We develop a formalism for parameterizing and evaluating the effects of lateral variations in the properties of Earth's atmosphere on the propagation of microwave signals. A parametric form is incorporated into our analysis of very long baseline interferometry (VLBI) data, and the estimated atmospheric delay gradients are compared with those calculated from three-dimensional weather analysis fields from the National Center for Environmental Prediction (NCEP). For a 12-day series of experiments in January 1994, the VLBI and the NCEP analyses show common atmospheric gradient delays with amplitudes of up to 30 mm at 10¿ elevation angle. Comparison of the characteristics over a longer period of time reveals common mean north-south gradients with amplitudes up to ≈10 mm at 10¿ elevation at midlatitudes. No discernible mean east-west gradients were found in either data set. The root-mean-square (RMS) variations of the gradient effects, determined from the NCEP analysis, are similar in the north-south and east-west directions, with a typical RMS scatter of 6--10 mm at 10¿ elevation. After accounting for gradients, detailed analysis of the January 1994 VLBI data shows clearly that the residual station height variations of ≈10 mm at Westford, Massachusetts, are almost totally explained by the effects of atmospheric pressure loading.¿ 1997 American Geophysical Union