The Westerbork radio telescope samples the wavefront from a radio source at 10 equidistant points over a 1300 m baseline. Observing frequencies are 0.6, 1.4, and 5.0 GHz. When observing a point source, the instrument senses the distortion of the wavefront resulting from atmospheric inhomogeneities. It is found that irregularities with sizes larger than the array predominate. In many cases they are seen to move over the array at speeds characteristic of tropospheric winds. Their effect can be described by a parabolic phase variation with position, the coefficients of which fluctuate with time. A number of examples illustrate the widely varying character the time dependences may exhibit. Analysis of a large number of observations shows diurnal and seasonal variations in the intensity of the fluctuations similar to those found by other workers and generally interpreted as indicating a tropospheric origin. The fact that the 1.4 GHz fluctuations are about twice as strong as those at 5.0 GHz is in disagreement with such an interpretation. Analysis of more observational material will be needed to resolve this matter.