Why does an aircraft experience lower performance at higher relative humidity?

The correct answer highlights the relationship between humidity and the density of air. Water vapor is indeed lighter than the nitrogen and oxygen that typically make up the atmosphere. When the air contains a higher concentration of water vapor, the overall density of the air decreases. This lower density air impacts aircraft performance in several critical ways.

When an aircraft operates in less dense air, it generates less lift because the lift produced by the wings is directly proportional to the air density. Lower air density also affects engine performance; internal combustion engines rely on the density of air to mix with fuel for optimal combustion. Consequently, a reduction in air density due to increased humidity can lead to a decrease in power output.

This understanding is fundamental for pilots, as it informs their decision-making regarding takeoff, climb, and overall aircraft performance in various weather conditions. Acknowledging how humidity influences air density allows pilots to better anticipate the aircraft's performance parameters in humid conditions, enhancing safety and operational efficiency.