A numerical experiment on falling particles arranged in zones, with slab symmetry, constant air density, and initially still air is performed whereby single-shed particles are treated by a Lagrangian method and the air motion by an Eulerian method. The results of this study of the zone's dynamics indicate that the zones fall considerably faster than their respective terminal velocity. This additional or convective velocity depends on loading and terminal speed. The spreading velocities, away from the axis of symmetry, appear to be dependent on terminal velocity such that a maximum occurs near the terminal velocity of 4 m sec−1. In all cases considered, the magnitude of the perturbation nonhydrostatic pressure gradient in the vertical is found to be an order of magnitude smaller than the perturbation hydrostatic pressure gradient.