What is urban heat island?

The majority of cities are sources of heat and pollution and the thermal structure of the atmosphere above them is affected by the so-called “heat island” effect. The heat that is absorbed during the day by the buildings, roads and other constructions in an urban area is re-emitted after sunset, creating high temperature differences between urban and rural areas (Santamouris, 2001).

Heat islands develop in areas that contain a high percentage of non-reflective, water-resistant surfaces and a low percentage of vegetated and moisture trapping surfaces. In particular, materials such as stone, concrete, and asphalt tend to trap heat at the surface (Landsberg, 1981; Oke, 1982; Quattrochi et al., 2000) and a lack of vegetation reduces heat lost due to evapotranspiration (Lougeay et al., 1996). Vegetation, especially in the presence of high moisture levels, plays a key role in the regulation of surface temperatures, even more than may non-reflective or low-albedo surfaces (Goward et al., 1985).

The addition of anthropogenic heat and pollutants into the urban atmosphere further contributes to the intensity of the UHI effect (Taha, 1997). Urban centres tend to have higher energy demands than surrounding areas as a result of their high population density. Though the heat island effect reduces the need for heating in the winter, this is outweighed by the increased demand for air-conditioning during the summer months (Landsberg, 1981), which in turn causes increased local and regional air pollution through fossil-fuel burning electric power generation. The pollution created by emissions from power generation increases absorption of radiation in the boundary layer (Oke, 1982) and contributes to the creation of inversion layers. Inversion layers prevent rising air from cooling at the normal rate and slow the dispersion of pollutants produced in urban areas (Sahashi et al., 2004).