Urban greening as a tool for urban heat island mitigation - a survey of research methodologies in different climatic regions

Urban Green Infrastructure (UGI), tree coverage in particular, possesses great mitigating potential for the urban heat island (UHI) effect and considerable capability of enhancement of human comfort. This is a survey of 90 studies dealing with UGI in Mediterranean, arid, Atlantic and Boreal climates. Quantification of the UGI cooling effect is the most popular methodology used, in particular the intensity of park cool islands, and the cooling effect of street trees. Field experiments serve as the dominant approach, mostly micro-scale, with time-scales of a few days at most. About 1/3 of the studies use numerical models, estimating UGI contribution to human comfort, but less than 20% apply questionnaires to evaluate subjective perception of this comfort. In spite of the dominant effectivity of urban trees, very few studies deal with evaluation of particular tree species, their shading effect and the influence of foliage type and extent. The most commonly measured variable is air temperature. Solar radiation, surface temperature, humidity and wind speed are used for studies on thermal comfort and modelling. The survey exposed the lack of research of UGI&UHI interplay on regional and city scales, as also on the cooling potential of green walls and roofs.