In contrast to the open landscape, the balance of energy – which is largely determined by shortwave radiation from the sun and by the longwave emanation of warmth – in a city is substantially altered. The relative influences on the urban heat budget are depicted schematically in Figure 2/1 (ROBEL et al, 1978). Solar radiation (dispersion and absorption) is reduced by particulate matter (pollutant gases and aerosols) in the city atmosphere. In the ultraviolet spectrum, the reduction ranges from 5% in summer up to 30% in winter. Global radiation (both direct solar radiation and diffuse celestial radiation) can be up to 20% less in cities. The duration of daily sunshine is further lessened up to 15% (LANDSBERG, 1981).

Soil capping and the correspondingly smaller proportion of green space reduces evaporation, contributing to increased temperatures in the city.

The built mass of the city accumulates heat from the incoming solar radiation, which indicates that daily maximum temperatures occur later in the day and that the temperature fluctuation range is narrower in a city versus in the surrounding land (Fig. 2/2a) (BRÜNDL et al, 1986) (Fig. 2/2b). The buildings release the stored heat only slowly overnight, and are still relatively warm in the morning.

Energy transfers in a city occur in large part not at ground level, but rather in the area of the roof level and the upper floors of buildings.

The effective dispersal of radiation is reduced by the relatively high proportion of pollutant gases (e.g. carbon dioxide), which can absorb longwave heat radiation and can thus lead to a warming of the city atmosphere – the local greenhouse effect.