Solar radiation is an essential parameter for the urban climate. This is particularly true for cities like Stuttgart with a large variety of orographic structures. As a result, solar radiation may vary significantly from one place to another, especially at slopes with different inclinations and direction to the sun.

Figure 3-1 illustrates the average annual values for global radiation in Stuttgart, figure 3-2 the local situation in the plan area for Stuttgart 21. The calculation is based on the monthly values of global radiation in Stuttgart. What was taken into account is direction, inclination and clouding of the area on the basis of the digital surface model by the regional survey office of Baden-Württemberg (GORETZKY, 1995). A monthly analysis for both global radiation and direct and diffuse solar radiation is available as well.

What stands out in the figures are south-facing slopes that benefit greatly from sunshine. These places are suitable for wine growing, which requires particularly sunny, warm and preferably frost-free microclimate conditions. Local sunshine conditions are also an important factor for energy-efficient development planning (BAUMÃœLLER et al., 1993). Considering the global greenhouse effect and the necessity to lower carbon dioxide emissions significantly, the use of renewable energy sources becomes more and more important. The most important measure is the use of passive solar energy. For this, the energy of the sun supports the heating of the building and of water in order to cover as much of the required heat as possible. Figure 3-1 makes clear that the amount of solar radiation differs considerably on a local basis (up to about 60 %), depending on the location of the building area in Stuttgart. Regardless of the topographic situation, the amount of solar radiation can be influenced and optimised through the alignment of the building itself and the windows as well as through avoiding shadows cast by other buildings. This aspect counts also for the development of plain grounds. Little attention was sometimes attached to radiation during planning in the past. In future, radiation is to play an important role in all planning activities.