Now we often have both the original and a struck particle leaping from the ground and moving downwind, and in a chain reaction, they will continue the saltation process as long as the wind speeds remain high. If the leap is sufficiently high, the particle may then become suspended and be pushed higher and farther along. When viewed from a distance, a field of saltating particles appears as if they are constantly suspended, creating a fuzzy layer next to the ground.
Note that I have not assigned any wind velocities to saltation and suspension. That is because the particle size and density factor into the equation. I can say that the ability of wind to lift and move particles varies as the cube of the wind's speed. Thus, if the wind speed doubles, it is eight times more effective in moving particles; tripling the wind gives a twenty-seven-fold increase to its ability. Therefore, under storm- and hurricane-force winds, even large objects such as bricks can be sent dangerously airborne.
Creep
When saltating particles strike particles too heavy to bump aloft, they may only nudge the larger grains (up to six times larger than saltating particles) along, a slow sliding and rolling movement known as creep. Creep usually requires wind speed exceeding 16 km/h (10 mph) to impart enough energy to the saltating particle to cause it to move the larger one.
Dirty Storms
If lofted dust forms a large cloud that sweeps forward across large areas, the blowing-dust event can be called a duststorm. Meteorological events can cause duststorms generally fall into two main categories: localized convective events and large-scale, non-convective events.
Convective-event duststorms arise during strong thunderstorm outflow or downburst winds. The duration of duststorms depends on the lifetime of the process causing them. A microburst duststorm may last only a few seconds, whereas one lofted by a downburst may last several minutes. Dust storms caused by dry squall-line winds or derechos may last many hours. Convective event dust storms are usually most common during the late afternoon in spring and summer when daytime heating is at its peak and are far less predictable than the large scale non-convective events.
Large-scale, non-convective duststorm events form under sustained high surface winds associated with cyclonic windstorms. Such duststorms can last from three to four hours, or they may last for several days, depending on the overriding weather situation. These duststorms generally occur in late winter or early spring when extreme pressure gradients produce high wind speeds and surfaces are dry and bare.
