What causes an aircraft to experience 'ground effect' when flying very close to the runway surface?

1. The physical ground restricts the flow of wingtip vortices and downwash, reducing induced drag and increasing lift.
2. The compressed air between the wing and the runway creates a physical cushion that increases parasite drag.
3. The engine thrust is reflected off the runway surface, creating a vertical upward force.
4. The runway's magnetic field interacts with the aircraft's steel landing gear, pulling it down.