Urban environments have been proven to have their own microclimates. The rapid urbanization experienced during the last several decades has modified the radiative, thermal, and aerodynamic performance of the urban climate. The progressive conversion of vegetated areas into buildings or paved surfaces has altered substantially the atmospheric characteristics of cities. Specifically, cities experience higher temperatures than natural areas. This effect is known as the urban heat island effect, and it is caused mainly by two urban characteristics: first is the fact that cities absorb and store more solar radiation in the form of heat due to their own morphology and the common use of dark materials such as asphalt; second is the fact that the predominance of impermeable materials does not allow for heat dissipation through evaporation. In addition, pollution and anthropogenic heat from heating, cooling, transportation, and industrial activities contribute to increase urban temperatures.
Thermal infrared satellite data measured by NASA’s Landsat Enhanced Thematic Mapper Plus on August 14, 2002 one of the hottest days in New York City's summer. Landsat also collected vegetation data. Image from NASA EArth Observatory web page.
One of the most important differences between the rural and urban energy balance is the minimization of water evaporation. This is due mainly to two reasons: urban areas are covered by impermeable materials that do not allow water to evaporate and the limited vegetated surfaces diminish heat release by evapotranspiration. Another important difference is reduced convection, as wind speeds are lower in urban settlements.
Many studies have demonstrated that various strategies can be used to diminish the heat island effect in our cities. Among them are: using cool-roofing materials, using vegetated surfaces and trees, increasing the albedo of the exposed surfaces, and covering the pavement with cool materials. Many of these strategies are cost-effective but still need more governmental and local community support. As Akbari noted, “in the US, it is estimated that about 20 percent of the national cooling demand can be avoided through a large-scale implementation of heat island mitigation measurements” (Akbari 2007, 84). For example, when planned properly, vegetated areas and trees can cost little and potentially decrease energy consumption in buildings while improving air quality and comfort.
This increase in temperature in urban areas has direct consequences, including discomfort, health problems, and higher energy consumption, affecting city habitability. The rapid increase in the urban population and the expected global warming exacerbate this issue and warrant its place as a main topic on urban agendas. In warmer climates, this effect acquires even more importance, as the dissipation of heat is crucial. Therefore, the study of this climatic alteration becomes crucial for the sustainable development of our cities. Further studies on urban climatology and predictive models considering all energy fluxes are needed to apply mitigation strategies by planners and policy makers. In addition, community involvement should be promoted as a way to help identify key issues and select indicators. A global and local view that takes into account all actors involved will be needed in order to achieve the goal of urban sustainability.