The coming together (usually horizontally) of air due to drag against the surface being different at different locations. A classic example is the wind blowing toward shore from over the ocean. The ocean is relatively smooth with little frictional drag, while the land, with trees and buildings, is rougher and has more drag. When the wind from the ocean reaches land, it will slow down due to the increased drag. Thus, air will be flowing toward the shore from the ocean faster than it will leave the shore over land, causing horizontal convergence. Mass continuity requires updrafts in these regions, thereby leading to enhanced cloudiness and possibly precipitation. See aerodynamic roughness length.