Abstract We computed zonal geopotimal coefficients from average seasonal variations in global air and water mass distributions. These coefficients are used to predict the seasonal variations of LAGEOS' and Starlette's orbital node, delta Omega, and the seasonal delta J3 for Starlette. A comparison of these predictions with the observed values indicates that air pressure and, to a lesser extent, water storage may be responsible for a large portion of the currently unmodeled variation in the earth's...