Effects of natural and artificial shade on human thermal comfort in residential neighborhood parks of Phoenix, Arizona, USA

This research studied the capacity of natural and artificial shade types to improve outdoor human comfort during normally hot, summer midday (1200 to 1400 h), desert conditions. Micrometeorological data were collected in full sun and under shade canopies of six landscape tree taxa and constructed ramadas in residential neighborhood parks of Phoenix, AZ, USA. The six tree taxa included Arizona ash (Fraxinus velutina Torr.), Mexican palo verde (Parkinsonia aculeata L.), Aleppo pine (Pines 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 natural shading from live trees and artificial shading from constructed ramadas were not similarly effective at improving outdoor human thermal comfort, represented by physiologically equivalent temperature (PET). The difference in PET between full sun and under shade canopies of Fraxinus and Quercus trees was greater than under shade canopies of Parkinsonia, Prosopis, trees or constructed ramadas by 2.9 to 4.3 degrees C. Radiation intensity was the most significant driver of lowered PET (p < 0.0001, R-2 = 0.69) and with the exception of ramadas, lower radiation intensities corresponded with lower PET. Our results suggest that selecting trees that attenuate the most solar radiation is the most effective urban design strategy for lowering PET during normally hot, summer midday, desert conditions.