In general, the larger the particle, the higher the threshold velocity required to move it. This law can sometimes be broken when clay sized particles are involved in the entraiment process. Clay particles have a general tendency to become cohesively bonded to each other. This aggregation results in the clumping of several particles into a mass of much larger size. As a result, the threshold velocity required to entrain clay is can as great as the wind speed required to move grains of sand .
Silt is usually the easiest type of particle to be entrain by wind.
Terminal fall velocity can be defined as velocity at which a particle being transported by wind or water falls out and is deposited on the ground surface. On this semi-log graph, a simple, some what linear, relationship is observed. The larger the particle the greater the wind speed that is required to keep it moving above the ground surface. Falling velocities for clay, silt, and sand sized particles for wind. Note the fall velocity for clay is many orders of magnitude less than the fall velocity for sand.