00 uhi report

POLITECNICO DI MILANO – Ingegneria dei Sistemi Edilizi PhD Program Sistemi Edilizi e compatibilita ambientale

URBAN HEAT ISLAND PHENOMENON (UHI) GENERATION | MITIGATION Individual Report Presented to: Prof. GATTONI Presented by: Amr ELESAWY (750084) This report provides an overview of the definition and description of the Urban Heat Island phenomenon, its causes, impacts, and factors that contribute to mitigating its effect.

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URBAN HEAT ISLAND PHENOMENON – Individual Report

750084 | Amr Elesawy 1

TABLE OF CONTENTS Introduction ........................................................................................................... 2 Report Objectives ................................................................................................... 2 URBAN HEAT ISLAND PHENOMENON .................................................................. 3 1. Definition | Description ..................................................................................... 3 -‐ Surface UHI: ........................................................................................................ 3 -‐ Atmospheric UHI: ................................................................................................ 4 2. Measurment ....................................................................................................... 5 3. Causes ................................................................................................................ 6 -‐ Buildings ............................................................................................................. 6 -‐ Properties of Surface Materials ............................................................................ 6 -‐ Reduction of Vegetation in Urban Areas ................................................................ 6 -‐ Anthropogenic heat ............................................................................................. 7 4. UHI Impact ......................................................................................................... 7 5. UHI Mitigation .................................................................................................... 8 -‐ Trees, vegetation, and green roofs ....................................................................... 8 -‐ Cool roofs ........................................................................................................... 9 -‐ Cool pavements .................................................................................................. 9 COOL ROOFS ........................................................................................................ 10 Properties of Cool Roofs ...................................................................................... 11 -‐ Solar Reflectance ............................................................................................... 11 -‐ Thermal Emittance ............................................................................................ 11 -‐ Temperature Effects .......................................................................................... 11 Cool Roof Types ................................................................................................... 12 -‐ Material ............................................................................................................ 12 -‐ Geometry ......................................................................................................... 13 Cool Roofs Contribution Conflict .......................................................................... 13 UHI and ENERGY BALANCE .................................................................................. 14 Energy Balance in the Urban Atmospheric layer ................................................. 14 Energy Balance in the Urban Surface layer .......................................................... 14 Storage and anthropogenic heat fluxes in the urban energy balance ................. 14 CONCLUSION ....................................................................................................... 16 BIBLIOGRAPHY ..................................................................................................... 17

LIST OF TABLES Tab. 1 Basic Characteristics of Surface and Atmospheric UHIs .............................. 4

LIST OF FIGURES Fig. 1 Thermal Image Depicting Surface UHI in Atlanta (Georgia), on September

28th, 2000. ..................................................................................................... 5 Fig. 2 Scheme showing the difference in behavior between Cool and Hot Roofs.

..................................................................................................................... 10



In t roduct ion In the modern age of urban climatology, much emphasis has been placed on observing and documenting heat island magnitudes in cities around the world. The first scientific observations were documented on this phenomena were by Luke Howard in 1833. His temperature analysis in and around London, England, have shown a city distinctly warmer than its countryside. These studies and their estimates of UHI magnitude are unrivalled (incomparable) in their contributions to urban climatology. Although the size of literature about this phenomenon is reasonable enough, recently, scholars have been questioning the authenticity with which heat island observations have been gathered and reported through history. To what extent does this literature serve the aims of science? Can its measurements be trusted? So far, the response to these questions is not obvious. Modern heat island investigators such as Parry (1956), Chandler (1962, 1970) and Bohm and Gabl (1978), for example, alluded to problems of methodology decades ago. In recent years, discussion around these same problems has been open and direct.1 Report Ob jec t ives This report provides an overview of different types of urban heat islands, their causes, impacts, and factors that contribute to mitigating their effect. The report will discuss:

-‐ Definition and a brief description of the phenomenon, and its types (Surface and Atmospheric);

-‐ Causes of urban heat island formation; -‐ Urban heat island impacts on energy consumption, environmental

quality, and human health; -‐ Methods of mitigating the negative impacts of the phenomenon,

focusing on the Cool Roofs as a possible solution; -‐ Discussing the relation between the energy balance and the UHI

Phenomenon. -‐ Highlighting the doubts and conflicts in the authenticity and correctness

of the information regarding the phenomenon, and how grave the impact of it on global warming.

1 Stewart, I. D., “A systematic review and scientific critique of methodology in modern urban heat island literature,” International Journal of Climatology, published online 15 Apr. 2010. DOI: 10.1002/joc.2141



URBAN HEAT ISLAND PHENOMENON 1 . Def in i t ion | Descr ip t ion In the urban development, a process of natural landscape replacement takes place. Buildings, roads, and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist generally become impermeable and dry. This development leads to the formation of urban heat islands; in other words, Urban Heat Island (UHI) is the phenomenon whereby urban regions experience warmer temperatures than their rural surroundings. The annual mean air temperature of a city with one million or more people can be (1 to 3°C) warmer than its surroundings, and on a clear, calm night, this temperature difference can be as much as (12°C). Even smaller cities and towns will produce heat islands, though the effect often decreases as city size decreases.2 With the increase of population, the urban areas tend to modify a greater and greater area of land and have a corresponding increase in the average temperature. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. Seasonally, UHI is seen during both summer and winter. The main causes of the urban heat island are:

1) The modification of the land surface by urban development which uses materials that effectively retain heat.

2) Waste heat generated by energy usage (e.g. Heating and cooling equipments in buildings…etc.) is a secondary contributor.3

Understanding urban heat island (UHI) contamination in the situ climate record is a complex task because the results are impacted by a wide variety of factors not related to urbanization. Two of the distinctive ways of forming HUI are the Surface UHI and the Atmospheric UHI. 4 These two heat island types differ in:

§ Their Formation; § The techniques used to identify and measure them; § Their impacts, and; § The methods available to moderate them.

-‐ Surface UHI:

On a hot, sunny summer day, the sun can heat and dry exposed urban surfaces, like roofs and pavement, to temperatures (27 to 50°C) hotter than the air5, while shaded or moist surfaces—often in more rural surroundings—remain close to air

2 Oke, T.R. 1982. The Energetic Basis of the Urban Heat Island. Quarterly Journal of the Royal Meteorological Society. 108:1-‐24. 3 Glossary of Meteorology (2009). "Urban Heat Island". American Meteorological Society. Retrieved 2009-‐06-‐19. 4 http://www.epa.gov/heatislands/about/index.htm 5 Berdahl P. and S. Bretz. 1997. Preliminary Survey of the Solar Reflectance of Cool Roofing Materials. Energy and Buildings 25:149-‐158.



temperatures. Surface UHI are present day and night, but they tend to be strongest during the day when the sun is shining. On average, the difference in daytime surface temperatures between developed and rural areas is (10 to 15°C); the difference in nighttime surface temperatures is typically smaller, at (5 to 10°C).6 Surface UHIs are typically largest in the summer and lowest in winter; because of the variation in radiation and temperature, due to the changes in the sun’s intensity with seasons.

-‐ Atmospheric UHI: Atmospheric urban heat islands refer to the existence of warmer air in urban areas compared to cooler air in nearby rural surroundings. It’s often divided into two different types:

1. Canopy layer UHI: It exists in the layer of air where people live, from the ground to below the tops of trees and roofs.

2. Boundary layer UHI: It starts from the rooftop and treetop level and extend up to the point where urban landscapes no longer influence the atmosphere.

Atmospheric urban heat islands are often weak during the late morning and throughout the day, and they become more marked after sunset due to the slow release of heat from urban infrastructure. The timing of this peak, however, depends on the properties of urban and rural surfaces, the season, and prevailing weather conditions. Following is a table summarizing the comparison between the main two types of the UHI phenomenon:

Tab. 1 Basic Characteristics of Surface and Atmospheric UHIs7

We could also mention that both Surface and atmospheric UHIs are interconnected and both have a significant impact on each other. Surfaces in the urban setting radiate heat which affects on the atmospheric temperature, which by turn reflects back on the surface materials (especially dark ones, such as 6 Numbers from Voogt, J.A. and T.R. Oke. 2003. Thermal Remote Sensing of Urban Areas. Remote Sensing of Environment. 86. (Special issue on Urban Areas): 370-‐384. 7 Oke. T.R. 1987. Boundary Layer Climates. New York, Routledge.



Asphalt) forcing them to store more heat than that in the normal temperatures. 2 . Measurment To identify urban heat islands, scientists use direct and indirect methods, numerical modeling, and estimates based on empirical models. Researchers often use remote sensing, an indirect measurement technique, to estimate surface temperatures. They use the data collected to produce thermal images, such as that shown in Fig. 1.

Fig. 1 Thermal Image Depicting Surface UHI in Atlanta (Georgia), on September 28th, 2000.8

8 http://earthobservatory.nasa.gov/IOTD/view.php?id=7205



3 . Causes There are several causes of an urban heat island (UHI). Briefly stated as follows:

-‐ Buildings § Buildings Block Surface Heat: The principal reason for the

nighttime warming, radiating into the relatively cold night sky. § Geometric Impact: The tall buildings within many urban areas

provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the "urban canyon effect". Urban geometry influences wind flow, energy absorption, and a given surface’s ability to emit long-‐wave radiation back to space. In developed areas, surfaces and structures are often at least partially obstructed by objects, such as neighboring buildings, and become large thermal masses that cannot release their heat very readily because of these obstructions. Especially at night, the air above urban centers is typically warmer than air over rural areas. Nighttime atmospheric heat islands can have serious health implications for urban residents during heat waves

-‐ Properties of Surface Materials

§ Materials commonly used in urban areas for pavement and roofs, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface “Radiative” properties (Albedo and Emissivity) than the surrounding rural areas.

§ Built up communities generally reflect less and absorb more of the sun’s energy. This absorbed heat results in an increase in surface temperatures and thus contribute to the formation of surface and atmospheric UHIs.

§ Materials such as solar reflectance, thermal emissivity or heat capacity control the ability of the material to have a lower or higher contribution to the increase of the UHI. For example, dark surfaces with high emittance values will stay cooler, because they will release heat more readily.

-‐ Reduction of Vegetation in Urban Areas

§ In rural areas, vegetation and open land are dominant. Trees and vegetation provide shade, which lowers surface temperatures and reduces temperatures through “evapotranspiration”.

§ In contrast, urban areas are characterized by dry, impervious surfaces, such as roofs, sidewalks, roads, and parking lots. This change in ground cover results in less shade and moisture to keep urban areas cool; also urban areas evaporate less water, which results in elevating surface and air temperatures.



-‐ Anthropogenic heat § Refers to the heat generated by cars, air conditioners, industrial

facilities, and a variety of other manmade sources, which contributes to the increase of the UHI as well as the urban energy budget, particularly in the winter.

-‐ High Pollution Levels § Various forms of pollution change the “Radiative” properties of

the atmosphere. This causes a change in the energy balance of the urban area, often leading to higher temperatures than surrounding rural areas.9

4 . UHI Impact Increased temperatures from UHIs, especially during summer, can affect a community’s environment and quality of life. While some heat island impacts seem positive, such as lengthening the plant-‐growing season, most impacts are negative and include:

-‐ Impact on Energy Consumption Increased summertime temperatures in cities increase energy demand for cooling. Research shows that electricity demand for cooling increases 1.5–2.0% for every (0.6°C) increase in air temperatures, starting from (20 to 25°C), suggesting that 5–10% of community-‐wide demand for electricity is used to compensate for the heat island effect.10 Urban heat islands increase overall electricity demand, as well as peak demand, which generally occurs on hot summer weekday afternoons, when offices and homes are running cooling systems, lights, and appliances.

-‐ Impact on Human Health and Comfort Increased daytime temperatures, reduced nighttime cooling, and higher air pollution levels associated with urban heat islands can affect human health by contributing to general discomfort, respiratory difficulties, heat cramps and exhaustion, non-‐fatal heat stroke, and heat-‐related mortality. Heat islands can also exacerbate the impact of heat waves, which are periods of unusually hot, and often humid, weather. Sensitive populations, such as children, older adults, and those with existing health conditions, are at particular risk from these events. Excessive heat events are particularly dangerous and can result in above-‐average rates of mortality. The Centers for Disease Control and Prevention estimates that from 1979–2003, excessive heat exposure contributed to more than 8,000 premature deaths in the United States. This figure exceeds the number of mortalities resulting from hurricanes, lightning, tornadoes, floods, and earthquakes combined.11

9 T. R. Oke (1982). "The energetic basis of the urban heat island". Quarterly Journal of the Royal Meteorological Society 108 (455): 1–24. 10 Akbari, H. 2005. Energy Saving Potentials and Air Quality Benefits of Urban Heat Island Mitigation (PDF) (19 pp, 251K). Lawrence Berkeley National Laboratory. 11 Center for Disease Control and Prevention. 2006. Extreme Heat: A Prevention Guide to Promote Your Personal Health and Safety.



-‐ Impact on Air Quality Urban heat islands raise demand for electrical energy in summer. Companies that supply electricity typically rely on fossil fuel power plants to meet much of this demand, which in turn leads to an increase in air pollutant and greenhouse gas emissions. The primary pollutants from power plants include sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM), carbon monoxide (CO), and mercury (Hg). These pollutants are harmful to human health and also contribute to complex air quality problems such as the formation of ground-‐level ozone (smog), fine particulate matter, and acid rain. Increased use of fossil-‐fuel-‐powered plants also increases emissions of greenhouse gases, such as carbon dioxide (CO2), which contribute to global climate change.12 In addition to their impact on energy-‐related emissions, increased temperatures can directly increase the rate of ground-‐level ozone formation. Ground-‐level ozone is formed when NOx and volatile organic compounds (VOCs) react in the presence of sunlight and hot weather. If all other variables are equal, such as the level of precursor emissions in the air and wind speed and direction, more ground-‐level ozone will form as the environment becomes sunnier and hotter.

-‐ Impact on Water Quality High pavement and rooftop surface temperatures can heat extra storm water. Tests have shown that pavements that are (38oC) can elevate initial rainwater temperature from roughly (21oC) to over (35oC).13 This heated storm water generally becomes excess, which drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Water temperature affects all aspects of aquatic life, especially the metabolism and reproduction of many aquatic species. Rapid temperature changes in aquatic ecosystems resulting from warm storm water runoff can be particularly stressful, even fatal to aquatic life. 5 . UHI Mi t igat ion Will the urban areas benefit from heat island reduction? The answer depends on a number of factors—some within and some outside of a community's control. Although prevailing weather patterns, climate, geography, and topography are beyond the influence of local policy, decision makers can select a range of energy-‐saving strategies that will generate multiple benefits, including vegetation, landscaping, and land use design projects, and improvements to building and road materials.14 Following are some of the most effective UHI mitigation strategies:

-‐ Trees, vegetation, and green roofs They can reduce heating and cooling energy use and associated air pollution and greenhouse gas emissions, remove air pollutants, help lower the risk of heat-‐related illnesses and deaths, improve storm-‐water control and water quality, reduce noise levels, create habitats, improve

12 http://www.epa.gov/heatislands/impacts/index.htm#2 13 James, W. 2002. Green roads: research into permeable pavers. Stormwater 3(2):48-‐40. 14 http://www.epa.gov/heatislands/mitigation/index.htm



aesthetic qualities, and increase property values. -‐ Cool roofs

They lower cooling energy use, peak electricity demand, air pollution and greenhouse gas emissions, heat-‐related incidents, and solid waste generation due to less frequent re-‐roofing. Later in the report, this mitigation technique will be discussed with more depth.

-‐ Cool pavements They have the potential of indirectly reducing energy consumption, air pollution, and greenhouse gas emissions. Depending on the technology used, cool pavements can improve storm-‐water management and water quality, increase surface durability, enhance nighttime illumination, and reduce noise.15

Using these strategies in combination can enhance their effectiveness. For example, installing a permeable pavement parking lot that includes shade trees can extend the longevity of the pavement and vegetation. Widespread implementation of these mitigation strategies also provides additional benefits. For example, a single cool roof will mainly result in benefits to the building owner and occupants. Community-‐wide cool roof installations, though, has the potential to provide savings to the building owner, occupants and to the community at large scales, as a large number of cool roofs can reduce air temperatures, resulting in multiple benefits associated with cooler summertime air.

15 Ibid.



COOL ROOFS Are the roofs characterized with high Albedo – Solar Reflectance— as well as high thermal emittance. These two characteristics help reflect sunlight and emit heat away from a building, reducing roof temperatures. Solar Reflectance of the cool roofs is the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun; while its Thermal Emittance is the ability to radiate absorbed, or non-‐reflected solar energy. Cool roofs may be installed on low-‐slope roofs (such as the flat or gently sloping roofs typically found on commercial, industrial, and office buildings) or the steep-‐sloped roofs used in many residences and retail buildings.

Fig. 2 Scheme showing the difference in behavior between Cool and Hot Roofs.16

Cool roofing products are made of highly reflective and emissive materials that can remain approximately 50 to 60°F (28-‐33°C) cooler than traditional materials during peak summer weather.17 Cool roofs achieve cooling energy savings in hot summers but can increase heating energy load during cold winters.18 Therefore, the net energy saving of cool roofs varies depending on climate. Without a proper maintenance program to keep the material clean, the energy savings of cool roofs can diminish over time due to Albedo degradation and soiling.19 In order to understand how cool roofing work, first I’ll start by explaining how their properties and roofing materials fit and contribute within the cycle of solar radiation, temperature and the Urban Heat Islands Phenomenon. This part of the report discusses the following: 16 http://www.custombiltmetals.com/cool-‐roof.php 17 Levinson, R., H. Akbari, S. Konopacki, and S. Bretz. 2002. Inclusion of Cool Roofs in Nonresidential Title 24 Prescriptive Requirements (PDF) (64 pp, 492K). Paper LBNL-‐50451. Lawrence Berkeley National Laboratory. 18 United States Environmental Protection Agency (2011). Reducing Urban Heat Islands: Compendium of Strategies. 19 Bretz, Sarah; Hashem Akbari (1997). "Long-‐term performance of high albedo roof coatings". Energy and Buildings 25 (2): 159-‐167. doi:10.1016/S0378-‐7788(96)01005-‐5.



Proper t ies o f Coo l Roofs -‐ Solar Reflectance

Solar reflectance, or albedo, is the percentage of solar energy reflected by a surface. Solar reflectance measurement methods have been developed in order to determine how well a material reflects energy at each solar energy wavelength, then calculating the weighted average of these values. Traditional roofing materials have low solar reflectance of 5 to 15 percent, which means they absorb 85 to 95 percent of the energy reaching them instead of reflecting the energy back out to the atmosphere. The coolest roof materials have a high solar reflectance of more than 65 percent, absorbing and transferring to the building 35 percent or less of the energy that reaches them.20 These materials reflect radiation across the entire solar spectrum, especially in the visible and infrared (heat) wavelengths.

-‐ Thermal Emittance Although solar reflectance is the most important property in determining a material’s contribution to urban heat islands, thermal emittance is also a part of the equation. Any surface exposed to radiant energy will get hotter until it reaches thermal equilibrium (i.e., it gives off as much heat as it receives). A material’s thermal emittance determines how much heat it will radiate per unit area at a given temperature, that is, how readily a surface gives up heat. When exposed to sunlight, a surface with high emittance will reach thermal equilibrium at a lower temperature than a surface with low emittance, because the high-‐emittance surface gives off its heat more readily.

-‐ Temperature Effects Solar reflectance and thermal emittance have noticeable effects on surface temperature. Conventional roof surfaces have low reflectance but high thermal emittance; standard black asphalt roofs can reach (74 -‐85°C) at midday during the summer. Bare metal or metallic surfaced roofs have high reflectance and low thermal emittance and can warm to (66 -‐77°C). Research has shown that cool roofs with both high reflectance and high emittance reach peak temperatures of only 110 to 115°F (43-‐46°C) in the summer sun. These peak values vary by local conditions. Nonetheless, research reveals that conventional roofs can be 55 to 85°F (31-‐47°C) hotter than the air on any given day, while cool roofs tend to stay within 10 to 20°F (6-‐11°C) of the background temperature.21

20 United States Environmental Protection Agency (2011). Reducing Urban Heat Islands: Compendium of Strategies: Cool Roofs. 21 These temperature ranges are compiled from the following individual reports:

Konopacki, S., L. Gartland, H. Akbari, and I. Rainer. 1998. Demonstration of Energy Savings of Cool Roofs. Paper LBNL-‐40673. Lawrence Berkeley National Laboratory, Berkeley, CA. Gartland, L. n.d. Cool Roof Energy Savings Evaluation for City of Tucson. Miller, W.A., A. Desjarlais, D.S. Parker, and S. Kriner. 2004. Cool Metal Roofing Tested for Energy Efficiency and Sustainability. CIB World Building Congress, May 1-‐7, 2004. Toronto, Ontario. Konopacki, S. and H. Akbari. 2001. Measured Energy Savings and Demand Reduction from a Reflective Roof Membrane on a Large Retail Store in Austin. Paper LBNL-‐47149. Lawrence Berkeley National Laboratory, Berkeley, CA.



These reduced surface temperatures from cool roofs can lower air temperature. For example, a New York City simulation predicted near-‐surface air temperature reductions for various cool roof mitigation scenarios. The study assumed 50-‐percent adoption of cool roofs on available roof space and ran models to evaluate the resulting temperature changes. Averaged over all times of day, the model predicted a city-‐wide temperature reduction of (0.2°C). The city-‐wide, 3:00 p.m. average reduction was (0.3°C) and ranged from 0.7 to (0.4 -‐ 0.8°C) in six specific study areas within the city.22 Cool Roof Types Cool roofs can be categorized in two difference manners:

-‐ Material Cool roofs for commercial and industrial buildings fall into one of three categories: roofs made from inherently cool roofing materials, roofs made of materials that have been coated with a solar reflective coating, or green planted roofs.

§ Inherently cool roofs White vinyl roofs, which are inherently reflective, achieve some of the highest reflectance and emittance measurements of which roofing materials are capable. A roof made of thermoplastic white vinyl, for example, can reflect 80 percent or more of the sun’s rays and emit at least 70% of the solar radiation that the building absorbs. An asphalt roof only reflects between 6 and 26% of solar radiation, resulting in greater heat transfer to the building interior and greater demand for air conditioning. 23

§ Coated roofs This type of intervention works also for retrofitting. The roof can be made reflective by applying a solar reflective coating to its surface. There are two main types of cool roof coatings: Cementitious and Elastomeric. Cementitious coatings contain cement particles. Elastomeric coatings include polymers, which are added to reduce brittleness and improve adhesion. Some coatings contain both cement particles and polymers. Both types have a solar reflectance of 65 percent or higher when new and have a thermal emittance of 80 to 90 percent or more. The important distinction is that elastomeric coatings provide a waterproofing membrane, while cementitious coatings are pervious and rely on the underlying roofing material for waterproofing.

§ Green roofs Green roofs provide a thermal mass layer which helps reducing the flow of heat into a building. The solar reflectance of green roofs varies depending on the

22 Rosenzweig, C., W. Solecki, L. Parshall, S. Gaffin, B. Lynn, R. Goldberg, J. Cox, and S. Hodges. 2006. Mitigating New York City’s Heat Island with Urban Forestry, Living Roofs, and Light Surfaces. Sixth Symposium on the Urban Environment and Forum on Managing our Physical and Natural Resources, American Meteorological Society. Atlanta, GA. 23 http://en.wikipedia.org/wiki/Cool_roof



plant types (generally 0.3-‐0.5).24 Because of the lower solar reflectance, green roofs reflect less sunlight and absorb more solar heat than white roofs. The absorbed heat in the green roofs is trapped by the greenhouse effect and then cooled by “evapotranspiration”.

-‐ Geometry Depending on the geometry of the roof, there are two categories: low-‐sloped and steep-‐sloped. A low-‐sloped roof is essentially flat, with only enough incline to provide drainage. It is usually defined as having no more than 2 inches (5 cm) of vertical rise over 12 inches (30 cm) of horizontal run. These roofs are found on the majority of commercial, industrial, warehouse, office, retail, and multi-‐family buildings, as well as some single-‐family homes. Steep-‐sloped roofs have inclines greater than a 2-‐inch rise over a 12-‐inch run. These roofs are found most often on residences and retail commercial buildings and are generally visible from the street. Low-‐sloped and steep-‐sloped roofs use different roofing materials. Traditionally, low-‐sloped roofs use built-‐up roofing or a membrane, and the primary cool roof options are coatings and single-‐ply membranes.

Cool Roofs Contr ibut ion Conf l i c t Recent works executed by researchers from Stanford University, regarding the Urban Heat Island phenomenon claim that, if all the roofs in urban areas were painted white, it would increase, not decrease, global warming.25 How correct or false this piece of information is, is a matter of scientific research and discussion, for it could change the orientation of studies implemented for remedying the phenomenon’s negative impact.

24 Levinson, Ronnen (2010). "Cool Roofs, Cool Cities, Cool Planet" (PowerPoint Slides). Retrieved 10 December 2011. 25 http://news.stanford.edu/news/2011/october/urban-‐heat-‐islands-‐101911.html



UHI and ENERGY BALANCE The following description of the relation between the UHI Phenomenon and ENERGY BALANCE is extracted and summarized from a paper by A.J Arnfield, titled “Two decades of Urban Climate Research: A review of Turbulence, Exchanges of Energy and Water, and the Urban heat Island”26 Understanding the urban energy balance is a complex issue and takes into account many interrelated factors on both the Urban Surface and urban atmospheric scale. Energy Ba lance in the Urban Atmospher i c l ayer Within the Urban Atmospheric Layer, energy balances are governed by micro-‐scale processes mediated by the site conditions of the immediate surroundings. These conditions, consisting of the specifics of a 3D surface geometry, substrate materials and wetness, wind exposure, shading and the like, are subject to countless variations within real cities. As a means of distilling what is common to many urban landscapes from what is unique to the particular architectural, cultural, and geographical milieu, much urban climate work has adopted the construct of the Urban Canyons (UC). The UC consists of the space between adjacent buildings, comprising the solid surfaces on the faces of those buildings and the street, the enclosed air volume, the open ‘top’ at roof level and the ‘ends’ of the canyon at street intersections, through which mass and energy fluxes may occur horizontally. The canyon aspect ratio (AR), the ratio of wall height to building separation, has been suggested by many as a major control on flow within the UC, on turbulent intensities, on radiative environments and, hence, on the total energy budget. Energy Ba lance in the Urban Sur face layer The understanding of urban energy balance cannot avoid the issue of the precise definition of the ‘surface’ to which the balance refers. This is a complex and interrelated issue, because, while it is generally accepted what constitutes the ‘surface’ to which the energy balance of a building wall, suburban lawn or warehouse roof applies, this becomes increasingly vague as we scale up through individual landscape units, like UCs, to land-‐use zones and even whole cities. Storage and anthropogen ic heat f luxes in the urban energy ba lance The energy balance for a simple plane facet may be written as: Q* = QH + QE + QG (1) Where: Q* is net radiation; QH and QE are the turbulent fluxes of sensible and latent heat respectively; QG is the (primarily) conductive heat flux into or out of the material that

26 Arnfield, A. John, “Two decades of Urban Climate Research: A review of Turbulence, Exchanges of Energy and Water, and the Urban heat Island,” International Journal of Climatology, published online 28 Aug. 2002. DOI: 10.1002/joc.859



constitutes the surface. Behavior of QG for simple facets can normally be evaluated using heat flux plates or by measuring time rates of temperature change if the heat capacity of the substrate is known. Oke (1988b) suggests that, at larger scales, for total urban landscapes, a useful approach is to evaluate the equivalent energy fluxes through the top of an imaginary volume, extending from a depth in the substrate below which energy exchanges are negligible at the time scale of consideration to a level roughly at roof level, at the upper margins of the UCL. The energy budget for this volume can be written as: Q* + QF = QH + QE + ΔQS + ΔQA (2) Where: QF is the anthropogenic energy releases within the volume; ΔQA is net advection through the sides of the volume; ΔQS is the storage heat flux. It represents all energy storage mechanisms within the volume, in air, trees, building fabric, soil, etc. In practice, by virtue of the sizes of heat capacities for air and solid fabric, ΔQS can normally be equated to the aggregate QG for all air–solid interfaces within the volume. Incorporation of anthropogenic heat flux in simulation models of urban climate is relatively straightforward, involving the addition of a (usually constant) term in the surface energy budget equation. The evaluation of QF at local scales, for incorporation into energy balances for suburban terrain; are given by Grimmond and Oke (1991) and Schmid et al. (1991). They compute this term as: QF = QFV + QFH + QFM Where: QFV is the heat released by vehicles; QFH is the heat released by stationary sources such as house furnaces; QFM is the heat released by metabolism. It should be noted that, in some cases, QF is included implicitly. For example, the Terjung and Louie (1974), Mills (1993) and Arnfield (2000b) energy budget simulation models incorporate constant internal building temperatures, which reflect the investment of anthropogenic energy in heating and cooling living spaces. These schemes influence external climate by conductive heat fluxes through building walls and roofs. However, they don’t incorporate energy releases to the external environment, like motor vehicle waste heat, chimney gases or air-‐conditioner heat output.



CONCLUSION Although urban climatologists have been studying urban heat islands for decades, community interest and concern regarding them has been more recent. This increased attention to heat-‐related environment and health issues has helped to advance the development of heat island reduction strategies, mainly trees and vegetation, green roofs, and cool roofs. Interest in cool pave-‐ments has been growing, and an emerging body of research and pilot projects are helping scientists, engineers, and practitioners to better understand the interactions between pavements and the urban climate. Cities release more heat to the atmosphere than the rural vegetated areas around them, but how much influence these urban "heat islands" have on global warming has been a matter of debate. Although the size of literature about this phenomenon is reasonable enough, recently, scholars have been questioning the authenticity with which heat island observations have been gathered and reported through history. To what extent does this literature serve the aims of science? Can its measurements be trusted? So far, the response to these questions is not obvious. Modern heat island investigators such as Parry (1956), Chandler (1962, 1970) and Bohm and Gabl (1978), for example, alluded to problems of methodology decades ago. In recent years, discussion around these same problems has been open and direct.27

Heat emanating from cities – called the "urban heat island" effect – is not a significant contributor to global warming, Stanford researchers have found. They also concluded that if all the roofs in urban areas were painted white, it would

increase, not decrease, global warming.28 This was quoted from the study by Stanford researchers, which has quantified the contribution of the heat islands for the first time, showing that it is modest compared with what greenhouse gases contribute to global warming. "Between 2 and 4 percent of the gross global warming since the Industrial Revolution may be due to urban heat islands," said Mark Z. Jacobson, a professor of civil and environmental engineering who led the study. He and his students compared this with the greenhouse gas contribution to gross warming of about 79 percent and the black carbon contribution of about 18 percent. Black carbon is a component of the soot created by burning fossil fuels and bio-‐fuels and is highly efficient at absorbing sunlight, which heats the atmosphere.

27 Stewart, I. D., “A systematic review and scientific critique of methodology in modern urban heat island literature,” International Journal of Climatology, published online 15 Apr. 2010. DOI: 10.1002/joc.2141 28 http://news.stanford.edu/news/2011/october/urban-‐heat-‐islands-‐101911.html



BIBLIOGRAPHY [1, ] Stewart, I. D., “A systematic review and scientific critique of methodology in modern urban heat island literature,” International Journal of Climatology, published online 15 Apr. 2010.

DOI: 10.1002/joc.2141

[2] Oke, T.R. 1982. The Energetic Basis of the Urban Heat Island. Quarterly Journal of the Royal Meteorological Society. 108:1-‐24.

[3] Glossary of Meteorology (2009). "Urban Heat Island". American Meteorological Society. Retrieved 2009-‐06-‐19.

[4] http://www.epa.gov/heatislands/about/index.htm

[5] Berdahl P. and S. Bretz. 1997. Preliminary Survey of the Solar Reflectance of Cool Roofing Materials. Energy and Buildings 25:149-‐158.

[6] Numbers from Voogt, J.A. and T.R. Oke. 2003. Thermal Remote Sensing of Urban Areas. Remote Sensing of Environment. 86. (Special issue on Urban Areas): 370-‐384.

[7] Oke. T.R. 1987. Boundary Layer Climates. New York, Routledge.

[8] http://earthobservatory.nasa.gov/IOTD/view.php?id=7205

[9] T. R. Oke (1982). "The energetic basis of the urban heat island". Quarterly Journal of the Royal Meteorological Society 108 (455): 1–24.

[10] Akbari, H. 2005. Energy Saving Potentials and Air Quality Benefits of Urban Heat Island Mitigation (PDF) (19 pp, 251K). Lawrence Berkeley National Laboratory.

[11] Center for Disease Control and Prevention. 2006. Extreme Heat: A Prevention Guide to Promote Your Personal Health and Safety.

[12] http://www.epa.gov/heatislands/impacts/index.htm#2

[13] James, W. 2002. Green roads: research into permeable pavers. Stormwater 3(2):48-‐40.

[14] http://www.epa.gov/heatislands/mitigation/index.htm

[15] Ibid.

[16] http://www.custombiltmetals.com/cool-‐roof.php

[17] Levinson, R., H. Akbari, S. Konopacki, and S. Bretz. 2002. Inclusion of Cool Roofs in Nonresidential Title 24 Prescriptive Requirements (PDF) (64 pp, 492K). Paper LBNL-‐50451. Lawrence Berkeley National Laboratory.

[18] United States Environmental Protection Agency (2011). Reducing Urban Heat Islands: Compendium of Strategies.

[19] Bretz, Sarah; Hashem Akbari (1997). "Long-‐term performance of high albedo roof coatings". Energy and Buildings 25 (2): 159-‐167. doi:10.1016/S0378-‐7788(96)01005-‐5.

[20] United States Environmental Protection Agency (2011). Reducing Urban Heat Islands: Compendium of Strategies: Cool Roofs.

[21] These temperature ranges are compiled from the following individual reports:

Konopacki, S., L. Gartland, H. Akbari, and I. Rainer. 1998. Demonstration of Energy Savings of Cool Roofs. Paper LBNL-‐40673. Lawrence Berkeley National Laboratory, Berkeley, CA.



Gartland, L. n.d. Cool Roof Energy Savings Evaluation for City of Tucson.

Miller, W.A., A. Desjarlais, D.S. Parker, and S. Kriner. 2004. Cool Metal Roofing Tested for Energy Efficiency and Sustainability. CIB World Building Congress, May 1-‐7, 2004. Toronto, Ontario.

Konopacki, S. and H. Akbari. 2001. Measured Energy Savings and Demand Reduction from a Reflective Roof Membrane on a Large Retail Store in Austin. Paper LBNL-‐47149. Lawrence Berkeley National Laboratory, Berkeley, CA.

[22] Rosenzweig, C., W. Solecki, L. Parshall, S. Gaffin, B. Lynn, R. Goldberg, J. Cox, and S. Hodges. 2006. Mitigating New York City’s Heat Island with Urban Forestry, Living Roofs, and Light Surfaces. Sixth Symposium on the Urban Environment and Forum on Managing our Physical and Natural Resources, American Meteorological Society. Atlanta, GA.

[23] http://en.wikipedia.org/wiki/Cool_roof

[24] Levinson, Ronnen (2010). "Cool Roofs, Cool Cities, Cool Planet" (PowerPoint Slides). Retrieved 10 December 2011.

[25] http://news.stanford.edu/news/2011/october/urban-‐heat-‐islands-‐101911.html

[26] Arnfield, A. John, “Two decades of Urban Climate Research: A review of Turbulence, Exchanges of Energy and Water, and the Urban heat Island,” International Journal of Climatology, published online 28 Aug. 2002.

DOI: 10.1002/joc.859

[27] Stewart, I. D., “A systematic review and scientific critique of methodology in modern urban heat island literature,” International Journal of Climatology, published online 15 Apr. 2010.

DOI: 10.1002/joc.2141

[28] http://news.stanford.edu/news/2011/october/urban-‐heat-‐islands-‐101911.html

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