In order to study possible perturbations of the ROBIN density calculations, a consistent set of velocities was computed between 70 km and 38 km for a standard atmosphere using simplified fall velocity equations. Consistency was achieved by iterating the calculations until variations in the drag coefficient produced insignificant changes in the fall velocities. Fall velocities were then computed using an equation that in cluded an arbitrary distribution of vertical atmospheric motions. The resulting fall velocities were used to compute air density, assuming, as is donhe ROBIN calculations, that there is no vertical air motion. These computations are used to demonstrate the error in density resulting from the neglect of the vertical-motion term. The effect of the vertical motion on the iterative process used to obtain the drag coefficients and on the process used to check the balloon's sphericity is also discussed.