Beyond UHI – how to build and use relevant indicators for heat mitigation studies

Five years ago the short article Is the Urban Heat Island intensity relevant for heat mitigation studies? (Martilli et al, Urban Climate, 2020) was published, detailing the limitations and shortcomings of using the Urban Heat Island Intensity (UHII) as an indicator of overheating in urban areas. This is because the rural reference, used to estimate the UHII, changes in space and time, and furthermore, it does not represent thermally comfortable conditions. It was also stressed in that paper that urban areas generate unique local climate signatures, not simply perturbations added on top of rural surface climates. Consequently, the UHII is not even a measure of the maximum impact that a heat mitigation/adaptation strategy can provide. On this basis, the aims of this contribution are to: 1) critically analyze the impact of the previously mentioned article on the field, based on the more than 200 citations it has received so far, 2) define features that relevant indexes for heat mitigation strategies should include, and 3) show how they can be used to evaluate the impacts of adaptation/mitigation strategies on negative aspects of urban overheating. To illustrate the last two points, examples from modelling studies over cities in different contexts are discussed.