=20

April 2003 =B7 Volume 85 =B7 Number 3 =

The Urban Heat Island Phenomenon:
How Its Effects Can = Influence=20 Environmental Decision Making in Your Community

Maurice Estes, Jr., Dale Quattrochi, and = Elizabeth=20 Stasiak

Reinvestment in urban centers is breathing new = life into=20 neighborhoods that have been languishing as a result of explosive = suburban=20 development over the past several decades. In communities all over = the=20 United States, adaptive reuse, brownfields redevelopment, = relocation of=20 entertainment venues into downtowns, and other infill initiatives = are=20 transforming urban landscapes, economies, and quality of life. The = way in=20 which this development occurs, however, could exacerbate the = urban heat=20 island (UHI) effect, an existing problem in many areas and one = that=20 poses a threat to the long-term sustainability and environmental = quality=20 of localities.

The UHI phenomenon is rooted in how landcovers respond to solar = heating=20 and how the heat from these surfaces affects the local = environment. This=20 phenomenon is responsible for urban centers=92 having higher air=20 temperatures and poorer air quality than suburban or rural areas. = The UHI=20 effect also forces the development of meteorological events = (increased=20 precipitation), boosts energy demands, poses threats to public = health, and=20 potentially contributes to global warming.

While the name of this phenomenon implies that it is solely an = urban=20 problem, research has shown that the effects of UHI also are = becoming=20 prevalent in suburbs. As suburban areas increasingly develop, = using=20 landcovers and building materials common to urban areas, they are=20 inheriting such urban problems as heat islands. For this reason, = it may=20 become necessary for nonurban communities to engage in heat island = mitigation. The good news is that, through education and planning, = the=20 effects of the UHI phenomenon can be mitigated.

Although this theory has not been not scientifically validated = as yet,=20 heat islands may be viewed more as products of urban design than = of the=20 density of development. [1]=20 Urban sprawl, for example, directly worsens UHI because there is = an=20 increase in built-up surfaces (pavement, buildings) and a = reduction in=20 natural surfaces (forests). Therefore, localities can continue to = grow and=20 develop without aggravating UHI by using sustainable development=20 strategies.

What Is the UHI Phenomenon?

The urban heat island effect results from the way in which = urban=20 landcovers (buildings, pavement) respond to solar radiation, = particular on=20 extremely warm, sunny days in the summer. While all materials = absorb heat=20 from the sun, materials from the built environment, like concrete = and=20 asphalt, absorb higher levels of solar radiation throughout the = day than=20 do natural landcovers.

After sunset, the built environment is no longer subjected to = intense=20 solar radiation. Surfaces that compose the urban landscape = re-radiate this=20 stored heat energy to the lower atmosphere. The total effect of = this=20 thermal energy re-radiation can cause the air temperature in = localities to=20 be elevated by three degrees Fahrenheit or more, in contrast to = rural=20 areas.

Figure 1. Diagram of the urban heat island = effect.

These elevated air temperatures effectively form a dome of = higher air=20 temperatures over a community, trapping air pollutants, degrading = air=20 quality, and preventing the heat waning as it would under natural=20 conditions. Figure 1 illustrates the causation of the UHI = effect.

Conversely, natural landcovers, such as grasses and trees, can = help to=20 mitigate the UHI phenomenon. Natural landcovers absorb heat from = the sun=20 and use it as energy via evapotranspiration, a process by = which=20 vegetation actually helps to cool the air. Few local governments = or highly=20 developed communities, however, maintain enough greenspace to = compensate=20 for all the heat radiated by the built environment. Such natural=20 landcovers as trees can be used to shade materials that, with = direct solar=20 radiation, would absorb and radiate high levels of heat.

Another factor that contributes to the development of UHI is = the=20 reflectivity of surfaces across the urban landscape. Reflectivity = of a=20 surface is known as its albedo. Human-induced surfaces = typical of=20 the city landscape, such as rooftops, asphalt, and concrete, have=20 relatively low reflectivity quotients=97a low albedo=97in = comparison with=20 natural surfaces.

Trees, grass, and other vegetated surfaces, therefore, absorb = sunlight=20 for use in evapotranspiration, as opposed to storing it like = nonnatural=20 surfaces. The water draws heat as it evaporates, cooling the air = in the=20 process. Surfaces ubiquitous to the urban landscape like rooftops = or=20 pavements, however, can be made more reflective through a number = of=20 manufacturing processes, thereby increasing their albedo and = reducing=20 their heat storage capacities.

How Is UHI Detected?

Through the science of remote sensing, information-gathering = devices on=20 satellites and aircraft can collect data on the state and = condition of the=20 land surface. Thermal infrared (TIR) sensors can obtain = quantitative=20 information on the amount of heat that is expressed by different = surfaces=20 across the landscape. These TIR data are critical for assessing = =93what=92s=20 hot and what=92s not=94 across the urban landscape and ultimately, = for=20 determining the magnitude of UHI over an urban area.

Figure 2. Thermal infrared (TIR) aircraft image = of the=20 Atlanta, Georgia, central busines district, showing surface = temperatures=20 across the urban landscape.

Figure 2, for example, shows TIR aircraft data collected during = the day=20 over the central business district (CBD) of Atlanta, Georgia, in = May 1997.=20 The temperature bar at the bottom of the figure depicts the amount = of=20 surface heat energy detected by the TIR sensor on board the = aircraft. The=20 air temperature at the time these data were obtained over Atlanta = was=20 barely 80o Fahrenheit.

As can be seen in the illustration, temperatures for built-up = surfaces=20 across the CBD were in excess of 100o Fahrenheit, with building = rooftops=20 having surface temperatures of almost 120o Fahrenheit. In = contrast,=20 residential areas adjacent to the CBD, where trees and grass were = more=20 extensive, had surface temperatures significantly lower than those = seen in=20 the CBD.

Thus, areas with high surface temperatures, such as those in = the=20 Atlanta CBD, are likely sources of the high surface thermal = radiation that=20 drives the UHI effect and may be viewed as =93hot spots=94 in = contrast to=20 cooler, vegetated areas like forests. TIR remote-sensing data, = therefore,=20 confirm and communicate the differences in heat absorption, = radiation, and=20 albedo between natural and human-made surfaces as they influence = the=20 development of UHI.

What Are the Effects of UHI?

The UHI phenomenon affects the environment and population in a = number=20 of ways, including through the degradation of air quality, threats = to=20 public health, and the triggering of meteorological occurrences. = Urban=20 environmental conditions also could threaten the viability of = local=20 governments, as fewer people will seek or choose to be in built-up = places=20 or downtowns for residential, business, or entertainment purposes. = These=20 shifts in lifestyle could reverse the current trend of urban = in-migration=20 and could cause an urban exodus similar to that of the mid-1900s, = which=20 left many traditional urban centers languishing for years and from = which=20 many American communities still are trying to recover.

Air quality. UHI may be a significant contributor to = elevated=20 ozone levels by adding excess background thermal radiation to the = overall=20 chemical reactions that form ground-level ozone (O3). Sunlight and = elevated heat levels can photochemically =93cook =93ozone to far = more=20 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=20 areas, ozone is formed in the presence of calm wind conditions, = intense=20 sunlight, and high air temperatures and as a result of the = chemical=20 interactions of two compounds: volatile organic compounds (VOCs) = and=20 nitrogen oxides (NOx).

VOCs come from a variety of nonpoint-source contributors, such = as paint=20 cans, gasoline cans, or even biological sources (vegetation). = Contributors=20 of NOx are primarily associated with point-source contributors = like=20 smokestacks, automobile exhaust systems, and other origins of=20 emissions.

The UHI effect, which under the right conditions can drive up = air=20 temperatures by 10o Fahrenheit or more, acts as an additive = background=20 effect to solar radiation and may contribute to the formation of = ozone. If=20 UHI can be mitigated, scientists hope that significant reductions = in ozone=20 levels can result. More important, a greater knowledge of the = relationship=20 between UHI and ozone levels can have significant budgetary = impacts on=20 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=20 will not meet the new air-quality standards for ground-level ozone = and=20 will be considered in a condition of nonattainment. This = designation=20 carries serious penalties to metropolitan areas. One of the most = severe of=20 these is the risk of losing funding for new highway development if = plans=20 are not made and enacted to reduce ozone levels to meet EPA = standards.

A state containing metropolitan areas not in attainment of EPA = ozone=20 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=20 uncomfortable but also can push temperatures from hot to = dangerously hot,=20 threatening public health. Poor air quality and high temperatures = will be=20 particularly harmful to children, the elderly, and those with = chronic and=20 respiratory illnesses. But even for those who are healthy, poor = air=20 quality and excessive heat can threaten good health and physical=20 condition.

On the average, 1,500 American city dwellers die each year = because of=20 the heat, which is more deaths than those from all other natural = disasters=20 combined. [3]=20 The compounding effect of heat waves and UHI could prove to be = even=20 deadlier to urban populations.

Global warming. The UHI phenomenon potentially = contributes to=20 global warming. The hotter the air temperatures, the higher the = demand for=20 electricity to generate air conditioning, which further boosts the = sulfur,=20 nitrogen, and particulate matter in the air. The vast quantity of=20 greenhouse gases emitted as a result of excess energy production = further=20 contributes to larger-scale climatic effects through the process = of global=20 warming. [4]

Meteorological effects. As UHI modifies urban climates, = its=20 effects change local meteorology. The UHI phenomenon is a proven = mechanism=20 that forces the development of precipitation events either over, = or=20 downwind of, communities. Furthermore, naturally occurring storms = often=20 intensify as they pass through cities with a UHI; moderate = rainstorms may=20 turn into full-blown thunder and lightning storms. Houston, for = example,=20 has realized a 40 percent increase in lightning strikes. [5]=20 Researchers have found that this lightning frequency is not = seasonal but=20 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=20 the effects of heat islands, [7]=20 localities are recognizing the need to address the urban heat = island. In=20 Chicago, for one, several municipal buildings have been designed = to=20 accommodate vegetated rooftops.

In addition, a number of cities of all sizes and geographic = locations=20 specifically mention mitigation of the UHI phenomenon or climatic = relief=20 as a rationale for enacting certain zoning ordinances, like those = relating=20 to landscaping standards, tree preservation, and parking lots. = Perhaps the=20 most progressive program is an ongoing effort in the Atlanta = metropolitan=20 area that seeks to observe, measure, model, and analyze how the = rapid=20 growth of greater Atlanta since the early 1970s has affected the = region=92s=20 climate and air quality.

The National Aeronautics and Space Administration (NASA) is = using its=20 technological capabilities to assist Atlanta in mitigating the UHI = phenomenon through an endeavor called Project ATLANTA (ATlanta = Land-use=20 ANalysis: Temperature and Air-quality). Project ATLANTA began as a = scientific research effort focused on how urban growth in the = Atlanta=20 metropolitan area over approximately the past 25 years has = affected the=20 region=92s meteorology and air quality.

Integral to Project ATLANTA=92s original research tasks has = been the=20 measurement and mapping of the extent of the area=92s UHI (using = TIR=20 remote-sensing data), in relation to the distribution and = composition of=20 various urban surface types common to Atlanta=92s urban landscape. = Soon=20 after the start of Project ATLANTA, however, it became obvious = that a=20 fundamental driver of the scientific research for the = investigation was=20 the goal of gaining accurate information on the characteristics = and=20 effects of the metropolitan area=92s UHI so that urban planners, = government=20 officials, and the like could use this information to make sound, = rational=20 decisions on the future of Atlanta=92s overall environment.

As a consequence, Project ATLANTA has been developing some = information=20 products for use by decisionmakers and the general public to = evaluate=20 possible strategies for mitigating the UHI effect. These have = included: 1)=20 TIR remote-sensing data in the form of thermal maps like the one = shown in=20 Figure 1, that depict the distribution of surface temperatures = over=20 Atlanta as derived from aircraft and satellite data; 2) landcover=20 classification maps (also derived from aircraft and satellite=20 remote-sensing data) that present the distribution of landcovers=20 (vegetation, high-density urban development, pavement) across the = Atlanta=20 urban landscape; and 3) an =93urban fabric analysis=94 (again = using=20 remote-sensing data) that could quantify both the surface heating=20 differences among urban landcovers and the distribution of thermal = hot=20 spots across the landscape.

What Can Be Done?

Communities can mitigate or prevent the UHI phenomenon by = employing=20 local planning initiatives and promoting sustainable development. = Some=20 common planning practices like the integration of greenspace can = actually=20 be crafted and bolstered to become community-wide strategies for=20 mitigating UHI. Strategies can be employed to retrofit currently = developed=20 sites (through any renovation or redevelopment process) and = integrated=20 into site plans for new construction.

Some mitigation strategies include:

Increasing greenspace and tree planting.=20
Increasing the reflectivity (albedo) of urban surfaces by = installing=20 highly reflective roofing materials or by =93lightening up=94 = the color of=20 pavements with a number of commercially available products and = methods.=20
Using =93green=94 building materials to improve the albedo = of these=20 surfaces and also raise energy efficiency.=20
Employing green building designs, such as those that include = vegetated rooftops. A number of local governments encourage = green=20 building development in their communities through programs that = offer=20 such incentives as density and height bonuses.=20
Designing site plans that minimize or eliminate expanses of = exposed,=20 paved surfaces. For example, instead of asphalt parking lots, = consider=20 underground parking.=20
Promoting the use of mass transit and reducing reliance on = the=20 automobile, as auto emissions can worsen UHI and its effects.=20
Decreasing expanses of impervious surface, which will reduce = runoff=20 in general and mitigate pollutant runoff in general into = waterways.=20
Installing =93porous paving=94 that filters more rainwater = into paved=20 surfaces, keeping these surfaces cooler and also helping to = lessen water=20 runoff.

Works Cited

Bahrampour, Tara. =93Most Deadly of the Natural Disasters: The = Heat=20 Wave.=94 The New York Times (13 August 2002).

Lyman, Frances. =93Survival Plan for Urban Heat Islands.=94 = MSNBC News.=20 Online: http://www.msnbc.com/ news/791658.asp (14 August = 2002).

Stone, Brian, and Michael O. Rodgers. =93Urban Form and Thermal = Efficiency: How the Design of Cities Influences the Urban Heat = Island=20 Effect.=94 APA Journal, Vol. 67, No. 2. Chicago: = American=20 Planning Association (Spring 2002), pp. 186=96198.

Texas A&M University, Office of University Relations. = =93Houston=20 Called =91Lightening Capital of Texas=92.=94 AggieDaily: = Internet:=20 http://rev. tamu.edu/stories/02/071002-12.html (12 December=20 2002).

Dramatic Benefits

While all the effects and challenges of mitigating the UHI = phenomenon=20 may seem insurmountable, research is showing that cooling the air=20 temperature of a city by as little as 5=B0 Fahrenheit can have = dramatic=20 benefits. [8]=20 Lower air temperatures could translate into less intensity in the=20 photochemical reactions that create ozone and smog.

Local mitigation efforts can be significant. The Lawrence = Berkeley=20 National Laboratory in California estimates that changing the = reflexivity=20 of pavements in Los Angeles alone could achieve up to $90 million = in=20 energy and smog reduction benefits each year. [9]=20 Successful UHI mitigation strategies can compound in their = benefits and=20 can improve urban environments, protect public health, preserve = quality of=20 life, and conserve energy resources.

Unchecked, the intensifying effects of the UHI phenomenon could = make=20 urbanized areas increasingly unpleasant places. Declining urban=20 environmental conditions and quality of life may essentially = encourage=20 sprawl, as citizens try to escape poor air quality and oppressive=20 temperatures. In fact, all the money and other resources that have = been or=20 are now dedicated to urban revitalization, sprawl management, = greenspace=20 protection, and infill development may come to naught if few = people choose=20 to live, work, or recreate in communities plagued by UHI.

By integrating mitigation strategies, communities can be = planned to=20 grow in a way that supports and sustains long-term investment, as = well as=20 protects citizens and their environment.

Notes

[1] Stone and Rodgers, 189. (See =93Works = Cited=94 for=20 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=20 planner/program manager for the Universities Space Research = Association at=20 the National Space Science and Technology Center, Huntsville, = Alabama.=20 Dale A. Quattrochi, Ph.D., is a geographer/senior research = scientist at=20 the NASA Marshall Space Flight Center, Huntsville, Alabama. = Elizabeth=20 Stasiak is a project manager at the International City/County = Management=20 Association, Washington, D.C (estasiak@icma.org).

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=A9 2003 International City/County Management = Association.=20 Please notify us if you = experience=20 any problems.

The Urban Heat Island Phenomenon:How Its Effects Can = Influence=20 Environmental Decision Making in Your Community