The effects of tree shading and ground surface characteristics on human thermal stress in urban spaces of Florence

This study evaluates human thermal comfort in different urban settings in Florence, using two thermal indices. The Index of Thermal Stress (ITS) is based on measured radiation fluxes for computation of the full energy balance between the human body and the environment, and the Universal Thermal Climate Index (UTCI), used here as a comparative index, is based on the mean radiant temperature (Tmrt). Field data collected during summer in a series of open spaces in the largest park of Florence included air temperature, global radiation, relative humidity, wind speed and albedo. In addition, ground surface temperature was measured for the calculation of long-wave radiation fluxes in ITS and black-globe thermometer measurements were made for the estimation of Tmrt in UTCI. ITS and UTCI were evaluated over exposed gravel, asphalt and grass as well as tree-shaded paving in order to investigate the effect of these different surface materials and levels of solar exposure on pedestrian thermal stress and sensation. Initial findings indicate clear differences in thermal stress induced by the characteristics of each landscape treatment. According to ITS, a pedestrian in the unshaded gravel area would experience the most stressful conditions, reaching the highest level of thermal sensation ("very hot") in the early afternoon, while a person standing on the unshaded asphalt paving would be exposed to slightly lower thermal stress (with a "hot" sensation during most daytime hours). This distinction highlights the effect of reflected solar radiation on the body from the higher-albedo gravel surface, which more than compensates for the higher surface temperature and long-wave radiation emitted from the asphalt. The low surface temperature of the transpiring grass lawn, meanwhile, affords conditions defined as "hot" for only a short time in the afternoon, with most daytime hours characterized as "warm." The most dramatic reduction in thermal stress, however, occurs in the area shaded by trees: here a pedestrian would be exposed to thermal conditions defined as "comfortable" for nearly all hours of the day.