4.1. Spatial distribution of the sums of annual sunshine duration and the average annual air temperature in Poland
Annual totals of sunshine duration are between around 1,460 hours in the south and south-west of the country (mountain areas) to more than 1,750 hours in the north − the area of the eastern Baltic coast (Fig. 2a). One area of large sunshine duration (over 1,700 hours) stretches in a wedge-like fashion to the south of the central part of the South Baltic Coast to the Silesian Lowland. Another area with the highest sunshine duration value (over 1,750) is located in the central-eastern part of Poland, including a fragment of the North Podlasie and the South Podlasie Lowlands, as well as the Western Polesie. In the largest area of the country, the central part of Poland, sunshine duration is between 1,650 to 1,700 hours per year, and gradually decreases from the centre towards the south-west, south, and north-east. A wedge-like area of reduced sunshine duration stretches from the mountainous areas to the centre of the country along the Vistula valley. The areas on the north-eastern edges of Poland have lower sunshine duration values compared to neighbouring areas.
The average annual air temperature in Poland is between 11oC in Lower Silesia to 0oC in the mountainous regions in the south and south-west of the country. Generally, the temperature decreases from the south-west towards the north-east. The spatial variability of the average annual temperature in Poland (except for mountainous areas) does not exceed 3oC. The area with the highest temperature values (above 10oC) stretches from the western end of the Pomeranian Lake District, through the Wielkopolska Lowland, to Lower Silesia. Apart from mountainous and upland regions, the lowest average annual temperature (below 8oC) occurs in north-eastern Poland, near Suwałki (Fig. 2b).
4.2. The annual variability of sunshine duration and air temperature
In the course of a year, day-time air temperature changes occur with about a one-month delay compared to changes in sunshine duration. The maximum annual sunshine duration falls on the transition between June and July (Fig. 3a), while the maximum temperature falls on the transition between July and August (Fig. 3b). The curves plotted on the basis of daily area-average values from the years 1971–2019 are irregular owing to the interference caused by variations in the supply of solar radiation and changes in air temperature. Irregularities can occur from day to day, which means that the effects of inter-day changes in sunshine duration and temperature have not been levelled even for the long-term curve. Some disturbances in the annual course are particularly large and occur over a few days in a row. In the case of sunshine duration, for example, these are days in April or June when there is a temporary decline in the value with a general trend of growth, followed by a rapid increase (Fig. 3a). It is similar with the course of temperature. A few days’ periods of cooling with rising temperature in the spring or periods of warming with falling temperature in the autumn occur regularly, though not necessarily every year.
4.3. The multi-annual course of area-averages
The multi-annual course of sunshine duration and air temperature on the basis of data from 25 stations in Poland shows an increasing trend. In the first research period (1971–1980) in the last decade of what is known in the literature as the “global dimming” (inter alia Norris and Wild 2007; Sanchez-Lorenzo et al. 2009), the values of both elements dropped to a minimum in 1980 (Fig. 4), and then gradually increased in line with the “global brightening” trend. There was a significant correspondence between the courses of sunshine duration and of temperature in shorter periods, and the lowest and highest values of both elements were recorded in the same years (Fig. 4). Until 1990, the totals of annual sunshine duration ranged from 1,278 hours (1980) to 1,795 hours (1982), and by the year 2000 they oscillated on average between 1,550 and 1,650 hours. In the following years of the 21st century, the annual values were often higher than 1,800 hours, and in 2018 they even exceeded 2,000 hours, reaching their maximum in the analysed multi-year period (Fig. 5). Like sunshine duration, the mean area annual air temperature also dropped until 1980 (6.7 °C). This was followed by the period of higher temperatures exceeding 9 °C and dropping to 7 °C in 1985, 1987, 1996. In 2000, the mean annual area air temperature for the first time exceeded 10oC and after a drop in 2010 (8oC) it did not fall below 9oC, and in 2014, 2015, 2018, 2019 it again exceeded 10oC, reaching a maximum of 10.9oC in the last year.
The values of the totals of sunshine duration in individual years are clearly reflected (Fig. 5) in the number of hot days (Tmax>25 °C), very hot days (Tmax>30 °C), and hot nights (Tmin>20 °C). This dependence applies to both area averages and data series from individual stations. In the first decade of research, the decrease in sunshine duration and temperature was accompanied by a lower incidence of the thermal characteristics mentioned, and in the following years there were increases and decreases in the number of characteristic days depending on the value of the total of sunshine duration. A clear increase in the incidence of hot days was recorded in most years in the 21st century, and very hot days after 2011. Hot nights started to appear sporadically after 1992, and the intensification of their incidence was marked in the last decade of the analysed multi-year period (Fig. 5). The correlation coefficients between sunshine duration and the discussed thermal characteristics were 0.82, 0.63 and 0.50, respectively.
The increase in the totals of sunshine duration and air temperature in individual months of the studied multi-year period is very clearly visible in the quantile classification (Fig. 6). From the last decade of the 20th century to the end of the studied period, the predominance of months with the highest quantile values at the same time for sunshine duration and temperature, is clearly noticeable (Fig. 6). The co-occurrence of the annual averages of both elements in the highest quintile had not happened until 2008. In recent years, however, this situation has been repeated more and more often. In the second decade of the 21st century, it has already occurred six times (in 2011, 2014, 2015, 2016, 2018, and 2019). Moreover, the simultaneous highest values of sunshine duration and temperature frequently occurred together in single months in the last two decades. They happened most frequently in: 2019 (February, April, June, August, October, December), 2018 (April, May, August, September, October), 2015 (March, August, September, October), and 2006 (June, July, September, December). It is worth noting that in each month since 1971, the highest quintile values often occurred simultaneously for both sunshine duration and temperature, most of them (9) in June and September, and least (1) in January. In the last three years of the examined period, August was the sunniest and warmest (Fig. 6).
The simultaneous annual values of sunshine duration and temperature were in the lowest quintile for the last time in 1995 (Fig. 6). The least sunny and coolest summer months (July and August) occurred in the years 1977–1981 and in June 1980. For the air temperature alone, despite the general trend of its increase, there are months in which the values are in the lowest quintile. This applies to all seasons except the summer, i.e. the months from January to May and from October to December. The spring months at the beginning of the surveyed multi-year period were particularly cold. From 1972, for ten consecutive years, air temperature values in April were classified in the lowest quintile. Moreover, the months of May in this decade were cold, except for 1975, 1979, 1981 (Fig. 6). On the other hand, not all winter months have been exceptionally warm in the last decade. Only in December (with the exception of 2012) high and very high temperatures occurred, while in January values below normal were most prevalent.
Sunshine durations also trend upwards, but there is a large variation in their totals in individual months. In a given year, a month belonging to the highest quintile may precede a month with the lowest quintile (e.g., November and December 2018). For example, in 2019, in April, May, and June, the months with the highest quintiles of both sunshine duration and temperature were separated by a month from the lowest quintile (Fig. 6).
4.4. Spatial variability of trends in sunshine duration and air temperature
Considering annual values, the trends in sunshine duration in Poland are more spatially varied than air temperature. The largest increase in sunshine duration (over 130 hr/10 years) occurred at the stations in Opole, Jelenia Góra, and Katowice (south-western Poland, except mountainous areas) and decreased to the east, with a minimum value at the north-eastern end of the country (Suwałki − 33 hr/10 years) (Fig. 7a). Air temperature increased most (0.5 °C/10 years) in Małopolska and Silesia Uplands (Kraków, Opole). Increase in temperature was stronger in the central and eastern parts of the country (Suwałki, Terespol) than in the west (Kołobrzeg, Śnieżka, Kłodzko), and in the south-east of Poland (Fig. 7b, Table 2).
Table 2
Directional coefficients of trends in sunshine duration (hr/10 years) and air temperature (°C/10 years) at the analysed meteorological stations in Poland (1971–2019)
Meteorological | Trends | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Year |
station |
Białystok | SDU | -1.7 | -0.7 | 2.4 | 14.6 | 10.5 | 18.1 | 8.9 | 9.3 | 9.4 | 6.9 | -0.3 | 0.8 | 78.3 |
(hr/10 y) |
T | 0.2 | 0.4 | 0.4 | 0.8 | 0.3 | 0.5 | 0.6 | 0.6 | 0.5 | 0.3 | 0.4 | 0.3 | 0.4 |
(°C/10 y) |
Chojnice | SDU | 1.0 | 4.8 | 7.7 | 18.6 | 8.0 | 13.5 | 7.7 | 6.9 | 15.3 | 6.9 | 1.4 | 0.7 | 92.4 |
(hr/10 y) |
T | 0.3 | 0.4 | 0.4 | 0.9 | 0.3 | 0.5 | 0.5 | 0.5 | 0.5 | 0.4 | 0.4 | 0.3 | 0.5 |
(°C/10 y) |
Gorzów Wlkp. | SDU | 2.8 | 5.8 | 6.6 | 17.9 | 4.7 | 16.0 | 12.1 | 9.5 | 12.2 | 5.2 | 2.6 | 3.2 | 98.5 |
(hr/10 y) |
T | 0.3 | 0.4 | 0.3 | 0.9 | 0.4 | 0.6 | 0.6 | 0.5 | 0.5 | 0.4 | 0.4 | 0.3 | 0.5 |
(°C/10 y) |
Jelenia Góra | SDU | 1.9 | 4.8 | 9.2 | 19.4 | 15.3 | 22.9 | 20.0 | 15.7 | 11.2 | 3.5 | 6.7 | 5.3 | 135.7 |
(hr/10 y) |
T | 0.2 | 0.2 | 0.1 | 0.9 | 0.4 | 0.7 | 0.7 | 0.6 | 0.4 | 0.3 | 0.5 | 0.2 | 0.4 |
(°C/10 y) |
Kalisz | SDU | 1.3 | 4.8 | 6.4 | 14.5 | 7.5 | 12.1 | 9.5 | 9.1 | 11.6 | 6.0 | 3.1 | 2.9 | 88.8 |
(hr/10 y) |
T | 0.2 | 0.3 | 0.2 | 0.8 | 0.3 | 0.6 | 0.7 | 0.6 | 0.5 | 0.4 | 0.5 | 0.3 | 0.5 |
(°C/10 y) |
Kasprowy Wierch | SDU | -3.6 | -3.9 | -1.3 | 15.0 | 5.2 | 12.1 | 12.0 | 13.1 | 3.9 | -0.5 | 1.3 | -0.8 | 52.5 |
(hr/10 y) |
T | -0.1 | 0.1 | 0.0 | 0.7 | 0.3 | 0.8 | 0.7 | 0.7 | 0.3 | 0.3 | 0.8 | 0.2 | 0.4 |
(°C/10 y) |
Katowice | SDU | 1.6 | 2.2 | 9.2 | 19.0 | 12.3 | 18.8 | 18.5 | 18.2 | 14.7 | 7.6 | 6.4 | 4.1 | 132.5 |
(hr/10 y) |
T | 0.2 | 0.3 | 0.2 | 0.9 | 0.4 | 0.8 | 0.7 | 0.7 | 0.5 | 0.4 | 0.7 | 0.3 | 0.5 |
(°C/10 y) |
Kłodzko | SDU | -1.1 | 3.2 | 6.1 | 15.0 | 7.6 | 12.2 | 10.0 | 8.2 | 9.1 | 0.4 | 2.2 | 0.3 | 73.2 |
(hr/10 y) |
T | 0.1 | 0.2 | 0.2 | 0.8 | 0.3 | 0.6 | 0.7 | 0.6 | 0.4 | 0.3 | 0.6 | 0.2 | 0.4 |
(°C/10 y) |
Kołobrzeg | SDU | 0.6 | 2.8 | 6.4 | 11.9 | 5.9 | 14.0 | 8.5 | 2.1 | 10.9 | 1.6 | 1.1 | -2.1 | 63.8 |
(hr/10 y) |
T | 0.3 | 0.3 | 0.3 | 0.7 | 0.4 | 0.5 | 0.4 | 0.4 | 0.4 | 0.3 | 0.4 | 0.3 | 0.4 |
(°C/10 y) |
Koszalin | SDU | 1.8 | 3.7 | 8.5 | 15.6 | 6.4 | 16.0 | 12.2 | 3.5 | 15.0 | 6.1 | 3.9 | 0.1 | 92.8 |
(hr/10 y) |
T | 0.3 | 0.4 | 0.3 | 0.8 | 0.3 | 0.5 | 0.5 | 0.5 | 0.5 | 0.4 | 0.4 | 0.3 | 0.4 |
(°C/10 y) |
Kraków | SDU | 2.3 | 2.6 | 8.8 | 15.9 | 9.5 | 12.5 | 13.8 | 13.1 | 11.4 | 5.6 | 2.8 | 4.9 | 103.0 |
(hr/10 y) |
T | 0.3 | 0.4 | 0.3 | 0.9 | 0.4 | 0.8 | 0.8 | 0.7 | 0.5 | 0.4 | 0.6 | 0.3 | 0.5 |
(°C/10 y) |
Lesko | SDU | -2.6 | -1.0 | 0.5 | 16.9 | 8.8 | 13.6 | 13.8 | 14.8 | 9.6 | 5.2 | 3.9 | 1.5 | 85.1 |
(hr/10 y) |
T | 0.0 | 0.2 | 0.1 | 0.8 | 0.3 | 0.7 | 0.6 | 0.7 | 0.4 | 0.4 | 0.8 | 0.2 | 0.4 |
(°C/10 y) |
Łódź-Lublinek | SDU | -1.1 | 1.9 | 4.1 | 15.3 | 7.2 | 15.6 | 8.0 | 8.8 | 10.9 | 6.1 | 2.2 | 2.3 | 81.3 |
(hr/10 y) |
T | 0.3 | 0.3 | 0.2 | 0.9 | 0.4 | 0.7 | 0.7 | 0.6 | 0.5 | 0.4 | 0.6 | 0.3 | 0.5 |
(°C/10 y) |
Mikołajki | SDU | -1.0 | 0.7 | 4.5 | 16.7 | 7.6 | 11.0 | 7.2 | 6.3 | 10.1 | 4.5 | -1.5 | -0.5 | 65.6 |
(hr/10 y) |
T | 0.1 | 0.3 | 0.4 | 0.8 | 0.5 | 0.5 | 0.9 | 0.9 | 0.9 | 0.3 | 0.3 | 0.2 | 0.5 |
(°C/10 y) |
Opole | SDU | 4.3 | 6.0 | 10.1 | 19.4 | 11.3 | 18.5 | 16.3 | 15.6 | 13.6 | 8.1 | 6.7 | 6.7 | 136.7 |
(hr/10 y) |
T | 0.2 | 0.2 | 0.2 | 0.9 | 0.4 | 0.8 | 0.8 | 0.7 | 0.5 | 0.4 | 0.7 | 0.2 | 0.5 |
(°C/10 y) |
Poznań | SDU | 1.4 | 2.8 | 4.1 | 18.8 | 6.5 | 15.8 | 13.8 | 14.0 | 12.0 | 4.0 | 3.5 | 0.1 | 96.7 |
(hr/10 y) |
T | 0.3 | 0.3 | 0.3 | 0.9 | 0.4 | 0.6 | 0.6 | 0.6 | 0.6 | 0.4 | 0.5 | 0.3 | 0.5 |
(°C/10 y) |
Suwałki | SDU | -1.8 | -3.0 | 1.9 | 12.5 | 3.4 | 7.7 | 2.9 | 2.1 | 7.8 | 2.9 | -1.7 | -1.3 | 33.4 |
(hr/10 y) |
T | 0.3 | 0.4 | 0.5 | 0.9 | 0.4 | 0.5 | 0.6 | 0.6 | 0.6 | 0.3 | 0.5 | 0.4 | 0.5 |
(°C/10 y) |
Szczecin Dąbie | SDU | 1.9 | 4.8 | 5.4 | 16.8 | 4.6 | 13.3 | 9.3 | 5.2 | 13.0 | 5.5 | 2.6 | 1.2 | 83.5 |
(hr/10 y) |
T | 0.3 | 0.4 | 0.3 | 0.8 | 0.3 | 0.5 | 0.4 | 0.4 | 0.4 | 0.4 | 0.4 | 0.3 | 0.4 |
(°C/10 y) |
Śnieżka | SDU | -2.5 | -1.5 | 2.3 | 13.3 | 5.1 | 15.2 | 13.8 | 8.9 | 3.5 | -2.4 | 1.9 | -0.8 | 56.8 |
(hr/10 y) |
T | 0.1 | 0.1 | 0.2 | 0.8 | 0.3 | 0.7 | 0.6 | 0.5 | 0.3 | 0.3 | 0.7 | 0.2 | 0.4 |
(°C/10 y) |
Terespol | SDU | -4.4 | -3.0 | 4.9 | 16.0 | 12.0 | 15.7 | 9.2 | 10.2 | 12.0 | 6.1 | -0.2 | -0.2 | 78.2 |
(hr/10 y) |
T | 0.3 | 0.4 | 0.4 | 0.8 | 0.4 | 0.6 | 0.7 | 0.7 | 0.6 | 0.4 | 0.5 | 0.3 | 0.5 |
(°C/10 y) |
Toruń | SDU | 0.2 | 3.4 | 5.2 | 16.0 | 2.9 | 10.5 | 3.7 | 5.0 | 9.3 | 5.7 | 0.7 | 1.0 | 63.5 |
(hr/10 y) |
T | 0.2 | 0.3 | 0.3 | 0.9 | 0.4 | 0.6 | 0.6 | 0.6 | 0.5 | 0.4 | 0.5 | 0.3 | 0.5 |
(°C/10 y) |
Warszawa | SDU | -0.5 | 0.5 | 3.8 | 13.9 | 8.3 | 16.1 | 10.2 | 10.2 | 10.8 | 5.4 | -0.1 | 0.8 | 79.4 |
(hr/10 y) |
T | 0.3 | 0.4 | 0.3 | 0.8 | 0.4 | 0.6 | 0.7 | 0.6 | 0.5 | 0.4 | 0.5 | 0.3 | 0.5 |
(°C/10 y) |
Włodawa | SDU | -2.6 | -1.7 | 2.0 | 11.5 | 8.1 | 13.4 | 5.7 | 5.8 | 7.7 | 7.4 | 0.8 | 0.9 | 59.0 |
(hr/10 y) |
T | 0.3 | 0.4 | 0.4 | 0.8 | 0.3 | 0.6 | 0.7 | 0.7 | 0.5 | 0.4 | 0.6 | 0.3 | 0.5 |
(°C/10 y) |
Zakopane | SDU | -0.4 | -1.8 | 2.1 | 13.8 | 4.5 | 7.3 | 8.1 | 7.4 | 2.0 | 0.7 | 3.2 | 1.7 | 48.5 |
(hr/10 y) |
T | 0.1 | 0.2 | 0.2 | 0.9 | 0.3 | 0.7 | 0.7 | 0.7 | 0.4 | 0.3 | 0.7 | 0.2 | 0.5 |
(°C/10 y) |
Zielona Góra | SDU | 1.0 | 5.0 | 6.7 | 19.0 | 6.7 | 16.9 | 12.8 | 10.6 | 11.2 | 4.6 | 3.9 | 2.4 | 100.8 |
(hr/10 y) |
T | 0.2 | 0.3 | 0.2 | 0.9 | 0.4 | 0.6 | 0.5 | 0.5 | 0.4 | 0.4 | 0.4 | 0.3 | 0.4 |
(°C/10 y) |
Note: Bold values indicate statistically significant at the level of p < 0.05; bold and underlined – p < 0.01 |
In January and February, trends in sunshine duration decrease longitudinally and are positive (4.3 hr/10 years, Opole) in the western half of Poland, and negative in the eastern half (-4.4 hr/10 years, Terespol). Air temperature increases during these months, but the trends are not statistically significant and change latitudinally, from the highest values in the north to the lowest in the south (Fig. 8, Table 2). In March, the largest trends in sunshine duration occur in the central Baltic coast (8.5 hr/10 years, Koszalin) and Silesian Upland (10.1 hr/10 years, Opole), while the smallest are in the Bieszczady Mountains and the belt in north-eastern and south-eastern Poland (Fig. 8). Temperature trends in March increase from the south-west and south to the north-east, but are small and not statistically significant. The largest increases in the length of sunshine duration and the value of temperature in a 10-year period in the year occur in April. For both elements they are the largest in the west of Poland (19.4 hr/10 years, 0.9 °C/10 years, Jelenia Góra and Opole) and decrease towards the east (Fig. 8). It is worth noting that a large increase in temperature also occurred in the mountains in April (Zakopane, Śnieżka, Kasprowy Wierch). In May, trend values do not vary much and are statistically significant only at very few stations (Table 2). In the summer months, the highest increases in sunshine duration are observed in the south-west of Poland (Jelenia Góra, Katowice, Opole) and decrease towards the north-east in June and July, and towards the north in August (Fig. 8). In these months, the temperature also increased the most in the south-west (0.8 °C/10 years Opole, Kraków), and the trends decreased towards the north-west. Interestingly, a large increase in temperature (0.8 − 0.7 °C/10 years) occurred in the mountains, at the stations on Kasprowy Wierch, Zakopane, and Śnieżka (Table 2). In September, trends in sunshine duration were the largest in Western Pomerania (Koszalin, Chojnice) and the Silesian Upland (Katowice, Opole), while the smallest occurred in the mountains and in the east of Poland (Fig. 8, Table 2). Temperature trends in September were the highest in the north-east (Mikołajki: 0.9 °C/10 years) and gradually decreased towards the south and south-west. In October, November, and December, slight increases in sunshine duration occurred at only very few stations in south-western Poland, while positive temperature trends were clearly marked in November (Fig. 8, Table 2). Also in November, the highest temperature increase was recorded in southern Poland (0.8 °C/10 years, Kasprowy Wierch, Lesko), and the values gradually decreased towards the north. In December, positive trends in sunshine duration in south-western Poland are not reflected in temperature trends. They increase slightly in the northern half of Poland, but the trends are not statistically significant (Table 2).
4.5. Correlation between sunshine duration and air temperature
A correlation analysis of the area-average monthly sunshine duration and air temperature shows a strong relationship between them in the period from April to October (Fig. 9). The highest correlation coefficients occur in July and September (r = 0.87). Sunshine duration is less related to the maximum temperature than to the average daily temperature, because the Tmax value relates only to one time during the day. The highest correlation coefficients between SDU and Tmax occur in June and August (r > 0.60), i.e. in months other than the ones with the highest correlation coefficients between SDU and the average air temperature (Fig. 9). A more pronounced relationship between sunshine duration and temperature in the warm half-year means that in this part of the year the radiation factor has a greater effect on air temperature than the circulation of the atmosphere.
The highest correlation coefficients between the average annual air temperature and sunshine duration (r > 0.80) were found at stations in the southern part of the country, i.e. in Kraków, Katowice, and Opole, while the least (r < 0.60) in Suwałki (north-eastern Poland) (Fig. 10a). In winter, at the stations located in the northern and eastern half of the country, the correlation coefficients are negative, however, they are statistically significant only in Suwałki and Białystok (r = − 0.46 and − 0.41, respectively) (Fig. 10b). In the other seasons, the association is positive; for example, in April, the correlation between sunshine duration and air temperature is statistically significant at all meteorological stations, and the highest correlation coefficient values occur in southern and western Poland (0.68 < r ≤ 0.72) while the lowest in the eastern part of the country (0.49 < r ≤ 0.55) (Fig. 10c). The correlation coefficients in July are the highest in the year, i.e., r > 0.80 at many stations (Fig. 10). In July, similarly to April, a slightly lower correlation occurs in eastern Poland (0.70 < r ≤ 0.75) (Fig. 10d). In October, the correlation of temperature and sunshine duration is low, especially in northern Poland (r < 0.30), and statistically significant only at the stations in the southern half of the country (r > 0.40) (Fig. 10e).
A strong statistically significant relationship between the average day-time air temperature and the sunshine duration facilitated development of linear regression models reflecting the course of air temperature in individual months in the years 1971–2019. The high degree of agreement of data calculated with the models with the values from meteorological stations from April-September is confirmed by the low values of the RMSE indicator (Table 3). The totals of sunshine duration account for from 41% (in May) to 76% (in July) of the air temperature variances in the warm half of the year. The average day-time air temperature values from the April-September period, both from meteorological stations and from the model, show a strong upward trend, with the highest values occurring in the last two years of the studied multi-year period, i.e., in 2018 and 2019 (Fig. 11).
Table 3
Estimation results of the linear regression model (1) of area-average monthly day-time air temperature in Poland in the years 1971–2019
Month | Model equation | R2 | F-value | p-value | RMSE (°C) | T (°C) |
Jan | \({T}_{1}=0.194-0.034{SDU}_{1}\) | 0.003 | 1.14 | 0.291 | 2.8 | -1.5 |
Feb | \({T}_{2}=-0.127-0.009{SDU}_{2}\) | 0.001 | 0.21 | 0.652 | 2.9 | -0.8 |
Mar | \({T}_{3}=0.223+0.024{SDU}_{3}\) | 0.062 | 3.95 | 0.053 | 2.2 | 3.1 |
Apr | \({T}_{4}=3.515+0.031{SDU}_{4}\) | 0.443 | 39.26 | 0.000 | 1.3 | 8.6 |
May | \({T}_{5}=8.506+0.025{SDU}_{5}\) | 0.412 | 34.64 | 0.000 | 1.2 | 14.1 |
Jun | \({T}_{6}=10.735+0.029{SDU}_{6}\) | 0.645 | 88.24 | 0.000 | 0.9 | 17.1 |
Jul | \({T}_{7}=11.544+0.033{SDU}_{7}\) | 0.757 | 150.58 | 0.000 | 0.9 | 19.1 |
Aug | \({T}_{8}=11.517+0.033{SDU}_{8}\) | 0.599 | 72.65 | 0.000 | 1.0 | 18.8 |
Sep | \({T}_{9}=8.147+0.041{SDU}_{9}\) | 0.746 | 142.16 | 0.000 | 0.9 | 14.1 |
Oct | \({T}_{10}=5.977+0.027{SDU}_{10}\) | 0.187 | 12.02 | 0.001 | 1.4 | 8.9 |
Nov | \({T}_{11}=3.051+0.017{SDU}_{11}\) | 0.013 | 0.99 | 0.324 | 1.9 | 4.0 |
Dec | \({T}_{12}=-0.559+0.018{SDU}_{12}\) | 0.005 | 0.56 | 0.458 | 2.2 | 0.1 |
Year | \(\varvec{T}=1.150+0.005\varvec{S}\varvec{D}\varvec{U}\) | 0.612 | 76.59 | 0.000 | 0.6 | 8.8 |
Note: SDU, 1971–2018 mean of area-average monthly sunshine duration; R2, coefficient of determination; RMSE – root mean square error; T, 1971–2019 mean of area-average monthly day-time air temperature |