A ground-based network of Global Positioning System (GPS) receivers can be used to determine the excess propagation path of the GPS signals. Using surface pressure data it is possible to derive the excess path caused by water vapour. This quantity is often referred to as the wet delay. We have studied the relationship between the wet delay and the integrated precipitable water vapour (IPWV), based on more than 120 000 radiosonde profiles from 38 sites in Europe. We have studied four different models based on the surface temperature, the latitude of the site, and the time of the year in different combinations. The accuracies of the models have been evaluated and also compared with the accuracy resulting from the use of published model parameters derived from radiosonde data acquired in the United States. The models using the surface temperature data yielded the smallest relative root mean square errors of 1.14% and 1.15% respectively. We find that the accuracy can sometimes be further improved, typically down to 1.02%, by adjusting the values of the model parameters to the area or site of interest.