.1.2.5).

A simple statistical model of daily precipitation based on the gamma distribution is applied to summer (JJA in Northern Hemisphere, DJF in Southern Hemisphere) data from eight countries: Canada, the United States, Mexico, the former Soviet Union, China, Australia, Norway, and Poland. These constitute more than 40% of the global land mass, and more than 80% of the extratropical land area. It is shown that the shape parameter of this distribution remains relatively stable, while the scale parameter is most variable spatially and temporally. This implies that the changes in mean monthly precipitation totals tend to have the most influence on the heavy precipitation rates in these countries. Observations show that in each country under consideration (except China), mean summer precipitation has increased by at least 5% in the past century. In the USA, Norway, and Australia the frequency of summer precipitation events has also increased, but there is little evidence of such increases in any of the countries considered during the past fiffty years. A scenario is considered, whereby mean summer precipitation increases by 5% with no change in the number of days with precipitation or the shape parameter. When applied in the statistical model, the probability of daily precipitation exceeding 25.4 mm (1 inch) in northern countries (Canada, Norway, Russia, and Poland) or 50.8 mm (2 inches) in mid-latitude countries (the USA, Mexico, China, and Australia) increases by about 20% (nearly four times the increase in mean). The contribution of heavy rains (above these thresholds) to the total 5% increase of precipitation is disproportionally high (up to 50%), while heavy rain usually constitutes a significantly smaller fraction of the precipitation events and totals in extratropical regions (but up to 40% in the tropics, e.g., in southern Mexico). Scenarios with moderate changes in the number of days with precipitation coupled with changes in the scale parameter were also investigated and found to produce smaller increases in heavy rainfall but still support the above conclusions. These scenarios give changes in heavy rainfall which are comparable to those observed and are consistent with the greenhouse-gas-induced increases in heavy precipitation simulated by some climate models for the next century. In regions with adequate data coverage such as the eastern two-thirds of contiguous United States, Norway, eastern Australia, and the European part of the former USSR, the statistical model helps to explain the disproportionate high changes in heavy precipitation which have been observed.