Linking shading patterns of trees in Phoenix, AZ to thermal comfort

Desert cities experience high daytime air temperatures and increased solar intensity in the summer, which decreases thermal comfort and poses serious health risks to vulnerable populations, especially during heat waves. Trees can play a significant role in reducing surface and ambient temperatures through evaporation and shading, therefore mitigating heat-related health impacts. This case study quantifies the thermal benefits of shade from trees using the Central Arizona-Phoenix Long-Term Ecological Research project's (CAP-LTER) North Desert Village (NDV) landscape experiment at the ASU Polytechnic campus as study site. We investigated microclimatic and thermal comfort differences experienced on two surface types, i.e. grass and inorganic mulch (gravel). Measurements were taken on June 21, 2012, representative of a typical heat wave day during the Phoenix pre-monsoon season with maximum temperature of 41.8 °C and a nighttime low of 26.2 °C. We observed hourly surface temperatures from 06:00 am to 10:00 pm under 5-6 selected trees for each surface type using a handheld FLIR i3 infrared (IR) camera. Average shaded surface temperatures were extracted from the IR images using a region growing algorithm. We then used the RayMan model to calculate the mean radiant temperature (MRT) and four thermal comfort indices: the predicted mean vote (PMV), the Standard Equivalent Temperature (SET), the Physiological Equivalent Temperature (PET), and the Universal Thermal Comfort Index (UTCI). The model outputs are validated using surface temperatures measured in each NDV neighborhood by IRR-PN infrared radiometer sensors, and outgoing long-wave radiation values in open space vs. under canopy from a pyranometer and net radiometer. Results show that daytime surface temperatures of inorganic mulch were higher by upwards of 5 °C, even under tree canopy, than temperatures of sun-exposed grass. Before sunrise and after sunset, surface temperatures were higher under the tree canopy than in the open, indicating that the canopies function as a trap for outgoing longwave radiation, retaining heat over both surface types on the order of 1-2 °C PET. PET values exceeded 40 °C for 9 consecutive hours (10:00 am – 06:00 pm) on shaded grass and 11 hours (09:00 am – 07:00 pm) on shaded inorganic mulch.