Jenni Vanos, Assistant Professor, School of Sustainability, College of Global Futures at Arizona State University

Urban Heat and Material Matters

A hot steering wheel. A sizzling bench. A scorching slide. These common objects are often the cause of some painful experiences for people in the summertime. The choice of materials in urban areas, and their exposure to the sun, importantly impacts the heat experienced by residents in the warm season. Certain properties of various materials can help the given item function properly and last longer (e.g., metal versus wood benches), yet may also result in extremely hot surface temperatures to the touch. Material choice and their thermal properties of non-natural surfaces, their orientation, reflectivity, and the fact that they do not evapo-transpire water (like plants) also enhances the development of the urban heat island (UHI), discussed in the March issue of the ULI newsletter. The image below shows an overhead shot of an urban area with various types of surfaces, where lighter colored surfaces and natural vegetation have cooler surface temperatures. With overhead photos like this, however, we cannot ‘see’ under the shade, which we know helps reduce surface temperatures!

Two common areas where we often study the impacts of different materials and exposure on various temperature metrics at different scales are playgrounds and streets. The material, the scale you are working at, and the type of use of the given area, matters a lot. 

Playgrounds

What if you walk barefoot across rubber, do you quickly run to the cooler grass? Playgrounds are spaces designed for children to play actively and engage in safe outdoor activity; however, often we find some of the hottest materials in playgrounds, as well as a lack of shade due to non-natural surface use, such as rubber, that may not support vegetation growth. Shade sails, (used to shade the slide in the image to the right), are also very costly! Much thought must go into designed thermally safe and climatically-appropriate playgrounds in addition to other safety considerations. The National Program for Playgrounds Safety (NPPS) recently added sun exposure and surface temperature in their guidelines alongside surfacing and material considerations – check them out here! 

At this small scale, properties of material high conductivity results in a metal bench feeling hotter than a wood bench, even at the same temperature. Yet a powder or paint coating on the metal helps insulate the material, and hence minimizes that quick conduction of heat to prevent fast burns. Did you know that different materials have different burn thresholds for human skin? A plastic slide at 170°F can burn human skin in 3 seconds, but uncoated metal only needs to get up to 140°F to burn skin. But, if you shade those surfaces that are certain to get hot in the mid-day sun, the temperatures can be kept to safe, non-burning temperatures, especially for more sensitive skin at younger ages. Shading considerations for hot materials by climate and time of day are also helpful for positioning or choosing other high-contact surfaces, like benches at bus  or transit stops and other outdoor seating areas.

Streets

Pavement surfaces, both asphalt and concrete, cover approximately 30–40% of the land in large urban areas and are some of the hottest surfaces found in cities. Cities are trying out various innovative heat mitigation technologies in an attempt to minimize the amount of heat retained in urban areas in hopes of bringing down surface and air temperatures and the intensity of the UHI. One such innovative technology, reflective pavement, aims to do just that: it reflects the sun’s radiation, rather than absorbing it, so that the surfaces don’t heat up so much.

The City of Phoenix ambitiously initiated the Cool Pavement Pilot Program in summer 2020, applying the product CoolSeal by GuardTop® to 36 miles of residential streets in––covering the most surface of anywhere globally. GuardTop® is a water-based, asphalt, emulsion seal coat designed to achieve lower pavement surface temperatures through its lighter color and reflectivity. Because streets need to be resealed periodically, applying a light, versus dark coat, can be tested and help lower surface temperatures. If it works, we could reduce energy and water use in the summer and create more comfortable spaces for people. Research is ongoing at ASU’s Urban Climate Research Center to determine the impact of cool (highly reflective) pavement on urban heat and various performance characteristics of the pavement.  Some research questions include:

• What are the differences in surface and air temperatures for traditional asphalt versus the GuardTop® reflective pavement?
• Will streets last longer because they stay cooler in their surface and subsurface layers? Can this result in lower long-term costs to cities?
• How does the solar reflectance degrade over time?
• What are the perceptions of city residents regarding the benefits and potential drawbacks of the cool seal?

We continue to crunch the data. Watch for our final report, and stay tuned for more work from our Urban Climate Research Center. 

References:

Vanos, J. K., Middel, A., McKercher, G. R., Kuras, E. R., & Ruddell, B. L. (2016). Hot playgrounds and children’s health: a multiscale analysis of surface temperatures in Arizona, USA. Landscape and Urban Planning, 146, 29-42.