Comparing the cooling effects of a tree and a concrete shelter using PET and UTCI

Trees could provide notable cooling by intercepting solar radiation and evapotranspiration. Human-made shelters in urban areas also serve as shading devices. However, few studies have compared the cooling efficacy of trees and artificial shelters. This study systematically quantified and compared the daytime and nighttime cooling effects of a large Chinese Banyan tree (Ficus microcarpa) with dense foliage and an extensive concrete shelter, in an urban park in Hong Kong's subtropical summer. Microclimatic parameters at the two sites were monitored to compare air temperature, and the computed values of PET (Physiological Equivalent Temperature) and UTCI (Universal Thermal Climate Index). The mean daytime cooling effects generated by the tree were 0.6 °C (air temperature), 3.9 °C (PET) and 2.5 °C (UTCI), which were higher than the shelter at 0.2 °C, 3.8 °C and 2.0 °C respectively. The differences were significant for air temperature and UTCI (p < .001 and p < .05 respectively, t-test) but not for PET (p = .261). The tree's mean daytime maximum cooling effects were 2.1 °C (air temperature), 18.8 °C (PET) and 10.3 °C (UTCI). The tree's mean nighttime cooling was significantly higher than the shelter for all three indices (p < .001, t-test). The thermal stress classifications by PET and UTCI were significantly different on the neutral or warmer side (p < .001, Chi-squared test), suggesting that they cannot be used interchangeably. The findings could inform decisions on natural versus artificial shelters in urban thermal design, and trigger comparative investigations in using PET and UTCI for outdoor thermal comfort assessment.