Launch Slideshow

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A hot night in the big city

A hot night in the big city

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    This nighttime thermal image shows mixed land use in a central urban area. At the top (purple) is a cooler vegetated area while the physical properties and spatial designs of the urban region control the localized temperatures.

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    A photovoltaic parking canopy can provide a larger volume of shading for a parking lot-with the added benefit of increasing the parking spaces per acre in comparison to a tree-lined parking lot. Photos: ASU SMART Program

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    This ASTER imagery shows the top 20% of surface temperatures for the Phoenix region at about 10:30 p.m. in October 2003. Photo: NASA MARS Space Flight Facility at Arizona State University

For more information, visit:

www.asusmart.org

http://eetd.lbl.gov/heatisland

www.epa.gov/heatisland

www.harc.edu/harc/Projects/CoolHouston

www.hotcities.org

www.urbanheat.org

Selecting cool pavements

Studies have shown that pavements—including those of highways, streets, and parking lots—cover 30% to 45% of the urban fabric. Because of this large area and because of the greater frequency at which pavement improvements occur compared to buildings, pavements have been identified as one of the most appropriate places for public works officials to explore mitigation strategies.

Conventional mitigation strategies for UHI have focused on increasing the pavement's reflectivity, known as albedo. While albedo is one important factor, other mitigation strategies like the use of porous or pervious pavements also are effective. Other issues include the aging process, where some pavements' reflectivity increases while others' decreases over time. Pavement selection must include considerations for material, traffic volumes and loads, safety, aesthetics, rideability, initial cost, maintenance costs, color, noise, availability of local materials, and air quality. When adding the UHI consideration, local governments need to consider the physical setting for the pavement structures.

An example is shown in the above right photo, indicating the surface temperatures of the different highway sections in relation to their material and location. Box A encompasses a section of Highway 101 on the east side of Phoenix. The pavement surface temperature for the middle section in Box A is much lower (lighter or no shade of red) compared to the top and bottom sections (darker shades of red) of the same highway segment and material type. The difference is attributed to the grade/elevation of the different segments and to the surrounding landscape. The top and bottom sections are depressed or below grade highway segments surrounded by residential areas and sound walls, which have a tendency to trap heat and lower the natural convection of winds. Convection of winds can serve to cool pavement surfaces. The surface temperatures of these locations remained higher at night compared to the middle section, which was constructed at grade with open fields on the east side.

Another consideration can be observed in the highway segment highlighted in Box B. Both the top and bottom segments are below grade with developments and sound walls on either side. The difference in surface temperatures between the sections was attributed to the use of a porous mixture (surface layer) for the bottom section, compared to a conventional dense surface for the top section. The concentrated bright (elevated surface temperatures) image in the middle represents Metro Center, which is a very large business and shopping area. From this figure, one can deduce that using a porous or pervious pavement to reduce the surface pavement temperature at night is one way to mitigate the UHI. However, optimization of the different variables in the final design should consider safety, performance, comfort measures, and costs.