Quantifying the cumulative cooling effects of 3D building and tree shade with high resolution thermal imagery in a hot arid urban climate

Abstract

Shading is an effective heat-mitigation strategy, with tree and building shade naturally cooling heated surfaces in hot-arid climates. However, increasing shade through tree planting and building arrangement has associated costs, such as tree maintenance and loss of solar access. To make strategic decisions, quantifying shade’s cooling effectiveness is crucial. This study examines the extent to which shade reduces land surface temperature (LST) in a hot-arid residential environment by considering shade’s areal coverage, overall duration, and time of day. Using fine-resolution 3D city models (1 m/pixel), land cover classification (1 m/pixel), and high-resolution thermal imagery (6 m/pixel) for Tempe, Arizona, USA, hourly shade calculations are conducted from 7:30 to 13:30 on July 12, 2011. The relationships among the observed LST at 13:30, land cover types, and shade characteristics are analyzed using linear and spatial regressions. The results reveal that shade’s cooling effect is cumulative, rather than instantaneous, as surface cooling by earlier shade persists to later times to some degree. Areas shaded for longer hours are cooler than those shaded only temporarily. The results further indicate that tree-shaded areas are significantly cooler than those shaded by other features, including buildings, with the difference increasing with shade duration. Shifting a 6 × 6 m area from 0% to 100% shade in the early morning generates a 2.3℃ LST reduction at 13:30. The same shift in shade for the whole morning and the entire period (7:30–13:30) would lower LST by 8.3℃ and 11.3℃, respectively. The importance of temporal shade planning and policy recommendation for urban heat mitigation is discussed.