**1. Introduction**

Today's climate warming is unmistakable and visible, among other manifestations, in the increase in global average air temperature [1]. The consequence of the aforementioned changes is the increasingly frequent occurrence of hot days and heat waves [2–6]. The waves of 2003, 2006, 2010 and 2015 are to be highlighted amongst the recent extreme heat waves of Poland and Europe. In the future, an increase in the frequency of heat waves and their duration is expected, which will be a consequence of the increase in air temperature [7,8]. It is estimated that in the Poland of 2071–2100, the increase in air temperature in the summer may reach 2.9 ◦C for the average maximum air temperature and 3.4 ◦C for the average minimum air temperature [9].

Urban areas are particularly vulnerable to prolonged and intense heat waves [10], which is a consequence of the transformation of the environment. The small proportion of natural vegetation, numerous vertical surfaces and human activity causes significant heat in the city during the day, which is then released into the atmosphere, causing it to cool down more slowly than the surrounding areas [11]. The above factors make bioclimatic conditions more burdensome in the city than in non-urban areas. As Gabriel and Endlicher's [12] studies in Brandenburg and Berlin show, the effects of heat are more pronounced in highly urbanised area. Numerous studies have shown an increase in the number of deaths during heat waves in large cities, including in Warsaw [13], Munich [14] and Paris [15].

In urban climate studies since the end of the 20th century, more and more investigations have been based on remote sensing data. For example, Landsat 5 TM was used by Kawashima et al. [16] to analyse relations between surface temperature and air temperature in the central part of Japan. Voogt and Oke [17] emphasised that the urban heat island observed from thermal remote sensing data is the surface urban heat island (SUHI).

An attempt at air temperature retrieval from Moderate Resolution Imaging Spectroradiometer (MODIS) data was made by Sun et al. [18] for the North China Plane. Taking into consideration land cover categories, vegetation cover, and building density Dobrovolny [19] studied the SUHI in Brno (Czech Republic). Some indicators obtained from remote sensing data were used by Schwarz et al. [20] to analyse differences between urban and agricultural areas. Ptak et al. [21] applied Lansat thermal images in an analysis of lake surface temperature in Poland. In the present study the temperature field on the area of Pozna ´n city was characterised by using Landsat 5 and 8 TM remote sensing data.

Due to the great risk to human health and life associated with the occurrence of heat waves, particularly in highly urbanised areas, the research results presented may be useful for a wide and diverse group of readers. The implementation of this research study is well founded, especially in the face of UN projections [22] of the increase in the number of people living in cities. With this information in mind, the purpose of the article was to:

