Once sand enters these systems, it does not leave. However, dune fields do shift across the Earth's surface from time to time. Periodic migrations of dune fields are normally caused by seasonal changes in wind direction. Over longer periods of time, dune fields may expand or contract because of climatic change. In the last few decades scientists have noticed a spatial expansion of deserts that may be correlated to human disturbance of natural vegetation cover because of agriculture.
Deposition occurs in areas where a pocket of slower moving air forms next to much faster moving air.
Such pockets typically form behind obstacles like the leeward sides of slopes. As the fast air slides over the calm zone, saltating grains fall out of the air stream and accumulate on the ground surface. Dunes first begin their life as a stationary pile of sand that forms behind some type of vertical obstacle. However, when they reach a certain size threshold continued growth may also be associated with active surface migration. When the upper leeward slope reaches an angle of about 30-34 degrees the accumulating pile becomes unstable, and small avalanches begin to occur, moving sand to the lower part of the leeward slope.