Solving Environmental Problems through Engineering

Background

There are two main types of urban heat islands: surface and atmospheric. A surface urban heat island refers to the difference in temperatures that is measured from dark areas, such as pavement and roofing, or right above them. An atmospheric urban heat island relates to measuring the ambient air in and around the tree canopy and above.⁸ For the purposes and teachings of my unit, I will solely focus on the characteristics and mitigation strategies for surface urban heat islands as there are more practical and grade appropriate activities for my students to engage in.

In addition to added impervious surfaces replacing naturally green areas, there are other qualities of cities that contribute to the Urban Heat Island effect such as the color of impervious surface materials, the shape of cities, and unique weather and geographic implications.⁹

Darker surfaces

Roofing, pavement, and parking lots tend to have lower solar reflectance values than lighter surfaces. Therefore, these surfaces absorb more radiation from the sun and heat the areas around them more through convection. Radiation from the sun is felt by city inhabitants in various forms: long-wave/infrared radiation, latent heat from evapotranspiration (evaporation + transpiration), heat that we feel as traditional temperature, and anthropogenic heat, referring to any excess heat created by human processes. This can include heat discharged by running AC window units during the day, driving a car back and forth to work or cooking.

Darker surfaces contain specific characteristics that makeup how much they contribute to the extra heating of their surrounding areas:

Albedo: an object’s ability to reflect solar radiation back into the atmosphere and fully to outer space, thus not contributing to the greenhouse gas effect and warming the surrounding areas and Earth as a whole. Solar reflectance is correlated with the color of its material, and 43% of the sun’s energy is attributed to visible wavelengths. As the sun’s energy increases, darker materials with lower wavelengths are less effective in reflecting heat.

Thermal emissivity: a material’s ability to shed heat away from the object. Most building materials, such as those used to produce cool roofing, have high thermal emittance values to keep their structures cool.

Heat Capacity: an object’s ability to store heat. Objects with low heat capacities that are able to displace heat effectively such as soil and sand, are found in rural areas more often. Higher heat capacities are found in materials such as steel or stone, which are more abundant in urbanized areas.

The Shape of Urban Spaces

The dimensions and spacing of various buildings and structures encompassed in the built up, urban, area can contribute to excessive warming as well. Obstructions, such as tall buildings and skyscrapers, can easily impact a building’s overall albedo. The effects of this can be felt through excess temperatures during the afternoon, as well as in the evening to night time. When there are copious amounts of skyscrapers or tall buildings on narrow streets, this is known known as an urban canyon.

Increased Energy Consumption associated with heat island effect

As our cities are getting hotter and temperatures continue to rise a result of the Urban Heat Island effect, there is an increase in overall energy consumption in communities within these localities. There is an increased demand for cooling in all settings, whether it be commercial or residential. As cooling increases, there is also an added demand on local electricity grids. During the later afternoon and into the evening, electricity grids are often working at full demand to cool homes, power lights, and run household appliances. At this peak, for every 1°F increase the electrical urban electrical demand increases between 1.5 to 2%. Adding additional surge puts an excessive strain on the overall capacity of the system. In worst cases, this can lead to brownouts and blackouts across cities, leaving residents in temporary health crises.

Elevated air pollutants and Greenhouse Gases

Higher temperatures and increased energy used across electricity grids only begin to scratch the surface of problems that the Urban Heat Island effect can cause localities. Pollutants that can be harmful to overall human health are produced as fossil fuels, which are combusted. Currently, fossil fuel combustion accounts for most electricity production in the United States. Fossil fueled power plants produce CO₂, the leading greenhouse gas which contributes to climate change. In addition, many other pollutants are produced in power plants including Mercury (Hg), Carbon Monoxide (CO), Sulfur Dioxide (SO₂), Particulate Matter 2.5 (PM), and Nitrogen Oxide (NOx). When Nitrogen Oxide reacts with sunlight, it yields the ground level formation of Volatile Organic Compounds (VOCs) such as Ozone, another greenhouse gas contributing significantly to climate change.

Compromised human health and comfort

Certain populations, especially those primarily made up of people living in low-income areas, can be made uncomfortable and even develop serious life-threatening conditions due to elevated temperatures. Factors such as higher daytime surface temperatures, higher evening temperatures without the reprieve of cooling, and added air pollution with dangerous pollutants greatly exacerbate this issue. This can lead to difficulty breathing, heat exhaustion, non-fatal heat strokes, and ultimately heat-related deaths. As heat waves roll across the country, which have become more regular as a result of climate change, the Urban Heat Island effect perpetuates these symptoms and can cause more sickness and deaths overall.

Water Quality of Surrounding Ecosystems

Rooftop and pavement surfaces are up to 50°F hotter as a result of surface urban heat islands, will residually heat excess runoff water and storm water as it moves through a collection system and into a body of water such as creek or stream that flows into a lake or river. This excess water can be heated on average up to 4°F more than normal. These rapid temperature changes can affect species’ metabolic rates and ability to reproduce.

Strategies for Building Heat-Resilient Communities

To limit these effects, there are research-based strategies that city residents should consider when it comes to their homes and land they own. On a larger scale, local governments should also take into account the following strategies with any urban planning. I will go into detail on research-based strategies examining the use of trees and vegetation, green roofing, cool roofing, cool pavements, and local community and policy efforts.

Trees and Vegetation

Strategic placement of trees, vines, and other plants in urban areas around various structures can have significant benefits overall in mitigating the aforementioned adverse outcomes due to the Urban Heat Island effect.¹⁰  There are two specific processes that support the use of trees and vegetation in this way.

Shading

When trees are placed near areas that radiate heat, such as roads, buildings, and parking lots, up to 90% of the sun’s energy is captured by the leaves of the tree before it can reach the ground below, and subsequently heat the surface. The light is both absorbed by leaves for photosynthesis, and reflected back into the atmosphere. This creates a more suitable temperature for people or materials below the tree.

Evapotranspiration

Evaporation + transpiration. This cools the air by using heat from the air to evaoprate water. This is essentially how a plant “sweats”.

Transpiration: as precipitation makes its way into soil and nutrients, the roots from trees and vegetation will absorb water, move the water through the stem, and eventually emit the water through their leaves. Evaporation: the chemical change of state in water from a liquid to a gas. The evaporation occurs in water that has been transpired from the plant, and from rainfall around  trees as it collects water. As water evaporates, the surrounding air cools. Rural areas with natural landscapes that are able to retain more moisture have more evapotranspiration than urban areas. As a result, impervious infrastructure reaches very high temperatures as heat is unable to be transferred. This leads to higher air temperatures.

Shading and evapotranspiration in conjunction can help address high temperatures during the day in the summer (see Figure 1). This can reduce temperatures up to 9°F of tree covered areas versus open terrain. Suburban living areas with trees can reduce the temperatures up to 6°F when compared to similar areas without trees. Open sports fields that are lined with trees can reduce bordering areas up to 4°F.

Figure 1: Demonstration of Evapotranspiration in a plant¹¹

Green Roofing

In most cities, rooftops account for about 20-25% of total land cover. Similar to how shading and evapotranspiration help cool ground layer surface temperatures, trees and vegetation can reduce temperatures when planned or retrofitted on buildings and homes. Conventional rooftop surfaces can exceed ambient air temperatures by up to 90°F. Growing a vegetative layer on a rooftop will mitigate these temperatures, and can reduce associated energy costs.¹²

The sun’s energy will be limited as specially engineered soils and plants on top of roofing blocks a portion of the sunlight that reaches the roof membrane. Through photosynthesis of leaves, heat and light energy is reflected back into the atmosphere. As a result, less heat is transmitted onto buildings that would excessively heat its surroundings, effectively keeping the area cooler. In addition, buildings are protected from ultraviolet radiation. There are two types of green roofing that can help buildings and homes stay cool, and use less energy.

Extensive Green Roofing

A low profile design system containing sedum plants, such as succulent and hardy plants that would tend to be suitable for an alpine environment (Figure 2). With a simple rugged design, little maintenance and upkeep is required over time. This type is perfect for retrofitting roofs, as little to no added structural support is required to house the added system. This system can be applied to diverse existing roof designs, such as roofs that feature a slope.

Figure 2: Virginia Commonwealth University Arts Pollak Building Rooftop Garden, Richmond, VA.¹³

Intensive Green Roofing

A high profile design considered to mirror a conventional garden or park, where people can utilize the space. These systems include a more diverse grouping of plants, such as small to medium size trees and shrubs. Intensive roofs will require more maintenance over time to upkeep the vegetation. Intensive roofs are more beneficial when they are planned into the original construction of a building. Larger vegetation and public use will require more structural support, which does not lend itself to being retrofitted onto an existing building. This will also often require a built in irrigation system.

Both intensive and extensive green roofing systems have a wide-range of benefits, while the negatives are mostly cost related. Vegetative roofing allows the building to reduce its overall energy consumption. When green roofs are wet following a precipitation event or routine irrigation, they’re able to store large amounts of heat due to evaporative cooling. Buildings are also cooled and protected when vegetative roofing layers are dry, as they act as an insulating barrier.

Cool Roofing

In the United States, traditional roofing can reach peak temperatures up to 185°F, which severely heats surface temperatures of suburban roofing and creates warmer ambient air temperatures in neighborhoods. Cool roofing materials, which have emerged in the last 30 years, can keep surfaces between 50-60°F cooler than traditional roofing. Cool roofing products are able to reflect and emit solar energy more effectively, thus keeping the surrounding areas cooler.¹⁴

Solar energy that reaches the Earth’s surface and roofing is comprised of 5% Ultraviolet rays, 43% visible light, and 52% infrared light. Ultraviolet rays are what we associate with getting a sunburn. Visible light solar energy accounts for colors from violet to red. Infrared light is felt as traditional heat to people. How do we keep all of these different types of damaging heat from penetrating our roofs? Simply, energy must be redirected. In this case, all three types of solar energies will be reflected away from Earth’s surface.

The solar reflectance of a surface material, also known as albedo, is the percentage of solar energy that is able to be reflected away from the surface. Solar reflectance is the most important property in determining how a material contributes to urban heat islands. The higher the percentage of albedo, the more effective cool roofing is able to be and keep ambient air temperatures lower. Scientists have devised many ways of calculating a material’s albedo by using specially designed equipment such as spectrophotometers, solar reflectometers, or pyranometers in both laboratories and in the field. Traditional roofing is able to reflect only about 5-15% of solar energy, while newer cool roofing materials are able to reflect up to 65%.

Another part of the equation in determining how materials, specifically roofing, contribute to urban heat islands is its thermal emittance. This term refers to a material’s ability to release any radiated heat that is absorbed back into the environment and atmosphere. Any surface exposed to radiated energy from the sun will get hotter until reaching thermal equilibrium, when a material releases as much heat as it absorbs. A surface with a higher, more efficient thermal emittance value reaches an equilibrium level at a lower temperature, thus releasing less heat into the surrounding environment. Below (Figure 3) is an image illustrating the combined effects of both solar reflectance and thermal emittance on roofing materials as it relates to their overall temperature. On a hot, sunny summer day, a black roof that reflects 5% of the sun’s energy and emits more than 90 percent of the heat it absorbs can reach 180°F (82°C). A metal roof will reflect the majority of the sun’s energy while releasing about a fourth of the heat that it absorbs and can warm to 160°F (71°C). A cool roof will reflect and emit the majority of the sun’s energy and reach a peak temperature of 120°F (49°C), see Figure 3.

Figure 3: Example of Combined Effects of Solar Reflectance and Thermal Emittance on Roof Surface Temperature¹⁵

The ASTM (the American Society for Testing and Materials) has devised a calculation to determine the solar reflectance index (SRI), a value that represents a material’s temperature in the sun.¹⁶ The calculation takes in to account both solar reflectance and thermal emittance of a material. The SRI is calculated as follows:

 Tblack= Temperature of a completely black surface

Tsurface: Temperature of the surface being tested

Twhite: Temperature of a completely white surface

This calculation compares how hot a material would get if you placed a completely black and a completely white one next to the material being tested, under radiant heat. The final value will be calculated as a percentage between 0 and 1. The higher the number, the higher the overall SRI, and more efficient it is in mitigating its contribution to urban heat islands.

Low-sloped commercial cool roofing

An essentially flat roof, with a minimal amount of slope to provide drainage. These roofs are defined as having more than 2 inches of vertical rise, over 12 inches of vertical run (a ratio of 2:12). These are often found on larger buildings such as commercial, industrial, warehouse, office, and multifamily housing.

As a heat mitigation strategy, large low-sloped roofs generally use two approaches: thick coatings or single-ply membranes. Roof coatings mimic the thickness of paint, with a few additives to make the layer more durable for a variety of temperatures and weather events. There are two types of coatings that are generally used: cementations and elastomeric.

Coatings

Cementitious coating contains particles of cement that are partially pervious, while relying on the underlying roof materials for water displacement and proofing. Elastomeric coatings use polymers for adhesion thus providing a waterproof membrane. Both cementitious and elastomeric coating have at least a 65% solar reflectance and 80-90% thermal emittance to keep the surrounding areas as cool as possible.

Single-ply membranes

This type of low-sloped cool roofing approach initially costs more, and is used when a roof is deemed as needing to be fully repaired for a variety of reasons. The system implores pre-fabricated sheets made with a cool surface and are placed together, connected, and fastened down to create a low sloping roof.

Steep-sloped residential roofing

While most cool roofing is focused on mitigating effects from low-sloped roofing, steep-sloped cool roofing options are becoming more readily available in the last 10 years. Asphalt tiles are most commonly used. Other materials that are used include metal roofing, tiles, and shakes. Generally, low-sloped roofing solar reflectance outcomes are more effective than steep-sloped, as they account for more area and conserve more energy proportionally. Traditional housing tiles only have about 10-30% of solar reflectance. Cool colored metal roofing has a wide variation of solar reflectance as different colors have quite different values from 20-90%. Regular asphalt tiles that are used on single family homes have 25-65% solar reflectance.

Cool Pavements

When compared with conventional paved surfaces, cool pavement solutions tend to store less heat and have lower surface temperatures than conventional pavement. Conventional pavements in the United States include impervious concrete and asphalt, which can reach summer temperatures of 120-150°F.¹⁷Conventional pavements have two detrimental contributing factors to the formation of urban heat islands: (1) Heat that is trapped below the subsurface of pavements, and re-released during the nighttime; and (2) Heated storm water runoff into local waterways impairs water quality.

Cool pavements resemble the process in which cool roofing uses to keep surrounding areas cooler. Understanding how solar energy is transmitted to Earth, solar reflectance, and thermal emittance are key to develop knowledge of cool pavements. For the purpose of not being repetitive, I will not reiterate these concepts again as they relate specifically to cool pavements.

Similar to cool roofing characteristics, reflective (increased solar reflectance) pavements tend to be the most widely used cool pavement. Conventional concrete typically has a high solar reflectance, but as time goes on and it is used more, the surface becomes dirty and is able to reflect less energy. Darker materials with lower albedo also leads to higher temperatures and the formation of Volatile Organic Compounds (VOCs) as air pollution. In general, the type of pavement used is based on the function it will serve such as roads, parking, sidewalks, etc. There is extensive research on new types of cool pavements by modeling parking lots, as all types can be used due to low impact nature and ability to maintain these spaces.

There are mixed results in cooling performance for permeable pavements. Originally developed for storm water management, permeable pavement allows air, water, and water vapor into the voids of the pavement. Various permeable pavements are being implemented in cities, when suitable for various structural requirements. These options include porous asphalt, pervious concrete, and grid pavements.

Similar to how evapotranspiration cools ambient air with trees, vegetation, and green roofing, so does permeable pavement when wet through evaporative cooling. Water passes down through openings into the soil and supporting material sub-surface. As the surface heats, water is drawn out of the pavement, thus drawing heat out as well and the ambient air. Although, when permeable pavement is dry, the effect is not nearly as efficient as temperatures of dry permeable pavements are on average about 9°F hotter.¹⁸

Local efforts and policy

From governments and citizens, to the scope of large corporations, many stakeholders can benefit from heat island mitigation across the United States. While heat island mitigation is typically focused on increasing sustainability and energy consumption, local communities can get involved at both a voluntary level and a policy level.¹⁹ The Environmental Protection Agency (EPA) lists many practices and initiatives for communities to engage in to help improve outcomes for their own communities.

Policy

Some state and local governments adopt and include urban heat island mitigation strategies in their own regulations and policies. This both provides wider access and incentives for implementing strategies.

Segregation and Heat Vulnerability

In 1933, the midst of the Great Depression in the United States, the federal government found themselves up against a national housing shortage. As a way to stabilize and encourage home ownership during this tumultuous financial time for families, the Home Owners’ Loan Corporation (HOLC) was established as a program under the New Deal. This program was instrumental because long-term federally backed mortgages were an option for homebuyers for the first time. According to Richard Rothstein, author of the book The Color of Law, this program was designed explicitly to not only increase, but segregate, the availability of equitable housing in America.

To further segregate housing in America, the HOLC examined and mapped cities across the country designating which neighborhoods were considered safe or risky to grant loans. Private lenders were provided with this information to increase the amount of white middle and lower class families moving into newer suburban areas. As African Americans were denied time and again the opportunity live in these areas, they were systemically pushed into urban housing projects in separate parts of their communities (Figure 4).

Figure 4: Geocorrectified image of the Residential Security “Redlined” map²⁸

In 1934, the Federal Housing Administration furthered segregation efforts by refusing to insure mortgages in and around these newly designated African American neighborhoods. While the Federal Housing Administration was subsidizing builders to produce suburban neighborhoods for whites only, the private security companies began quantitatively grading residential areas according to their lending risk. This discriminatory practice is known as redlining. The lending risk factors were assessed by the quality of the housing, race, and ethnicity. Neighborhoods were graded from A – D, an A rating consisting of the green lined, safest, whitest, high quality neighborhoods,  and  a D rating made up of risky, redlined, people of color, with lesser quality of homes.²⁹"The segregation of our metropolitan areas today leads to stagnant inequality, because families are much less able to be upwardly mobile when they're living in segregated neighborhoods where opportunity is absent. If we want greater equality in this society, if we want a lowering of the hostility between police and young African-American men, we need to take steps to desegregate.”³⁰

While there have been lasting financial and social equity implications for African Americans across the United States as a result of these redlining practices, there have also been implications for the environmental quality in these neighborhoods. It is no coincidence that the historically redlined neighborhoods lack access to green spaces, green roofing, cool roofing, and cool pavements to mitigate excessive heat in their communities. Today, excessive heat disproportionally affects people who can’t afford to cool their homes and live in predominantly low income areas as a result of government instituted segregation. 74% of the neighborhoods that the HOLC deemed as hazardous over 80 years ago are still considered to be low to moderate income today. In addition, 64% of neighborhoods that were deemed hazardous are home to minorities.³¹

The bar graph below (Figure 5) represents the overall environmental risk in correlation with original HOLC neighborhood grades. As the neighborhood grade decreases, the average land surface temperature increases, the tree canopy cover decreases, and the amount of impervious surfaces increase as well. As it relates to the Urban Heat Island effect, these three factors are the main determinants in how much hotter these areas will get. There is a significant relationship between the practice of redlining, community segregation, and overall environmental risk in many cities today, including Richmond, Virginia. A neighborhood that was rated “D” over 80 years ago, will be on average 4°F hotter than a neighborhood designated “A.” This leaves certain neighborhoods and communities more intensely impacted by urban heat and more vulnerable to excessive heat events as a result.

Figure 5: HOLC Neighborhood Grades in Richmond, Virginia³²

Calculating Heat Vulnerability