April 2003 · Volume 85 · Number 3

The Urban Heat Island Phenomenon:
How Its Effects Can Influence Environmental Decision Making in Your Community

Maurice Estes, Jr., Dale Quattrochi, and Elizabeth Stasiak

Reinvestment in urban centers is breathing new life into neighborhoods that have been languishing as a result of explosive suburban development over the past several decades. In communities all over the United States, adaptive reuse, brownfields redevelopment, relocation of entertainment venues into downtowns, and other infill initiatives are transforming urban landscapes, economies, and quality of life. The way in which this development occurs, however, could exacerbate the urban heat island (UHI) effect, an existing problem in many areas and one that poses a threat to the long-term sustainability and environmental quality of localities.

The UHI phenomenon is rooted in how landcovers respond to solar heating and how the heat from these surfaces affects the local environment. This phenomenon is responsible for urban centers’ having higher air temperatures and poorer air quality than suburban or rural areas. The UHI effect also forces the development of meteorological events (increased precipitation), boosts energy demands, poses threats to public health, and potentially contributes to global warming.

While the name of this phenomenon implies that it is solely an urban problem, research has shown that the effects of UHI also are becoming prevalent in suburbs. As suburban areas increasingly develop, using landcovers and building materials common to urban areas, they are inheriting such urban problems as heat islands. For this reason, it may become necessary for nonurban communities to engage in heat island mitigation. The good news is that, through education and planning, the effects of the UHI phenomenon can be mitigated.

Although this theory has not been not scientifically validated as yet, heat islands may be viewed more as products of urban design than of the density of development. [1] Urban sprawl, for example, directly worsens UHI because there is an increase in built-up surfaces (pavement, buildings) and a reduction in natural surfaces (forests). Therefore, localities can continue to grow and develop without aggravating UHI by using sustainable development strategies.

What Is the UHI Phenomenon?

The urban heat island effect results from the way in which urban landcovers (buildings, pavement) respond to solar radiation, particular on extremely warm, sunny days in the summer. While all materials absorb heat from the sun, materials from the built environment, like concrete and asphalt, absorb higher levels of solar radiation throughout the day than do natural landcovers.

After sunset, the built environment is no longer subjected to intense solar radiation. Surfaces that compose the urban landscape re-radiate this stored heat energy to the lower atmosphere. The total effect of this thermal energy re-radiation can cause the air temperature in localities to be elevated by three degrees Fahrenheit or more, in contrast to rural areas.

Figure 1. Diagram of the urban heat island effect.

These elevated air temperatures effectively form a dome of higher air temperatures over a community, trapping air pollutants, degrading air quality, and preventing the heat waning as it would under natural conditions. Figure 1 illustrates the causation of the UHI effect.

Conversely, natural landcovers, such as grasses and trees, can help to mitigate the UHI phenomenon. Natural landcovers absorb heat from the sun and use it as energy via evapotranspiration, a process by which vegetation actually helps to cool the air. Few local governments or highly developed communities, however, maintain enough greenspace to compensate for all the heat radiated by the built environment. Such natural landcovers as trees can be used to shade materials that, with direct solar radiation, would absorb and radiate high levels of heat.

Another factor that contributes to the development of UHI is the reflectivity of surfaces across the urban landscape. Reflectivity of a surface is known as its albedo. Human-induced surfaces typical of the city landscape, such as rooftops, asphalt, and concrete, have relatively low reflectivity quotients—a low albedo—in comparison with natural surfaces.

Trees, grass, and other vegetated surfaces, therefore, absorb sunlight for use in evapotranspiration, as opposed to storing it like nonnatural surfaces. The water draws heat as it evaporates, cooling the air in the process. Surfaces ubiquitous to the urban landscape like rooftops or pavements, however, can be made more reflective through a number of manufacturing processes, thereby increasing their albedo and reducing their heat storage capacities.

How Is UHI Detected?

Through the science of remote sensing, information-gathering devices on satellites and aircraft can collect data on the state and condition of the land surface. Thermal infrared (TIR) sensors can obtain quantitative information on the amount of heat that is expressed by different surfaces across the landscape. These TIR data are critical for assessing “what’s hot and what’s not” across the urban landscape and ultimately, for determining the magnitude of UHI over an urban area.

Figure 2. Thermal infrared (TIR) aircraft image of the Atlanta, Georgia, central busines district, showing surface temperatures across the urban landscape.

Figure 2, for example, shows TIR aircraft data collected during the day over the central business district (CBD) of Atlanta, Georgia, in May 1997. The temperature bar at the bottom of the figure depicts the amount of surface heat energy detected by the TIR sensor on board the aircraft. The air temperature at the time these data were obtained over Atlanta was barely 80o Fahrenheit.

As can be seen in the illustration, temperatures for built-up surfaces across the CBD were in excess of 100o Fahrenheit, with building rooftops having surface temperatures of almost 120o Fahrenheit. In contrast, residential areas adjacent to the CBD, where trees and grass were more extensive, had surface temperatures significantly lower than those seen in the CBD.

Thus, areas with high surface temperatures, such as those in the Atlanta CBD, are likely sources of the high surface thermal radiation that drives the UHI effect and may be viewed as “hot spots” in contrast to cooler, vegetated areas like forests. TIR remote-sensing data, therefore, confirm and communicate the differences in heat absorption, radiation, and albedo between natural and human-made surfaces as they influence the development of UHI.

What Are the Effects of UHI?

The UHI phenomenon affects the environment and population in a number of ways, including through the degradation of air quality, threats to public health, and the triggering of meteorological occurrences. Urban environmental conditions also could threaten the viability of local governments, as fewer people will seek or choose to be in built-up places or downtowns for residential, business, or entertainment purposes. These shifts in lifestyle could reverse the current trend of urban in-migration and could cause an urban exodus similar to that of the mid-1900s, which left many traditional urban centers languishing for years and from which many American communities still are trying to recover.

Air quality. UHI may be a significant contributor to elevated ozone levels by adding excess background thermal radiation to the overall chemical reactions that form ground-level ozone (O3). Sunlight and elevated heat levels can photochemically “cook “ozone to far more dangerous levels when the air temperature is higher than 90 degrees. [2]

Toxic to humans at ground level, ozone inflames lung tissue and aggravates a range of respiratory ailments, including asthma. Over urban areas, ozone is formed in the presence of calm wind conditions, intense sunlight, and high air temperatures and as a result of the chemical interactions of two compounds: volatile organic compounds (VOCs) and nitrogen oxides (NOx).

VOCs come from a variety of nonpoint-source contributors, such as paint cans, gasoline cans, or even biological sources (vegetation). Contributors of NOx are primarily associated with point-source contributors like smokestacks, automobile exhaust systems, and other origins of emissions.

The UHI effect, which under the right conditions can drive up air temperatures by 10o Fahrenheit or more, acts as an additive background effect to solar radiation and may contribute to the formation of ozone. If UHI can be mitigated, scientists hope that significant reductions in ozone levels can result. More important, a greater knowledge of the relationship between UHI and ozone levels can have significant budgetary impacts on state and local governments.

Under the more stringent air-quality guidelines set by the U.S. Environmental Protection Agency in 1997, nearly 300 counties in 34 states will not meet the new air-quality standards for ground-level ozone and will be considered in a condition of nonattainment. This designation carries serious penalties to metropolitan areas. One of the most severe of these is the risk of losing funding for new highway development if plans are not made and enacted to reduce ozone levels to meet EPA standards.

A state containing metropolitan areas not in attainment of EPA ozone levels must develop an implementation plan (SIP) to illustrate the measures it will take to bring these areas into attainment.

Public health. Elevated air temperatures are not only uncomfortable but also can push temperatures from hot to dangerously hot, threatening public health. Poor air quality and high temperatures will be particularly harmful to children, the elderly, and those with chronic and respiratory illnesses. But even for those who are healthy, poor air quality and excessive heat can threaten good health and physical condition.

On the average, 1,500 American city dwellers die each year because of the heat, which is more deaths than those from all other natural disasters combined. [3] The compounding effect of heat waves and UHI could prove to be even deadlier to urban populations.

Global warming. The UHI phenomenon potentially contributes to global warming. The hotter the air temperatures, the higher the demand for electricity to generate air conditioning, which further boosts the sulfur, nitrogen, and particulate matter in the air. The vast quantity of greenhouse gases emitted as a result of excess energy production further contributes to larger-scale climatic effects through the process of global warming. [4]

Meteorological effects. As UHI modifies urban climates, its effects change local meteorology. The UHI phenomenon is a proven mechanism that forces the development of precipitation events either over, or downwind of, communities. Furthermore, naturally occurring storms often intensify as they pass through cities with a UHI; moderate rainstorms may turn into full-blown thunder and lightning storms. Houston, for example, has realized a 40 percent increase in lightning strikes. [5] Researchers have found that this lightning frequency is not seasonal but is rather a result of the urban heat island effect and air pollution. [6]

Addressing the UHI Phenomenon

While no communities have developed comprehensive programs to mitigate the effects of heat islands, [7] localities are recognizing the need to address the urban heat island. In Chicago, for one, several municipal buildings have been designed to accommodate vegetated rooftops.

In addition, a number of cities of all sizes and geographic locations specifically mention mitigation of the UHI phenomenon or climatic relief as a rationale for enacting certain zoning ordinances, like those relating to landscaping standards, tree preservation, and parking lots. Perhaps the most progressive program is an ongoing effort in the Atlanta metropolitan area that seeks to observe, measure, model, and analyze how the rapid growth of greater Atlanta since the early 1970s has affected the region’s climate and air quality.

The National Aeronautics and Space Administration (NASA) is using its technological capabilities to assist Atlanta in mitigating the UHI phenomenon through an endeavor called Project ATLANTA (ATlanta Land-use ANalysis: Temperature and Air-quality). Project ATLANTA began as a scientific research effort focused on how urban growth in the Atlanta metropolitan area over approximately the past 25 years has affected the region’s meteorology and air quality.

Integral to Project ATLANTA’s original research tasks has been the measurement and mapping of the extent of the area’s UHI (using TIR remote-sensing data), in relation to the distribution and composition of various urban surface types common to Atlanta’s urban landscape. Soon after the start of Project ATLANTA, however, it became obvious that a fundamental driver of the scientific research for the investigation was the goal of gaining accurate information on the characteristics and effects of the metropolitan area’s UHI so that urban planners, government officials, and the like could use this information to make sound, rational decisions on the future of Atlanta’s overall environment.

As a consequence, Project ATLANTA has been developing some information products for use by decisionmakers and the general public to evaluate possible strategies for mitigating the UHI effect. These have included: 1) TIR remote-sensing data in the form of thermal maps like the one shown in Figure 1, that depict the distribution of surface temperatures over Atlanta as derived from aircraft and satellite data; 2) landcover classification maps (also derived from aircraft and satellite remote-sensing data) that present the distribution of landcovers (vegetation, high-density urban development, pavement) across the Atlanta urban landscape; and 3) an “urban fabric analysis” (again using remote-sensing data) that could quantify both the surface heating differences among urban landcovers and the distribution of thermal hot spots across the landscape.

What Can Be Done?

Communities can mitigate or prevent the UHI phenomenon by employing local planning initiatives and promoting sustainable development. Some common planning practices like the integration of greenspace can actually be crafted and bolstered to become community-wide strategies for mitigating UHI. Strategies can be employed to retrofit currently developed sites (through any renovation or redevelopment process) and integrated into site plans for new construction.

Some mitigation strategies include:

Increasing greenspace and tree planting.
Increasing the reflectivity (albedo) of urban surfaces by installing highly reflective roofing materials or by “lightening up” the color of pavements with a number of commercially available products and methods.
Using “green” building materials to improve the albedo of these surfaces and also raise energy efficiency.
Employing green building designs, such as those that include vegetated rooftops. A number of local governments encourage green building development in their communities through programs that offer such incentives as density and height bonuses.
Designing site plans that minimize or eliminate expanses of exposed, paved surfaces. For example, instead of asphalt parking lots, consider underground parking.
Promoting the use of mass transit and reducing reliance on the automobile, as auto emissions can worsen UHI and its effects.
Decreasing expanses of impervious surface, which will reduce runoff in general and mitigate pollutant runoff in general into waterways.
Installing “porous paving” that filters more rainwater into paved surfaces, keeping these surfaces cooler and also helping to lessen water runoff.

Works Cited

Bahrampour, Tara. “Most Deadly of the Natural Disasters: The Heat Wave.” The New York Times (13 August 2002).

Lyman, Frances. “Survival Plan for Urban Heat Islands.” MSNBC News. Online: http://www.msnbc.com/ news/791658.asp (14 August 2002).

Stone, Brian, and Michael O. Rodgers. “Urban Form and Thermal Efficiency: How the Design of Cities Influences the Urban Heat Island Effect.” APA Journal, Vol. 67, No. 2. Chicago: American Planning Association (Spring 2002), pp. 186–198.

Texas A&M University, Office of University Relations. “Houston Called ‘Lightening Capital of Texas’.” AggieDaily: Internet: http://rev. tamu.edu/stories/02/071002-12.html (12 December 2002).

Dramatic Benefits

While all the effects and challenges of mitigating the UHI phenomenon may seem insurmountable, research is showing that cooling the air temperature of a city by as little as 5° Fahrenheit can have dramatic benefits. [8] Lower air temperatures could translate into less intensity in the photochemical reactions that create ozone and smog.

Local mitigation efforts can be significant. The Lawrence Berkeley National Laboratory in California estimates that changing the reflexivity of pavements in Los Angeles alone could achieve up to $90 million in energy and smog reduction benefits each year. [9] Successful UHI mitigation strategies can compound in their benefits and can improve urban environments, protect public health, preserve quality of life, and conserve energy resources.

Unchecked, the intensifying effects of the UHI phenomenon could make urbanized areas increasingly unpleasant places. Declining urban environmental conditions and quality of life may essentially encourage sprawl, as citizens try to escape poor air quality and oppressive temperatures. In fact, all the money and other resources that have been or are now dedicated to urban revitalization, sprawl management, greenspace protection, and infill development may come to naught if few people choose to live, work, or recreate in communities plagued by UHI.

By integrating mitigation strategies, communities can be planned to grow in a way that supports and sustains long-term investment, as well as protects citizens and their environment.

Notes

[1] Stone and Rodgers, 189. (See “Works Cited” for complete footnote information for all.)

[2] Lyman.

[3] Bahrampour, 1.

[4] Stone and Rodgers, 188.

[5] Lyman.

[6] Texas A&M University.

[7] Stone and Rodgers, 189.

[8] Lyman.

[9] Lyman.

Maurice G. Estes, Jr., is an environmental planner/program manager for the Universities Space Research Association at the National Space Science and Technology Center, Huntsville, Alabama. Dale A. Quattrochi, Ph.D., is a geographer/senior research scientist at the NASA Marshall Space Flight Center, Huntsville, Alabama. Elizabeth Stasiak is a project manager at the International City/County Management Association, Washington, D.C ([email-masked]).

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