As wind blows over open water, it generates surface waves. An island acts as an obstacle to wind flow, which in turn makes a "wind shadow" downwind of the obstacle.

  !   Move your cursor over the downwind side of Oahu to highlight the wind shadow in this image.

The ocean surface in the inner part of the wind shadow zone is relatively smooth. Along the edges of the shadow zone, turbulence is visible where the smooth water meets the choppy, windblown surface outside the shadow.