Measured nighttime effect of green, blue, and grey features on thermal exposure: A pilot study in Czech cities

Abstract

Human thermal comfort has been the focus of research in urban areas of temperate climates due to the need for the implementation of adequate climate change adaptation measures in urban design. Public spaces have been mostly assessed in terms of the effect of blue and green infrastructure on thermal stress during hot summer days, however, the increasing number of hot summer nights also draws attention to the so far understudied nighttime period. In this study, we focused on the nighttime effect of blue, green, and grey elements on thermal exposure. We performed a series of nighttime measurements to obtain data on air temperature (ta), mean radiant temperature (MRT), UTCI, and PET over various types of surfaces and under trees and shade sails. Trees lead to higher nighttime ta (up to +1.0 °C), MRT (up to +3.9 °C), UTCI (up to +2.4 °C), and PET (up to 4.3 °C) in comparison to open areas, yet without changes in the category of thermal stress. A less pronounced effect was observed under shade sails (up to + 0.6 °C in ta, +1.0 °C in MRT, +0.4 °C in UTCI, +0.6 °C in PET). Pervious surfaces, such as grass or gravel, supported nighttime cooling more than impervious pavement. The cooling effect of a splash fountain continued into the nighttime even after the fountain had been turned off, but elevated fountains did not have any measurable effect on thermal exposure during the night. The PET values and heat stress categories were generally more variable at night than UTCI, which deserves further attention.