What causes the phenomenon known as "ground effect"?

Ground effect is primarily caused by the aerodynamic alterations that occur when an aircraft is flying close to the surface. As an aircraft approaches the ground, the airflow around its wings is significantly influenced by the proximity to the surface. This results in an increase in air pressure beneath the wings, which in turn enhances lift. The enhanced lift occurs because the reduced distance between the wings and the ground limits the amount of airflow disruption that typically happens when flying at higher altitudes.

Additionally, as the aircraft is closer to the ground, a decrease in induced drag occurs. This is because the upwash from the ground leads to a more efficient airflow over the wings, allowing the aircraft to maintain lift while requiring less power. This coupling of increased lift and reduced drag makes it easier for the aircraft to fly just above the runway during landing and takeoff, requiring careful attention from pilots to manage altitude and speed appropriately during this phase of flight. Thus, the correct answer reflects the dual characteristics of increased lift and decreased drag that define the ground effect phenomenon.