50 Grades of Shade

Abstract

Cities increasingly recognize the importance of shade to reduce heat stress and adopt urban forestry plans with ambitious canopy goals. Yet, the implementation of tree and shade plans often faces maintenance, water use, and infrastructure challenges. Understanding the performance of natural and non-natural shade is critical to support active shade management in the built environment. We conducted hourly transects in Tempe, Arizona with the mobile human-biometeorological station MaRTy on hot summer days to quantify the efficacy of various shade types. We sampled sun-exposed reference locations and shade types grouped by urban form, lightweight/engineered shade, and tree species over multiple ground surfaces. We investigated shade performance during the day, at peak incoming solar, peak air temperature, and after sunset using three thermal metrics: the difference between a shaded and sun-exposed location in air temperature (ΔTa), surface temperature (ΔTs), and mean radiant temperature (ΔTMRT). ΔTa did not vary significantly between shade groups, but ΔTMRT spanned a 50°C range across observations. At daytime, shade from urban form most effectively reduced Ts and TMRT, followed by trees and lightweight structures. Shade from urban form performed differently with changing orientation. Tree shade performance varied widely; native and palm trees were least effective, while non-native trees were most effective. All shade types exhibited heat retention (positive ΔTMRT) after sunset. Based on the observations, we developed characteristic shade performance curves that will inform the City of Tempe’s design guidelines towards using “the right shade in the right place” and form the basis for the development of microclimate zones (MCSz).

Urbanization Impact on Regional Climate and Extreme Weather: Current Understanding, Uncertainties, and Future Research Directions

“…However, there is a potentially higher cost of tree planting and maintenance plus water requirements, which would become even more expensive in the future due to water stress in many cities around the world (He et al, 2021). Moreover, other urban structures can also provide shade, with many being more effective than urban vegetation (Middel et al, 2021).…”

Feasibility of Afforestation as an Equitable Nature-Based Solution in Urban Areas

“…However, while the relationship between air temperature and tree canopy coverage may be somewhat different in strength than that between LST and tree cover, we expect the direction of these relationships to be similar (Novick and Katul, 2020). Tree cover can also reduce heat exposure and improve pedestrian comfort through its shading effect (Middel et al, 2021;Zhao et al, 2018), which is difficult to estimate using satellite observations. Second, it is evident that multiple strategies need to be combined for maximum local-scale heat mitigation.…”

How are cities planning for heat? Analysis of United States municipal plans

“…The everyday lived experience of heat is one of the most direct ways that cities can influence urban climate by providing shade (Middel et al 2016). Recent work even provides cities with guidance on the relative effectiveness of different shade interventions (Rahman et al 2020, Middel et al 2021. This study focused on outdoor interventions and, potentially, plans might include indoor interventions like air conditioning as the central strategy for addressing thermal comfort.…”