Which atmospheric condition is described as "ground effect"?

The term "ground effect" refers specifically to the aerodynamic phenomenon where an aircraft experiences increased lift and decreased drag when flying close to the ground. This occurs due to the compression of air between the wings of the aircraft and the ground surface, resulting in a more efficient airflow around the wings. When an aircraft is operating within a certain altitude, usually within one wingspan of the ground, it benefits from this effect, allowing it to take off and land more efficiently.

In contrast to this concept, the other options describe unrelated atmospheric conditions. Reduced visibility during takeoff pertains to weather conditions that can affect flight safety but does not relate to aerodynamics. Sudden changes in wind direction refer to turbulence and wind shear, which can affect flight dynamics but are not classified under ground effect. Higher air density at altitude deals with the relationship between altitude and the density of air, impacting lift and engine performance but is not connected to ground effect. Therefore, the correct understanding of ground effect centers around the benefits gained in lift and drag while operating near the surface.