3.4.3. Land Surface Temperature and Land Use

On 9 February 2010 at 9:34 UTC over the study area, considerably high variability of the value of the LST field occurred because of variable land cover and land use (Figure 7a). The highest values of the LST median (−3.2 ◦C) are characteristic of continuous urban fabric and discontinuous dense urban fabric (−3.5 ◦C). For both types of land use, statistically significant medians are different from the medians calculated for all the remaining types, directly suggesting evident distinctiveness of the LST field in the areas. The lowest values of the LST median occur in the following areas: agricultural + semi-natural areas and wetlands as well as pastures (−6.4 ◦C, −6.6 ◦C, respectively) (Figure 8a).

21 February 2011 was cooler than the day described above. Similarly, as previously, the highest values of the LST median (excluding water areas) occurred in urbanized areas to the greatest degree, i.e., continuous urban fabric (−5.4 ◦C) and discontinuous dense urban fabric (−5.5 ◦C) (Figure 7b). Also, in this case, LST medians were statistically significantly different from medians for the remaining types of land use. The lowest values of the LST median (−7.4 ◦C) occurred in the following areas: agricultural + semi-natural areas, wetlands, and pastures (Figure 8b). On that day, a higher value of the LST median (−4.3 ◦C) over water areas in comparison to medians in the remaining types of land use draws attention. The comparison of values of medians for water areas from 21 February 2011 and 9 February 2010 shows a slight difference in LST (0.7 ◦C), whereas for the remaining types of land use the differences are considerably higher and amount to almost 2.0 ◦C. The direct cause of such a situation is presumably substantially higher heat capacity of water areas in comparison to land areas and their resulting slower response to changes in air temperature.

**Figure 7.** Land surface temperature pattern in Pozna ´n ((**a**)—9 February 2010, (**b**)—21 February 2011).

**Figure 8.** Statistic of land surface temperature in Pozna ´n ((**a**)—9 February 2010, (**b**)—21 February 2011) based on Landsat images according to Urban Atlas 2012 types (colors and order of types according to legend in Figure 1). On the boxplot, the middle values denote medians, the box extends to the Q1 (first quartile) and Q3 (third quartile), and the whiskers show the range (99.3%): the upper whisker shows Q3 + 1.5 × IQR (the interquartile range), the lower shows Q1 − 1.5 × IQR. The notches extend to ±1.58 IQR/sqrt(n) and the dots represent outlier.

3.4.4. Weather situation on 9 February 2010 and 21 February 2011

On 9 February, Poland remained under the influence of a retreating high from over Russia, gradually replaced by a low from over Scandinavia (Figure 9a). Old polar-marine air flew in over Poland. On that day, the weather was cloudy with clear spells and local clearings. Maximum temperature varied from −7.0 ◦C in the east and center of the country to −1.0 ◦C on the coast. The wind was weak, from eastern directions. In Pozna ´n, the wind was north and north-east, and its average daily speed was 6.8 km/h.

On 21 February, Poland remained within the range of an extensive high pressure system with the center over Russia, providing for advection of arctic air masses (Figure 9b). Over a major part of the country, the weather was cloudy or slightly cloudy, the cloudiness grew to moderate only locally. Maximum temperature varied from approximately −12.0 ◦C in north-east to −4.0 ◦C on the coast. The wind was weak, northern, and north-eastern. In Pozna ´n, the wind was north and north-east, and its average daily speed was 9.4 km/h.

**Figure 9.** Synoptic maps on 9 February 2010 (**a**) and 21 February 2011 (**b**). Source: Met Office.
