We describe two ways in which horizontal atmospheric structure affects GPS observations. For a single site, such structure results in azimuthal variations in the atmospheric propagation delay. For a network of sites, the structure will induce an intersite variability in the atmospheric propagation delays. The former effect is more sensitive to high-than low-altitude atmospheric gradients above the site, whereas the latter is insensitive to the altitude of the gradient above the network. This difference in sensitivity can be utilized to probe the local vertical structure of the atmosphere. We demonstrate this technique using GPS data from a small network on the Swedish west coast. We infer for one observing session the presence of a strong horizontal gradient which varies with height. ¿ 1999 American Geophysical Union