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The grids for 1961-1990 are based on homogenized station data [Herzog und Müller-Westermeier, 1998]. The 30 year averages were calculated for or each calendar month of the year before the spatial interpolation. The gridding method is based on height regression and Inverse Distance Weight (IDW), seeMüller-Westermeier, 1995: The station density allows for a linear regression between topographic height and climatological parameters within a region, and varies somewhat between the regions in Germany [Maier und Müller-Westermeier, 2010]. The regression coefficients were determined seperately for each month, based on the monthly means recorded 1951-1980.

Using these interpolated regression coefficients, in a first step, the station values are reduced to the reference height and attributed to the grid cells. In case several stations refer to a grid cell, the mean was taken.

In a second step, the values at reference height were interpolated horizontally to cover the grid (weighted with the inverse square distance).

Finally, in a third step, the values at reference height are transformed to values corresponding to the topographic elevation using again the spatially variable regression function. This is done with the DWD digital topographic height model. When grid cells contain a station,the value of the latter is simply interpolated vertically to the height of the grid cell.