Why does an airplane perform less effectively in high humidity conditions?

An airplane performs less effectively in high humidity conditions primarily because the air is less dense. When humidity increases, the amount of water vapor in the air rises, which displaces some of the oxygen and nitrogen that make up dry air. This results in a lower air density.

Lower air density is significant for several reasons. Aircraft generate lift based on the principles of aerodynamics, which rely on the density of the air interacting with the wings. When the air is less dense, the wings generate less lift at a given speed, forcing the aircraft to either fly faster or require a longer takeoff and landing distance. Additionally, the engines may not perform as efficiently because they depend on a certain mass flow of air for combustion. Overall, high humidity can impair the aircraft's ability to perform optimally due to these factors tied to air density.