Diurnal variations in the Integrated Precipitable Water Vapour (IPWV) are studied from GPS observations acquired at 32 sites in the Baltic region during 1996-2005. The seasonal means for spring and summer show a diurnal sinusoidal pattern of the IPWV with the maximum value in the afternoon. The peak-to-peak (PtP) value of the average diurnal IPWV cycle was 0.5 mm in the spring and 0.6 mm in the summer. In the autumn and in the winter the diurnal variations in IPWV show no clear patterns and the average PtP values of the noise-like signal are only 0.2-0.3 mm. The diurnal IPWV cycle can only be estimated by averaging data from many years because the IPWV can show fast and large variations, reaching up to 5 mm/hour during several hours. These are explained exclusively by changes in the synoptic situation and substitution of airmasses above the location of observations; two case studies with analyses of the vertical humidity profiles are presented. The impact on the transmittance of the direct solar radiation is evaluated
