Effect of Tree Species and Shade Structures on Outdoor Human Thermal Comfort

The objective of this research was to evaluate overhead shade densities as a means of mitigating outdoor human heat stress during midday, normally-dry, desert summer conditions. During June and July 2015, micrometeorological data were collected in full sun and under shade of six landscape tree species and constructed ramadas in neighborhood parks of Phoenix, AZ, during early afternoons (1200 to 1430 Hours). The six landscape tree species included Arizona ash (Fraxinus velutina Torr.), Mexican palo verde (Parkinsonia aculeata L.), Aleppo pine (Pinus halepensis Mill.), South American mesquite (Prosopis spp. L.), Texas live oak (Quercus virginiana for. fusiformis Mill.), and Chinese elm (Ulmus parvifolia Jacq.). Results showed that shading from live trees and constructed ramadas were not similarly effective at improving outdoor human thermal comfort, represented by physiologically equivalent temperature (PET). The difference between PET in full sun and under shade was greater under Fraxinus and Quercus than under Parkinsonia, Prosopis, and constructed ramadas by 2.9 to 4.3°C. Radiation was a significant driver of PET (p<0.0001, R²=0.69) and with the exception of ramadas, lower radiation corresponded with lower PET. Variations observed in this study suggest selecting trees or structures that attenuate the most solar radiation is a good urban design strategy for lowering PET during summer midday, desert conditions.