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

The study area is the city of Pozna ´n. It is the largest city in the Wielkopolskie Voivodship, inhabited by 545.7 thousand people. In terms of the size of population, Pozna ´n is the fifth city in Poland, and eighth in terms of surface area. The city is in three physiographic mesoregions, namely: Pozna ´nskie Lakeland, Wrzesi ´nska Plain, and Pozna ´nski Warta River Gorge. The three areas are parts of the Wielkopolskie Lakeland [22,23].

The paper is based on two sets of data concerning air temperature. The first set covered daily maximum (Tmax) and minimum (Tmin) air temperature values for the station in Pozna ´n (Pozna ´n-Ławica) from the multi-annual period 1966–2016 provided by the Institute of Meteorology and Water Management–National Research Institute. The second data set includes daily air temperature values from the years 2008–2016 for eight measurement points located in Pozna ´n in different types of land use (Table 1, Figure 1). The data were obtained from the resources of the Department of Climatology of the University of Adam Mickiewicz in Pozna ´n. At the one of the measurement points, namely Collegium Geographicum, season 2008/2009 was excluded from the analysis due to the lack of a complete sequence of data.


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

**Figure 1.** Location of Pozna ´n (**a**) as well as Landsat 5 (thermal) 8-bit greyscale image of the study area (**b**) and image composite of three infrared bands of the ETM+ radiometer RGB = 654 (**c**) (acquisition date: 9 January 2010) and location of measurement points against type of land use (**d**) (Urban Atlas 2012).

The first data set provided the basis for the calculation of mean Tmax for particular winter seasons (December–February). Cold days and cold waves were also identified. Then, for the aforementioned characteristics, the direction of changes and statistical significance of such changes was determined. This involved the application of a non-parametric Mann-Kendall test, detecting a trend in time series. The strength of tendencies of characteristics was determined based on the non-parametric Sen's method [24]. Next, based on threshold values from station Pozna ´n-Ławica, cold days were designated at all measurement points in the years 2008/09–2015/16.

A cold day was defined as a day with maximum daily air temperature below 5th annual percentile of Tmax (from 1966–2015), and a cold wave as a continuous sequence of at least five such days.

For detailed determination of thermal conditions in the city, land surface temperature (LST) was calculated with the application of two satellite images registered during days included in a cold wave. The analysis of satellite images is intended to illustrate the differentiation of thermal conditions of the city at a resolution of 30 × 30 m. The used images were registered by satellite LANDSAT-5 TM on 9 February 2010 and 21 February 2011 at 9:34 UTC. A sample of a thermal image showing the study area in shades of grey is presented in Figure 1b (darker pixels refer to colder objects and brighter ones to warmer objects). Figure 1c is much easier to interpret by the human eye. It can be obtained by mixing three infrared bands in RGB composite (Red = 6 band, Green = 5 band, Blue = 4 band). In this image, the interpretation is easier and more intuitive because warmer objects are in yellow, the coldest

in dark grey, blue, and violet. Figure 1c clearly shows the trail of smoke and its shadow covering some of the pixels that were masked before any calculations.

The calculation of basic LST statistics for each type of land use required applying a layer with information on the range of each of the types on the LST raster map for the purpose of reading the LST value for overlapping pixels. Due to this, the Urban Atlas [25] vector layer was rasterized to a raster layer with a size of a pixel corresponding to a surface of 10 × 10 m in the field. Moreover, due to the presence of a smoke streak and the shadow of the streak coming out of the chimney of the heat and power plant Karolin right outside the eastern boundary of Pozna ´n, some of the pixels were masked before performing the calculations.

A detailed description of the performed calculations and used methods and tools was presented in a paper concerning thermal conditions in the city during selected heat waves [20].

The last stage of work involved the determination of weather conditions based on weather forecast from Daily Meteorological Bulletin of the Institute of Meteorology and Water management—National Research Institute. The presented weather maps were obtained from websites of Met Office.
