The Challenge

In the desert Southwest, shade is one of the most important urban design features to lower Mean Radiant Temperature (MRT) and keep people comfortable during the hot, dry summer months. Many cities in Arizona have recognized the significance of shade for healthy urban environments and developed tree and shade master plans with city-wide canopy goals. For example, the City of Tempe’s Urban Forestry Plan envisions 25 percent tree canopy cover by 2040. The plan strongly ties into the City’s goal to become a 20-minute city, i.e., a community where residents can comfortably walk or bike to major urban hubs and amenities within 20 minutes of their homes.

Trees cool the urban ecosystem through evapotranspiration and yield substantial thermal comfort benefits by providing shade. However, planting trees poses several challenges to cities in the Southwest. Engineered systems such as underground water utilities, communication cables, and overhead power lines stand in direct competition for limited space in the city’s rights-of-way (Figure 1). Drought conditions and increased irrigation demands further create a cooling – water use tradeoff that raises water conservation concerns. 

The Question

Alternative shade solutions are needed, but little is known about the thermal comfort benefits of those solutions. Is all shade created equal? Or are there significant differences in shade performance? Could a shade sail or photovoltaic canopy replace a tree?

Looking for the Answer

In 2019, the HUE Initiative supported a pilot study to test the performance of natural and engineered shade in a living laboratory setting using the mobile weather station MaRTy. Researchers conducted measurements in Tempe, Arizona on hot summer days to quantify the efficacy of various shade types. They selected a wide range of shade types that cover diverse ground surfaces (concrete, asphalt, gravel, grass) and grouped them into three categories: 1) lightweight or engineered shade, 2) shade from urban form, and 3) natural shade from trees. Lightweight or engineered shade includes non-permanent structures such as umbrellas and shade sails, pergolas, and engineered canopies (e.g., roofs and photovoltaic structures). Shade from urban form consists of building-integrated shade (e.g., overhangs, arcades, tunnels, breezeways, and shade from street canyon geometry). Lastly, natural shade encompasses various native and desert-adapted trees that are common in Tempe. In addition, several sun-exposed locations were selected to serve as reference locations.

Researchers measured the shade performance of 159 unique locations at midday, in the late afternoon during the hottest time of day, and after sunset. They were interested in three thermal metrics: the difference between a shaded and sun-exposed location in air temperature (DTa), surface temperature (DTs), and mean radiant temperature (DMRT).

Results

Air temperature did not vary significantly under shade types, but all shade significantly reduced the heat load on the human body, quantified as MRT. During the day, shade from urban form most effectively reduced Ts and MRT, 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.

Based on the MaRTy observations, researchers developed characteristic shade performance curves that illustrate how the shade benefit varies for each shade type (Figure 3). Each graph includes two reference lines: 1) the solid horizontal yellow line represents a sun-exposed location; 2) the solid black line represents a long tunnel in which the standing person does not receive direct shortwave radiation all day. The blue and red dashed lines with arrows in between show two extreme cases of a shade type, for example, an urban canyon with high and low aspect ratio. The brown and green dashed lines represent the shade performance of high canopy, low leaf area density (LAD) trees and low canopy, high LAD trees.

Figure 3: Characteristic shade performance curves for courtyards and trees. Curves show how the shade benefit (ΔMRT) varies under the shade on a day in mid-July.  Curves refer to a person standing in the center of the fisheye photo (Middel et al., 2021).

The curves show the characteristic timing and magnitude of ΔMRT, which represents the shade performance over the course of a day. The complete set of curves and a more detailed description of the study findings can be found here. The research team at Arizona State University is currently working on a web-based tool that will allow users to generate their own customized shade performance curves to inform decisions on effective shade deployment and implement the “right shade in the right place” depending on urban context and function of space.

Source: Ariane Middel, Saud AlKhaled, Florian Arwed Schneider, Björn Hagen, Paul Coseo. (in press). 50 Grades of Shade. Bulletin of the American Meteorological Society (BAMS). https://doi.org/10.1175/BAMS-D-20-0193.1