In winter a difference between the air temperature above green and gravel area either at 30 cm and 5 cm was observed. Above the gravel, the temperature at 30 cm reached up to 5.7 oc more than in the green area, in the early afternoon (15:40). The difference was more accentuated at -5 cm proving the evaporative cooling effect of the green roof. No difference in the minimum values was observed, fact that can be explained considering the role of the relative humidity (RH). During the night due to the low temperature the outdoor air completely saturated and therefore the relative humidity is recorded to be 100%, while during the day the gradual increase in the air temperature induces a fall in the relative humidity so that evapotranspiration allows the evaporative process to take place. It can be easily recognized that RH assumed higher values above the green area more than in the gravel, owing to the mentioned processes. Accordingly, in Refs. (135,371) it was shown that relative humidity is the most important climatic factor affecting evapotranspiration. During 2010, in low plant cover conditions, similar results were found, but the gravel air temperature was between 1 oc and 3 oc higher than green area. The greater fractional vegetative cover(80%) in 2012 resulted in an increased evapotranspiration; so it can be deducted that plants traspiration effect added to the substrate evaporation provided an overall enhanced air cooling above the green area.