The purpose of this work is to understand the thermal environment of cities by the analysis of the thermal phenomena in urban spaces. It aims to analyse how formal features of urban space influence the thermal environment. Firstly, it depends on the calculation of surface temperatures thanks to the "Solene" software previously developed at the CERMA Laboratory. The simulation takes into account shadows in meshed complex geometry, temporal and spatial angular position of the sun, sky income, thermal exchanges, physical characteristics of walls and interior conditions of buildings. Secondly, it is based on the estimation of the "zone of influence", the area in which the formal and physical characteristics of walls can influence the temperature of their surrounding surfaces. Our method consists of studying the thermal phenomena in urban spaces by means of the spatial distribution of the view factor values, and of surface temperatures calculated by Solene. We remind the reader that the view factor is a non-dimensional figure which represents the radiative flux fraction leaving a given surface and reaching an other one.
The interpretation of the view factors distribution is based on the definition of the minimum view factor's value which allows us to say that there is no significant thermal exchange between a surface and its environment. We call this value "limit view factor", which depends on the geometric and physical characteristics of the space, and on the surface temperatures which in turn depend on air condition, solar fluxes and the walls insulation. The limit view factor is estimated by an energetic balance, for horizontal and vertical surfaces in different cases of latitude, physical and formal characteristics of walls (orientation, position,…), under simplified assumptions.
The "zone of influence" is an area that gathers all facets for which the view factor versus a given facet is greater than the limit view factor defined before. This area presents an interest for the comprehension of the role played by urban form in modelling microclimate conditions. We can also define the sensitive area in which one can intervene in changing the thermal conditions of urban spaces. Indeed, surface temperature variations can be observed when sensitive changes are performed in this area. The zone of influence is spatially defined by the analytics expressions of view factors, and by a numerical algorithm integrated in Solene. The radiative exchange increases proportionally with the view factor value. But the zone of influence shrinks when the limit view factor increases.
The examination of the urban form versus zone of influence allows us to underline the specific formel features of spaces intervening in citie's climate.