Urban Heat Islands: An Investigation of the Causes, Consequences and Solutions

By Emily Cleare

Abstract

This paper is directed to those people who are interested in the relationship between the urban setting and the urban climate. It serves to explain the urban heat island phenomenon in specific detail. This includes an explanation of their formation, as well as a description of the three different kinds of heat islands and their relationship to the urban atmosphere. Furthermore, with the city of Los Angeles, California^* as a model, the environmental factors associated with heat islands are described in detail. This includes the effects on energy use, environmental pollution and the general health of the city-dwelling population. Finally, this paper discusses potential mitigation strategies to reduce the heat island effect. These strategies are analyzed from an environmental and economic perspective using a hypothetical computer model concerning the city of Los Angeles.

The reader should take away from this paper the knowledge that there is more to heat islands than an increase in temperature. In fact, heat islands initiate a chain reaction that has significant costs both economically and environmentally.

^*Nota Bene: Although this paper uses Los Angeles as a model, many of the topics addressed can be applied to urban heat islands throughout the world. Los Angeles is used because it has been extensively studied. Additionally, it is a good example of the problems associated with an urban heat island, as well as the steps that can be taken to solving these problems.

It may seem obvious to the average person that the inner city feels a lot hotter and more uncomfortable than the countryside. What may not seem obvious is the reasoning behind this temperature rise: the phenomenon known as the urban heat island effect. This urban climate change has some serious consequences concerning the atmosphere and environment. Heat islands affect people’s health as well as the health of the environment. And, although they may seem trivial now, they will continue to expand their influence over regional climates as population growth and subsequent urbanization continues.

With these things in mind, this paper is directed to those people who are interested in the relationship between the urban setting and the urban climate. It serves to explain urban heat islands and their formation in specific detail. This includes an explanation for the ambient temperature increase, as well as a description of the three different kinds of heat islands and their relationship to the urban atmosphere. Furthermore, with the city of Los Angeles, California^* as a model, the environmental factors associated with heat islands are described in detail. This includes the effects on energy use, environmental pollution and the general health of the city-dwelling population. Finally, this paper discusses potential mitigation strategies to reduce the heat island effect. These strategies are analyzed from an environmental and economic perspective using a hypothetical computer model concerning the city of Los Angeles.

^*Nota Bene: Although this paper uses Los Angeles as a model, many of the topics addressed can be applied to urban heat islands throughout the world. Los Angeles is used because it has been extensively studied. Additionally, it is a good example of the problems associated with an urban heat island, as well as the steps that can be taken to solving these problems.

What is an Urban Heat Island?

In the simplest term, a heat island is a metropolitan area that is at a warmer temperature than the surrounding countryside (Wikipedia, 2005). According to the EPA (2005), “on hot summer days, urban air can be 2-10°F (2-6°C) hotter than the surrounding countryside.” This heat island phenomenon is “an example of an unintentional climate modification when urbanization changes the characteristics of the Earth’s surface and temperature” (Voogt, 2004). Not only is city air significantly hotter in the late afternoon, but the temperature of various surfaces also increases (Voogt, 2004). A sketch of a typical heat island is shown in Figure 1 below.

Figure 1:

Source: http://geography.about.com/library/weekly/aa121500a.htm

A typical urban heat island. Notice the increase in temperature towards center city.

What are the Causes of Urban Heat Islands?

The two major causes of urban heat islands are the presence of more dark surfaces and the absence of vegetation (The Heat Island Group, 2005). Dark surfaces contribute to heat islands due to their low “albedo” or reflectivity (Voogt, 2004). According to the Heat Island Group, “albedo is ratio of the amount of light reflected from a material to the amount of light shone on the material.” Dark and dry surfaces, such as pavements and buildings, absorb sunlight. This produces thermal energy, causing the surface to become hotter (The Heat Island Group, 2005). Light and dry surfaces, such as natural ground and forest, have a high albedo (Voogt, 2004). They reflect sunlight and therefore, have a cooler surface temperature (Voogt, 2004). The low overall albedo of the urban fabric is a major cause of the heat island effect. The heat that is stored in dark surfaces causes the overall ambient temperature to rise (EPA 2005). Figure 2 illustrates the albedo values of some common surfaces in the urban environment.

Figure 2:

Source: http://www.ghcc.msfc.nasa.gov/urban/urban_heat_island.html

The albedo of some common city surfaces. Low albedo surfaces absorb sunlight.

High albedo surfaces reflect sunlight.

Notice that the lighter colored surfaces tend to have a higher albedo than darker surfaces, such as the asphalt.

The absence of vegetation contributes to the formation of urban heat islands because it can no longer provide two important cooling mechanisms: shade and evapotranspiration (The Heat Island Group, 2005). Shade cools the air by blocking solar radiation from low albedo surfaces (The Heat Island Group, 2005). This reduces thermal energy and prevents the surface and ambient temperature from greatly increasing (The Urban Heat Island Phenomenon and Potential Mitigation Strategies, 1999). Vegetations’ evapotranspiration cools the atmosphere because as the leaves of vegetation sweat water in their natural processes, they remove heat from the air (The Heat Island Group, 2005). According to the Heat Island Group (2005), “a single mature, properly watered tree with a crown of 30 feet can 'evapotranspire' up to 40 gallons of water in a day, which is like removing all the heat produced in four hours by a small electric space heater.” Figure 3 shows a diagram of a tree and its various functions in environmental processes, including climate regulation.

Figure 3:

Source: http://eetd.lbl.gov/heatisland/Vegetation/Evapotranspiration.html

A tree and its various functions in environmental regulation, including

climate control.

The Urban Atmosphere and Specific Characteristics of Heat Islands

There are three kinds of heat islands that can form due to the factors discussed above: the canopy layer heat island, the boundary layer heat island and the surface layer heat island (Voogt, 2004). The surface layer heat island, on the other hand, refers to the warming of surfaces and is not directly associated with the layers of the urban atmosphere (Voogt, 2004). The canopy and boundary layer heat islands refer to the increasing temperature of the air in urban settings (Voogt, 2004). These heat islands occur in different layers or sections of the urban atmosphere (Figure 4). The canopy layer heat island occurs in the urban canopy layer (Voogt, 2004). This refers to the layer of air closest to the city surface extending up to the average building height (Voogt, 2004). The boundary layer heat island occurs in the urban boundary layer (Voogt, 2004). This atmospheric layer can range in composition from over one kilometer in thickness during the day to 100 meters or less at night (Voogt, 2004).

Figure 4:

Source: http://www.actionbioscience.org/environment/voogt.html

The layers of the urban atmosphere.

These three types of heat islands not only differ in their placement in the urban atmosphere but they also vary in intensity. Intensity is “a measure of the strength or magnitude of the heat island” (Voogt, 2004). For example, when comparing the heat intensity of a canopy layer heat island at different areas in a city, there is a typical “island” shape that forms (Voogt, 2004). In Figure 5, the isotherms (lines of equal temperature) show this island shape, with the warmest air or most intense regions in the downtown areas (Voogt, 2004). The boundary layer heat island and the surface layer heat island do not form this distinct shape (Voogt, 2004).

Figure 5:

Source: http://www.actionbioscience.org/environment/voogt.html

The typical “island” shape of a canopy layer heat island. Notice the increasing

intensity towards the downtown areas.

There is also a temporal feature associated with these heat islands. As Voogt (2004) says, the three different heat islands form and vary in intensity based on different rates of warming and cooling. For example, the canopy layer heat island is most intense at night, from sunset to the predawn hours (Voogt, 2004). It can be anywhere from 1-3°C in intensity, to a record 12°C at optimal conditions (Voogt, 2004). This excessive warming occurs because the buildings in urban areas block the view to the cooler night sky. As a result, warm surfaces are not able to lose heat through radiation (Wikipedia, 2005). The optimal conditions for this excessive warming of the urban canopy layer occur when there are no clouds and the wind speed is below 1.5 m/s (Morris, 2005). By contrast, the surface layer heat island is usually most intense during the day. This is because of direct solar radiation on the surfaces (Voogt, 2004). The boundary layer heat island is mildly intense during both day and night, with no significant temporal feature (Voogt, 2004).

Consequences of Urban Heat Islands: The LA Model

Heat islands have several impacts on the cities in which they occur. They affect the city dwellers as well as the ecosystems within and surrounding the city. As Figure 6 shows, a variety of urban heat islands are spread throughout the world. They all illustrate a significant increase in annual urban temperatures over the last decade (The Heat Island Group, 2005).

Figure 6:

Source: http://eetd.lbl.gov/heatisland/HighTemps/IncreasingTemps.html

The increase in annual urban temperatures over the last decade in several heat

islands throughout the world.

Of these cities, however, Los Angeles, California (LA) represents an excellent model for an urban heat island. The transition from a natural environment to an urbanized environment can be clearly demonstrated. Furthermore, the LA heat island clearly exemplifies the environmental and economical impacts associated with an ambient temperature increase.

In the 1930’s, the area now referred to as Los Angeles was nothing like it is today. It was covered by irrigated orchards with a high temperature of around 97 °F (The Heat Island Group, 2005). However, as urbanization and industrialization began and vegetation and trees were replaced with concrete and metal, the average temperature of Los Angeles increased steadily (Figure 7) (The Heat Island Group, 2005). The average temperature reached 105 °F and higher in the 1990’s (The Heat Island Group, 2005).

Figure 7:

Source: http://eetd.lbl.gov/heatisland/HighTemps/IncreasingTemps.html

The steady increase in LA temperatures as a result of urbanization

and industrialization.

There are several problems associated with this temperature increase in LA. First of all, energy use has been greatly affected. As temperature increases, the demand for air conditioning also increases (The Heat Island Group, 2005). As Figure 8 shows, in LA the demand for electricity increases about 2% for every degree Fahrenheit that the temperature increases (The Heat Island Group, 2005). This increase in energy use results in about 1-1.5 gigawatts of power being used to compensate for the heat island in Los Angeles (The Urban Heat Island Group). In financial terms, that is about 100,000$ per hour and 100 million dollars per year in increased power costs (The Heat Island Group, 2005).

Figure 8:

This is data for Southern CA Edison in 1988

Source: http://eetd.lbl.gov/HeatIsland/LEARN/LAIsland/

The increased electricity use as a result of a steady temperature rise in LA.

The electricity use appears to increase by about 2% for every 1 °F

increase in temperature.

The LA heat island also has a significant impact on air quality (The Heat Island Group, 2005). As energy use increases, so does the emission of heat and greenhouse gases (Voogt, 2004). In addition, a heat island with an intensity of five degrees Fahrenheit greatly increases the rate at which ozone forms from nitrogen oxides and other volatile organic compounds (Rosenfeld et. al., 1997). Figure 9 illustrates the importance of heat in catalyzing this reaction. The higher the temperature is outside, the faster the formation of surface level smog (The Heat Island Group, 2005).

Figure 9:

Source: http://www.epa.gov/heatisland/about/healthenv.html

This diagram illustrates the necessary components for the formation of

surface level ozone or smog. The more heat there is radiating from city surfaces,

the faster this reaction is catalyzed. Therefore, increased temperature means

more smog.

Consequently, the heat island is partially responsible for LA being considered the “smog capital of the United States” (Rosenberg et. al., 1997). At 70°F, the ozone is at an acceptable level within national standards (Rosenfeld et al., 1997). However, as Figure 10 illustrates, at about 90°F the ozone is no longer at an acceptable level (Rosenfeld et al., 1997). In fact, the LA heat island raises the ozone levels by about 10-15 percent (Rosenfeld et al., 1997). For every increase in one degree above 70°F, the amount of smog increases by 3% (The Heat Island Group, 2005).

Figure 10:

Source: http://eetd.lbl.gov/heatisland/AirQuality/

As this graph illustrates, the amount of smog in LA increases with

increasing temperature.

This increasing smog, as a result of the LA heat island, has a significant impact on the general health of the population (Rosenfeld et. al., 1997). Exposure to large amounts of ozone can result in a variety of health problems (EPA, 2005). For example, ozone can irritate the eyes (Rosenfeld et. al., 1997), cause serious lung damage, reduced lung capacity, aggravated asthma, and increased susceptibility to other respiratory illnesses (EPA, 2005). Elderly peoples and children are especially susceptible to these conditions (EPA, 2005). As the EPA (2005) says, “studies have linked hospital admissions and emergency room visits to ground-level ozone exposure.”

Smog also affects the vegetation within cities and in surrounding ecosystems (EPA, 2005). The excess ozone interferes with the growth and food storage of plants (EPA, 2005). Furthermore, the wind carries the ozone to the crops and forests outside of the city, making them more susceptible to disease and other pollutant conditions (EPA, 2005). The visual appeal of vegetation is also affected, taking on a tarnished appearance (EPA, 2005).

The Los Angeles heat island illustrates a variety of concerns associated with the urban heat island effect. Not only is the health of city-dwellers affected, but the increase in energy use has considerable environmental and economic impacts on the city.

Solutions to the Problem: The LA Model

There are a variety of things that can be done to minimize the urban heat island phenomenon. One of the most effective solutions is to increase the amount of vegetation in the urban fabric (The Heat Island Group, 2005). This can be achieved through the proper planting of trees to increase the amount of shade and stimulate evapotranspiration (The Heat Island Group, 2005). A second solution is to increase the albedo of city surfaces (The Heat Island Group, 2005). Making surfaces in the urban fabric more reflective will result in less heat storage and a subsequent decrease in the ambient temperature. This can be done by changing the composition of roofs and pavements to a lighter color. For example, white roofs and concrete colored pavements have been shown to significantly lower the temperature in cities experiencing an urban heat island (The Heat Island Group, 2005).

To measure the effects these kinds of mitigation strategies would have on the LA heat island in particular, several computer simulations have been performed. In this particular model (performed by Rosenfeld et. al., 1997), LA’s albedo was increased by 7.5% and 5% of the surface area was covered with 10 million trees. The results of this simulations indicated that, if these strategies were implemented throughout LA, the heat island could decrease by as much as 5°F (Rosenfeld et. al., 1997). This would cut the need for air conditioning by 18% (Rosenfeld et. al., 1997). As Table 1 indicates, this would save about 175 million dollars per year in air conditioning bills (Rosenfeld et. al., 1997). Furthermore, the levels of smog in Los Angeles would also decline. Table 1 illustrates that 360 million dollars could be saved in smog related health costs (Rosenfeld et. al., 1997). This would result in ozone levels no longer increasing above the national standard in the LA afternoon (Rosenfeld et. al.,1997).

Table 1:

	Direct Energy Savings
	Avoided peak power (MW)	A/C cost savings ($M/yr)
Cooler roofs	400	46
Trees	600	58
Cooler pavement	0	0
Total	1000	104
	Indirect Energy Savings
	Avoided peak power (MW)	A/C cost savings ($M/yr)
Cooler roofs	200	21
Trees	300	35
Cooler pavement	100	15
Total	600	71
	Totals
	Total avoided peak power (MW)	Total cost savings ($M/yr)
Cooler roofs	600	171
Trees	900	273
Cooler pavement	100	91
Total	1600	535
	Smog Benefit
	Avoided medical costs, 12% ozone reduction ($M/yr)
Cooler roofs	104
Trees	180
Cooler pavement	76
Total	360

Source: http://eetd.lbl.gov/HeatIsland/PUBS/PAINTING/

These are the results from the LA computer model. Direct savings refers to individual buildings that are directly affected by these changes (Rosenfeld et. al., 1997). Indirect savings refers to buildings that are not directly affected but experience a decrease in air conditioning use due to a lower ambient temperature (Rosenfeld et. al., 1997).

Through computer simulations such as these, one can see the incredible environmental and economical effects that reducing the heat island would have on urban areas. As Rosenfeld et. al (1997) says, organizations concerned with global warming should not only urge planting trees in the forests but they should also stimulate tree planting programs in cities. Not only would carbon dioxide be removed from the atmosphere, but the temperature and smog of urban areas would also be significantly reduced (Rosenfeld et. al., 1997).

Conclusion

So, cities are hot and rural areas are cool? Not quite. There is much more to the urban heat island phenomenon than a simple temperature swell. The ambient temperature rise drives a chain reaction that affects both the city-dwellers and the ecosystem in and surrounding the city. However, this is not a problem without a solution. As the Los Angeles model illustrates, there are many steps that can be taken to reduce urban heat islands. These solutions are not only economically viable but they are also environmentally friendly and beneficial. However, it is necessary to take action now. The problems associated with urban heat islands will only continue to increase in magnitude as urbanization accelerates with the population growth. Therefore, it is our responsibility to do everything we can to prevent the expansion of urban heat islands, for the sake of the urban environment as well as the people living in it.

References

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Retrieved February 22, 2005 from http://www.earthsci.unimelb.edu.au/~jon/WWW/uhi-melb.html

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http://eetd/lbl/gov/HeatIsland/

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