Evaluating the effectiveness of tree canopy and building shade in urban heat mitigation using solar radiation transmittance

Abstract

Urban Heat Island (UHI) effects challenge pedestrian thermal comfort and urban livability, particularly in high-density cities. This study employs Solar Radiation Transmittance (SRT) mapping and Physiologically Equivalent Temperature (PET) analysis to quantify urban shading benefits, with in-situ measurements in Tainan and Taipei, Taiwan. A GIS-based approach standardizes tree shade and building shadow effects on solar radiation, identifying shade-deficient areas.

Results indicate that tree shade provides stable cooling effects, while building shadow significantly reduces solar radiation but may exacerbate UHI. Field measurements show SRT values ranging from 0.18 to 0.60 for tree shade and 0.02 to 0.25 for building shadow, with a strong negative correlation between Leaf Area Index (LAI) and SRT (R² = 0.95). Increased shading reduced PET by 1–2 °C, enhancing thermal comfort and walkability.

This study introduces the concept of micro-scale and urban grid-scale SRT. In Taiwan, common tree species exhibit SRT values of 0.2–0.5 (mean = 0.3), while building shadow SRT ranges from 0.02 to 0.18 (mean = 0.14). Using SRT = 0.3, ArcGIS solar radiation simulations corrected sub-canopy radiation, and grid-based SRT_j assessed baseline urban radiation conditions.

Findings provide a spatially explicit approach for urban planners to prioritize shading improvements in high-exposure areas, contributing to sustainable heat adaptation strategies.