How does humidity affect lift on an aircraft?

Humidity actually affects lift by changing the density of the air. When humidity increases, the amount of water vapor in the air increases, which replaces some of the heavier nitrogen and oxygen molecules. As a result, the air becomes less dense, not denser. This lower air density leads to a decrease in lift generation because lift is directly related to the air density: the more dense the air, the more lift is produced as the aircraft passes through it.

The reasoning for why the correct choice states that humidity decreases lift due to increased air density is flawed. Instead, the correct understanding is that increased humidity reduces air density, which in turn decreases lift. Consequently, pilots must account for these variations when calculating takeoff and landing distances, as higher humidity conditions can impact an aircraft's performance.

While the options that mention no effect or only affect during landing may touch on different aspects of performance, they do not accurately encapsulate the relationship between humidity and lift generation.