Ground-based GPS receivers measure the Zenith Total Delay (ZTD) in mm which is the sum of the Zenith Hydrostatic Delay (ZHD) caused by the atmospheres dry components and the Zenith Wet Delay (ZWD) resulting mainly from atmospheric water vapor. The 15 min resolution IWV estimates for central Europe are processed by GFZ Potsdam. The dense swiss network with a hourly resolution is kindly provided by University of Bern.