Directional Mean Radiant Temperature Decomposition Reveals Micro-Scale Drivers of Urban Heat Stress in Novi Sad (Serbia)

Urban heat stress is an increasing issue with large implications for health and comfort. This study investigates micro-scale drivers of heat stress in Novi Sad, Serbia, using mobile meteorological carts to obtain measurements at 29 locations with diverse urban design during a hot, cloud-free summer day. Air temperature, relative humidity, wind speed and six-directional short- and longwave radiation were recorded and used to compute Mean Radiant Temperature (MRT) and Physiological Equivalent Temperature (PET). PET values ranged from 25°C to 56°C, indicating slight to extreme heat stress. Sensitivity analysis showed MRT as the dominant driver of spatial differences in PET during daytime. Decomposition of radiant load revealed that longwave lateral radiation contributed most to radiant load, but lateral shortwave contributions were most variable, because of the variability in local shading. Sky View Factor (SVF) appeared a strong predictor of MRT and PET, with a 10 % reduction in SVF lowering MRT up to 5°C. In contrast, surface type (grass vs. impervious) had only minor effects, limited to reduced upward longwave radiation in the afternoon and evening. Buildings and trees achieved comparable maximum MRT reductions, though tree-induced shading exhibited greater variability. These results show the importance of directional radiant load decomposition for identifying which radiative pathways contribute most to heat stress and when. By distinguishing the dominant solar and longwave components—and their dependence on SVF, shading type, and surface materials—this approach provides site-specific empirical evidence on the radiative mechanisms that shaped pedestrian heat stress under clear-sky, low-wind conditions in a temperate climate city.