The beginning of sand movement is entrainment, when wind "picks up" sand grains and puts them into motion.

As a wind blows over a surface, the moving air exerts lift and drag forces on loose grains. The lift force is similar to the lift given to airplane wings as fast-moving air flows over them (the Bernoulli effect). Drag forces exert a forward push on objects (like loose grains) sticking up from the surface.

The lift and drag forces on a sand grain must overcome two types of resistance to movement. The resistance of gravity acting on a grain depends on particle size: larger grains with more mass are more resistant to movement. Frictional and cohesive forces between grains also produce resistance to movement.

The term "threshold of movement" refers to the point when wind forces overcome resistance to movement--and the grain leaves its position to bounce, roll or slide across the ground. A vital concept in understanding aeolian processes is that there is a minimum force needed to move particles of a given size. The threshold velocity indicates the minimum force.

The graph shows two threshold velocity lines for sand. The fluid threshold applies to wind moving over a surface when no particles are in motion. Once sand movement has begun, the impacts of grains colliding with the surface puts other grains into motion. Less wind energy is needed to keep sand moving after some grains are in motion, and the impact threshold is approximately 80% of the fluid threshold.