The precision of vertical position and atmospheric water vapor content determined by the Global Positioning System (GPS) is limited by errors due to tropospheric delay. One factor is the spatial and temporal variability in tropospheric refractivity caused by passing weather fronts. We can explain some of the temporal characteristics of estimated tropospheric delay in terms of a simple path delay model as a function of frontal parameters. These results suggest that GPS could be used to estimate the geometry and passage time of a frontal zone. We have developed indices which detect tropospheric variability from GPS data alone; the detection rate of fronts with this approach is up to 70%. Once detected, we eliminated days affected by fronts or other tropospheric variability from the time series of station height estimates, resulting in improved long-term repeatability. The additional variance attributable to fronts is estimated to be up to 10 mm2 at Herstmonceux, England, where fronts occur every 2--3 days. The effect of fronts on the horizontal station component is up to 80% smaller than for the vertical. Studies in the field of GPS meteorology may be improved by estimating frontal parameters. ¿ 1999 American Geophysical Union