The zone theory explains the ground effect flawlessly, and proved its correctness by accurately describing how this effect is created during landing or takeoff. Ground effect refers to a slight increase in lift when an aircraft is close to the ground at an altitude comparable to its wingspan. As usual, all sorts of wrong theories have been used thoughtlessly to explain this phenomenon, without mentioning one single scientific evidence, involved in its creation. Vidoes and text abound with the frivolous and defunct concepts like air flows, downwashes, and wingtip vortices, and so forth, thus totally misleading the public.

First lets observe an aircraft during landing as it starts nearing the ground. All aircraft almost inevitably maintain a nose-up attitude during this approach. The bottom or Lifter surface of the wings is not co-planar with the runway and the wings push the air downwards towards the ground in the direction perpendicular to the Lifter surface, creating a high pressure bottom subduction zone similar to the cushion of a hovercraft, as shown in figure below.

The airspeed vector of an aircraft in descent, has both a forward horizontal component and a downward vertical component. Analogous to a piston, the wings start compressing the air below them till the time aircraft is near ground, when this compression increases due to reaction of the ground surface or runway on air above it. The air that was easily dissipated in all directions below the aircraft, when it was at a higher altitude, does not find an easy path to get out the aircraft-runway sandwich, and begins to build up pressure as the gap decreases. This creates a high pressure cushion or a subduction zone, this time below the airframe, and generates the upwards acting reaction force, or extra lift. Same happens during takeoff, starting from the time aircraft pitches its nose up at V1, till it has sufficiently cleared the runway by more then the wingspan.

The extra ground effect lift is maximum at touch down when the nose is still pitched up at maximum angle, and the gap between runway and airframe is now minimum, see figure below. For the wings, this gap is now approx. equal to the height of the landing gear. Therefore the wings generate the largest fraction of the ground effect extra lift. Most important aspect of ground effect is to understand that this lift is the result of the reaction force of the air cushion on the airframe, and is not due to induction zone lift created by airfoil action.

For aircraft landing with nose level attitude, the ground effect will still exist, though to a lesser degree, as it is still pushing the air below it downwards towards the runway. An upwards reaction force is experienced similar to a nose-up landing though smaller then nose-up.

It is also worth noting that the ground effect force is produced by the whole air frame, i.e. the fuselage, wings, and the elevators. The entire structure is fanning the air downwards, creating the subduction zone below it. The effect of the reaction from ground becomes dominant when its close enough to the ground, and a backwards (or upwards in our case) reaction is generated which acts on the airframe pushing it up during landing. Once again the scientific robustness of the zone theory is established again, as it completely explains the ground effect.