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Urban Heat Island: History and Causes
In this chapter of the thesis, a detailed account of the history and definition of the urban heat island is provided as well as the different types of heat islands that exist, such as the urban canopy layer, the urban boundary layer and the surface heat islands, all based on the available literature on the topic. In addition, this chapter discusses the underlying causes that lead to the formation and exacerbation of the heat island, namely natural factors such as geographic locations, proximity to major water bodies, climatic conditions and so on, and anthropogenic factors such as industrial and traffic-associated heat emissions, as well as urban geometry of cities, solar reflectivity of urban materials and presence or absence of vegetation.
History and Definition
In 1818, Luke Howard – a chemist turned meteorologist, renowned for his seminal work on clouds’ classification – published the first volume of “The Climate of London”, a documentation of the urban climate of London with continuous daily meteorological records of wind direction, temperature, precipitation, and atmospheric precipitation (International Association for Urban Climate [IAUC], 2014). Howard was the first to recognize that urban areas can have significant effects on the local climate by identifying how London’s urban climate was much hotter than that of the English countryside, a phenomenon that was later coined as the “Urban Heat Island (UHI)”.
Similar observations were then made in Paris in the second half of the 19th century, in Vienna in the early 20th century, and in the U.S. in the first half of the 20th century (Gartland, 2008). Thereafter, several investigations have been made uncovering the causes that contribute to the UHI effect, putting forth various methods to modeling the UHI, and suggesting effective measures to mitigate and alleviate the negative impacts of UHI on cities and their dwellers.
The term « heat island » therefore describes urbanized areas that are hotter than nearby non-urbanized areas due to the fact that urban areas have typically darker surfaces and less vegetation than semi-urban and non-urban surroundings. This difference in daily temperatures between urban and non-urban areas affects not only the microclimate but also the energy use and habitability of cities. At the building scale, dark roofs heat up more, thus increasing the summertime cooling demands of buildings. For example, a study conducted in the city of Athens, with an urban heat island intensity of 10oC, showed that the cooling load, or cooling energy demand, of urban buildings may be doubled thus reflecting the strong impact of the UHI on energy demands in urbanized areas (Santamouris, et al, 2001). Similar effects can be expected in Beirut due to the resemblances in urban characteristics between these two Mediterranean cities. It is therefore important to study the effects of urbanization on the urban heat island in Beirut to determine to what extent energy demands are affected and what the potential consequences are on the surrounding environment and urban microclimate.
Moreover, dark surfaces and lack of vegetation collectively warm the air over urban areas leading to the creation of urban heat sinks. It has been reported, that on a clear summer afternoon, the air temperature in a typical city is as much as 2.5oC higher than in the surrounding rural areas (Akbari, Pomerantz, & Taha, 2001).This urban heating or UHI can therefore also result in mortality like in the case of the 2003 summer heat wave in Paris which caused over 10,000 heat-related deaths as recorded by the French National Institute of Health thus reflecting the strong impact of the UHI on human health and quality of living in urbanized areas (INVS, 2015). In Beirut, the number of hot days5 has increased during the second half of the last century, at a rate of about two days per decade. In fact the largest number of hot days has been recorded from the late 1980s onwards, during which time urbanization developments and urban population records in Beirut increased significantly making the living thermal conditions extremely uncomfortable and unhealthy for the city dwellers. Hot summer days can accordingly have negative effects on human comfort levels and can contribute to significant heat stress. The number of hot summer days has increased significantly in Beirut in this past decade (Hatzaki, Giannakopoulos, Hadjinicolaou, & Kastopoulou; CIRCE project, 2010), which lowers thermal comfort levels dramatically consequently affecting human health and overall quality of life (this is described in more detail in Part III of this thesis – climatic conditions).
Types of heat islands.
Looking closely at this phenomenon, Oke (1982) distinguished 3 types of heat islands; these are: the urban canopy layer (UCL), the urban boundary layer (UBL), and the surface heat island (SHI). It’s important to distinguish between these different heat islands as their underlying mechanisms are also different. The first two layers refer to a warming of the urban atmosphere and are referred to as “atmospheric urban heat islands” whereas the last refers to the relative warmth of urban surfaces (see Figure 3).
The UCL is the layer of air closest to the surface in cities, extending upwards to approximately the mean building height. It is in fact within this layer, the urban canyon layer, in which numerical analysis for this research is carried out by using the Town Energy Balance (TEB) urban surface exchange model developed by Masson (2000) which aims to realistically represent the energy balance of the 3-dimensional urban canyon representing roofs, walls and roads (Masson, 2006) (see Chapter 3 for more details on existing modeling tools of the urban heat island).
Above the urban canopy layer lies the UBL, which varies in thicknesses from 1 kilometer or more during the day to hundreds of meters or less at night. It is a dome-shaped heat sink of warmer air that extends downwind of the city which may sometimes take a plume shape depending on the wind gust. Atmospheric heat islands are typically weak during the late morning and throughout the day but become more pronounced after sunset due to the slow release of heat from urban mineralized surfaces. Also, atmospheric heat islands vary much less in intensity than SHIs. On an annual mean basis, air temperatures in cities might be 1 to 3oC warmer than their rural surroundings. On the other hand, SHIs are typically present throughout the day and night but tend to be strongest during the day. On average, the difference in daytime temperatures between urban and rural areas is about 10 to 15oC while the difference in surface temperatures during the nighttime is typically smaller, between 5 to 10oC showing therefore a greater intensity than atmospheric heat islands (USEPA, 2014).
It is also worth noting that heat islands may vary in spatial form or shape, in temporal characteristics, and in some of the underlying physical processes that contribute to their development. Air temperatures at the UCL and UBL are measured directly using thermometers; whereas, the SHI is measured either indirectly using remote sensing techniques and technologies or directly through handheld or aircraft mounted thermal scanners (Voogt, 2004).
Therefore, out of the three types of urban heat islands identified, this research focuses on the urban canyon layer (UCL). More specifically, investigations in this research within the context of Beirut are concentrated on the effects of the various complex interactions of the existing urban conditions on the urban heat island air temperatures, as opposed to surface temperatures or boundary layer air temperatures, in the urban canyon layer. A canyon is defined as a street which is flanked on both sides by buildings and therefore investigations in an urban canyon layer extends upwards to the average building height in a street. This specific layer was selected since the purpose of this research is to ultimately find mitigation solutions to ensure the thermal comfort of the urban dwellers who are more directly affected by air temperatures within the canyon layer in which they frequent as opposed to the boundary layer. While surface temperatures may also have an impact on surrounding air temperatures, measurements for urban surfaces require thermal remote sensing data which is somewhat limited for Beirut city as explained in more detail in Chapter 4 of this thesis.
Causes of the Heat Island
There is no single cause of the heat island phenomena. Instead, many factors combine to increase urban and suburban temperatures including 1) meteorological or natural factors and 2) urban or anthropogenic parameters examples of which are described in more detail as follows:
1) Meteorological parameters such as temperature, cloud cover and wind have a significant impact on the UHI. The natural location of a city also plays a role since its physical characteristics that include topography, mountain ranges and hills, rivers and/or other water bodies can determine the extent to which UHI can be affected.
2) As for anthropogenic factors, these include urban parameters such as city size, influenced by urban populations and densities, density of built-up areas including land coverage, distance between buildings and average height of buildings, urban geometry that includes street orientations, aspect or height (H) to width (W) ratio (H/W) of buildings, and sky view factor (SVF), which is the visible area of the sky from a given surface. Other urban parameters that exacerbate the UHI include man-made surfaces including buildings and pavements that are generally composed of dark materials that readily absorb and store the sun’s heat. Most building materials are also impermeable thus further exacerbating the warming trend in cities. Other man-made reasons for the heat island formation include urban heat generations as well as high levels of urban air pollution. These natural and man-made causes of the heat island, which are relevant for the Beirut case, are described in more detail in the following sections.
Natural factors. Meteorological factors such as winds and cloud cover also have an impact on the development of UHI. For instance, during periods of calm winds and clear skies, UHI may be intensified by the amount of solar energy retained by urban surfaces which is not easily convected away from the city, whereas the opposite effect occurs during periods of strong winds and increased cloud cover (Che-Ani, Shahmohamadi, Sairi, Mohd-Nor, Zain, and Surat, 2009). Alonso, Labajo & Fidalgo (2003) defined five weather types based on variations of wind speeds and cloud cover (see Table 1) and found that the UHI is more intense under conditions of atmospheric stability than under conditions of instability. Cloud cover and wind speed can also exacerbate the UHI by intensifying pollution and smog episodes in cities since particles in the air absorb and emit heat to a city’s surfaces.
The time of day can also affect the UHI intensity. Oke (1982) and Voogt (2004) demonstrated that the heat island intensity increases from the sunset hours till the predawn and typically reaches its maximum values when the temperature is at a daily minimum (Karl, Diaz, & Kukla, 1988). Alonso et al. (2003) also studied the intensity of the UHI during a three year period between 1996 and 1998 in the city of Salamanca in Spain comparing the temperatures in an urban area and those in a nearby rural area and found that the UHI is more intense during the night-time hours. A similar study carried out in Athens which studies the hourly variability of the UHI effect also shows that the hottest temperatures are recorded in the afternoon hours (Kourtidis, et al., 2015). It is expected that Beirut city would have similar variability of the UHI effect during the day. This is attributed to the fact that the city of Beirut has urban characteristics similar to those of Athens; they are both coastal situated along the Mediterranean Sea, they are both characterized by a dense built environment with mainly artificial dark urban surfaces, few green or natural spaces, major road networks, a skyline that is gradually changing with the construction of newer and taller buildings6, and both are densely populated (Beirut has a population density of about 21,000 persons/km2 while Athens has a population density of about 23,000 persons/km2). Indeed this research aims to investigate the UHI effect in the city of Beirut in the view to identify the urban parameters that have the most significant effect on the UHI diurnal variability and to consequently find mitigating solutions from an urban planning and design perspective (see Part IV of this research).
Therefore the available literature shows that the natural factors that characterize a city play an important role on impacting the UHI. For the case of Beirut, which is a coastal city and which is characterized by a relatively flat topography with two hill-tops at 90m elevations above sea-level (A.S.L.), and a mild winter season with typically low average wind speeds throughout the year, the UHI effect can be exacerbated especially when these natural factors are combined with the existing artificial (as opposed to natural) urban fabric and the characteristic geometry of the city (see subsequent sections). The natural factors characterizing the city of Beirut, namely the defining weather characteristics like minimum and maximum temperatures and wind speed and so on, are described in more detail in Part III of this thesis.
Therefore urbanization and anthropogenic heat sources have been found to have significant impacts on the UHI effect according to the available literature. The city of Beirut, which is a dense city with many road networks, significant traffic congestion, and haphazardly located industrial areas which generate uncontrolled and high levels of air polluting emissions (MOE/UNDP/ECODIT, 2011), is without doubt a major source of anthropogenic heat emissions. However this research focuses primarily on the artificial urban surfaces and their respective impacts on the UHI in Beirut as opposed to anthropogenic heat sources since the aim of this research is primarily to identify the urban planning and design schemes that can be modified or corrected in future sustainable planning solutions for the city. Having said this, the effects of anthropogenic heat on the UHI in Beirut can be a potential topic of research to be considered in future works for the city.
Albedo and urban surface materials. Another significant anthropogenic factor that has a major impact on the UHI phenomenon is the albedo property of urban surface materials. Simply put, the “albedo” – from Latin “albus” meaning whiteness – is the ratio of the amount of light or energy that is reflected back into the atmosphere by any particular surface. A low albedo means a surface reflects a small amount of the incoming radiation and absorbs the rest, for example for trees the albedo (α) is between 0.15 and 0.18. On the other hand, a high albedo means a surface absorbs a small amount of the incoming radiation and reflects the rest, for example a white paint albedo (α) may range between 0.5 and 0.95 (see Table 2). Therefore, depending on the type of urban material and its associated albedo property, the UHI phenomenon can be affected significantly.
Table of contents :
PART I: INTRODUCTION
1.1 Background Information on Lebanon
1.2 Urbanization Effects on UHI and implications for Beirut city
1.3 Scale of Study for Beirut City
1.4 Research Objectives
1.5 Thesis Structure
PART II: LITERATURE REVIEW AND ANALYSIS OF THE URBAN HEAT ISLAND
Chapter 2: Urban Heat Island: History and Causes
2.1 History and Definition
2.2 Types of heat islands.
2.3 Causes of the Heat Island
2.3.1 Natural factors.
2.3.2 Urbanization and anthropogenic heat.
2.3.3 Albedo and urban surface materials.
2.3.4 Urban geometry.
2.3.5 Lack of vegetation
Chapter 3: Climate Change, Impacts and Characteristics of Urban Heat Islands and the Energy Balance Equation
3.1 Climate Change and Urban Heat Islands
3.2 Impacts and Characteristics of the Urban Heat Island.
3.2.1 Impacts of the Urban Heat Island.
3.2.2 Characteristics of the Urban Heat Island.
3.3 The Energy Balance
3.3.1 Radiation characteristics.
3.3.2 Convection and conduction
3.3.3 Modification by urban areas.
3.3.4 The energy balance equation.
Chapter 4: Measuring, Modeling and Mitigating Heat Islands
4.1 Measuring the Urban Heat Island
4.1.1 Weather stations.
4.1.2 High resolution aerial ortho-photos.
4.1.3 Thermal remote sensing.
4.1.4 Summary.
4.2 Modeling the Urban Heat Island
4.2.1 Micro-scale and local scale modeling schemes.
4.2.2 Mescoscale modeling schemes.
4.2.3 Other modeling schemes: mesoscale and macroscale.
4.2.4 Summary.
4.3 Mitigation Strategies to Combat Urban Heat Island
4.3.1 Planting of urban vegetation.
4.3.2 Implementing cool surfaces.
4.3.3 Combination of urban vegetation planting and cool surfaces implementation
4.3.4 Pavement watering.
4.3.5 Summary.
PART III: BEIRUT CASE STUDY
Chapter 5: Beirut Case Study
5.1 Introduction
5.2 Physical and Climatic Characteristics of Beirut City
5.2.1 Physical Characteristics.
5.2.2 Climatic Conditions.
5.3 Cadastral Districts of Beirut
5.4 Population of Beirut City
5.5 Beirut, an artificial city
5.5.1 History of Urbanization, Urban Planning and Environmental Considerations in Beirut
5.5.3 Urban Morphology of Beirut.
5.5.4 Albedo values of dominating artificial urban surfaces in Beirut.
5.6 Importance of Investigating Urban Heat Island in Beirut
5.6.1 Sustainable urban planning practices in developing countries worldwide.
5.6.2 Haphazard urban planning practices in Beirut.
5.6.3 Previous climatic and UHI studies in Beirut
5.6.4 Conclusions and justification for UHI analysis in Beirut.
PART IV: UHI IN BEIRUT
Chapter 6: Research Methodology
6.1 Selection of Measurement Type for Beirut Case
6.1.1 Elimination process.
6.1.2 Selection of records from existing weather stations in Beirut.
6.2 Selection of Modeling Scheme for Beirut Case
6.3 Data Collection and Compilation for Beirut
6.3.1 Nature of data required to run simulations on TEB.
6.3.2 Collected data for Beirut case.
6.3.3 Data extrapolation for Beirut case.
6.4 Challenges during the data collection process
Chapter 7: Numerical Analysis
7.1 Control Run
7.1.1 Selected cells for simulation runs.
7.1.2 Selected outputs for the control run.
7.1.3 Summary of control run results in Beirut.
7.2 Developing Scenarios
7.2.1 Scenario 1: Modification of Roof Albedo.
7.2.2 Scenario 2: Modification of Road Albedo.
7.2.3 Scenario 3: Modification of Glass Ratio.
7.2.4 Scenario 4: Modification of Building Height.
7.2.5 Scenario 5: Modification of HVEG height.
7.2.6 Scenario 6: Modification of Garden Fraction.
7.3 Summary of developing scenarios results for Beirut
PART V: DISCUSSION, RESEARCH OBJECTIVES AND CONCLUSIONS
Chapter 8: Discussion
8.1 Overview of Research Methodology
8.2 Discussion of Numerical Analysis Results
Chapter 9: Achieving Research Objectives
9.1 To measure and model the effect of UHI in Beirut and identify major contributing factors
9.2 To assess whether and to what extent the urban microclimate is considered in urban planning processes for Beirut
9.3 To find most suitable measures to alleviate the effects of UHI in Beirut from a technical perspective
9.4 To assess implications for urban planning process from an institutional and administrative perspective
9.5 To find opportunities at all planning and design levels to help achieve more sustainable design and planning practices in Beirut
Chapter 10: Conclusions
