Analysis of the Spatio-Temporal Variability of Air Temperature Near the Ground Surface in the Central Baltic Area from 2005 to 2019

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Article Analysis of the Spatio-Temporal Variability of Air Temperature Near the Ground Surface in the Central Baltic Area from 2005 to 2019

Agu Eensaar

Centre for Sciences, Tallinn University of Applied Sciences, Pärnu mnt 62, 10135 Tallinn, Estonia; [email protected]

Abstract: In this study, we analyzed the changes in the average daily, monthly, seasonal, and annual surface air temperatures based on the temperature data obtained from seven stations (1 January 2005– 31 December 2019; 15 years) belonging to the central Baltic area (Stockholm, Tallinn, Helsinki, Narva, Pärnu, Tartu, and Võru). The statistical analysis revealed that there was a strong correlation between the daily average surface air temperature of the studied cities (range: 0.95–0.99). We analyzed the frequency distribution of the average surface air temperatures in addition to the Kruskal–Wallis and Dunn’s tests (significance level of 0.05) to demonstrate that the difference in air temperatures between Narva, Tallinn, Tartu, and Stockholm are critical. The Welch’s t-test (significance level 0.05), used to study the differences in the average monthly air temperature of the cities in question, showed that the surface air temperatures in Stockholm do not differ from Tallinn air temperatures from May to August. However, the surface air temperatures of Narva were similar to those of Tallinn in September. According to our results, the trends in the changes of monthly average surface air temperatures have a certain course during the year (ranging from 1.8 ◦C (Stockholm) to 4.5 ◦C (Võru and Tartu) per decade in February). During the entire study period, in addition to February, the surface air temperature increased in all the studied cities in March, May, June, and December, and the surface air temperature did not increase in January or from July to October. During the study period, the Citation: Eensaar, A. Analysis of the average annual surface air temperature in the cities of the central Baltic area increased by 0.43 ◦C Spatio-Temporal Variability of Air per decade. The results also confirm that the surface air temperature in the study area is changing Temperature Near the Ground differently in different cities. The acceleration of the surface air temperature is very alarming and Surface in the Central Baltic Area requires a significant intensification of the measures taken to slow down the temperature rise. from 2005 to 2019. Atmosphere 2021, 12, 60. https://doi.org/10.3390/ Keywords: air temperature; trends; Welch’s t-test; Kruskal–Wallis test; Dunn’s test; Baltic area atmos12010060

Received: 19 November 2020 Accepted: 29 December 2020 Published: 31 December 2020 1. Introduction To forecast climate change and its potential environmental and societal impact, it is Publisher’s Note: MDPI stays neu- important to examine the evolution of changes in various components of the atmosphere. tral with regard to jurisdictional clai- One of the most significant parameters characterizing the condition of the atmosphere is ms in published maps and institutio- surface air temperature [1,2]. Many researchers have used various methods and modeling nal affiliations. techniques and confirmed that global warming is an ongoing phenomenon [3–8]. These studies have fueled further interest among various groups of researchers who study the effect of global warming on trends in air temperature [9]. Changes in surface air temperature have a significant effect, direct or indirect, on the ecological balance of the environment, Copyright: © 2020 by the author. Li- censee MDPI, Basel, Switzerland. biodiversity, and the extent of environmental pollution. This also affects the functioning of a This article is an open access article society, including the functioning of agriculture, and consequently, food supply, healthcare, distributed under the terms and con- industry, transport, and other infrastructure components [10–15]. Climate change, due to ditions of the Creative Commons At- global warming, is having significant societal and economic consequences, which are more tribution (CC BY) license (https:// severe in some areas than in others. Furthermore, it can have a greater negative impact creativecommons.org/licenses/by/ on certain social groups, for example, the elderly and people with disabilities. Therefore, 4.0/). the member states of the European Union, including Estonia, are developing strategies

Atmosphere 2021, 12, 60. https://doi.org/10.3390/atmos12010060 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, 60 2 of 20

for the adaptation to climate change [16], which can only be effective when they are based on comprehensive studies of changes in the condition of the atmosphere (including air temperature). Knowing the current condition of the environment and trends in its changes will help predict what developments are to be expected in the upcoming decades, which will have to be considered while drafting strategies on adaptation to changes. Meteorological studies in the vicinity of the central part of the Baltic Sea have been conducted for quite a long time [17–26]. This allowed for the evaluation of the regularities of the changes in the condition of the atmosphere of the coastal area both in the long- and short-term [27–41]. The change in the thermal regime of the Baltic Sea is extremely important, as it has a significant impact on economic activity, especially during the cold season. In the central part of the Baltic Sea, the formation of ice cover and the extent of its distribution have a particularly strong impact, affecting fishery and port activities and the safety of navigation in Russia, Finland, Estonia, and Sweden [42–44]. The changing trend in the surface air temperatures is quite a complicated process to be assessed, which, in addition to its random aspect, has certain temporal and spatial characteristics. Observation places, measurement techniques, and types of equipment have changed over time; therefore, it is very important to analyze the results of the latest more precise measurements of surface air temperatures. Various studies have shown that on average, the increase in surface air temperature in the central Baltic area, reaching as much as 0.5 ◦C per decade, is faster than that in the Northern Hemisphere [32,33]. Thus, the increase in surface air temperature has primarily taken place in the spring and autumn. Despite the relatively small size of the central Baltic area, we can still observe significant regional differences due to the influence of the Baltic Sea, as well as the Atlantic and Arctic oceans. North Atlantic oscillation plays an important role in the formation of the local climate of the Baltic Sea area (especially from November to April) [26]. Studying surface air temperatures in the central Baltic area will allow the characteristics of climate change to be understood on a regional scale and the nature of global warming can be defined more accurately [33].

2. Materials and Methods In this study, we aimed to analyze the changes in surface air temperatures and their relationship between cities of the central Baltic area (Tallinn, Narva, Pärnu, Tartu, Võru, Stockholm, and Helsinki) at various time points (daily, monthly, seasonal, and annual) between 2005 and 2019. It is noteworthy that in some cases, the meteorological stations are located outside the city limits in order to eliminate the general thermal effect of the urban environment. For the sake of brevity, hereinafter, we will be using the names of the cities instead of the names of the stations where air temperatures were measured. We selected the area and the speciﬁc cities by considering their location in relation to the Baltic Sea and the availability of the long-term results of reliable measurements of air temperature. Figure1 displays the map of the study area. In the study area, the coastal cities namely, Stockholm, Tallinn, and Narva are located in the west-to-east direction, whereas Helsinki, Tallinn, and Pärnu are located in the north-to-south direction. Tartu and Võru are inland cities located somewhat off the coast of the Baltic Sea. Previous studies show greater differences in the changes in the air temperature of various cities in the area being researched in this study. Tallinn’s location is central in relation to the area being researched; therefore, in many cases, it is reasonable to look for correlation and difference in surface air temperatures in relation to Tallinn’s surface air temperatures. The operation of meteorological stations in Estonia is coordinated by the Estonian Weather Service (EWS), which publishes the results of meteorological observations in its yearbooks; the latest data available are from the year 2019 [45]. The Tallinn meteorological station began its operation in 1805, and since 1946, air temperatures in Tallinn have been measured by the Tallinn–Harku meteorological station. Narva meteorological station has been operating since 1866. Between 2000 and 2013, the station was in Narva–Jõesuu, and at the end of 2013, the station was moved to Soldina village, which is under the Atmosphere 2021, 12, 60 3 of 20

station began its operation in 1805, and since 1946, air temperatures in Tallinn have been measured by the Tallinn–Harku meteorological station. Narva meteorological station has been operating since 1866. Between 2000 and 2013, the station was in Narva–Jõesuu, and at the end of 2013, the station was moved to Soldina village, which is under the municipal administration of Narva–Jõesuu. Pärnu meteorological station began recording measurements in 1842. In Tartu, meteorological measurements have been recorded since 1865, and since 1964, air temperatures for Tartu have been measured by the Tartu–Tõravere mete- Atmosphere 2021, 12, 60 orological station. Võru meteorological station has been operating since 1868. The data3 of on 20 the daily average surface air temperatures in Tallinn, Narva, Pärnu, Tartu, and Võru for the period from 1 January 2005 to 31 December 2019 (15 years) has been obtained from the Estonian Environment Agency [46]. municipalMeteorological administration observation of Narva–J in Stockholmõesuu. Pärnu was meteorological started in 1754, station and the began data recording on daily andmeasurements monthly average in 1842. surface In Tartu, air meteorologicaltemperatures have measurements been recorded have by been the old recorded astronomi- since cal1865, observatory and since 1964,of Stockholm air temperatures since 1756. for The Tartu current have been observation measured location by the in Tartu–T Stockholmõravere is ameteorological meteorological station. station Vofõ theru meteorologicalnational station station network has operated been operating by the Swedish since 1868. Meteor- The ologicaldata on and the dailyHydrological average Institute surface air(SMHI). temperatures Data on the in Tallinn, daily av Narva,erage air Pärnu, temperatures Tartu, and in StockholmVõru for the between period from2005 and 1 January 2019 have 2005 been to 31 obtained December from 2019 the (15 database years) has of the been Bolin obtained Cen- trefrom for the Climate Estonian Research Environment [47]. Agency [46].

Figure 1. Map of the central Baltic area.

TheMeteorological oldest meteorological observation station in Stockholm in Finland was is Helsinki started in–Kaisaniemi, 1754, and the which data has on dailybeen operatingand monthly since average 1844. We surface obtained air temperatures the daily average have been surface recorded air temperature by the old astronomicaldata of Hel- sinkiobservatory–Kaisaniemi of Stockholm from 2005 since to 2019 1756. from The the current database observation of the Finnish location Meteorological in Stockholm Ser- is vicea meteorological [48]. Table 1 stationshows the of the locations national of stationthe meteorological network operated stations by. Supplementary the Swedish Meteo- Ma- terialrological (Table and S1) Hydrological shows the daily Institute average (SMHI). surface Data air on temperatures the daily average in the airrelevant temperatures cities of thein Stockholm central Baltic between area during 2005 and the 2019period have 2005 been–2019. obtained from the database of the Bolin Centre for Climate Research [47]. The oldest meteorological station in Finland is Helsinki–Kaisaniemi, which has been operating since 1844. We obtained the daily average surface air temperature data of Helsinki–Kaisaniemi from 2005 to 2019 from the database of the Finnish Meteorological Service [48]. Table1 shows the locations of the meteorological stations. Supplementary Material (Table S1) shows the daily average surface air temperatures in the relevant cities of the central Baltic area during the period 2005–2019. To compare the peculiarities of the changes in air temperatures in the central Baltic area, the daily mean surface air temperatures in central England were used [49,50]. The central England surface air temperature series is the longest existing meteorological record. These daily temperatures are representative of a roughly triangular area of the United Kingdom enclosed by Lancashire, London and Bristol. Atmosphere 2021, 12, 60 4 of 20

Table 1. Locations of meteorological stations.

Meteorological Station Country Coordinates Tallinn–Harku Estonia 59◦240 N; 24◦360 E Narva Estonia 59◦230 N; 28◦070 E Pärnu Estonia 58◦230 N; 24◦290 E Tartu–Tõravere Estonia 59◦240 N; 24◦360 E Võru Estonia 57◦510 N; 27◦010 E Stockholm Sweden 59◦210 N; 18◦030 E Helsinki–Kaisaniemi Finland 60◦120 N; 24◦570 E

The daily, monthly, seasonal, and annual average surface air temperatures were determined as weighted averages based on the data on daily average air temperatures:

∑i nixtix tax = , (1) ∑i nix

where tax is the average surface air temperature for the period x, x is the period (date, month, season, or year), and nix is the number of periods x. To describe changes in the average surface air temperatures, the trend in changes in air temperatures was calculated using the method of least squares [51]. As the area being studied is relatively small, surface air temperatures are mostly influenced by the same synoptic processes. Consequently, surface air temperatures in different cities are not completely independent. Therefore, we used regression analysis to assess the relationship between them. The formation of specific meteorological conditions depends on a multitude of factors, which makes cause-and-effect relationships difficult to determine, which is only possible in individual-specific situations. Each individual meteorological situation can be regarded as one realization of all kinds of situations in a given location that have a probability of occurrence. In this study, we employed statistical methods to analyze the trend in changes in surface air temperatures as they change to a great extent over time [51,52]. Random variables were characterized by a certain distribution function. Depending on the distribution function, different methods can be used for the assessment and verification of the significance of changes and differences. Welch’s t-test allows changes of values to be tested when the changes being studied correspond to a normal distribution. The correspondence of the distribution of surface air temperatures of a specific period to the normal distribution is verified by Shapiro–Wilk, Jarque–Bera, and Smirnov–Kolmogorov tests [53]. The difference between the average temperatures x1 and x2, in case the variance of the comparable temperatures is different, can be tested with Welch’s t-test, for which the test statistic is calculated using the following equation:

x1 − x2 t = r , (2) s2 s2 1 + 2 n1 n2

where n1 and n2 are sample sizes, and s1 and s2 are standard deviations of the measured temperatures (samples). The number of degrees of freedom ν is calculated using the following equation: 2 s2 s2 1 + 2 n1 n2 ν = 2 2 (3) (s2/n ) (s2/n ) 1 1 + 2 2 n1−1 n2−1

There is no signiﬁcant difference if |t| < tα,ν. tα,ν is the value of the Student’s t-test or critical value of the test statistic corresponding to the signiﬁcance level or error probability α and the number of degrees of freedom ν. Atmosphere 2021, 12, 60 5 of 20

In cases where the data do not follow the normal distribution, we used the Kruskal– Wallis test to compare the medians of two or more samples of equal or different sizes (temperatures at different measuring points) [54,55]. All observations are ranked together in the test. Each sample is characterized by the sum of ranks. The test statistic H is calculated using the following equation:

12 k R2 H = i − 3(N − 1), (4) ( + ) ∑ N N 1 i=1 ni

k where ni is the number of data in sample i, k is the number of samples, N = ∑ ni is the i=1 size of total data set, and Ri is the sum of ranks in the ith sample. For the null hypothesis (that the medians of all the samples are equal) to be rejected, the calculated H = χ2 must be greater than the critical value of the chi-square distribution at the given signiﬁcance level. The number of degrees of freedom is determined using the following equation: ν = k − 1 (5) A negative result of the Kruskal–Wallis test means that the data being analyzed are not from the same data set. In this case, the medians of different samples (temperatures at different time points) can be compared in pairs with Dunn’s test [56], which compares the mean ranking of different samples Wi = Wi/ni, where Wi is the sum of ranks of the ith sample, and ni is the sample size for the ith group. To compare sample A to B, the test statistic zi is calculated using the following equation:

yi zi = , (6) σi

where yi = W A − WB, and σi is the standard deviation of yi. Dunn’s test can be used to identify the samples that are more similar to each other and group them by similarity. The results can be represented by a diagram, using the critical difference diagrams developed by Demšar [57]. The diagram combines the samples (different measuring points) that do not have a critical difference of temperatures into groups. A sample can be in several different groups at the same time if there are no critical differences.

3. Results and Discussion 3.1. Changes in Average Daily Surface Air Temperatures 3.1.1. Annual Changes in Average Daily Surface Air Temperatures The annual average daily surface air temperatures changed noticeably during the 15-year study period. Changes in surface air temperature follow a certain course during a year, with the maximum surface air temperatures recorded in July and the minimum in February. Changes in the surface air temperatures in different cities are quite similar, and differences are greater during the winter. Figure2 shows that the short-term changes in surface air temperatures have occurred synchronously in different cities over most of the time. Table2 displays the extreme and average values of daily surface air temperatures in the cities of the central Baltic area. The minimum average daily air temperature between 2005 and 2019 was measured in Võru (−28.1 ◦C), and the maximum average daily air temperature was also recorded in Võru (28.5 ◦C). Atmosphere 2021, 12, 60 6 of 20 Atmosphere 2021, 12, 60 6 of 20

Figure 2. Annual changes in average daily temperatures as an average of the years 2005–2019. Figure 2. Annual changes in average daily temperatures as an average of the years 2005–2019. Table 2 displays the extreme and average values of daily surface air temperatures in theTable cities 2. Statistical of the central indicators Baltic of air area. temperatures The minimum (◦C) in average the cities ofdaily the centralair temperature Baltic area frombetween 2005 2005to 2019. and 2019 was measured in Võru (−28.1 °C), and the maximum average daily air temperature was also recorded in Võru (28.5 °C). Tallinn Narva Pärnu Tartu Võru Stockholm Helsinki TableMaximum 2. Statistical27.3 indicators 28.1of air temperatures 26.4 (°C) 27.1in the cities 28.5of the central 25.9 Baltic area 26.4from 2005Minimum to 2019. −23.7 −27.3 −25.1 −27.2 −28.1 −18.0 −22.7 Average 6.7 6.3 6.8 6.6 6.7 7.4 6.9

SD 8.5Tallinn 9.2Narva Pärnu 8.8 Tartu 9.2 Võru 9.4 Stockholm 7.8 Helsinki 8.6 Maximum 27.3 28.1 26.4 27.1 28.5 25.9 26.4 Abbreviation: SD, standard deviation. Minimum −23.7 −27.3 −25.1 −27.2 −28.1 −18.0 −22.7 Average 6.7 6.3 6.8 6.6 6.7 7.4 6.9 Figure3 shows the changes in average daily surface air temperatures (5478 observa- SD 8.5 9.2 8.8 9.2 9.4 7.8 8.6 tions in each location) in terms of the frequency of the occurrence of different temperatures. Abbreviation: SD, standard deviation. The frequency distribution of the surface air temperature in the cities of the central Baltic area has two local maximums: one at approximately 2 ◦C and the other at 16 ◦C. The local Figure 3 shows the changes in average daily ◦surface air temperatures (5478 observa- tionsminimum in each temperature location) in between terms of them the isfrequency about 10 ofC. the The occurrence distribution of isdifferent asymmetric tempera- from the local maximums toward extreme temperatures; a change of about 30 ◦C is possible tures. The frequency distribution of the surface air temperature in the cities of the central toward lower temperatures and of about 12 ◦C toward higher temperatures. Two local Baltic area has two local maximums: one at approximately 2 °C and the other at 16 °C. The maximum frequencies mean that there are two most probable surface air temperature local minimum temperature between them is about 10 °C. The distribution is asymmetric situations (cool and warm). Frequency distributions in different cities are similar, and the from the local maximums toward extreme temperatures; a change of about 30 °C is possi- visual shows that the frequency of lower air temperatures in Stockholm is lower than that ble toward lower temperatures and of about 12 °C toward higher temperatures. Two local in the other cities. maximum frequencies mean that there are two most probable surface air temperature sit- The frequency distribution of the daily mean air temperatures in central England uations (cool and warm). Frequency distributions in different cities are similar, and the differs from the frequency distribution in the central Baltic area. At higher temperatures, visual shows that the frequency of lower air temperatures in Stockholm is lower than that the frequency distribution practically coincides with the frequency distribution of air tem- in the other cities. peratures in the central Baltic region, but there is a signiﬁcant difference in the occurrence of lowerThe airfrequency temperatures. distribution of the daily mean air temperatures in central England differs from the frequency distribution in the central Baltic area. At higher temperatures, the frequency distribution practically coincides with the frequency distribution of air temperatures in the central Baltic region, but there is a significant difference in the occurrence of lower air temperatures.

Atmosphere 2021, 12, 60 7 of 20 Atmosphere 2021, 12, 60 7 of 20

Figure 3. Frequency distributions of average daily air temperatures during the period 2005–2019. Percentages are calcu- Figure 3. Frequency distributions of average daily air temperatures during the period 2005–2019. Percentages are calculated lated for 2 °C intervals. for 2 ◦C intervals.

The frequency distribution of the average daily surface air temperatures in the cities of the central Baltic are areaa during the 15-year15-year study period does not correspond to normal distributiondistribution,, either visually or according to the Jarque–BeraJarque–Bera andand Smirnov–KolmogorovSmirnov–Kolmogorov tests. Therefore, Therefore, we we used the nonparametric Kruskal Kruskal–Wallis–Wallis test to compare the average daily surface air temperaturestemperatures of the cities. The analysis reveal revealeded that the average daily surface air air temperatures temperatures of of all all the the cities cities cannot cannot be beconsidered considered to come to come from from the same the same data setdata at set the at significance the significance level levelof 0.05 of 0.05(as the (as observed the observed value value of H of (26.75)H (26.75) is higher is higher than than the ctheritical critical value value (12.59)), (12.59)), i.e., i.e.,the surface the surface air temperatures air temperatures of different of different cities cities differ differ from from one another.one another. Using Using Dunn’s Dunn’s test, we test, can we determine can determine in which in which cities there cities is there no significant is no significant differ- encedifference between between the average the average daily dailysurface surface air temperatures air temperatures during during the per theiod period 2005– 2005–2019.2019. Ac- cordingAccording to the to the results results of Dunn’s of Dunn’s test, test, at the at thesignificance significance level level of 0.05, of 0.05, the thecities cities can canbe di- be videddivided into into two two groups groups (A (A and and B) B) based based on on the the average average daily daily air air temperature, of which Narva (A) and Stockholm (B) differ the most. Figure 44 showsshows aa diagramdiagram ofof thethe criticalcritical differences in surface air temperatures. On the diagram, the cities are ranked according to the mean rank of the variation series. It It is is interesting interesting to to note note that that three three cities (i.e., Pärnu, Võru,Võru, andand Helsinki)Helsinki) belongbelong to to both both groups, groups i.e.,, i.e., their their average average daily daily air air temperatures temperatures do notdo notshow show a critical a critical difference difference in the in Dunn’s the Dunn’s test at test the at significance the significance level oflevel 0.05 of compared 0.05 compared to the tosurface the surface air temperature air temperature of any of other any citiesother incities the centralin the central Baltic area.Baltic area. The cities of the central Baltic area in question are located within a small area: the maximum distance between the cities does not exceed 600 km, i.e., they remain within the limits of the average synoptic scale. Consequently, the condition of the atmosphere in these cities can, in many cases, be affected by the same synoptic processes. Therefore, it is important to perform a correlation analysis between the average daily surface air temperatures of the cities in question. As Tallinn is in the center of the study area, we compared the correlations between the average daily surface air temperatures of the different cities with that of Tallinn (Figure5). The analysis (Table3) revealed that the strongest correlation between average daily surface air temperatures was between Tallinn and Pärnu (Pearson

Atmosphere 2021, 12, 60 8 of 20

Atmosphere 2021, 12, 60 8 of 20 correlation coefﬁcient 0.990), and the weakest correlation between surface air temperatures was between Tallinn and Stockholm (Pearson correlation coefﬁcient 0.947).

AtmosphereFigure 2021 4., 12FigureDivision, 60 4. Division of the of cities the cities into into groups groups based based on the the critical critical difference difference (Dunn’s (Dunn’s test) in average test) in daily average surface daily air tem- surface air9 of 20 temperaturesperatures from from 2005 2005 to 2019. to 2019.

The cities of the central Baltic area in question are located within a small area: the maximum distance between the cities does not exceed 600 km, i.e., they remain within the (a) limits of the average synoptic( b scale.) Consequently, the condition of the atmosphere in these cities can, in many cases, be affected by the same synoptic processes. Therefore, it is important to perform a correlation analysis between the average daily surface air temperatures of the cities in question. As Tallinn is in the center of the study area, we compared the correlations between the average daily surface air temperatures of the different cities with that of Tallinn (Figure 5). The analysis (Table 3) revealed that the strongest correlation between average daily surface air temperatures was between Tallinn and Pärnu (Pearson correlation coefficient 0.990), and the weakest correlation between surface air temperatures was between Tallinn and Stockholm (Pearson correlation coefficient 0.947).

Table 3. Correlation of the average daily surface air temperatures in the different cities with the average daily air temperature in Tallinn—correlation statistics.

Narva Pärnu Tartu Võru Stockholm Helsinki (c) Pearson correlation 0.9862(d ) 0.9897 0.9856 0.9801 0.9468 0.9796 Coefficient of deter- 0.9726 0.9796 0.9714 0.9607 0.8965 0.9596 mination Standard error 1.5231 1.2556 1.5555 1.8642 2.5219 1.7301 Table 4 presents the regression parameters characterizing the correlations between surface air temperatures in different cities and the average daily air temperature in Tal- linn. Changes in surface air temperatures in Narva, Pärnu, Tartu, and Võru have a higher rate than in Tallinn (the slope is greater than 1.0). In Stockholm and Helsinki, the changes in surface air temperatures have a lower rate than those in Tallinn (the slope is less than 1.0). The intercept shows the average daily surface air temperature that is being compared to the average air temperature in Tallinn. The average surface air temperature in Narva, Pärnu, Tartu, and Võru is, respectively, 0.79, 0.05, 0.56, and 0.50 °C lower. The average (e) daily surface air temperature in(f Stockholm) is, on an average, 1.52 °C higher, and in Hel- sinki, it is 0.25 °C higher. The higher surface air temperature in Stockholm is associated with its proximity to the Atlantic Ocean (Gulf Stream). The reason for the higher surface air temperature in Helsinki, which lies to the north of Tallinn, requires further understanding.

Figure 5. Correlation of average daily air temperatures in different cities with the average daily air temperature in Tallinn Figure 5. Correlation of average daily air temperatures in different cities with the average daily air temperature in Tallinn during the period 2005 2005–2019–2019 with a linear regression and 95% conﬁdenceconfidence bounds: ( aa))—Narva,—Narva, ( (bb))—Pärnu,—Pärnu, ( (cc))—Tartu,—Tartu, (d))—V—Võru,õru, (e)—Stockholm,)—Stockholm, and (f)—Helsinki.)—Helsinki.

Table 4. Correlations between the average daily surface air temperatures in the different cities and the average daily air temperature in Tallinn—regression parameters.

Standard Parameter Value t-statistic P-value Lower 95% Upper 95% Error Narva Slope a 1.0699 0.0024 440.76 0.0000 1.0652 1.0747 Intercept b −0.7887 0.0262 −30.15 0.0000 −0.8400 −0.7375 Pärnu Slope a 1.0255 0.0020 512.47 0.0000 1.0216 1.0294 Intercept b −0.0509 0.0216 −2.36 0.0183 −0.0932 −0.0086 Tartu Slope a 1.0699 0.0025 431.55 0.0000 1.0650 1.0747 Intercept b −0.5571 0.0267 −20.85 0.0000 −0.6095 −0.5047 Võru Slope a 1.0865 0.0030 365.71 0.0000 1.0807 1.0924 Intercept b −0.5014 0.0320 −15.66 0.0000 −0.5641 −0.4386 Stockholm Slope a 0.8754 0.0040 217.81 0.0000 0.8676 0.8833 Intercept b 1.5240 0.0433 35.19 0.0000 1.4391 1.6089 Helsinki Slope a 0.9939 0.0028 360.45 0.0000 0.9885 0.9993 Intercept b 0.2516 0.0297 8.47 0.0000 0.1933 0.3098

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Table 3. Correlation of the average daily surface air temperatures in the different cities with the average daily air temperature in Tallinn—correlation statistics.

Narva Pärnu Tartu Võru Stockholm Helsinki Pearson 0.9862 0.9897 0.9856 0.9801 0.9468 0.9796 correlation Coefﬁcient of 0.9726 0.9796 0.9714 0.9607 0.8965 0.9596 determination Standard error 1.5231 1.2556 1.5555 1.8642 2.5219 1.7301

Table4 presents the regression parameters characterizing the correlations between surface air temperatures in different cities and the average daily air temperature in Tallinn. Changes in surface air temperatures in Narva, Pärnu, Tartu, and Võru have a higher rate than in Tallinn (the slope is greater than 1.0). In Stockholm and Helsinki, the changes in surface air temperatures have a lower rate than those in Tallinn (the slope is less than 1.0). The intercept shows the average daily surface air temperature that is being compared to the average air temperature in Tallinn. The average surface air temperature in Narva, Pärnu, Tartu, and Võru is, respectively, 0.79, 0.05, 0.56, and 0.50 ◦C lower. The average daily surface air temperature in Stockholm is, on an average, 1.52 ◦C higher, and in Helsinki, it is 0.25 ◦C higher. The higher surface air temperature in Stockholm is associated with its proximity to the Atlantic Ocean (Gulf Stream). The reason for the higher surface air temperature in Helsinki, which lies to the north of Tallinn, requires further understanding.

Table 4. Correlations between the average daily surface air temperatures in the different cities and the average daily air temperature in Tallinn—regression parameters.

Standard Lower Upper Parameter Value t-Statistic P-Value Error 95% 95% Narva Slope a 1.0699 0.0024 440.76 0.0000 1.0652 1.0747 Intercept b −0.7887 0.0262 −30.15 0.0000 −0.8400 −0.7375 Pärnu Slope a 1.0255 0.0020 512.47 0.0000 1.0216 1.0294 Intercept b −0.0509 0.0216 −2.36 0.0183 −0.0932 −0.0086 Tartu Slope a 1.0699 0.0025 431.55 0.0000 1.0650 1.0747 Intercept b −0.5571 0.0267 −20.85 0.0000 −0.6095 −0.5047 Võru Slope a 1.0865 0.0030 365.71 0.0000 1.0807 1.0924 Intercept b −0.5014 0.0320 −15.66 0.0000 −0.5641 −0.4386 Stockholm Slope a 0.8754 0.0040 217.81 0.0000 0.8676 0.8833 Intercept b 1.5240 0.0433 35.19 0.0000 1.4391 1.6089 Helsinki Slope a 0.9939 0.0028 360.45 0.0000 0.9885 0.9993 Intercept b 0.2516 0.0297 8.47 0.0000 0.1933 0.3098

3.1.2. Changes in Average Daily Surface Air Temperatures in Different Seasons Changes in the average daily surface air temperatures in the cities of the central Baltic area in different seasons are characterized by the frequency distributions that are shown in Figure6, which demonstrates that the changes in air temperatures are virtually coincident in summer (June to August, 1380 observations in each city). In winter (December to February, 1353 observations in each city), Stockholm stands out due to its more moderate temperatures. The most signiﬁcant differences are in the frequency distributions of surface air temperatures in spring (March to May, 1380 observations in each city) and autumn (September to November, 1365 observations in each city). Atmosphere 2021, 12, 60 10 of 20

3.1.2. Changes in Average Daily Surface Air Temperatures in Different Seasons Changes in the average daily surface air temperatures in the cities of the central Baltic area in different seasons are characterized by the frequency distributions that are shown in Figure 6, which demonstrates that the changes in air temperatures are virtually coincident in summer (June to August, 1380 observations in each city). In winter (December to February, 1353 observations in each city), Stockholm stands out due to its more moderate Atmosphere 2021, 12, 60 temperatures. The most significant differences are in the frequency distributions of10 ofsur- 20 face air temperatures in spring (March to May, 1380 observations in each city) and autumn (September to November, 1365 observations in each city).

(a) (b)

Figure 6. Frequency distributionFigure of 6. the Frequency average distribution daily surface of air the temperatures average daily during surface different air temperatures periods: (dura)—Decembering different to February, (b)—March toperiods: May, (c ()—Junea)—December to August, to February, (d)—September (b)—March to November.to May, (c)— PercentagesJune to August, are ( calculatedd)—September for to November. Percentages are calculated for 2 °C intervals. 2 ◦C intervals.

The frequencyfrequency distributiondistribution of of the the average average daily daily surface surface air air temperatures temperatures in thein the summer, sum- autumn,mer, autumn, and winterand winter did not did conform not conform to the to normal the normal distribution distribution based based on the on Jarque–Bera the Jarque– test.Bera Basedtest. Based on the on data the ofdata Tallinn, of Tallinn, Narva, Narva, Pärnu, Pärnu, Stockholm, Stockholm, and Helsinki, and Helsinki, the frequency the fre- distributionquency distribution of the average of the average daily surface daily surface air temperatures air temperatures in the springin the spring correspond correspond to the normalto the normal distribution distribution only based only on based the Jarque–Bera on the Jarque test and–Bera not test based and on not the basedShapiro–Wilk on the test.Shapiro Therefore,–Wilk test. it is Therefore, appropriate it tois useappropriate the Kruskal–Wallis to use the testKruskal instead–Wallis of Welch’s test insteadt-test toof compareWelch’s the-test surface to compare air temperatures the surface ofair the temperatures different seasons. of the different seasons. According to to the the Kruskal–Wallis Kruskal–Wallis test, test, average average daily daily surface surface air temperatures air temperatures in spring in (Marchspring (March to May) to inMay) the in cities the ofcities the of central the central Baltic Baltic area werearea were not fromnot from the samethe same data data set atset theat the significance significance level level of of 0.05 0.05 (since (since the theH observed observed value value 43.76 43.76 is is higher higher thanthan the H critical value 12.59). Using Using Dunn’s Dunn’s test, test, we we can determine in which cities there waswas nono critical difference between the average daily surface air temperaturestemperatures during spring from 2005 to 2019. According to to the results of the Dunn’s test, the cities can be divideddivided intointo three groups (A, B, and C) depending on the average dailydaily surfacesurface airair temperaturetemperature inin thethe spring, ofof whichwhich NarvaNarva andand Helsinki Helsinki (A) (A) and and Tartu Tartu and and V õVõruru (C) (C) differ differ the the most. most. Figure Figure7a shows7a shows a diagram a diagram of of the the critical critical difference difference in in spring spring air air temperatures. temperatures. Dunn’sDunn’s testtest showsshows no critical difference (at the significancesignificance level of 0.05) betweenbetween spring air temperatures inin Pärnu, Stockholm, Tartu,Tartu, and and V Võru.õru. The The same same can can be be said said about about surface surface air air temperatures temperatures in Tallinn,in Tallinn, Pärnu, Pärnu, and and Stockholm. Stockholm. In addition,In addition, there there was was no significantno significant difference difference between between the air temperatures in Narva, Helsinki, Tallinn, and Pärnu. In spring, surface air temperatures in Pärnu show no critical difference from surface air temperatures in any other city in the central Baltic area.

Atmosphere 2021, 12, 60 11 of 20

the air temperatures in Narva, Helsinki, Tallinn, and Pärnu. In spring, surface air temperatures in Pärnu show no critical difference from surface air temperatures in any other city in the central Baltic area. According to the Kruskal–Wallis test, the average daily surface air temperatures in the summer (June to August) in the cities of the central Baltic area are not from the same data set either (since the observed value 54.69 is higher than the critical value 12.59). As a result of the Dunn’s test, the cities can be divided into three groups (A, B, and C) depending on the average daily surface air temperature in the summer, of which Tallinn (A) and Võru (C) differ the most. Figure 7b shows the diagram of the critical difference in summer air temperatures. The surface air temperatures in Tartu (group B) differ significantly from the summer air temperatures in both Tallinn (A) and Võru (C). Unlike in other seasons, surface air temperatures in Stockholm in the summer are among the lowest in the central Baltic area. According to the Kruskal–Wallis test, average daily surface air temperatures in autumn (September to November) in the cities of the central Baltic area were not from the same data set either (since the observed value 45.59 is higher than the critical value 12.59). Dunn’s test (Figure 7c) revealed that the cities can be divided into three groups (A, B, and C) depending on the average daily surface air temperature in the autumn, of which Tartu and Võru (A) and Stockholm (C) differ the most. In autumn, surface air temperatures in Pärnu show no critical difference from surface air temperatures in any other city in the central Baltic area. According to the Kruskal–Wallis test, average daily surface air temperatures in winter (December to February) in the cities of the central Baltic area were not from the same data set either (since the observed value 140.58 is higher than the critical value 12.59). Dunn’s test (Figure 7d) revealed that the cities can be divided into three completely different groups (A, B, and C) depending on the average daily surface air temperature in the winter, of which Narva (A) and Stockholm (C) differ the most. In winter, the distribution into groups coincides with the location of the cities in the east-to-west direction. Un-

Atmosphere 2021, 12, 60 like in other seasons, no city belongs to several groups at the same time in winter.11 ofThis 20 means that the cities of the central Baltic area can be divided into three significantly different groups depending on the average daily surface air temperatures in the winter.

(a) (b)

Figure 7. Division of the cities in the central Baltic area into groups on the basis of the critical difference in average daily Figure 7. Division of the cities in the central Baltic area into groups on the basis of the critical difference in average daily surface air temperatures according to the Dunn’s test: (a)—spring, (b)—summer, (c)—autumn, and (d)—winter. surface air temperatures according to the Dunn’s test: (a)—spring, (b)—summer, (c)—autumn, and (d)—winter. According to the Kruskal–Wallis test, the average daily surface air temperatures in the summer (June to August) in the cities of the central Baltic area are not from the same data

set either (since the H observed value 54.69 is higher than the H critical value 12.59). As a result of the Dunn’s test, the cities can be divided into three groups (A, B, and C) depending on the average daily surface air temperature in the summer, of which Tallinn (A) and Võru (C) differ the most. Figure7b shows the diagram of the critical difference in summer air temperatures. The surface air temperatures in Tartu (group B) differ signiﬁcantly from the summer air temperatures in both Tallinn (A) and Võru (C). Unlike in other seasons, surface air temperatures in Stockholm in the summer are among the lowest in the central Baltic area. According to the Kruskal–Wallis test, average daily surface air temperatures in autumn (September to November) in the cities of the central Baltic area were not from the same data set either (since the H observed value 45.59 is higher than the H critical value 12.59). Dunn’s test (Figure7c) revealed that the cities can be divided into three groups (A, B, and C) depending on the average daily surface air temperature in the autumn, of which Tartu and Võru (A) and Stockholm (C) differ the most. In autumn, surface air temperatures in Pärnu show no critical difference from surface air temperatures in any other city in the central Baltic area. According to the Kruskal–Wallis test, average daily surface air temperatures in winter (December to February) in the cities of the central Baltic area were not from the same data set either (since the H observed value 140.58 is higher than the H critical value 12.59). Dunn’s test (Figure7d) revealed that the cities can be divided into three completely different groups (A, B, and C) depending on the average daily surface air temperature in the winter, of which Narva (A) and Stockholm (C) differ the most. In winter, the distribution into groups coincides with the location of the cities in the east-to-west direction. Unlike in other seasons, no city belongs to several groups at the same time in winter. This means that the cities of the central Baltic area can be divided into three signiﬁcantly different groups depending on the average daily surface air temperatures in the winter. The distribution of temperatures into groups varies from season to season because of the lack of critical difference. The division of cities in the central Baltic region into groups based on the critical difference in average daily surface air temperature in different months Atmosphere 2021, 12, 60 12 of 20

according to the Dunn’s test is presented in the Supplementary Material Figure S1. This might be associated with the varying nature of atmospheric circulation at different seasons and with the impact of the Atlantic Ocean, the Baltic Sea, and the Arctic Ocean.

3.2. Changes in Average Monthly and Annual Air Temperatures Changes in the surface air temperatures vary greatly from year to year. For example, the difference in average monthly surface air temperatures during the winter months in different years can be as great as 16 ◦C. In August, these changes are the smallest, staying at around 5 ◦C. Figure8 shows the changes in average monthly surface air temperatures during the period 2005–2019. Changes in air temperatures in different years were not synchronous. For example, lower temperatures in were observed in January of 2010, in February of 2013, and in September of 2008. However, the changes in different cities were rather similar. Average monthly surface air temperatures in the different cities during the 15-year study period (2005–2019) are quite close to one another despite the high variability of average monthly surface air temperatures (Table5).

Table 5. Average monthly and annual surface air temperatures (◦C) during the period 2005–2019.

Month Tallinn Narva Pärnu Tartu Võru Stockholm Helsinki January −3.6 −4.9 −3.8 −4.9 −5.0 −2.0 −3.6 February −3.9 −5.3 −4.3 −5.0 −5.0 −2.0 −3.8 March −0.8 −1.4 −0.9 −0.7 −0.6 0.6 −0.8 April 5.0 4.7 5.3 5.9 6.1 5.9 4.8 May 10.9 11.2 11.6 12.1 12.5 10.7 11.1 June 14.6 15.2 15.2 15.6 16.1 14.8 15.0 July 18.0 18.5 18.1 18.2 18.6 18.0 18.5 August 16.7 17.0 17.0 16.8 17.2 16.8 17.2 September 12.6 12.5 12.7 12.4 12.5 13.1 12.8 October 6.8 6.4 6.8 6.3 6.4 7.7 7.0 November 2.9 2.2 3.0 2.3 2.3 3.6 3.3 December −0.1 −1.1 −0.1 −1.1 −1.1 0.4 0.5 Year 6.7 6.3 6.8 6.6 6.7 7.4 6.9

Surface air temperatures changed drastically throughout the year in the different cities. The values were close to each other; therefore, studying the relative difference in surface air temperatures is reasonable. The comparison can be based on the average monthly surface air temperatures in Tallinn due to the city’s central location in the area being studied. Figure9 shows the difference between the average monthly surface air temperatures in other cities being studied and the average monthly surface air temperature in Tallinn throughout the year. Depending on the nature of changes in the average monthly surface air temperatures, the cities of the central Baltic area can be divided into three groups. The ﬁrst group comprises Helsinki and Pärnu, where the average monthly surface air temperature does not differ signiﬁcantly from that of Tallinn, especially in the winter. The second group comprises Tartu, Võru, and Narva, which are characterized by lower temperatures in the winter (up to −1.5 ◦C), but with equally higher temperatures in the summer than in Tallinn. The third group comprises Stockholm, where the surface air temperature in the winter is up to 2 ◦C higher than in Tallinn but nearly the same as in Tallinn in the summer. Atmosphere 2021, 12, 60 13 of 20 Atmosphere 2021, 12, 60 13 of 20

(a) (b)

(e) (f)

(g) (h)

(i) (j)

(k) (l)

Figure 8. ChangesChanges in in the the average average monthly monthly surface surface air airtemperatures temperatures from from 2005 2005 to 2019: to 2019: (a)—January, (a)—January, (b)—February, (b)—February, (c)— (March,c)—March, (d)— (April,d)—April, (e)— (May,e)—May, (f)— (June,f)—June, (g)— (July,g)—July, (h)— (August,h)—August, (i)—September, (i)—September, (j)—October, (j)—October, (k)— (November,k)—November, and ( andl)— (December.l)—December.

Atmosphere 2021, 12, 60 14 of 20

Depending on the nature of changes in the average monthly surface air temperatures, the cities of the central Baltic area can be divided into three groups. The first group comprises Helsinki and Pärnu, where the average monthly surface air temperature does not Atmosphere 2021, 12, 60 differ significantly from that of Tallinn, especially in the winter. The second group com-14 of 20 prises Tartu, Võru, and Narva, which are characterized by lower temperatures in the winter (up to −1.5 °C), but with equally higher temperatures in the summer than in Tallinn. The third group comprises Stockholm, where the surface air temperature in the winter is up to 2 °C higher than in Tallinn but nearly the same as in Tallinn in the summer.

Figure 9. The difference of average monthly air temperatures from surface air temperatures in Tallinn from 2005 to 2019. Figure 9. The difference of average monthly air temperatures from surface air temperatures in Tallinn from 2005 to 2019. According to the Jarque–Bera test, the frequency of distribution of the average According to the Jarque–Bera test, the frequency of distribution of the average monthly monthly surface air temperatures corresponds to normal distribution. Therefore, the dif- surfaceference air between temperatures the average corresponds monthly surface to normal air temperatures distribution. in Tallinn Therefore, and in the other difference betweencities can the be average assessed monthly using Welch’s surface t-test. airtemperatures in Tallinn and in the other cities can be assessedTo test using the null Welch’s hypothesist-test. (the average monthly surface air temperature in different citiesTo testis equal the to null the hypothesissurface air temperature (the average in Tallinn monthly in any surface given air month), temperature the test statis- in different citiestic is found equal using to the the surface correlation air temperature coefficient (2). in The Tallinn null inhypothesis any given is month),valid if the the value test of statistic is foundthe test using statistic the correlationis in the range coefﬁcient of − (2). to The null. hypothesisFigure 10 shows is valid test ifstatistic the value val- of the Atmosphere 2021, 12, 60 testues statistic for all thet is cities in the in range different of −months.t to t . Figure 10 shows test statistic15 valuesof 20 for critical critical all the cities in different months.

Figure 10. Testing the hypothesis (surface air temperatures in the different cities are equal to the surface air temperature Figurein 10. TallinnTesting in any the given hypothesis month) (surfaceusing Welch’s air temperatures t-test. The hypothesis in the different is valid if cities the value are equalof the totest the statistic surface is air in temperaturethe range in Tallinnof in − any given to month). using Welch’s t-test. The hypothesis is valid if the value of the test statistic t is in the range of −tcritical to tcritical. The analysis shows that the average monthly surface air temperatures in Helsinki are the closest to Tallinn air temperatures at the significance set, with the differences practically being noticeable in the summer months alone. Surface air temperatures in Stockholm are not statistically different from those in Tallinn from May to August. Surface air temperatures in Tartu do not differ from those in Tallinn in March, July, and August alone. Surface air temperatures in Võru do not differ from those in Tallinn only in March and August. Surface air temperatures in Narva are only equal to Tallinn air temperatures in September. Surface air temperatures in Pärnu are equal to those in Tallinn from September to January, as well as in March and April. Average monthly surface air temperatures did not change evenly over time in the studied central Baltic cities. The trends characterizing changes in the average monthly surface air temperature have a certain course during the year (Figure 11). The rate of increase in surface air temperature is slightly different in different cities, but the nature of changes is virtually the same in all the cities. The air temperature increased the most in February, with the increase ranging from 1.8 °C (Stockholm) to 4.5 °C (Võru and Tartu) per decade. In addition to February, during the 15-year study period, the surface air temperature increased in all the studied cities in March, May, June, and December. There was virtually no increase in surface air temperature from July to November. There was no increase in air temperature in January either. Consequently, the changes in surface air temperatures in the central Baltic area are virtually nonexistent during the summer period and are smaller in autumn. Significant changes have occurred in late autumn, late winter, and in spring. Changes in average monthly surface air temperatures in central England differ significantly from the changes in average monthly air temperatures in the central Baltic area. The monthly average air temperature trends in central England ranged from −0.77 °C per decade to 0.54 °C per decade, except in December where the average monthly air temperature increased by 1.88 °C per decade (Figure 11).

Atmosphere 2021, 12, 60 15 of 20

The analysis shows that the average monthly surface air temperatures in Helsinki are the closest to Tallinn air temperatures at the signiﬁcance set, with the differences practically being noticeable in the summer months alone. Surface air temperatures in Stockholm are not statistically different from those in Tallinn from May to August. Surface air temperatures in Tartu do not differ from those in Tallinn in March, July, and August alone. Surface air temperatures in Võru do not differ from those in Tallinn only in March and August. Surface air temperatures in Narva are only equal to Tallinn air temperatures in September. Surface air temperatures in Pärnu are equal to those in Tallinn from September to January, as well as in March and April. Average monthly surface air temperatures did not change evenly over time in the studied central Baltic cities. The trends characterizing changes in the average monthly surface air temperature have a certain course during the year (Figure 11). The rate of increase in surface air temperature is slightly different in different cities, but the nature of changes is virtually the same in all the cities. The air temperature increased the most in February, with the increase ranging from 1.8 ◦C (Stockholm) to 4.5 ◦C (Võru and Tartu) per decade. In addition to February, during the 15-year study period, the surface air temperature increased in all the studied cities in March, May, June, and December. There was virtually no increase in surface air temperature from July to November. There was no increase in air temperature in January either. Consequently, the changes in surface air temperatures in the central Baltic area are virtually nonexistent during the summer period Atmosphere 2021, 12, 60 and are smaller in autumn. Signiﬁcant changes have occurred in late autumn, late16 winter, of 20 and in spring.

Figure 11. Trends of average monthly surface air temperatures from 2005 to 2019 in the different cities of the central Baltic Figure 11. Trends of average monthly surface air temperatures from 2005 to 2019 in the different cities of the central Baltic area and in central England. area and in central England.

TheChanges increase in average in the average monthly monthly surface surface air temperatures air temperatu in centralres resulted England in an differ increase sig- inniﬁcantly averagefrom annual the surface changes air in temperature. average monthly Although air temperatures the average annual in the central temperature Baltic area.was lowerThe monthly in some average years, it air increased temperature steadily trends during in central the 15- Englandyear study ranged period. from The− average0.77 ◦C annualper decade surface to 0.54air temperature◦C per decade, in the except cities of in the December central Baltic where area the increased average monthly by an aver- air agetemperature of 0.43 °C increased per decade by between 1.88 ◦C per 2005 decade and 2019. (Figure The 11 average). annual surface air temperature in central England increased significantly less between 2005 and 2019, with an average of 0.12 °C per decade (Figure 12).

Figure 12. Changes in the average annual surface air temperatures from 2005 to 2019.

Atmosphere 2021, 12, 60 16 of 20

Atmosphere 2021, 12, 60 16 of 20 Figure 11. Trends of average monthly surface air temperatures from 2005 to 2019 in the different cities of the central Baltic area and in central England.

TheThe increase increase in in the the average monthly surface air temperatu temperaturesres resulted in an increase inin average average annual annual surface surface air air temperature. temperature. Although Although the the average average annual annual temperature temperature was lowerlower in in some some years, it increased steadily during the 15-year15-year study period. The average annual surface air temperature in the cities of the central Baltic area increased by an average annual ◦surface air temperature in the cities of the central Baltic area increased by an aver- ageof 0.43 of 0.43C per°C per decade decade between between 2005 2005 and and 2019. 2019. The The average average annual annual surface surface air temperature air temper- aturein central in central England England increased increased signiﬁcantly significantly less between less between 2005 and2005 2019, and 2019, with anwith average an aver- of 0.12 ◦C per decade (Figure 12). age of 0.12 °C per decade (Figure 12).

FFigureigure 12. ChangesChanges in the average annual surface air temperatures from 2005 to 2019.

It is noteworthy that 15 years is a relatively short period of time to ﬁnd a reliable trend but compared to the results of a previous study [28], the trend did not change significantly (approximately 0.4 ◦C per decade in Tallinn and 0.5 ◦C per decade in Stockholm and Helsinki).

4. Conclusions Average daily surface air temperatures in the cities of the central Baltic area differed relatively little between 2005 and 2019. The maximum difference between the daily average temperatures during the 2005–2019 is 1.0 ◦C, and the frequency distributions of all the average daily surface air temperatures in the period being studied virtually coincide. Based on the critical difference calculated at the confidence level of 0.05 by Dunn’s test for all the average daily surface air temperatures from 2005 to 2019, the central Baltic cities can be divided into two groups (A and B). Group A comprises Narva, Tallinn, and Tartu with only lower temperatures. Stockholm is the only city in group B. Air temperatures in Pärnu, Võru, and Helsinki are not critically different from the surface air temperatures in Stockholm, Narva, Tallinn, and Tartu. There is a significant correlation between the average daily surface air temperatures in different cities; the Pearson correlation coefficient ranges from 0.94 (Stockholm) to 0.99 (Narva, Tartu, Võru). The results of the regression analysis show that there are some differences in the correlation between the daily average surface air temperatures. Changes in surface air temperatures in Narva, Pärnu, Tartu, and Võru are greater (slope from 1.026 to 1.09) than those in Tallinn, whereas changes in the surface air temperature in Stockholm are moderate Atmosphere 2021, 12, 60 17 of 20

(slope 0.88). Changes in the surface air temperatures in Helsinki are similar to those in Tallinn (slope 0.99). Frequency distribution shows that changes in the average daily surface air temperatures virtually coincide in the summer (June to August). In winter (December to February), Stockholm stands out due to its more moderate temperatures. According to the results of Dunn’s test, the cities of the central Baltic area can be divided into three groups based on the critical difference in surface air temperatures in different seasons. The distribution into groups varies from season to season. Stockholm, where surface air temperatures are generally higher, is one of the cities with the lower surface air temperatures in summer. It is only in the winter that the three groups of cities differ completely from one another. Surface air temperatures in Pärnu in the spring and autumn do not differ critically from the surface air temperatures in any other studied city. Similarly, in the summer and winter, the mean rank of the Pärnu surface air temperatures differs little from the mean of the entire data set, by 43 and 102, respectively. The annual changes in surface air temperatures vary greatly from year to year. Average monthly surface air temperatures in the winter months vary by up to 16 ◦C in different years. These changes are the smallest in August and remain at about 5 ◦C. Changes in surface air temperatures in different years did not happen synchronously. However, changes in different cities were rather similar. Depending on the nature of the changes in the average monthly surface air temperatures, the cities of the central Baltic area can be divided into three groups. The first group comprises Helsinki and Pärnu, where the average monthly surface air temperature does not differ significantly from that of Tallinn, especially in the winter. The second group comprises Tartu, Võru, and Narva, which are characterized by lower temperatures in the winter (up to −1.5 ◦C), but with equally higher temperatures in the summer than in Tallinn. It is up to 2 ◦C warmer in the winter in Stockholm than in Tallinn, but in the summer, the surface air temperatures are quite similar to Tallinn. Welch’s t-test shows that, statistically, average monthly surface air temperatures in Helsinki at the significance level of 0.05 are closest to that of Tallinn, and differences occur practically in the summer months only. Surface air temperature in Stockholm does not differ from that of Tallinn from May to August. Surface air temperature in Tartu does not differ from that of Tallinn in only March, July, and August. Surface air temperature in Võru do not differ from that of in Tallinn in March and August. Surface air temperatures in Narva are only equal to Tallinn air temperatures in September. Air temperatures in Pärnu are equal to Tallinn from September to January and in March and April. Average monthly surface air temperatures did not change in the same way in different months. The trends characterizing changes in the average monthly surface air temperature have a certain course during the year. The rate of increase in surface air temperature is slightly different in the different cities, but the nature of changes is virtually the same in all the cities. The surface air temperature increased the most in February, with the increase ranging from 1.8 ◦C (Stockholm) to 4.5 ◦C (Võru and Tartu) per decade. In addition to February, the surface air temperature increased over the 15-year study period in all the studied cities in March, May, June, and December. There was virtually no increase in surface air temperatures from July to November and in January. Consequently, changes in surface air temperatures are nonexistent during the summer period and are smaller in autumn. Significant changes took place in late autumn, late winter, and in spring. The increase in the average monthly surface air temperatures resulted in an increase in the average annual surface air temperature. The average annual surface air temperature in the cities of the central Baltic area increased by an average of 0.43 ◦C per decade between 2005 and 2019. Changes in the surface air temperature in the cities of the central Baltic coastal area did not occur evenly on either the spatial or the temporal scale. The rate of increase in surface air temperature was higher in Pärnu and Tartu and lower in Narva than that of Atmosphere 2021, 12, 60 18 of 20

other cities, where the increase in surface air temperature per decade was 0.59, 0.53, and 0.04 ◦C, respectively. It is noteworthy that 15 years is a relatively short period, which does not allow a reliable assessment of climate change; nonetheless, it does conﬁrm the continuing increase in air temperature.

Supplementary Materials: The following are available online at https://www.mdpi.com/2073-4 433/12/1/60/s1, Table S1: Daily average air temperatures (◦C) in the cities of the central Baltic area during the period 2005–2019. Figure S1: Division of the cities in the central Baltic area into groups on the basis of the critical difference in average daily air temperatures according to the Dunn’s test: (a)—January, (b)—February, (c)—March, (d)—April (e)—May, (f)—June, (g)—July, (h)—August. (i)—September, (j)—October, (k)—November, and (l)—December. Funding: This research received no external funding. Conﬂicts of Interest: The author declares no conﬂict of interest.

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