Directional Contributions to the Radiant Temperature Environment Measured During Record-Breaking Extreme Heat in Tempe, AZ

Excessive heat leads to nearly 2000 emergency room visits and more than 100 deaths in Arizona every year. In coming decades, heat waves are expected to be longer, more intense, and more frequent, increasing the population’s vulnerability to heat-related illnesses, mortality, and discomfort. Current summer heat waves in hot desert cities could soon become the new normal for those cities; at the same time, they may indicate the extremes of severity of future heat wave conditions in more temperate climates.

Three-dimensional radiation budgets are decomposed to determine the main drivers of mean radiant temperature (MRT) and thermal comfort outcomes, using an unprecedented human-biometeorological data set collected during a record breaking extreme heat wave day in Tempe, Arizona.

On June 19, 2016, we conducted hourly transects of 22 sites on Arizona State University’s main campus from 9:00 h to 21:00 h with a mobile human-biometeorological station that logged air temperature, humidity, shortwave and longwave radiation in six directions, and 2D wind at 2 s intervals. Sampled locations include a variety of heterogeneous environments, i.e. fully shaded and partially shaded sites; artificial and natural shade; sun-exposed sites; and varying ground cover (concrete, grass, and asphalt).

We time-detrended the observations for cross-site comparison. The sky view factor and sun duration for each location were determined from fisheye photographs. MRT was calculated using the six-directional method, applying angular factors for a standing reference person. We then decomposed the observed daily average, peak, and post sunset MRT into its weighted shortwave and longwave radiation components for the lateral, up, and down facing sensors to create a distinct MRT profile for each site.

Average daily MRT was highest on a sun-exposed concrete path (62.5°C) and peak hourly MRT across sites was 76.4°C at 15:00 h in an east-west building canyon. During the day, MRT was lowest in a building tunnel (37.6°C); after sunset, it was lowest over turf grass (28.9°C). Lateral longwave radiation was found to be the dominant factor in the radiation budget, while lateral shortwave radiation was most variable across all MRT profiles, reflecting the various shade conditions and sky view factors.