During a short experiment we have investigated the vertical variability of water vapor in the lower part of the atmosphere with the help of small network of GPS stations positioned on the eastern slopes of the Puy de Dôme in central France. We have found out that the urban layer exhibits somewhat constant water vapor content. In contrast, the major IWV variations arise in the upper troposphere level, in particular in the presence of westerly flows that bring elevated water vapor content over the mountain ridge. Finally, the transition layer situated between these lower and upper levels presents quite variable water vapor content, acting as a buffer zone for the boundary layer. Comparing two episodes of higher water vapor contents, one being associated with a sharp frontal passage, we have shown that the contrasted behavior of the different layers revealed the possible formation of clouds before the advent of rain.