Turbulence caused by isolated surface features
As a breeze moves over an isolated hill that rises above the rest of the landscape, wind velocity in the boundary layer is modified by the additional resistance to airflow created by the hill. Not surprisingly, wind speeds on the leeward side of the hill are diminished relative to wind velocities elsewhere in the boundary layer. Less obvious is that wind velocities on the windward side of a hill are also slower. This is because the hill blocks the column of air immediately in front of it.