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.