We have investigated the spatial and temporal variations of atmospheric water vapor using estimates of differential zenith wet delay. These estimates were obtained from an experiment involving Global Positioning System (GPS) receivers at five sites near Madrid, Spain. Data were acquired for 14 consecutive days in December 1996. The intersite horizontal separation varied from 5 km to 50 km (with a maximum altitude difference between sites of 400 m). The sampling rate for the GPS observations was 30 s, except for 2 days during which we used a sampling rate of 10 s. The GPS data were used to estimate relative zenith wet delays. One of the GPS receiving systems was colocated with a continuously operating dual-channel Water Vapor Radiometer (WVR). During the entire experiment the WVR obtained observations from a fixed pattern of directions intended to “map” the water-vapor distribution. We have found that these estimates of differential zenith wet delay are highly correlated with the altitude of the site and that the wet refractivity, on average, followed an exponential distribution with an assumed scale height of 1.5 km. We present results from this experiment, including the high degrees of correlation observed both in the spatial and temporal domains once the estimates of differential zenith wet delay were normalized after this exponential law.