Vertical motions of dust grains in the absence of collisions for particles with sizes ﹩s=0.1\;\mu {\rm{m}},\;1\;\mathrm{mm},\;1\;\mathrm{cm}﹩. Trajectories are simulated at ﹩r=5\;\mathrm{au}﹩, assuming ﹩\alpha ={10}^{-3}﹩. Left: three typical trajectories plotted as a function of time. Right: normalized histograms of the time spent at different heights. For each histogram, the histories of 10³ particles are combined. The shaded curves show predictions of Equation (9) using the corresponding midplane Stokes numbers.