Figure 5.6 shows the reduction in the forecast error for a German 100 MW wind farm as the forecast horizon decreases. Geographical spread will depend on the distance between windmills and the size of the observed area. Comparing Germany and Ireland, for instance, Ireland will have higher variations of wind output than Germany as the Irish land surface is smaller than Germany’s (Bach, 2010). Thus, geographical spread is less important in Ireland, which implies a greater variability of wind output.