A wind gradient describes the change in velocity and/or direction of the wind in a certain direction. There is always a significant wind gradient just above the ground, where the wind gets stronger with increasing altitude. The technique of dynamic soaring allows birds and sailplanes to gain energy from wind gradients. Vertical wind gradients affect starting and landing aircraft: It is part of the reason why airplanes land and take off into the wind. During climb into increasing wind, the airspeed increases, allowing the plane to gain energy and climb faster. During landing, it helps the aircraft to lose energy and get down. Landing with the wind causes aircraft tendency to overshoot because of the energy gain associated with the process. These effects are more pronounced for lighter and slower planes.