What is the reason for lower aircraft performance at higher relative humidity?

The correct answer identifies the relationship between water vapor and air density, which is crucial in understanding aircraft performance at higher relative humidity. When the relative humidity increases, there is indeed more water vapor present in the air, but the key factor is that water vapor is less dense than the nitrogen and oxygen molecules that make up the majority of the atmosphere.

As a result, when the air is saturated with moisture, the overall density of the air decreases. Lower air density means fewer air molecules are available for lift, which can reduce the performance of an aircraft in terms of lift generation, engine performance, and propeller efficiency. This reduction in density can cause longer takeoff distances, decreased climb rates, and lower overall performance, especially at high altitudes where air density is already reduced.

Factors like the weight of water vapor in relation to other components of air are critical to understanding why aircraft performance diminishes as relative humidity increases. As humidity rises, the volume of air for a given weight of air decreases, resulting in less effective lift and performance for the aircraft.