Mean radiant temperature in outdoor urban environments: From radiative physics to data-driven and hybrid modelling frameworks

Mean Radiant Temperature (MRT) is a fundamental variable in outdoor thermal comfort (OTC) assessment, integrating shortwave and longwave radiative exchanges between the human body and the urban environment. Unlike air temperature, MRT captures the combined effects of urban geometry, surface thermal state, shading, and vegetation on pedestrian-level heat exposure. Numerous studies demonstrate that under sun-exposed conditions MRT can exceed air temperature by several tens of degrees Celsius, while in shaded or enclosed environments elevated MRT may persist due to longwave radiation emitted by heated surfaces. Despite its importance, MRT research has evolved across largely disconnected methodological domains, limiting consistency, transferability, and operational use. This review provides a comprehensive, physics-informed synthesis of outdoor MRT modelling, integrating physics-based formulations, measurement-based evidence, application-oriented studies, and emerging data-driven and hybrid approaches within a unified conceptual framework. Physics-based studies show that accurate MRT estimation requires explicit resolution of radiative exchange and urban geometry, with simplified schemes exhibiting systematic bias under complex morphologies. Field measurements confirm that radiative forcing dominates outdoor thermal exposure and reveal that globe-thermometer–based MRT estimates can deviate by several degrees Celsius due to wind sensitivity and radiative anisotropy. Application-oriented studies indicate that urban interventions—particularly tree shading—commonly reduce MRT by 5–15 °C locally, with spatially averaged reductions of 2–7 °C depending on urban form. Recent data-driven and hybrid approaches enable scalable MRT prediction with typical errors of approximately 1–3 °C (RMSE), supporting city-scale assessment at substantially reduced computational cost when physically meaningful features are retained. By reframing MRT modelling as a continuum that reflects the synthetic interaction of radiation, heat storage, shading, and geometry, this review provides a roadmap for physically grounded and scalable assessment of outdoor thermal environments.