Refer to figure.
In a still air hover, the airflow through the rotor disc begins at zero velocity above the disc and is accelerated by the rotors through the disc into the air below. Acceleration starts well above the rotor and continues through and slightly beyond it until the air is outside the influence of the high pressure region under the blades. Thereafter, the airflow diverges and slows down as it loses energy to the surrounding air.

When the helicopter hovers close to the ground, the downwash meets the ground and is forced to diverge more sharply, this increases the pressure in the wake below the helicopter. The increased pressure opposes and thus reduces the induced flow.

Factors affecting ground effect:

• Slope: Air flows downhill leading to an uneven, reduced ground effect.
• Nature of the ground: Rough ground tends to dissipate the air below the helicopter, reducing the divergent duct effect, reducing the effectiveness of the ground effect. A hard surface i.e concrete pad will have the most effective ground effect.
• Hover height: Ground effect reduces with increasing height and is negligible above one rotor diameter distance from the ground. Therefore, the closer to the ground, the more effective the ground effect.
• Wind: Ground effect tends to be displaced downwind, however the translational lift due to wind is beneficial for reduction of power required.

Note: The answer options showing "less than 4 ft skid height" is the closest to the ground and therefore the correct answer.