Forehead temperatures as an indicator of outdoor thermal comfort and the influence of tree shade

Abstract

Due to climate change and a growing urban population worldwide, cities as heat islands are of increasing concern for the health and living comfort of urban populations. Accordingly, thermal comfort is an important element of bioclimatic design within cities, and areas with an abundance of hard surfaces, and a lack of tree shade, can become uncomfortably hot in summer periods, which can negatively alter usage patterns of these spaces and even become an issue of human health. In this context, a pilot study was undertaken to assess if thermal photography of the forehead of an individual can be a reliable, non-invasive, rapid tool to predict outdoor thermal comfort. A combination of street interviews, micrometeorological measurements and facial thermography was used with 775 volunteers on warm summer days in the city of Bolzano (N Italy). Two locations were chosen with contrasting sky view factors and tree shade amounts. A large difference in the mean radiant temperature, and Universal Thermal Comfort Index (UTCI), between the two locations resulted in higher thermal sensation votes and higher levels of discomfort in the space without tree shade. The mean average forehead temperature was 1.5 °C higher in the treeless space (35.2 °C) compared to the shaded space (33.7 °C). Forehead temperatures correlated significantly positively with UTCI (treeless: r = 0.30, p < 0.001; shaded: r = 0.21, p < 0.001) and air temperature (treeless: r = 0.28, p < 0.001; shaded: r = 0.22, p < 0.001). Thermal comfort was successfully predicted from forehead temperatures in the treeless space using a logistic regression model, however low R2 values limit the predictive power of these models. Facial thermography shows promise as a proxy for thermal comfort in thermally stressful spaces.