Shade and Thermal Comfort

Walking outside during the warm season in Maricopa County is a physically demanding activity, even for short distances. High temperatures and abundant sunshine place significant demands on the body’s cooling mechanisms. These mechanisms become more stressed with body heat production associated with walking. Heat exposure while walking in the warm season can be a physically uncomfortable experience for many. In severe cases, it can lead to adverse health effects like heat exhaustion and dehydration. Providing thermally comfortable routes can make walking and biking more viable and appealing to individuals who are vehicle-reliant as well as those who depend on active transportation modes. Increased use of active transportation modes can help to reduce the demand for sources of urban heat such as waste heat from cars and heat storage by asphalt.

Thermal comfort refers to people’s subjective sense of how hot or cold they feel as they move throughout their environment. It is formally defined in ASHRAE Standard 55 as “the condition of mind that expresses satisfaction with the thermal environment.” There are many variables that influence thermal comfort, some of which are about the physical environment (e.g., air temperature, sunlight, radiated heat, humidity, air movement), and others which are about people (e.g., their health status, thermal preference, clothing, or activity level). Urban planners and designers are thus key players in shaping the thermal comfort of people in cities. The decisions and recommendations of urban planners and designers ultimately determine the mixture of landscapes and infrastructure that produce environments with different thermal characteristics.

A Thermally Comfortable Pedestrian Route (TCPR) is defined as one that can be walked from start to finish without a risk of increasing human body core temperature and/or perceived discomfort. The analysis below guides a project planner in identifying a need for either cooling features and/or places of respite. Features may include street trees or engineered shade structure, materials that absorb and/or reflect heat, seating, and drinking water access. In addition, reducing waiting and walking times between destinations, transit stops, and at street crossings are strategies to increase thermal comfort.

Quantifying a Thermally Comfortable Pedestrian Route⁷

For a route to be considered thermally comfortable, residents should be able to walk 20 minutes (about one mile) from origin to destination without risk of heat illness or personal discomfort from heat exposure. This 20-minute standard derives from walkability standards in public health literature and municipal sustainability goals. Recommendations for shade coverage along walking routes below are based on well-established standards for occupational heat exposure according to the Wet Bulb Globe Thermometer⁸ (WBGT) Index. Using contemporary observations, the WBGT was calculated in both full sun and shaded conditions at a representative urban weather station in Maricopa County under varying weather conditions. The necessary fraction of a 20-minute route for the average WBGT to meet the recommended standard of a maximum of 87.9F was then determined. Assumptions used in this calculation are that pedestrians are walking at light to moderate intensity (the metabolic equivalent of “light work,”) and that the population is not fully acclimatized to heat, to account for a wide range of physiological needs, health statuses, and lifestyles.

The recommendations presented below are based on weather data collected over the period 2005–2015 for the afternoon hours, defined as noon through 6pm. Summer months in this analysis are considered May through October. Recommended shade fractions should be revisited on a decadal basis to account for projected changes in climate.

Recognizing that it may be impractical or impossible to provide sufficient shade coverage that fully protects the population throughout the entire summer, additional recommendations are presented below that provide targets that will keep residents safe on all but the most extreme summer days.

To achieve a walking route that is safe during the entire summer, target shade coverage on walking corridors (as measured during the hottest times of the day) should be greater than or equal to 62%.

To achieve a walking route that is safe for 95% of summer afternoon hours, target shade coverage should be greater than or equal to 30%. Based on historical weather data, this shade coverage would have yielded unsafe conditions for at least one hour on an average of 12 days over the past decade.

To achieve a walking route that is safe for 90% of summer afternoon hours, target shade coverage should be greater than or equal to 20%. Based on historical weather data, this shade coverage would have yielded unsafe conditions for at least one hour on an average of 25 days over the past decade.

Therefore, along a route that takes a pedestrian 20 minutes to complete (including walking and waiting times):

• Minimum acceptable shade coverage: 20%
• Good shade coverage: 30%
• Excellent shade coverage: 60%

Trees planted on both sides of sidewalk offer shade throughout the day. Peoria, AZ.

Simple fabric shade devices, supported by columns in this instance, require minimal structure. Canopy can easily be removed in cooler months, replaced when needed for maintenance, or traded out for seasonal color.

Tall sculptural features should leverage their height to provide shade, but designers should seek creative ways to shape its form to maximize shade area for multiple times of day. These examples provide an identity feature for the neighborhood, but only provide shade on a few seats for a very short time around solar noon, when foot traffic in the area is lighter.

Perforated screen parallel with roadway provides shade on one side at all times of day. The perforation is necessary to allow views into the shaded space for security. Photo Source: Kristian Kelley

Shade with seating. Temporary shade devices employed when needed. Photo Source: Kristian Kelley

Overhead perforated steel shade structure at intersections provides relief for pedestrians waiting for the light to change. These structures must be designed and placed in a manner that does not interfere with traffic signal visibility. Photo Source: Kristian Kelley

Shade Design Considerations

Many shade studies only examine shade patterns around noon and that has limited impact on thermal comfort. LEED ND v4 provides some specific standards for increasing shade to minimize the urban heat island effects. In contrast, the principles below can be applied contextually when selecting and designing shade at the site or corridor-level. The design principles are followed by examples illustrating those guiding concepts. These shade design principles are intended to be complementary to other existing meta-principles (adopted by the City of Phoenix) for reducing the impacts of urban heat.

Design Principles for Shade⁹

1. Simulate worst case scenario: Create shade simulations through use of tools such as Revit, SketchUp, Rhino, or ENVI-met for worst case scenarios for time of year and time of day. In Maricopa County, that is during May through October in afternoon hours.
2. Connectivity: Through modeling and/or by inspecting shade percentage for achieving a Thermally Comfortable Pedestrian Route (TCPR) as described above, ensure pedestrian pathways adjacent to the project site have opportunities to occur in shaded conditions, especially in late afternoon.
3. Solar orientation: Strive to maximize shading between May to October during afternoon hours. Also:
   • Rights-of-way orientation:
     • East-west (E-W) oriented rights-of-way are the most difficult to shade with buildings. However, the southside of E-W streets, on the northside of buildings, provide the greatest shading opportunity. The effectiveness of this northside building shade is minimized around the summer solstice and becomes more effective as the sun shifts south over the solar cycle, creating longer shadows. E-W streets may need the most additional shade elements to provide effective shade early and late in the day. On east-west oriented streets, consider placement of trees and shade structures that maximize shade over pedestrian walkways. This may result in unsymmetrical treatment of streetscape elements to ensure shade is falling on pedestrian walks. Vertical shading elements can take advantage of steep early morning and late afternoon sun angles to provide effective long shadows.
     • North-south (N-S) oriented rights-of-way provide more opportunities to shade with buildings than on E-W streets. On streets with taller buildings (2+ stories), prioritize the shaded areas produced from buildings. On these streets, the challenge will be providing shade on either side of solar noon, for which horizontal structures might work best. If the N-S right of way has low or no buildings, apply principles for E-W oriented streets.
4. Sidewalk location: On either E-W or N-S oriented streets, locate sidewalks as closely as possible to the building to maximize building shade for as much of the day as possible and provide ease of access to building entrances. Locating walks adjacent to buildings also may minimize walking distance. Providing a buffer between the sidewalk and the roadway creates space for trees and other shade structures while increasing the perceived safety and comfort of people walking.
5. Use of space to address heat equity: Prioritize shade on neighborhood walks that serve as critical pathways for essential life activities (e.g. commuting to work, school, day care, and grocery) for people more vulnerable to heat. Consider when people use the space and select designs that prioritize shade during times of use. Also consider volumes of pedestrian traffic in determining how much shade to provide. A remote technique to identify heat vulnerable locations is to combine data on residents’ health, surface temperatures, and frequency of use including time of day to identify areas of critical heat vulnerability for transit. This may not depend on the total volume of people, but rather the frequency of use of walks by vulnerable residents. For more effective assessment of how people move through and use spaces, conduct on-the-ground investigations during different times of day and different days of the week to confirm movement and use patterns.¹⁰.
6. Types of shade: Select shade types that optimize shade during worst case scenarios; some options include trees and vegetation, canvas/sails, awnings, vertical panels, solar panels, and shade by buildings. Shade may complement other cooling strategies to maximize thermal comfort.
7. Materials & coatings: Select materials that decrease heat absorption. LEED-ND requires a three-year aged solar reflectance (SR) value of at least 0.28. Be cautious as to not utilize highly reflective materials that will redirect light onto pedestrian pathways as it adds to thermal discomfort (see Principle #11).
8. Vegetation: Incorporate trees to provide multiple benefits in addition to shade. Follow local guidance for the right tree in the right place, minimizing sidewalk maintenance issues, and water demand where possible by utilizing fit-for-purpose water sources, low-water demand plants, or bioretention. Vines and green walls can provide increased opportunities for cooling via evapotranspiration. Trees Matter maintains an online tree database for Maricopa County. Design guidance for a bioretention system with trees is available in the Greater Phoenix Green Infrastructure Handbook: Low Impact Development (LID) Details for Alternative Stormwater Management. Other decision support tools for green infrastructure planning include the Central Arizona Conservation Alliance natural infrastructure viewer (including an urban heat vulnerability assessment), the Arizona Department of Forestry and Fire Management Urban and Community Forestry 2016 Shade Tree Planting Prioritization, and the Arizona Department of Health Services Environmental Public Health Tracking Explorer.
9. Mixing & Layering: Combine multiple layers of shade to maximize coverage.
10. Working with constraints: In compact development, complement building shade with trees and building awnings.
11. Reflected Light Mitigation: Consider reflected light as an additional source of exposure whether it comes from adjacent structures, passing vehicles, paved surfaces, or other off-site sources. Mitigate these conditions by shading adjacent high albedo surfaces, blocking incoming reflected light using shade structures or ground level vegetation, or through the modification of context materials.
12. Art: Aesthetic and artistic approaches to shade can convey community identity or create a sense of safety or play. Designs may include cut-outs and perforations for dappled shade, sculpture, and/or interactive elements.
13. Design for human comfort: Finally, it is critical to underscore for whom shade is being developed. Many of the above principles could be adhered to and it could still be uncomfortable or unusable for people. For successfully shaded spaces, shade on pedestrian walks and waiting spaces should address comprehensive comfort including: 1) separating modes of movement, 2) designing for neighborhood sociability, and 3) constructing overall high-quality and intuitive spaces.

Shade Examples

Figure 8 shows Adams Street looking west in Downtown Phoenix at about solar noon on April 3, 2018. Sidewalks on east-west streets are of particular concern because of the length of the sun exposure. The metal shade awning on the Renaissance Hotel is designed to allow some light through and to be combined with tree shade. The material heats up more efficiently than other materials, but the alternating metal slots allows for more cooling than a solid sheet of metal. Coatings can be added to the metal to decrease absorption of shortwave radiation. The bioretention basins on the south side of the sidewalk have small ornamental tree species that will grow to 15’-0” to enhance shade as they grow. The vegetation also provides transpiration up until 104⦁F, after which evaporative cooling only comes from water evaporating from the soil.

Figure 8: Awning and tree shade (Photo Source: Mark Skalny / The Nature Conservancy; Illustration: Paul Coseo)

Combining multiple types of shade is highly recommended. Figure 9 illustrates the Roosevelt Light Rail stop with a north-south orientation (image looking north). Shade is provided from a combination of vertical structures, horizontal structures, angled canvas, and trees. Vertical shade structures are critical to provide shade in the early morning and late afternoon when sun angle is low.

Figure 9: Layered shade strategies (Illustration: Paul Coseo)

In areas of compact development, sidewalks on north-south streets next to buildings provide shaded pathways. Figure 10 shows Central Avenue looking south toward Garfield Street, where pedestrians walking before noon can find shade on the eastern side sidewalk even as the western side of the street is in full sun. Later in the afternoon the entire street canyon is shaded by a nineteen-story mid-rise building. The benefits can be enhanced with street trees and building awnings.

Figure 10: Building Shade (Illustration: Paul Coseo)

^{7. [Developed by Arizona State University researchers, David Hondula and Ariane Middel, and Maggie Messerschmidt (The Nature Conservancy)]↩}

^{8. [The Wet Bulb Globe Temperature (WBGT) Index, which is the most widely used index for heat stress assessment in occupational and recreational settings. The WBGT index takes into account air temperature, humidity, air movement, and radiation and can be measured in the sun and shade (unlike, for example, the heat index, which is measured in shade only). The WBGT is a more comprehensive measure for assessing the thermal environment than other commonly-reported variables like the heat index (which only includes temperature and humidity) or the wet bulb temperature (which is a measure of only humidity). ]↩}

^{9. [Developed by Arizona State University landscape architects Paul Coseo and Kristian Kelley, Arizona State University researchers Ariane Middel and David Hondula, and Maggie Messerschmidt (The Nature Conservancy)]↩}

^{10. [Gehl, J., & Svarre, B. (2013). How to study public life. Island press.]↩}