TreyFitz
Visible propeller vortices stream from the twin propellers of a Leonardo-Finmeccanica C-27J Spartan military transport, thanks to humid air.
Moist Air is Less Dense
When you see condensation forming, you should know all that water came out of the air. While it might seem reasonable to conclude that the air is now lighter from losing that water mass, you would be flat wrong. That’s because when water is dew, it is liquid. When it is in the air, it is a gas. The two states—liquid and gas—are different physical phases of an H2O molecule and follow totally different laws.
When humidity is high, the air you breathe and the air that flows over the wing and into the engine is much less dense. The upshot of that low density is that a molecule of H2O hitting the wing has much less effect than the much heavier N2 and O2 in ambient air. Beyond that, humid air contains less combustible O2 per unit volume, meaning our engines will have less of it to combine with fuel to burn and create power.