On the definition of urban heat island intensity: the “rural” reference

One of the fundamental objectives of many studies on urban heat islands (UHI) involves establishing their intensity on determined nights or, if there is a sufficiently high number of observations, their maximum intensity and average intensity can also be ascertained. Regardless of the methodology employed, whether it refers to (1) differences between two fixed observatories, one urban and another peripheral or non-urban, (2) mobile urban transects or (3) remote sensing, in the end it is a question of providing a value of thermal differences between contrasted points or sectors, one urban and another that could be termed non-urban. Thus, the intensity of the UHI is seen in the temperature difference expressed at a given time between the hottest sector of the city and the non-urban space surrounding this. The intensity of the heat island is the simplest and most quantitative indicator of the thermal modification imposed by the city upon the territory in which it is situated and of its relative warming in relation to the surrounding rural environment at night time (Kim and Baik, 2002; Memon et al., 2009).