ABSTRACT
Urban terrain, in which built and natural forms are extruded from the ground, is often highly geometrically complex, having greater rugosity than is found in rural settings where the climatic measurements that form the basis of standard meteorological databases tend to be made. Due to inter-reflections between the surfaces constituting this terrain, its albedo tends to be lower, so that shortwave radiation is more efficiently absorbed relative to rural terrain. Furthermore, obstructions between surfaces diminish views to the sky, so that heat is less efficiently transferred at night by longwave exchange to the relatively cold sky. In consequence, more heat is absorbed and retained in urban than in rural terrain. Added to this is the heat gain due to anthropogenic sources, for example due to transport systems and heat sources in buildings, although this may be partially offset by evapotranspiration from vegetation.
