41 What is ground effect?
A wing generates lift by deflecting the oncoming airmass (relative wind) downward. This is identified as lift. Flying close to a surface increases air pressure on the lower wing surface. This is called ground effect, and thereby improves the aircraft lift-to-drag ratio. The nearer the wing is with regards to the ground, the more pronounced the ground effect becomes. While in the ground effect, the wing requires a lower angle of attack to produce the same amount of lift. If the angle of attack and velocity remain constant, an increase in the lift coefficient occurs, which accounts for the "floating" effect.
A) The result of the interference of the surface of the Earth with the airflow patterns about an aircraft.
Due to the reduced drag in ground effect, the airplane may seem able to take off below the recommended airspeed. However, as the airplane rises out of ground effect with an insufficient airspeed, initial climb performance may prove to be marginal because of the increased drag. Under extreme conditions such as high density altitude, high temperature, and maximum gross weight, the airplane may be able to become airborne at an insufficient airspeed, but unable to fly out of ground effect. Consequently, the airplane may not be able to clear an obstruction, or may settle back on the runway.
During the landing flare when the airplane is brought into ground effect at a constant angle of attack, the airplane will experience an increase in lift coefficient. Thus, a “floating” sensation may be experienced.
From: FAA-H-8083-25B "Pilot's Handbook of Aeronautical Knowledge"
Page 5-11, Ground Effect
"Ever since the beginning of manned flight, pilots realized that just before touchdown it would suddenly feel like the aircraft did not want to go lower, and it would just want to go on and on. This is due to the air that is trapped between the wing and the landing surface, as if there were an air cushion. This phenomenon is called ground effect."