**1. Introduction**

The urbanization processes and related environmental effects are increasingly steered to the study of the urban heat island (UHI) phenomenon. The UHI is a typical form of anthropogenic climate modification mainly due to the growing urbanization process and accompanied by an increase of air pollution and anthropogenic heat sources [1]. The city expansion generally reduces the presence of green areas, with building structures and materials trapping solar radiation, determining significant temperature differences between urban and rural areas [1–4]. Urban texture and construction materials can cause an increase of air and surface temperatures of several degrees with respect to the surrounding rural areas. The UHI effect worsen during the hot season: high temperatures may affect human health, the quality of life of urban residents, energy consumption and air pollution. The UHI effect has been a concern for several decades, affecting not only large metropolitan areas, but also smaller cities; therefore, their planning and design represent the main step for the solution to the problem. The UHI typologies may be grouped in three categories [2]: the surface urban heat island (SUHI); the boundary layer heat island, and; the canopy layer heat island. The canopy layer UHI is affected

by the air temperature heating in the atmosphere extending up to the top of buildings, while the boundary layer UHI extends up to 1 km [5]. The SUHI concerns the land surface temperature (LST) pattern, having greater variability and higher thermal peaks with respect to the air temperature. In the paradigm of urban sustainable development, the mitigation of the urban heat island is a key point [6]. Different strategies to reduce UHI effects can be adopted, based on properly designing the urban texture in order to obtain energy savings and health benefits [7–9], such as the increase of urban surface reflectivity [10–12] and urban vegetation (green roofs, street trees, and green spaces) [13–15].

Urbanised areas are characterised by an irregular thermal pattern, with the LST variations generally linked to the impervious surfaces reflectivity: albedo describes quite completely the surface behaviour in terms of reflectivity, as it represents its hemispherical reflection in the wavelength range of the solar spectrum. For instance, a simple way to increase the albedo of building roofs, and consequently to mitigate the UHI, is the adoption of cool roof paint.

Earth observation data from space-borne sensors have been widely exploited to examine the SUHI effects. Unlike in situ measurements, providing uneven distributed data, satellite observations have the advantages of covering large areas at the same time, and during different temporal intervals, ensuring a more effective analysis of the intra-urban SUHI spatial variability, closely related to the building distribution, surface materials and vegetation density. Different space-borne platforms, such as AVHRR [16–18], MODIS [19,20], ASTER [21–23], and Landsat [24–26], were used to retrieve the SUHI. Furthermore, satellite sensor measurements of surface reflectivities make it possible to retrieve albedo maps, both at the local and global spatial scale.

In this work, the retrieval from Landsat 7 satellite data of the urban LST and albedo is carried out over the city of Terni (Central Italy), characterized by several urban changes during the last 10–15 years. In particular, a district located in the historical city centre, Corso del Popolo, is analysed, where a clear construction intervention was realized starting from 2006; the intervention ended in June 2014. This district is an example of urban texture modification in the construction sector planned for the regeneration of the ancient part of the town. An analysis of the SUHI maps of this district before and after the interventions using different satellite images is provided. The 60 m pixel size of Landsat 7 ETM+ thermal channel proved to be suitable to monitor SUHI changes at the district level, making it possible to point out if urban construction changes move towards an urban sustainable criteria. Moreover, an analysis of the correspondent albedo maps is proposed, with a study of the parameters affecting the LST variations through an analytical model. The synergy of satellite techniques and analytical studies is aimed at assessing whether the planning and design principles fulfil the sustainability requirements in terms of urban heat island mitigation, and the more evident reasons for the heating variations.
