A Study of Urban Heat Island Effect in Dhaka City
1.1 Background of the Study
Urbanization and industrialization improve our material lives and comfort but brings many devastating consequences such as global warming, industrial waste, and air pollution. Apart from adverse global impacts, the regional effects of urbanization are rather serious in particular in the areas where industrial activities flourishes or synthetic construction material is heavily used. As a result, the natural environment and ecology are tremendously affected and lost necessary balance in the urbanized areas. Three billions of people (i.e., 48% of the world population) living in urban areas are directly exposed to urban heating problems and more people will be vulnerable to these problems as the number of people living in urban areas is expected to grow to five billion by 2030 (World Urbanization Prospectus, 2004). One major phenomenon is observed in large cities as compared to the surrounding rural areas i.e., a higher temperature or heat contents called urban heat island (UHI).UHI is reported to be the response of many controllable and uncontrollable factors (Memon et al., 2008). These factors are manmade such as decreased sky view, low albedo, lack of vegetation, higher roughness of structures and thermal properties of materials etc. and natural such as wind speed, cloud cover and solar heating.
1.2 Objective of the Study
To study and analysis the heat island effect in Dhaka City
1.3 Rationale of the Study
The adverse effects of urban heating includes deterioration of living environment, increase in energy consumption (Konopacki and Akbari, 2002), elevation in ground-level ozone (Rosenfeld et al., 1998) and even an increase in mortality rates (Changnon et al., 1996). The field of urban heating has been highly interesting for scientists and engineers due to its adverse environmental and economic impacts on the society and promising benefits associated with its mitigation. Why should we care about heat islands? Because their negative impacts affect so many people in so many ways. Heat islands do not just cause a bit of additional, minor discomfort. Their higher temperatures, lack of shade and role in increasing air pollution have serious effects on human mortality and disease. They waste money by increasing the need for energy use, for building and infrastructure maintenance, for the management of storm-water run-off and for the disposal of waste. In addition, the barren construction techniques that foster heat islands tend to be unattractive, unappealing and unhealthy for urban flora and fauna.
Whoever has experienced the sweltering summer days of Dhaka will agree that average temperature of Dhaka City has increased over the decades. The scorching heat during daytime and hot, see thing nights coupled with load shedding are the bane of the city dweller’s life. Poor urban design in Dhaka is the biggest cause of heat island in our city. Heat islands are created when city growth alters the urban fabric by manmade asphalt roads and tar roofs and other features substituting forest growth. These surfaces absorb – rather than reflect – the sun’s heat, causing surface temperatures and overall ambient temperatures to rise.
In Bangladesh there is no comprehensive study on urban heat Island for Dhaka City: its cause and impacts. Dhaka City is experiencing an increasing influx of people from across the nation; this has made Dhaka the fastest growing city in the world. Apartment, schools, colleges, offices and institutions in Dhaka are increasing to meet the needs of the rapidly the population of Dhaka City. With mass migration going unabated and the number of vehicles steadily increasing, Dhaka’s skyline is covered with soot. More infrastructure are created, industrial activities are increasing leading to unpaved areas. So there is an urgent need for Dhaka to study Urban Heat Island.
1.4 Limitation of the study
An important limitation of this study is that the investigations have mostly covered air-temperature based analysis. Due to lack of instrument, surface temperature could not be collected. Besides, this study has been conducted in wet season. If data were collected in summer season then better result could be found. Another limitation of this study is time and manpower. Due to time limitation, temperature data have been collected at only three locations in Dhaka City. Needless to so that if temperature data were collected from whole Dhaka, then the result could be more significant, meaningful and representative. Another limitation in this study is the unavailability of secondary data on air temperature at various locations in Dhaka.
Urban heating has been considered a major problem of the inhabitants of cities and areas in particular with tropical environment. The specific characteristics of urban-structures (i.e., related to design and planning e.g., building aspect ratio and related to material properties e.g., thermal admittance) enable them to capture, store and release higher quantities of heat as compared to their counterparts in rural areas. The presence of normally abundant sources of anthropogenic heat in urban areas is the second driving force of urban heating. The urban heating, caused by the specific characteristics of urban structures and anthropogenic heat sources, increases urban area temperatures as compared to surrounding rural areas. The problem is worse in the cities or metropolises of tropical environment with large population and extensive economic activities. Due to the severity of the problem, vast research effort has been dedicated and a wide range of literature is available for the subject. The literature available in this area includes latest research approaches, concepts, methodologies, investigation tools and mitigation measures. This chapter reviews and summarizes this research area aiming at comprehending the most important factor(s) of the subject.
2.2 Role of different factors in urban heating
UHI is a mutual response of many factors which could broadly be categorized as controllable and uncontrollable as shown in Figure 2.1. The controllable and uncontrollable factors could further be categorized as temporary effect variables, such as wind speed and cloud cover, permanent effect variables such as green areas, building material, and sky view factor and cyclic effect variables such as solar radiation and anthropogenic heat sources. The basic sources of heating are the sun and anthropogenic heat sources (such as power plants, automobiles and air-conditioners). Most of the anthropogenic heat would enter into the environment instantly and directly. It has been reported that UHI is positively correlated with city population. Hung et al. (2006) have studied UHI in twelve Asian mega cities and reported that the magnitude and extent of UHI was positively correlated with city population. However, they have noted a maximum UHI of 8oC in Bangkok, Thailand with a population of 11 million and maximum UHI of 7oC in Shanghai, China with a population of 12.5 million.
|Sky View Factor||Green Areas||Building Material||Street Canyon Design||Population Related|
|Anticyclonic Conditions||Season||Diurnal Conditions||Wind Speed||Cloud Cover|
|Figure 2.1: A view of interaction among different factors in generating urban heat island|
They have also reported maximum UHI of 7oC in Manila, Philippines with a population density of 15,617 (persons/km2) and maximum UHI of 12oC with a population density of only 6,218(persons/km2) in Tokyo, Japan. It seems that the population could have twofold effects on heat generation, a direct effect as more people would result in higher metabolisms (i.e., heat) and an indirect effect as number of buildings, vehicles, factories etc. would be increased. However, other variables such as sky view factor, anthropogenic heat, building design, material etc. do play important part in increasing heat contents. These factors are not population dependent and may or may not favor in increasing heat with increased population. Therefore, population (population density seems to be a better term for comparison) could, but not necessarily, increase the heat contents of an area as compared to another area.
UHI has also been affected by temporary effect variables. Pongracz et al. (2006) reported that anticyclonic conditions increase UHI. Many studies have reported the influence of wind speed and cloud cover on UHI. The reported findings revealed that UHI is negatively correlated with wind speed and cloud cover (Kim and Baik, 2005; Oke, 1982). There has been some research on quantifying the significance of temporary effect variables. An example is the study conducted by Kim and Baik (2002) who reported that a decrease in maximum UHII could be visible with wind speed greater than 0.8 m/s. They illustrated further that maximum UHII of 0.3oC could not be recorded at a wind speed of over 7 m/s. In another study Klysik and Fortuniak (1999) reported that an UHII of higher than 1oC could still be observed when city average wind speed is 4 m/s during night-time and 2 m/s during daytime. Morris et al. (2001) reported that UHII is approximately fourth root of both the wind speed and cloud cover.
2.3 Determination of UHI
Magee et al. (1999) have quantified urban heat island magnitude by calculating simultaneous temperature difference between an urban and nearby rural area with similar geographic features. The same concept is applied to calculate the intensity of air or surface temperature based UHI in an urbanized area. However, UHI is not limited to finding a temperature difference between an urban and a rural area. It covers a range of diversified ideas that includes finding temperature difference between a well and rarely developed area or temperature difference between two differently built-up areas (Giridharan et al., 2004, 2005; Wong and Yu, 2005; Ezber et al., 2007). Wong and Yu (2005) reported a maximum UHII of 4oC between a well planted and a highly built-up region of central business district in Singapore. Giridharan et al. (2004 and 2005) reported UHII of 0.4oC to 1.5oC within and between three housing estates in Hong Kong.
2.4 Employed Techniques
2.4.1 Space technology
Space technology has been successfully employed in determining surface-temperature and SEB. Many studies have demonstrated wide and versatile applications of space technology. An example is the study conducted by Voogt and Grimmond (2000) who have determined sensible heat flux in Canada using remotely sensed surface-temperature. Kondoh and Nishiyama (1999) have studied changes in evaporatization in Japan using space technology.
2.4.2 Numerical modeling
Numerical modeling is another important tool with a wide area of successful applications in studying UHI. Lemonsu and Masson (2002) used numerical models and reported that conduction of heat in buildings is the most significant contributor in SEB. Arnfield and Grimmond (1998) also used numerical models and reported that wall thermal properties and building height to separation ratio have the most important influence in SEB. Ashie et al. (1999) used numerical modeling techniques and reported that artificial heat released by air-conditioners is 0.5oC along the building.
2.4.3 Small-scale physical models
In a comparatively less applicable technique, small-scale physical models have been employed to study UHI and related urbanization factors (Poreh, 1995). Spronken-Smith and Oke (1999) have used small-scale physical modeling technique and reported that the sky view factor and thermal admittance are the main properties for developing park cool island.
2.4.4 Manual data collection method
In this method manually temperature data are collected through various instruments like Hobometer. In this study this method has been used to collect temperature data.
2.5 Mitigation of UHI
2.5.1 Mitigation measures
Many studies have reported widely and successfully applied measures for mitigating UHI with promising financial and environmental benefits. The possible mitigating measures could broadly be categorized as those: (1) related to reducing anthropogenic heat release (e.g., switch off air-conditioners); (2) related to better roof design (e.g., green roofs, roof spray cooling, reflective roofs etc.); (3) other design factors (e.g., humidification, increased albedo, photovoltaic canopies etc.).
2.5.2 Potential savings and practicability
Although literature covers a wide range of different mitigation measures with huge financial and environmental benefits, most of the proposed measures were based on the numerical simulations and have not been implemented practically. An example is the study conducted by Kikegawa et al. (2006) who have carried out computer simulations and reported that reductions in anthropogenic heat and planting vegetation on the side walls of buildings can reduce air-temperatures up to 1.2oC and space cooling energy demand up to 40%. Ashie et al. (1999) have also used computer modeling and reported an air-temperature reduction of 0.4oC to 1.3oC with building cooling energy savings of up to 25% through planting vegetation. Yu and Hien (2006) have used computer simulations to report that parks and green areas could achieve 10% reduction in cooling load. Experimental studies were also conducted to estimate potential benefits of mitigation. An example is the field investigation carried out by Ca et al. (1998) who have reported that planting a 0.6 km2, park could reduce air temperatures by 1.5oC and achieve potential savings of 4000 kWh in an hour in a summer day.
2.6 Summary and Conclusions
This chapter reviewed the latest research approaches, concepts, methodologies and tools employed in studying UHI. The role and importance of various factors involved in the generation of UHI has been outlined. The design and planning related factors which could humanly be controlled to some extent are classified as ‘controllable’ whereas the environment and nature related factors which are beyond human control are designated as ‘uncontrollable’. The heat emitted from anthropogenic heat sources and radiated from sun has been identified as the main reason of heating of an area. Both the heat released by anthropogenic heat sources and the sun would ultimately exist in the form of latent heat flux, sensible heat flux and stored heat in an area. It was noted that the solar heating which enters into the environment directly, would affect both the urban and rural areas simultaneously and equally. The mitigation measures could be divided into those which could not be implemented in anyway and those which have been implemented either through simulation or via experimental means. However, practical implementation needs to focus on design and planning parameters as most of the promising benefits of mitigation have been reported through controllable part of ISH (i.e., design and planning parameters). This again hints that design and planning parameters are most important factors in studying urban heating. It is therefore recommended that more effort should be directed to study this aspect and quantify the significance of design and planning parameters.
Before to conduct this study on analysis of Heat Island much literature review has been made with the course teacher to conceptualize ourselves the term Heat Island – a much talked global issue – and why it is necessary to study Heat Island and what could be the methodology for this study to have a meaningful result.
3.2 Site Selection
Generally the term Heat Island describe the phenomenon of urban area or metropolitan area having higher temperature compared to its surrounding area as a result of increasing population, more buildings and structures, increasing paved area, industrial and commercial activity, increasing number of vehicles and trips etc in urban area. So to get the better result it should consider whole urban area while conducting such sort of study, but due to time limitation and complexities of data collection in this study only three wards in Dhaka city have been considered. These are ward No. 20 (Mohakhali), Ward No. 46 (Mohammadpur) and Ward No. 56 (Ramna).
3.3 Literature review
Various literatures such as books, journal papers, articles, proceedings, and internet have been searched to have in depth knowledge and state of the art of methodology to conduct this study. Another purpose of this literature review is to find and understand the other study on Heat Island, their methodology, result and implication.
3.4 Data Collection
To conduct this study two kind of date have been collected. These are Primary data and Secondary data.
3.4.1 Primary data collection
Primary data has been collected directly from field at three locations in Dhaka. In this study mainly temperature data has been collected across the day starting from morning to evening. Data has been collected on the basis of land cover; these are paved (road), open space/unpaved, water body and building area. The collected data have been presented in tabular form in Appendix-A.
3.4.2 Secondary data collection
In this study some secondary data has also been collected. These are mainly the yearly temperature data in Dhaka, and land use map of Dhaka. The land use map of Dhaka has been collected from the project of Preparation of Detail Area Plan (DAP) for Dhaka Metropolitan Development Plan under RAJUK. Past temperature data has been collected to see the temperature trend and land use map has been collected to relate temperature with land use/land cover of selected location in Dhaka.
3.5 Instrument used
In this study air temperature data has been collected by Lab Thermometer.
3.6 Data Processing and Analysis
After collecting the raw data from field, these are firstly entrusted into Microsoft Excel Software and Arc GIS 9.2 software for convenient of analysis. Then various spatial and temporal analyses have been conducted on these data to meet the goals of the study.
After conducting analysis, data and information have been presented in tabular formats, graphs and maps. Then the outcome of the study has been interpreted, discussed and explained meaningfully. A power point presentation was also made in the presence of the class teacher, students and environmental specialist to include critical analysis and recommendation for the fine adjustment of the final report.
3.8 Preparation of Final Report
After conduction all analysis, presentation and interpretation a well-organized report has been made.
Study Area Profile
4.1 Location and Area of the Study Area:
In this study three wards have been selected. These are Ward no 20 (Mohakhali), Ward no 46 (Mohammadpur) and ward no 56 (Ramna). Mohakhali is centrally located in Dhaka City Mohammadpur is located in the western part of DCC and Ramna is located in the south-east part of DCC. Latitude and Longitude of Ward 20 (Mohakhali), Ward 46 (Mohammadpur) and Ward 56 (Ramna) are 23°46’43.019”N, 90°24’9.793”E; 23°45’25.932”N, 90°20’58.645”E and 23°45’45.96”N, 90°24’27.02”E respectively and the area of aforesaid wards are 1.85 sq. km, 5.44 sq. km and 2.09 sq. km. respectively. Location of these wards has been presented by the Map No 4.1
4.2 General Description of the Study Area:
Mohakhali is one of the busiest places in Dhaka, Bangladesh. It has the main bus station of Dhaka. It also has several gas stations. On its north there is Banani. On its south there is Moghbazaar. Some important offices and institutions are located here, such as the Health Ministry office, BCPS (Bangladesh College of Physicians and Surgeons), and the Paramedical College of Dentistry. The 2nd flyover of Bangladesh is located here. A private Bangla TV channel Boishakhi increases new importance of this place. Mohakhali Bus terminal is one of the important terminals of Dhaka city. Every day thousands of people, particularly from greater Mymensingh region, travel by this bus terminal. Moreover, the first flyover-bridge of the country was built at the Mohakhali rail crossing to reduce traffic-jams, which characterize the Bangladesh transportation sector.
Mohammadpur is a Thana of Dhaka District in the Division of Dhaka, Bangladesh. Though initially Mohammadpur has grown as a residential area, nowadays many commercial places can be found here. The area has become more crowded than it was before. Massive urbanization has turned Mohammadpur into a miniature city and has resulted in the loss of natural environment including swamps, wetlands. As of 1991 Bangladesh census, Mohammadpur has a population of 316203. Males constitute are 55.02% of the population, and females 44.98%. This Upazila’s eighteen up population is 185413. Mohammadpur has an average literacy rate of 56.2% (7+ years), and the national average of 32.4% literate. Mostly middle class people live here. Mohammadpur is a small prototype of Dhaka city. Everything is available here and the communication system is very good. It is smoothly connected to both Sadar Ghath and Gabtali by the City Protection Dam. Many bus services operate from Mohammadpur, linking it with Motijheel, New Market, Gulshan and others parts of Dhaka City. Here population density is moderate, unlike other parts of Dhaka. Many hospitals, clinics, educational institutions, shopping complexes, wholesale markets, housing societies etc. are here. One of the largest apartment blocks in the capital, Japan Garden City is situated here. Besides, PC Culture Housing Society, Mohammadia Housing Society, Probal Housing and a number of residential areas have grown here. This has resulted in a real estate construction boom accompanied with different markets and shopping complex erection.
Ramna is a large park and recreation area situated at the heart of Dhaka, the capital city of Bangladesh. This park is one of the most beautiful areas in Dhaka with lots of trees and a lake near its center. The history of Ramna starts about 1610 AD during Mughal rule, when the city of Dhaka was founded by Subehdar Islam Khan under Emperor Jahangir. At that time two beautiful residential areas were developed in the northern suburb of Dhaka city. New residential houses, gardens, mosques, tombs and temples were built in this area during that period. Ramna Park was officially inaugurated in 1949 with an area of 88.50 acres (358,100 m2) of land with 71 species of plants. The large open spaces on the southwest facing the lake were used for holding National Fairs and Exhibitions. Ramna Park now boasts with 71 species of flowering plant, 36 species fruit bearing plant, 33 species medicinal plant and 41 species of forestry and 11 other species. Walkways inside park have been widened and five new gates built for entry from different sides. The Park features lot of beautiful and modern venues for relaxation.
Analysis and Discussion
This chapter analyzes in detail the temperature profile of three locations in Dhaka considering time and land cover. Along with this it cover the local and global temperature trend
5.2 Average Minimum and Maximum Temperature at Various Land Covers
The following figure shows average minimum and maximum temperature at different land covers. From the figure it is seen that the temperature differences for road, building area, open space and water body are 3.8, 4, 3.8 and 3.7oC respectively. The figure shows that there exist highest temperature difference for building area and lowest temperature difference exists for water body. From the figure it is also seen that the average temperature is highest for road and lowest for water body. There are several reasons why average temperature is higher for road compared to other land covers. One reason behind this is the movement of motorized vehicles those are already heated due to their engines. Besides road is paved with asphalt of black color receiving and trapping more heat than others.
Figure 5.1: Average minimum and maximum temperature of different land covers.
5.3 Temperature variation of various land cover across the day
In this section, there are several figures. These figures show the temperature variation of different land covers across the day. From the following figures it is seen that at 9 am temperature at all land covers remain lower and it tends to be increase with time up to 1pm. Then the temperature tends to decrease with time. It is observed that the slope at increasing portion of the trend line is much steeper than the slope at decreasing portion. Which means that temperature increasing rate is higher than temperature decreasing rate.
Figure 5.2: Temperature variation on road across the day
Figure 5.3: Temperature variation on Building Area across the day
Figure 5.4: Temperature variation on Open Space across the day
Figure 5.5: Temperature variation on Water Body across the day
From the Figure 5.6 it is also notice that there exists a little variation of temperature at different land covers or 3 different locations the figure shows that at Ward No 20 (Mohakhali) are higher compared to other locations. It is because temperature also depends on surrounding land uses and land covers. As Mohakhali is a predominantly a commercial area with agglomeration of tallest establishments and high rate of vehicle movement, the temperatures are higher compared to other locations. The similar scenario is found in case of average temperature for the three locations as shown in Figure: 5.7. In ward no. 56 (Ramna area), there is a major park— Ramna park which is a major open space in this ward. The area of total open space is also high in this ward. That is why temperature is lower in ward no 56 than that of ward 20 which have a greater building and paved areas.
Figure 5.6: Temperature comparison at 1pm of different land covers of different wards
Figure 5.7: Average temperature comparison of different land covers of different wards
5.4 Identification of Heat Island
In a very general sense, Urban Heat Island refers to the phenomenon where an area or corridor has higher temperature compared to its surrounding area. In this study it has been tried to identify the Urban Heat Island for three different locations. Following three maps show that Urban Heat Island scenario for three different locations. As it has already been mentioned that temperature at road is highest compared to other land covers. The study finds the Urban Heat Islands mainly along roads and then around building areas.
Map 5.1 Temperature Variation with Land cover at Ward No 20 (Mohakhali)
Map 5.2 Temperature Variation with Land cover at Ward No 46 (Mohammadpur)
Map 5.3 Temperature Variation with Land cover at Ward No 56 (Ramna)
5.5 Local and Global Temperature Trend
In the figure 5.8, a comparison has been made between yearly mean temperature deviation of global and northern hemisphere over 55 years. The deviation is computed from the temperature data of 1979 which is a median year in the range. It is seen that the average yearly temperature is increasing globally. The same pattern is observable for the northern hemisphere where our country situated.
Figure 5.8: Trend of temperature change in the world and in the northern hemisphere
Figure 5.9: Trend of temperature change in Bangladesh and Dhaka
In the figure 5.9, the position of Dhaka has been presented in the context of Bangladesh. In overall Bangladesh, after 1979 temperature was gone below the base year temperature for some years. But for Dhaka temperature never came back to the base year temperature. It is also observeable that the increase in temperature is higher in Dhaka than in Bangladesh.
Figure 5.10 (a) Trend of average temperature for Dhaka
From the above figure it is noticeable that in the last 10 years, average increase in temperature is higher in Bangladesh than that of Dhaka. From this scenario, it is apparently seemed that Dhaka does not form a heat island with respect to the other parts of Bangladesh. But the study of urban heat island requires different data collection method than that the
Figure 5.10 (b) Trend of average temperatures for Dhaka
Bangladesh Meteorological Department (BMD) is using. The weather stations of BMD from where temperature data are recorded mostly situated in the core urban areas and so those data do not represent the actual temperature profile for the whole area under consideration with respect to different land covers. The study of heat island must include this side and temperature data have to be collected on different land covers in the same geographic condition. This is why the graphs are not the actual representation to describe heat island phenomenon. So when appropriate manual methodology is selected and data are collected accordingly, Dhaka city appears as a heat island.
Figure 5.10 (c) Trend of average temperature for last 5 years in Dhaka
The figure 5.10 (a, b, c) show the trend of average temperature for Dhaka. From the figure it is seen that temperature of Dhaka is increasing over the decade. And increase in last decade is higher than previous decades illustrating the indication of Climate Change Effect.
5.6 Effect of Urban Heat Island
Elevated temperatures from urban heat islands, particularly during the summer, can affect a community’s environment and quality of life. Most impacts of Urban Heat Island are discussed below.
5.6.1 Energy Consumption
Elevated summertime temperatures in cities increase energy demand for cooling and add pressure to the electricity grid during peak periods of demand, which generally occur on hot, summer weekday afternoons, when offices and homes are running cooling systems, lights, and appliances.
5.6.2 Air Quality and Greenhouse Gases
As discussed in Section 5.6.1, higher temperatures can increases energy demand, which generally causes higher levels of air pollution and greenhouse gas emissions. Thus, pollutants from most 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 contribute to complex air quality problems such as acid rain. Further, fossil-fuel-powered plants emit greenhouse gases, particularly carbon dioxide (CO2), which contributes to global climate change.
5.6.3 Human Health and Comfort
Increased daytime surface 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. Urban heat islands can also exacerbate the impact of heat waves, which are periods of abnormally 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.
5.6.4 Water Quality
Surface urban heat islands degrade water quality, mainly by thermal pollution. Pavement and rooftop surfaces that reach temperatures 50 to 90°F (27 to 50°C) higher than air temperatures transfer this excess heat to storm water. Field measurements from one study showed that runoff from urban areas was about 20-30°F (11-17°C) hotter than runoff from a nearby rural
area on summer days when pavement temperatures at midday were 20-35°F (11-19°C) above air temperature. When the rain came before the pavement had a chance to heat up, runoff temperatures from the rural and urban areas differed by less than 4°F (2°C).
5.7 Urban Heat Island Mitigation
There are several actions to reduce urban heat islands using four main strategies: 1) increasing tree and vegetative cover, 2) installing green roofs (also called “rooftop gardens” or “eco-roofs”), 3) installing cool—mainly reflective—roofs, and 4) using cool pavements. These are discussed bellow.
5.7.1 Trees and Vegetation
Trees and other plants help cool the environment, making vegetation a simple and effective way to reduce urban heat islands.
Trees and vegetation lower surface and air temperatures by providing shade and through <href=”#Evapotranspiration”>evapo-transpiration. Shaded surfaces, for example, may be 20–45°F (11–25°C) cooler than the peak temperatures of unshaded materials. Evapotranspiration, alone or in combination with shading, can help reduce peak summer temperatures by 2–9°F (1–5°C) (Akbaria and et al., 1997)
Trees and vegetation are most useful as a mitigation strategy when planted in strategic locations around buildings or to shade pavement in parking lots and on streets. Researchers have found that planting deciduous trees or vines to the west is typically most effective for cooling a building, especially if they shade windows and part of the building’s roof.
5.7.2 Green Roofs
A green roof, or rooftop garden, is a vegetative layer grown on a rooftop. Green roofs provide shade and remove heat from the air through <href=”#Evapotranspiration”>evapotranspiration, reducing temperatures of the roof surface and the surrounding air. On hot summer days, the surface temperature of a green roof can be cooler than the air temperature, whereas the surface of a conventional rooftop can be up to 90°F (50°C) warmer (http://www.greenroofs.com/projects/plist.php).
5.7.3 Cool roof
A high <href=”#SolarReflectance”>solar reflectance—or <href=”#Albedo”>albedo—is the most important characteristic of a cool roof as it helps to reflect sunlight and heat away from a building, reducing roof temperatures. A high thermal <href=”#Emittance”>emittance also plays a role, particularly in climates that are warm and sunny. Together, these properties help roofs to absorb less heat and stay up to 50–60°F (28–33°C) cooler than conventional materials during peak summer weather.
Cool pavements include a range of established and emerging technologies that communities are exploring as part of their heat island reduction efforts. The term currently refers to paving materials that reflect more solar energy, enhance water evaporation, or have been otherwise modified to remain cooler than conventional pavements.
5.7.4 Cool Pavements
Conventional paving materials can reach peak summertime temperatures of 120–150°F (48–67°C), transferring excess heat to the air above them and heating storm water as it runs off the pavement into local waterways. Due to the large area covered by pavements in urban areas (nearly 30–45% of land cover based on an analysis of four geographically diverse cities1), they are an important element to consider in heat island mitigation.
As urban areas develop, changes occur in the landscape. 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—the phenomenon whereby urban regions experience warmer temperatures than their rural surroundings. This chapter provides an overview of urban heat islands, methods for identifying them, and factors that contribute to their development. It introduces key concepts that are important to understanding and mitigating this phenomenon.
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 pavements 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. In case Dhaka we should take remedial measures immediately to control Heat Island Effect.
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