Assessing Uncertainties in Mean Radiant Temperature Measurements in Controlled or Outdoor Conditions.

Urban heatwaves have a significant impact on human health and thermal comfort in cities. The Universal Thermal Climate Index (UTCI) is widely used to evaluate outdoor thermal comfort in cities.  UTCI is based on meteorological inputs (air temperature, relative humidity, solar radiation…), clothing characteristics and a human physiological model. Accurate estimation of UTCI requires an accurate assessment of radiative heat exchanges between the human body and the surrounding environment. The mean radiant temperature (Tmrt) is the primary input of UTCI. Tmrt represents a simplified parameterization of the combined shortwave and longwave of radiative exchanges between the human body and its environment, expressed as a single equivalent value corresponding to a hypothetical uniform radiative enclosure.  Under outdoor conditions, the estimation of radiative heat exchanges, and thus of Tmrt, remains complex due to the spatial non-uniformity of the surrounding environment and the complexity of human body geometry. In this context, the three-direction radiometer method is commonly used to measure incoming shortwaves and longwaves radiation, and based on assumptions regarding human geometry and emissivity, Tmrt can thus be reliably evaluated. However, because radiometer method is expensive, an alternative cost-effective, smaller, along with associated analytical methods have been developed. These approaches are mainly based on black and grey globes of various diameters and materials and are widely used to characterize the effect of strategies to mitigate the impacts of urban heat waves on the microclimate of cities. The accuracy, response time and representativeness of these probes with respect to human body perception of radiative effects are often questioned. This study focuses on the experimental evaluation of the uncertainties associated with the use of these cost-effective devices for estimating Tmrt. A new cylindrical probe has been designed to better represent human body geometry; its accuracy is evaluated and compared with the classical radiometer method and with black and grey globes commonly used. The experimental campaigns include tests conducted in a controlled environment (wind tunnel) as well as outdoor measurements. The influences of surface emissivity, globe diameter, and globe material on Tmrt estimation are investigated. The wind tunnel setup, combined with a xenon lamp to simulate solar radiation, allows precise control over airflow, radiation, and thermal conditions affecting globe temperature measurements. This setup is used to evaluate the sensitivity of the different probes to the controlled variables. Outdoor experiments investigate real thermal radiation conditions and a wider range of meteorological variables, including cloud cover, wind regimes, and solar angles. Using experimental results obtained from the outdoor campaign, Tmrt values derived from globe measurements are compared with reference values.