**1.2. Urban surface and morphology**

The urban surface is composed of a large number of man-made materials arranged in a complex three-dimensional (3D) structure. Cities are built with artificial materials, such as cement, asphalt, brick, pebbles, or aggregates, which absorb and store radiation throughout the day and slowly release heat through the night. Moreover, streets, sidewalks, and parking lots are generally impervious, meaning that they do not allow the water to infiltrate into the soil. Since the urban environment is predominantly covered by artificial pavement, it is important to study the types of materials used and their individual characteristics. Impervious surfaces not only absorb high heat loads, which increase air temperatures through heat convection, but also increase the rate and temperature of runoff during storms [5].

While land changes from forests, grasslands, and croplands to impervious surfaces, the energy balance changes. The larger amount of solar radiation reaching the Earth's surface is reflected, absorbed, and transformed into sensible and latent heat. A small percentage of the solar radiation is also used in photosynthesis. The atmosphere close to the surface is mainly heated by energy radiating off the Earth's surface and not by direct solar heating. The surface materials affect largely the amount of solar radiation reflected or absorbed and, thus, they affect the heat flux from the surface to the atmosphere. The impervious surfaces alter the local energy balances through changes in the albedo, the emissivity, the specific heat capacities, and the thermal conductivities of the surfaces, as well as the ratio of sensible to latent heat fluxes from the surface to the atmosphere. Therefore, this impacts the temperature and humidity of the overlying air. Cool pavements are made from advanced materials and surface types that are used for decreasing the surface temperature in urban environments. These are mainly based on the use of materials with high albedo combined and high emissivity or techniques that exploit the latent heat to decrease the surface and ambient temperature [6]. Nevertheless, not only the materials, but also the 3D structures of the city have important impact on its radiational balance and thus its temperature. The shape of the cities can be described by several measures, each of which has effect on the city climate. The buildings and trees height affects the reflectivity, the flow regimes, and the heat dispersion above ground. The surface properties as well as the 3D structure of the cities can both be assessed using remote sensing methods and EO data.
