Assessment of aerial thermography as a method of in situ measurement of radiant heat transfer in urban public spaces

Abstract

Urban public spaces are an essential part of the urban environment, supporting social relationships and pro- moting a healthy lifestyle among citizens. However, the high value of urban land has led to an over-urbanisation of cities, increasing urban heat stress and decreasing the number and size of public spaces. Rising air temper- atures in cities – known as the urban heat island effect (UHI) - combined with global warming, make public spaces less comfortable. For these reasons, there has been a growing concern to improve the thermal comfort of urban spaces. Thermal radiation is a determining factor in urban thermal comfort and is normally summarised in a value called mean radiant temperature (TMRT). In the past, conventional methods have been used to calculate it, such as net radiometers and globe thermometers. In recent years, the scientific community has used ground- based handheld thermal cameras for its quantification. However, there is a lack of literature on the use of aerial thermography for this purpose (i.e. an unmanned aerial vehicle (UAV) equipped with a thermal infrared device). Given this gap in the literature and the advantages in time, versatility and accuracy of these systems, this paper presents a new method for assessing the measurement of radiant heat transfer in a pedestrian urban space using aerial thermography. From the surface temperatures of the infrared imagery collected by the UAV, TMRT was estimated at multiple points in a pedestrian area of a subtropical city (Huelva, Spain) during a typical summer day. In order to verify accuracy of the proposed method to estimate the TMRT, a microclimate urban simulation was carried out using ENVI-met v5. The comparative analysis of the measured and simulated dataset verified the applicability of aerial thermography for the measurement of radiant heat transfer (with R2 values of 0.98 for the data set and 0.8 for the data of each time period). To conclude, new strategies were proposed to improve urban thermal comfort and to make cities more sustainable.