Ground effect is the increased efficiency of the rotor system caused by interference of the airflow when near the ground. Ground effect permits relative wind to be more horizontal, lift vector to be more vertical, and induced drag to be reduced. These allow the rotor system to be more efficient. The aviator achieves maximum ground effect when hovering over smooth hard surfaces. When the aviator hovers over such terrain as tall grass, trees, bushes, rough terrain, and water, maximum ground effect is reduced. Two reasons for this phenomenon are induced flow and vortex generation.