Impact of shade on outdoor thermal comfort-a seasonal field study in Tempe, Arizona

Abstract

Shade plays an important role in designing pedestrian-friendly outdoor spaces in hot desert cities. This study investigates the impact of photovoltaic canopy shade and tree shade on thermal comfort through meteorological observations and field surveys at a pedestrian mall on Arizona State University's Tempe campus. During the course of 1 year, on selected clear calm days representative of each season, we conducted hourly meteorological transects from 7:00 a.m. to 6:00 p.m. and surveyed 1284 people about their thermal perception, comfort, and preferences. Shade lowered thermal sensation votes by approximately 1 point on a semantic differential 9-point scale, increasing thermal comfort in all seasons except winter. Shade type (tree or solar canopy) did not significantly impact perceived comfort, suggesting that artificial and natural shades are equally efficient in hot dry climates. Globe temperature explained 51 % of the variance in thermal sensation votes and was the only statistically significant meteorological predictor. Important non-meteorological factors included adaptation, thermal comfort vote, thermal preference, gender, season, and time of day. A regression of subjective thermal sensation on physiological equivalent temperature yielded a neutral temperature of 28.6 °C. The acceptable comfort range was 19.1 °C-38.1 °C with a preferred temperature of 20.8 °C. Respondents exposed to above neutral temperature felt more comfortable if they had been in air-conditioning 5 min prior to the survey, indicating a lagged response to outdoor conditions. Our study highlights the importance of active solar access management in hot urban areas to reduce thermal stress.

Fig 1:

(From left to right) Geographic location of the City of Tempe in Maricopa County, Arizona, USA; Arizona State University’s Tempe Campus; Solar canopy structures provide shade at the Memorial Union on campus

Fig. 2

Frequency distribution (in percent) of subjective seasonal thermal sensation votes (TSV) for shaded and sun-exposed survey samples; seasonal mean thermal sensation votes (MTSV), sun vs. shade

Fig. 3

Observed air temperature (1.1 m height) vs. estimated air temperature for all samples (N = 1284) (left). Air temperature estimation error for shaded and sun-exposed survey samples by season (right)

Fig. 4

a Relationship between mean thermal sensation votes (MTSV) and binned PET (all seasons): linear regression yields a neutral temperature of 28.6 °C. b Subjective thermal comfort vs. binned PET (all seasons) reveals an acceptable comfort range of 19.1–38.1 °C

Fig. 5

Probit analysis yields 20.8 °C as preferred temperature

Fig. 6

Frequency distribution (in percent) of subjective seasonal thermal sensation votes (TSV) for respondents who were in an air-conditioned space 5 min before the survey vs. respondents who were not; samples from summer survey with conditions above neutral temperature only