Primary controls over a city's thermal climate are the character and density of urban surfaces; that is, the total surface area of buildings and roads, as well as the building geometry. Table 12.2 shows the relatively high heat absorption of the city surface. A problem of measurement is that the stronger the urban thermal influence, the weaker the heat absorption at street level, and, consequently, observations made only in streets may lead to erroneous results. The geometry of urban canyons is particularly important. It involves an increase in effective surface area and the trapping by multiple reflection of short-wave radiation, as well as a reduced 'sky view' (proportional to the areas of the hemisphere open to the sky), which decreases the loss of infra-red radiation. From analyses by T. R. Oke, there appears to be an inverse linear relationship on calm, clear summer nights between the sky view factor (0 to 1.0) and the maximum urban-rural temperature difference. The difference is 10 to 12°C for a sky view factor of 0.3, but only 3°C for a sky view factor of 0.8 to 0.9.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.