Quantifying outdoor thermal comfort in different street vegetation arrangements: Insights from a study in tropical and subtropical Australian cities

Street vegetation serves as a nature-based strategy for urban heat mitigation leading to improved outdoor thermal comfort (OTC). However, the improvement of OTC through street vegetation is restricted by the limited availability of developable urban land. A sound understanding of spatial and temporal OTC variation in different street vegetation arrangements is essential to strategically develop urban heat mitigation planning and design. This study introduces a methodology framework that quantifies the thermal improvement in different street vegetation arrangements by using three indices - cooling intensity (CI), cooling area (CA) and cooling duration (CD). The framework is applied to four streets in tropical and subtropical Australian cities to quantify Physiological Equivalent Temperature (PET) over a 24-hour period. Results indicate that the street with trees on the west-facing side experienced a longer duration of street thermal enhancement (up to 46% CD improvement). At 2pm, the OTC improvement in the planting scenarios with additional trees in the street centre (median strip) incorporating multiple vertical layers of vegetation is more pronounced in the studied subtropical urban area (up to 3.22°C CI) than that in the studied tropical urban area (up to 0.9°C CI). These findings can contribute to urban heat mitigation planning and design in tropical and subtropical cities worldwide. The methodology framework proposed in this study provides a foundation for quantifying spatial and temporal OTC variation in different street vegetation arrangements, and establishes an adaptive framework for landscape architects, urban designers and planners, and decision-makers worldwide to evaluate urban street thermal environments.