**2. Research Area, Data and Methods**

Pozna ´n is located in western Poland in the area of the Wielkopolska Lakeland. The city's area is 262 km2 and the population is 545,700 people. Over 58% of the city area is above 80 m a.s.l.; 7% is in the floodplain of the Warta River Valley, and the rest (35%) in upper river terrains. Within the limits of Pozna ´n, there are natural and artificial lakes covering a total of 1.9% of the city area. Developed and urbanised land constitutes 44.6% of the city's area (of which about 28.6% is residential areas, 30.3% of communication and 9.1% of industrial areas), while forested and wooded land constitutes 15.3% [23].

Pozna ´n is located in a temperate transition zone between the sea and the continental climate. The average annual air temperature is 8.3 ◦C. On average, the coldest month is January with an average temperature of −1.6 ◦C, and the warmest is July with an average of 18.1 ◦C. The average annual rainfall is 517 mm, with the lowest observed in February (26 mm) and the highest in July (75 mm). Over the year, winds from the western sector (SW, W, NW) dominate the study area and about 5% of the day is calm. The average wind speed is 4.1 m/s and it ranges from 3.5 m/s in August to 4.8 m/s in March [24].

This article has used daily values of the maximum (Tmax) and minimum (Tmin) air temperature for the station in Pozna ´n (Pozna ´n–Ławica) from 1966 to 2015. The data were obtained from the records of the Institute of Meteorology and Water Management–National Research Institute (IMGW). In addition, daily air temperature data for the years 2008–2015 were used from eight measuring points located in the Pozna ´n area on various types of land (Table 1, Figure 1). The analysis excluded the year 2012 due to the lack of complete data for all the measuring points. Data were obtained from the Department of Climatology of Adam Mickiewicz University in Pozna ´n. The air temperature is measured 2 m above ground level with HOBO U23-001 recorders with 30-min resolution and 0.2 ◦C accuracy.


**Table 1.** Location and characteristics of measuring points in Pozna ´n.


**Table 1.** *Cont.*

**Figure 1.** Location of measuring points against the type of land use (Urban Atlas 2012).

Based on the data from the Pozna ´n–Ławica station, the average Tmax for each summer season (June–August) was calculated and the hot days and heat waves were differentiated. A hot day was defined as a day with Tmax above the 95th annual percentile (from 1966–2015), and a heat wave was assumed to be at least five consecutive hot days. The aforementioned assumption was based on the definition of an extreme weather event included in IPCC reports [1], according to which a weather phenomenon is so rare within the particular area that it lies within the range of 10th or 90th percentile of an observed probability density function or rarer. It is hence defined as an extreme weather event. The value of the 95th percentile of the air temperature was often used in previous studies on the occurrence of heat waves [25–28].

The next step was to examine changes in the above characteristics in the years 1966–2015 and determine their statistical significance. To this end, the non-parametric Mann–Kendall test was used to detect the trend in the time series. The strength of trends in temperature characteristics and the number of days in the multiannual period were determined by Sen's non-parametric method. Sen's method adopts a linear trend model, f (t) = Qt + B, where Q is an estimator of the linear regression coefficient (trend strength); B is a free term. The calculations were done using the MAKSESENS 1.0 application (freeware) developed by researchers from the Finnish Meteorological Institute [29].

Moreover, with the threshold values from Pozna ´n–Ławica station, hot days and heat waves were distinguished at every measuring point in the city.

Thus, to characterise the thermal conditions in the city accurately, two satellite images were used, i.e., the image recorded by the LANDSAT-5 TM satellite on 12 July 2010 at 9:34 UTC and LANDSAT-8 OLI/TRIS on 11 August 2015 at 9:34 UTC. Both days were characterised by the heat wave occurrence. In addition, on 12 July 2010, there was clear and calm weather with a clearly marked thermal diversification of the urban surface; 11 August 2015 was thus characterised by the passage of warm weather front and less spatial thermal diversification of the city.

The image processing involved the data processing procedure described in detail by Sobrino and Raissouni [30], Jiménez-Muñoz and Sobrino [31], Walawender [32] and Majkowska et al. [33], including separate patterns for LANDSAT-5 TM and LANDSAT-8 OLI/TRIS images. The aforementioned procedure consisted of the following steps:


$$\text{Tasl} = \mathbf{a} \times \text{LST} + \mathbf{b} \dots$$

a, b—linear regression coefficient.

Maps of the air temperature distribution were then constructed for the area analysed for the days and the air temperature course analysed along the designated profiles on 12 July 2010. This day was chosen because of the large variation in air temperature in the city area and the apparent impact of land development on the thermal conditions. Daily sea level pressure (SLP), heights of isobaric surface 500 hPa (z500 hPa) and temperatures on isobaric surface 850 hPa (T850) were used to characterise the weather conditions. The data were obtained from the collection of the National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) Reanalysis [34], which are available at the Climate Research Unit resources. Based on the aforementioned data, the maps of SLP, z500 hPa and SLP, z500 hPa and T850 anomalies maps were plotted for the analysed days. In addition, weather maps and comments from the Daily Meteorological Bulletin from IMGW resources were used.

What is more, the Urban Atlas 2012 [35] database for Poland was used. For each type of land use, the average values of LST and their median, first and third quartiles as well as outliers and ranges were calculated. The aforementioned data was used to evaluate the impact of land use on the thermal conditions in the city during the hot days analysed. The following were used in the research: ESRI ArcGiS Desktop 10.5 software with Spatial Analyst, Quantum GIS 2.8 and R software (R Core Team, Vienna, Austria, 2015) and its packages: "raster" [36] and "rgdal" [37] dedicated to the spatial analysis.
