Characterization of the Urban Heat and Dry Island effects in the Turin metropolitan area

The urban metropolitan area of Turin (Italy) was studied to characterize its seasonal thermal and moisture behaviour with respect to the rural surroundings. The area exhibits a complex topography and a set of ground level stations was selected in the urban and rural environment for a period of 14 years (2007–2020). The Urban Heat Island (UHI) effect clearly emerges from the analysis, with the largest values, up to 4–5 °C on average, occurring during the night. Also a light Urban Cool Island (UCI) effect, between 0.5 °C and 1 °C, occurs in some conditions. An Urban Dry Island (UDI) effect during daytime up to 3–4 g/m3 was identified by the analysis of absolute humidity. Moreover, the interaction between UHI, UDI and wind intensity showed that the urban–rural differences are swept up when wind velocity is higher than ≈4 m/s. The availability of temperature data measured by three radiometers, one in the city center and two in the rural area (north and south of Turin) between 2007–2020, allowed the investigation of the city boundary layer. The vertical limit of UHI was found at 150–200 m above the ground level, surmounted in some cases by a very weak cross-over or cold lens effect.