Global warming is a process that is currently happening with little doubts. Urban heat island (UHI) phenomenon amplifies its effects by additional heat coming from urbanised areas. Its negative effects can impact both human health and the comfort of living. European researchers include Bucharest UHI among the most intense phenomena on the continent, especially during the summer time, when temperature extremes present high risk for public health. High intensity of Bucharest UHI urges the need of implementation of mitigation measures. Our paper focuses on quantification of the contribution of active urban surface to formation of UHI, in order to provide adequate input in public policies related to extreme heat mitigation. We use hierarchical agglomerative clustering and linear regression analysis for measuring the relationship between the two variables: density of urban fabric and average monthly temperature. Our results reveal that independent variable explains about 20% of variation in dependent variable. In addition, we found that there is an important effect of scale that makes this relationship significant. In such a way, one of the UHI mitigation policies must be focused on changing the characteristics of active surface in urbanised area.