The main windiness index in the Netherlands (Windex-CBS) has been subject to a decreasing trend since the systematic recording started in 1988. This seriously complicates the estimation of future long-term wind supply. Therefore, it is necessary to know the main causes of the reported decline.
For this purpose, Windex-CBS, derived from actual wind turbine yields, is compared with two alternative windiness indices. The first index is based on wind speed observations Wland and the second on geostrophic wind speed Gland. Similar indices are formulated for offshore wind supply in the North Sea, Woff and Goff. High mutual correlations between the indices indicate that they do efficiently account for natural variability. Yet, the trends of the different indices do substantially differ from each other.
The decrease of Windex-CBS is twice as large as (200%) the decrease of Wland. The trend of the difference between both indices is highly significant. This suggests that the Windex-CBS is contaminated by operational or methodological factors. Wland does not suffer from such factors and is therefore the preferred measure of wind supply. Copyright © 2012 John Wiley & Sons, Ltd.