Everyone knows that winds on the Earth are commonly strong enough to erode, transport, and deposit sediment. What is perhaps less obvious is that the modes of sediment transport by the wind are greatly different from those of sediment transport by water flows. This great difference does not arise from any great difference in the structure of the wind at the lowermost levels in the atmosphere: you saw in Chapter 7 that low in the atmospheric boundary layer the dynamics of flow are the same in all essential respects as in turbulent shear flows above a solid boundary in water. The difference lies in the greatly different ratio of sediment density to fluid density, which is almost eight hundred times greater in air than in water; go back and look at Figure 8.1.5 in Chapter 8 to see where the point for $$\rho_{s}/\rho$$ lies for quartz-density particles in air, relative to the point for quartz-density particles in water. This difference has profound effects on the nature of particle movement in the two fluid media. As discussed briefly in Chapter 8, the very large ratio of particle density to air density means that the trajectories of particles that are in transport by the wind are largely independent of the fluid turbulence, except for fine particles, in the silt and clay size range.