Mitigation of urban heat island effect and greenroofs

The anthropogenic developments have been causing a warming phenomenon of city centres called urban heat islands (UHI). The effect is made worse by the urban canopy characteristics of concrete, facade glazing and asphalt surfaces of building and road structures. Buildings would absorb shortwave radiation and store heat during daytime, and release heat into atmosphere at night, thus increasing the ambient temperature. The wide spread use of HVAC air-conditioners in the densely urbanised areas, as well as increasing commercial developments and massive rise in automobile traffics are also important contributing factors, which can lead to formation of ozone and smog. Urban temperatures can be up to 5-12? warmer than the surrounding countryside under extreme meteorological conditions of calm and clear anticyclonic weather. High temperatures in cities can exacerbate energy demand for building air conditioning, therefore higher emission of CO2 contributing further to green house warming. There are a number of UHI mitigation strategies including increasing the surface reflectivity and vegetation density in urban areas. This can be achieved by greening the urban environment by planting trees and vegetation; incorporating better roof designs such as cool roofs, greenroofs, reflective roofs and cool pavements. The purpose of this paper is to provide a review of UHI effects in city centres and examine the mitigation strategies and the thermal benefits of greenroofs.