In the 1950s, 30% of the world's population lived in urban areas. By 2030, 60% of the world's population will live in cities. The phenomenon of rapid urbanization is resulting in the transition of native vegetation to manmade engineered paved surfaces for roads, parking areas, sidewalks, and building structures.

The urban environment—with its impervious paved surfaces, varied building geometries, and reduced vegetation—causes less of the incoming radiant energy from the sun to be reflected or re-released. This heat island effect in urban areas also reduces the conversion of energy to latent energy that is associated with evaporation or transpiration of moisture. Compounding this effect is the larger volume of paving and building materials, which gives urban areas a much higher thermal storage capacity than natural surfaces.

This can be observed by satellite images showing the impact of engineered materials on surface and ambient temperatures. Satellite (ASTER) imagery of the Phoenix region captured in October 2003 provides a coarse visual representation of the paved surfaces, including local roads, highways, and parking lots. The image shows that these areas contribute significantly to the Urban Heat Island (UHI) and exhibit variability of surface temperatures related to spatial patterns and pavement designs.

When the urban region's temperatures exceed a rural setting's temperature, the result is known as the UHI effect, which is generally thought of as a nocturnal phenomenon, though many regions are hotter both day and night. As surfaces throughout a community or city become more abundant and urban geometry from buildings traps energy and prevents it from re-radiating to the atmosphere, the overall ambient air temperatures increase in comparison to the surrounding rural region. In Phoenix, the difference has been documented to be as large as 12° F.

The UHI can adversely impact the sustainability of regions by increasing the dependence on mechanical cooling, which results in increased greenhouse gas emissions, consumption of water to make electricity, and increased cost of living for residents. The UHI also can increase the incidence and severity of heat-related illnesses. Summertime heat is known to have a greater impact on human health than any other form of severe weather in the United States. Heat waves claim more lives each year than floods, tornadoes, and hurricanes combined.

Extremes of heat-related impacts are exemplified by the European heat wave of August 2003 where thousands died, and the Chicago heat wave in July 1995, which is thought to have caused 465 deaths. Although the UHI was not the sole cause of the deaths, the elevated nighttime temperatures played a critical role.

The UHI also has serious financial impacts for local governments. According to a U.S. Environmental Protection Agency report, the heat island in Los Angeles raises ozone concentrations by 10% to 15%; the report indicates that heat-island reduction measures could lower the city's smog-related expenses by $360 million per year. The EPA also has recently promulgated guidance for incorporating UHI mitigation strategies as part of the State Implementation Program (SIP) process.

The federal Clean Air Act requires states with counties failing to meet national ambient air quality standards to produce an SIP. If a state fails to submit or implement an SIP, or if it submits an SIP that is unacceptable, the EPA has the power to impose sanctions or other penalties on that state. Typical sanctions include the threat of cutting off federal highway funds. A recent study by Arizona State University's Sustainable Materials and Renewable Technologies (SMART) Institute has shown a steady and increased annual peak electricity demand on residential electricity consumption due to the UHI in the Phoenix region. This took into consideration more efficient building design and HVAC systems.

Mitigation

Historically, conventional mitigation strategies have focused on cool pavements, cool roofs, and urban forestry. However, each option has to be evaluated in a "systems" approach due to the complexities of the causes of the UHI. And we also need to select materials based on other factors, such as first and operating costs, performance, aesthetics, and environmental considerations of noise, surface water, emissions, and location.

To simplify the UHI approach by committing to planting more trees, increasing reflectivity, or changing pavement types in a region is to grossly overstate and simplify the value of those mitigation strategies. First, consider the urban setting. The strategies to mitigate the UHI for a region must be developed and applied in more localized settings as it is the summation of the localized UHI "pockets" (of about 2 km) that contribute to the larger regional UHI (greater than 50 km). A city can include a sprawling suburban region, a dense urban core, or a mixture of the two. The spatial design will affect the appropriate mitigation strategies.

A strategy that might primarily provide the maximum amount of shade to keep surfaces cool during the daytime might be rejected for a similar setting where the primary consideration is for nighttime activities. The UHI must be addressed while also meeting human comfort needs. Trees and buildings that provide shade during the day can serve to trap the energy and modify air flow, all of which can contribute to increased temperatures at night.

Another example of a systems approach is the practice of using urban forestry as a primary means to reduce the UHI. Many local governments have developed ordinances to require a specified volume of trees per parking space in commercial parking lots. However, trees located within the boundaries of an impervious surface of parking lots and sidewalks have a high mortality rate and an increased incidence of "dwarfing," which reduces the benefits of the prescribed mitigation strategy. System issues for trees in parking areas include increased temperature of the rhizosphere (root-zone) as a result of the increased thermal storage of the pavement and low soil moisture due to the lack of permeability. To adjust for this, large diamond-shaped planting boxes are constructed, which increases the land required to meet the parking needs and does not resolve the tree dwarfing and viability issues.

A systems approach could include the use of a pervious (or porous) pavement to reduce surface temperatures and in some eases, increase soil moisture content. Such a system can provide the added benefit of reducing the size of the parking lot required and supporting stormwater management.

Another approach is a project that used solar photovoltaic (PV) parking canopies that was undertaken by the nation's third largest public utility (Salt River Project) and the city of Phoenix. Researchers at the SMART Institute found that the PV canopies were more effective in reducing surface temperatures of pavement throughout the diurnal cycle and reduced the required parking lot size since planter space for the trees was not required. And as a bonus, the PV canopies provide an ongoing source of renewable energy.

Trees may, in fact, be a cost-effective and appropriate mitigation strategy for residential structures to reduce the effects of the UHI and energy consumption. However, urban forestry for parking lots and streets must be examined as part of a larger pavement, forestry system, or urban structures system.

In conclusion, mitigation strategies for UHI are complex and considerations of urban form and function need to be considered. The goal should be to identify materials and material designs that can optimize performance and provide both daytime and nighttime mitigation of surface and ambient temperatures.

Materials can be designed that consider properties of thermal conductivity, heat capacity, density, absorption, and emissivity to reduce surface temperatures during the diurnal cycle and also ensure a functional consideration or optimization for the actual function of the material. A new generation of pavement and building materials that use nanomaterials, microfibers, and composites are being developed by researchers at the SMART Institute and similar organizations to reduce the UHI impacts and improve material performance.

Golden is director and Kaloush is co-director of the SMART Program at Arizona State University.

The National Cool Pavements Conference will be April 24, 2006, at Arizona State University. The conference will be in cooperation with the U.S. EPA and national pavement associations. For more information, visit www.asusmart.org.

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.