Factors Affecting Changes in Urban Temperature

Factors Affecting Changes in urban Temperature1.1 urban Climatologyurban climatology is a branch ofclimatologythat examines interactions between the urbanarea and the atmospheric condition conditions around it, their rivals on each other(a), and the variant spatial and temporal scales at which these interactions occur.There are many passings between the urban and boorish moods these differences by and large entangle the quality of the wrinkle as strong as the pencil lead and rainfall patterns. However, the most observable difference is theurban horninessing system Island(UHI) put up which represents the temperature difference between the rural and urban sites. This fields main implementation is its use for optimal urban design and planning of cities 21.urban areas have a significant result on the overlying air due to a variations in the nature of surface cover (urban form) and emissions of warmness, pissing vapor and materials that are involved human activities (u rban function). Although the urban-rural differences have been recognized for a long occlusion of time, only recently urban climatology has started collecting veritable urban observations, developing urban models and validating these models using the required data.1.1.1 Urban Heat Island EffectUrban warming, also called Urban Heat Island effect (UHI), is a well-established phenomenon. The intensiveness of the UHI has been measured essentially as macrocosm the temperature difference between rural and urban locations. Many studies have been conducted in coiffe to quantify the UHI in large cities, mainly in Europe and other areas 16. This phenomenon is considered as a representation of all the microclimatic differences actord by man-made modifications of the urban surface (Landsberg, 1981). The Urban Heat Island effect was first identified in 1820 by Luke Howard who noticed that in London, urban temperatures were high by 3.7F at night and lower by 0.34F during the daylight comp ared to the rural surroundings. The intensity of Heat island varies depending on the engrossment of the urban area, with the highest values of UHI being found in the most densely built areas. Moreover, in cities hardened in the high-latitude region, and having relatively cold weather, heat islands are considered as an proceeds since it contributes to a reduction in heating loads however, in cities fit(p) in mid- and low-latitude regions, heat islands are a major source of out-of-door thermal discomfort for the urban existence, and indoor thermal discomfort of mental synthesiss occupants (and whence higher indoor cooling loads), specially in the summer point in time 22.On the other hand, there is a phenomenon called the urban cool island (UCI) which is an antagonist effect to the UHI, where urban areas are found to be colder than the rural dry (desert) surrounding environments. The UCI is the highest during the daytime, where the effect of increased shading (from building s) and evapotranspiration (from artificial, man-made body of water system bodies and vegetation) in urban areas contributes to more or less reduction in local temperatures compared to the unshaded rural arid surroundings. This phenomenon is mainly found in countries where rural areas are mainly deserts, much(prenominal) as the case of the UAE 19. As a matter of fact, the UHI intensity is influenced by the location of the urban, as well as by the climate regime, season and synoptic patterns 36. Evidently, UHI shows higher intensities during the summer season due to the larger amounts of solar radiation received by the surface and leading to higher absorption and exonerate of heat through urban structures compared to the winter period 37.1.1.1.1 Factors Affecting Urban Temperature contemporaries of UHIUHI is the results of the interactions among several factors which can be assort as controllable and uncontrollable factors as shown in Fig.4. Figure 4. Generation of Urban Heat Island (UHI) xxThe uncontrollable factors of UHI include climatic variables (synoptic scale) and weather conditions (local scale) such as air speed and cloud covers. Some studies show that the UHI is negatively correlated with wind speed and cloud cover (Kim and Baik, 2005 Oke, 1982). The controllable factors include urban design and structure related variables such as vegetation, building construction material, and sky prospect factor and race related variables such as anthropogenic heat sources (power plants, automobiles, air-conditioners). The main source of heat produced and envelop in an area originates from the sun which emits this heat in the form of solar radiation. The major dexterity conservation and heat exile processes (through conduction, convection and radiation) have a dominant role in the heat exchange within an urban area. The structures that are located in the canopy layer level, such as walls, roofs and green spaces absorb and reflect solar radiation in differ ent ways. The absorption and storage of solar radiation (in the form of heat energy) occurs from morn till sunset, then the environment starts cooling down. The stored heat energy in urban structures is then released to the surrounding environment, based on the sky view factor and the building material of these urban structures. An urban area is typically characterized by a decreased sky view (due to the presence of folding buildings), as a result, the ability of heat release by long-wave radiation is decrease in all cities, leading to high heat storage in building surfaces. Surface absorptivity, which is a thermal property of any material, represents the member of total incident light that is effectively absorbed by a surface, and is believed to be high in cities and is considered to be one of the main reasons of UHI. Moreover, as a result of the lack of vegetation in most of the cities, possible heat due to evapotranspiration is also reduced in these areas. Convective heat r emoval and transfer by wind are also found to be negatively affected by the high roughness of structures in urban areas. In addition, air pollutants that are found in polluted urban areas, are able to capture and re-radiate long wave radiation and obstruct the resulting radiative surface cooling. This leads to the formation of a greenhouse-like effect, causing UHI. Furthermore, the magnitude of UHI has been positively correlated with the size of the urban population of a city in some studies (Hung et al., 2005), while it was found to have be independent of urban population density in other studies (Kim and Baik, 2004). Hung et al. (2005) have found a supreme UHI of 8C in the city of Bangkok where the population is of 11 million, while they have observed a maximum UHI of 7C in the city of Shanghai where the population density reaches 12.55 million. The population affects heat generation in 2 ways directly, as an increased density of people results in increased human metabolisms and indirectly, as an increased population is typically correlated with a higher number of buildings and vehicles, as well as an increased industrial activity and pollutants concentration 18.To summarize the factors modify the UHI, Oke et al.7 indicates the following key factors a decrease in radiative heat injustice (canyon effect), an increase in thermal storage within the buildings of the urban areas, the release of anthropogenic heat, the reduction of evaporative cooling and turbulent heat transfer in street canyons, in addition to some other small factors 4.Following is a list of the factors affecting urban temperatureCity home (S Site Area in meter square)The UHI is believed to be more consuming in large cities where the cumulative effect of the urban warming of legion(predicate) street canyons is combined and increases the intensity of the UHI compared to cities of a smaller scale.Thermal Properties of the body structure Material (surface absorptivity (m) and albedo) of b uilds and of the Street Surface MaterialA essay that was conducted in Singapore in order to investigate the most strategic factors causing the UHI, found that the buildings frontlet materials and colors had a significant impact on the local climate by increasing the temperature in the concern of a canyon by up to 2.5C, in the case where the facade material had high surface absorptivity (Rajagopalan et al, 2008) 22.Geometry Orientation of the urban area, includes Building Density (FA/S), Aspect Ratio (the ratio of the Height of Building over the Width of Street), and the Orientation of the street canyon, relative to the incident solar radiation. Urban geometry has a major role in the heat build-up in urban areas. Urban canyon can be designed in a way that improves natural ventilation within the city, hence enhancing heat release. Moreover, the taste of a street canyon has the key role in ascertain the quantity of solar radiation that the canyon surfaces receive 22. As the gri mace ratio increases, shading increases, and air temperatures subsequently decrease, especially during some hours of the day 31.VegetationThe evapotranspiration process from vegetation is another means of urban surfaces cooling, especially in mid and low latitudes having warm arid climatic conditions. Urban areas having abundant impervious surfaces have usually more runoff water than their rural surroundings. The runoff water quickly drains and less surface water becomes available for evapotranspiration in the long run, consequently influencing the urban surface energy balance. This results in a decreased evapotranspiration rate in urban areas which is a main contributor in higher daytime temperatures 22.Anthropogenic Heat (population density, Number of cars, HVAC system) It mainly originates from heat emissions coming from vehicles and air conditioners. Even though indoor cooling (using air conditioners) improves the indoor thermal comfort of residents in a building, the waste heat dissipated into the outside environment negatively affects the outdoor urban thermal environment. Studies show that air conditioners can cause significant heat accumulation (Chow et al, 2000). Nevertheless, this issue is currently addressed by implementing central air-conditioning systems in many commercial buildings of major cities. Using this method, the heat dissipation takes place using cooling towers (Kikegawa et al, 2003 Kolokotroni et al, 2006) 22.