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Urban heat island and mitigation strategies at city and building level
Extensive urbanization has resulted to economic, social, energy & environmental challenges. The global population increase led to an increasing demand for housing. Natural land has been replaced with artificial surfaces in most cities around the world with undesirable thermal effects. This, together with industrialization growth, has caused a deterioration of the urban environment. Urban heat island (UHI) phenomenon has become a common problem in many major cities worldwide. Several factors influence the urban heat island phenomenon, such as the continuous reduction of green spaces, the changes of wind velocity due to high buildings' density, the anthropogenic heat release and the alteration of surfaces' albedo. The aforementioned factors lead to overheating problems in cities due to the absorption of solar radiation by the various surfaces and buildings. Hence, urban climate is one of the most important elements of urban physical environment, which is often ignored in urban planning. To design a sustainable city, it is necessary to take into account the climatic conditions holistically and strategically during the planning process. Since the 1970s, German researchers have developed the concept of urban climate map (UC-Map) that has a strong focus on applied urban climatology. UC-Map is an appropriate tool for translating climatic phenomena and problems into 2-D images including symbols for land use and spatial information suitable for the urban planner. Therefore this map is a useful tool for urban planners, architects and governors in order for them to understand more accurately and evaluate the effects of urban climatic issues on decision-making and environmental control. At the micro-climate level, several UHI mitigations can be implemented to reduce the UHI severity. First is greenery. The benefits of greenery to the built environment have been widely investigated. Greenery dissipates the incoming solar radiation on the building structures through its effective shading; it reduces longwave radiation exchange between buildings due to the low surface temperatures created by plants' shading; it reduces the ambient air temperature through evapotranspiration. The role of building's materials, mainly determined by their optical and thermal characteristics, is crucial in reducing the thermal and solar hear gains, in the urban environment. The so-called 'cool' materials, characterized by high reflectivity and high emissivity, can improve the thermal conditions in cities by lowering the surface temperatures that affect the thermal exchanges with the surrounding air. Urban ventilation is another important strategy of UHI mitigation. It is important to understand the nature of air flow regimes within urban canyons in order to make further progress in describing the complex interactions between mesoscale forces and the built environment that create the urban boundary layer.