Quantifying the Cumulative Cooling Effects of 3D Building and Tree Shades with High Resolution Thermal Imagery in a Hot Arid Urban Climate

Shading is an effective heat-mitigation strategy, with tree and building shades naturally cooling down heated surfaces, especially in a hot-arid climate. However, increasing shades through tree planting and building arrangement often implies opportunity costs, such as water/maintenance costs for trees and loss of solar access for buildings. It is thus important to better quantify the cooling effectiveness of shade to make strategic decisions. Urban landscape involving trees and buildings creates shades of varying size and location at different times. This study examines the extent to which shade reduces land surface temperature (LST) by considering its areal coverage, overall duration, and time of day in a hot arid residential environment. Based on a fine-resolution 3D surface model (1m/pixel), land cover classification (1m/pixel), and high-resolution thermal imagery (6m/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 regression. The results show that the cooling effect of shade is cumulative, rather than instantaneous, as surface cooling by earlier shade persists to some degree to later times. An area of 6×6m can generate a LST reduction of 2.3℃ at 13:30 if that area shifted from zero to 100% shade in the early morning. 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 for urban heat mitigation is discussed.