Factors Affecting Changes in Urban Temperature
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Urban climatology is a branch of climatology that examines interactions between the urban area and the weather conditions around it, their impacts on each other, and the different spatial and temporal scales at which these interactions occur.
There are many differences between the urban and rural climates; these differences generally include the quality of the air as well as the wind and rainfall patterns. However, the most observable difference is the Urban Heat Island (UHI) effect which represents the temperature difference between the rural and urban sites. This field’s main implementation is its use for optimal urban design and planning of cities .
Urban areas have a significant effect on the overlying air due to a variations in the nature of surface cover (urban form) and emissions of heat, water vapor and materials that are involved human activities (urban function). Although the urban-rural differences have been recognized for a long period of time, only recently urban climatology has started collecting actual urban observations, developing urban models and validating these models using the required data.
Urban warming, also called ‘Urban Heat Island’ effect (UHI), is a well-established phenomenon. The intensity of the UHI has been measured essentially as being the temperature difference between rural and urban locations. Many studies have been conducted in order to quantify the UHI in large cities, mainly in Europe and other areas . This phenomenon is considered as a representation of all the microclimatic differences caused 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 higher by 3.7°F at night and lower by 0.34°F during the day compared to the rural surroundings. The intensity of Heat island varies depending on the density of the urban area, with the highest values of UHI being found in the most densely built areas. Moreover, in cities located in the high-latitude region, and having relatively cold weather, heat islands are considered as an advantage since it contributes to a reduction in heating loads; however, in cities located in mid- and low-latitude regions, heat islands are a major source of outdoor thermal discomfort for the urban population, and indoor thermal discomfort of buildings’ occupants (and consequently higher indoor cooling loads), especially in the summer period .
On the other hand, there is a phenomenon called the urban cool island (UCI) which is an opposite effect to the UHI, where urban areas are found to be colder than the rural arid (desert) surrounding environments. The UCI is the highest during the daytime, where the effect of increased shading (from buildings) and evapotranspiration (from artificial, man-made water bodies and vegetation) in urban areas contributes to some reduction in local temperatures compared to the unshaded rural arid surroundings. This phenomenon is mainly found in countries where rural areas are primarily deserts, such as the case of the UAE . 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 . 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 release of heat through urban structures compared to the winter period .
Generation of UHI
UHI is the results of the interactions among several factors which can be classified as controllable and uncontrollable factors as shown in Fig.4.
Figure 4. Generation of Urban Heat Island (UHI) [xx]
The 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 view factor and population related variables such as anthropogenic heat sources (power plants, automobiles, air-conditioners). The main source of heat produced and enclosed in an area originates from the sun which emits this heat in the form of solar radiation. The major energy conservation and heat transfer 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 different ways. The absorption and storage of solar radiation (in the form of heat energy) occurs from sunrise 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 obstructing buildings), as a result, the ability of heat release by long-wave radiation is reduced in all cities, leading to high heat storage in building surfaces. Surface absorptivity, which is a thermal property of any material, represents the fraction 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, latent heat due to evapotranspiration is also reduced in these areas. Convective heat removal 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 maximum UHI of 8°C in the city of Bangkok where the population is of 11 million, while they have observed a maximum UHI of 7°C 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 .
To summarize the factors affecting the UHI, Oke et al. indicates the following key factors: a decrease in radiative heat loss (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 minor factors .
Following is a list of the factors affecting urban temperature
- City Scale (S: Site Area in meter square)
The UHI is believed to be more intense in large cities where the cumulative effect of the urban warming of numerous street canyons is combined and increases the intensity of the UHI compared to cities of a smaller scale.
- Thermal Properties of the Construction Material (surface absorptivity (m) and albedo) of Buildings and of the Street Surface Material
A study that was conducted in Singapore in order to investigate the most important factors causing the UHI, found that the buildings’ facade materials and colors had a significant impact on the local climate by increasing the temperature in the center of a canyon by up to 2.5°C, in the case where the facade material had high surface absorptivity (Rajagopalan et al, 2008) .
- 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 orientation of a street canyon has the key role in determining the quantity of solar radiation that the canyon surfaces receive . As the aspect ratio increases, shading increases, and air temperatures subsequently decrease, especially during some hours of the day .
The 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 .
- 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) .
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