Midland 63, good question, but there is a rather simple reason why.
Most people find it hard to believe that humid air is lighter, or less dense, than dry air. How can the air become lighter if we add water vapour to it?
Scientists have known this for a long time. The first was Isaac Newton, who stated that humid air is less dense than dry air.
To see why humid air is less dense than dry air, we need to turn to one of the laws of nature the Italian physicist Amadeo Avogadro discovered in the early 1800s. In simple terms, he found that a fixed volume of gas, say one cubic meter, at the same temperature and pressure, would always have the same number of molecules no matter what gas is in the container.
Imagine a cubic meter of perfectly dry air. It contains about 78% nitrogen molecules, which each have a molecular weight of 28 (2 atoms with atomic weight 14) . Another 21% of the air is oxygen, with each molecule having a molecular weight of 32 (2 stoms with atomic weight 16). The final one percent is a mixture of other gases, which we won't worry about.
Molecules are free to move in and out of our cubic meter of air. What Avogadro discovered leads us to conclude that if we added water vapor molecules to our cubic meter of air, some of the nitrogen and oxygen molecules would leave — remember, the total number of molecules in our cubic meter of air stays the same.
The water molecules, which replace nitrogen or oxygen, have a molecular weight of 18. (One oxygen atom with atomic weight of 16, and two hydrogen atoms each with atomic weight of 1). This is lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapor decreases the weight of the air in the cubic foot; that is, it's density decreases.
You might say, "I know water's heavier than air." True, liquid water is heavier, or more dense, than air. But, the water that makes the air humid isn't liquid. It's water vapor, which is a gas that is lighter than nitrogen or oxygen.
So, in effect we know from Bernoulis theory, "In the streamlined flow of an ideal fluid, the sum of the pressures are constant", in this case, the fluid is the air moving over the surface of the wing. So we have reduced the density of the air, therefore we have reduced the pressure it exerts, that is the lift on the wing.
Same with the engine. The air is less dense, produces less work.
Hope this answers it for you.
Cheers.
