Spatiotemporal Variability, Drivers and Extreme-Heat Response of Surface Urban Heat Islands Across Italian Cities

This study presents a comprehensive assessment of land surface temperature (LST) and surface urban heat island intensity (SUHII) in Italian cities, integrating multi-year thermal observations from ECOSTRESS (2018–2025), land-use data from CORINE Land Cover and globally available morphological, biophysical, radiative and climatic datasets.

A multiple linear regression (MLR) model combined with partial R² analysis was applied to quantify the relative contribution of radiative, biophysical and morphological factors controlling the SUHII during the diurnal cycle. Furthermore, extreme heat conditions were identified using the Warm Spell Day Index (WSDI), allowing for a comparison between normal summer conditions and extreme heat periods.

The results reveal spatial and temporal variability of the SUHII in Italian cities. While several northern and inland cities exhibit persistent positive SUHII values, many Mediterranean and coastal cities record a weak or even negative diurnal SUHII due to the greater warming of surrounding rural areas, characterized by limited vegetation cover and low soil moisture.

A key finding is the identification of a systematic, but spatially heterogeneous, diurnal reorganization of SUHII controls. During the day, vegetation and surface moisture exacerbate the strongest influence on the surface energy balance, generally mitigating urban heat gain. As solar forcing decreases, the relative importance of morphological and surface-related factors increases, with urban geometry and imperviousness becoming dominant controls of heat accumulation and nocturnal heat release. The timing and magnitude of this transition vary considerably from one city to another, reflecting local climatic conditions and urban structure.

The analysis also demonstrates that extreme heat events do not necessarily result in more intense SUHII conditions. Although nocturnal SUHII generally intensifies during extreme heat conditions, diurnal responses are highly city-dependent, with both amplification and attenuation of thermal contrasts between urban and rural areas observed.