The urban heat budget derived from satellite data

In Central Europe up to 85% of the population lives in cities or urban agglomerations (United Nations 2000). This makes the urban climate and its anthropogenic modifications an important issue for planners, scientists and policy makers. The urban climate differs completely from that of rural or forested areas. Since urban surfaces are extremely heterogeneous the interaction with the urban boundary layer is very complex and there are only a few micro-meteorological meas¬ urements existing providing us with sufficient data of the urban radiation and energy balance. For many years, remotely sensed data have been used in urban climate studies. Most of these data are in the thermal infrared band between 8 and 14 um where the spatially distributed surface temperature can be meas¬ ured. In many publications this is often used as an indicator for the urban heat island (UHI). Long-wave emission, calculated from surface temperature data, is however only one variable among others impor¬ tant for the characterisation of net radiation and heat balance. The UHI is a characteristic feature of the urban climate. Used alone, it can thus lead to inconsistent results. In general. mean air temperatures in cities are several degrees higher than in rural environments or in forested areas. This is true not only of big cities, but also of provincial towns. A more detailed analysis shows that in most cases the UHI-effects can be detected in annual, monthly and daily means and in mobile measurements made at night. Detailed investigations during daytime are very seldomly carried out (Alexander 1988). Satellite or aerial thermal imagery always show surface temperatures of urban areas which are 10 to 15 K higher than the surrounding rural areas. This is one reason why high surface temperatures are often used as an indicator of high air temperatures but this is not necessarily the case for urban surfaces.