Once a particle is lofted into the atmosphere and leaves the eruption plume it is affected by three principal forces:

1. Gravity

2. Local wind 3. Atmospheric drag

Gravity causes a particle to settle directly to the ground, but the atmospheric drag will slow its descent. The wind typically moves a particle in a direction parallel with the ground. The net effect of these forces is to produce a curved trajectory for the particle during it descent to the surface. Watch the animation below to see this effect.