La variabilità termica della città di Firenze: il ruolo della vegetazione

The world's urban population is increasing as well as urbanized areas. The growth of urban populations means that most people will experience urban climate, that is significantly different from the rural one. Urban climate is a very complex field of study because of the great number of characteristics that affect weather variables in the urban environment and because of the different kind of morphologies and materials used in cities all over the world. Furthermore, cities have an important role to play in mitigating climate change but the present climatic knowledge is not yet applied by urban planners. There is a need to increase the climatic knowledge of urban areas all over the world and to translate it into a planning language in order to facilitate the design of more sustainable cities. The aim of this PhD thesis is to quantify the intra-urban thermal variability of the city of Florence and to study the influence of vegetation and other urban indicators on air temperature and the results could be an useful tool for urban planners and policymakers. A network of air temperature sensor was used to monitor the thermal trend in various part of the city. Data were collected every fifteen minutes since December 2005. The thermal variability was quantified by applying some climatological and degree-day indices. The role of vegetation and of other urban characteristics was evaluated by analyzing the relationship between air temperature and some urban indicators, such as green cover ratio, street cover ratio and building volume. The effect of vegetation on maximum and minimum air temperature during the summer period was also evaluated during particular synoptic conditions in parks and streets with different vegetation type. Results show an intra-urban thermal variability comparable to the urban rural one. Four thermal level were found inside the city in all seasons and the average difference between maximum and minimum air temperature was of 2 °C in winter, spring and autumn and of 3 °C in summer. An average difference of more than ten frost days in winter and of more than 12 summer days (days with maximum air temperature higher than 25 °C) in summer was found. The intra-urban difference of the tropical nights (days with minimum temperature higher than 20 °C) was even more evident, with a maximum of 42 days versus 10 days during the summer period (total of 92 days). Concerning the role of vegetation, a significant negative relationship between green cover ratio and summer minimum and maximum temperatures was found. The reduction of maximum air temperature (0.18 °C for each 10% increase of tree cover ratio in an area of 250 meters radius centred in the thermal sensor) determined by vegetation is significant only on its proximity (less than 50 meters) and it is related to the presence of trees. The reduction of minimum air temperature is higher (0.58 °C for each 10% increase of green cover ratio in an area of 250 meters radius centred in the thermal sensor) and related to the urban morphology both near the temperature sensor and at larger areas, confirming previous studies on the relationship between the magnitude of the urban heat island phenomenon and the size of the city . Finally, the effect of the presence of trees in the urban environment was analysed. In strong urbanized context, such as a parking lot, it was shown that the presence of a single tree reduced the maximum air temperature of almost 1 °C near of it during summer. In addition, the tree cover intensity was also analysed and the results show that green areas with a total tree cover had lower maximum air temperatures and higher minimum temperatures than grass green areas. A mean 3 °C difference was registered in maximum values and a 2 °C difference was found in minimum temperatures. Areas with a medium tree coverage showed a midway trend. Gaps in areas with a medium tree coverage had a similar trend for maximum values to areas with a total tree cover and for minimum values to grass areas. The results of this study can find application in several field of study, such as phenology and human health, such as the production of biometeorological maps for plant flowering and to classify areas at higher risk during the heat waves or cold spells. Furthermore, these results could be useful for planners and policymakers to take action towards urban temperature mitigation.