As old friends we are always talking and visiting. Yesterday with foggy conditions, the subject of how humidity affects the flight of a bullet and density of the air in which the bullet flies. There were two schools of thought: One that the humidity increased the density of the air; and one that the humidity decreased the density of the air.
Here is the answer:
"The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear contrary to logic.
This occurs because the molecular mass of water (18 g/mol) is less than the molecular mass of dry air (around 29 g/mol). For any gas, at a given temperature and pressure, the number of molecules present is constant for a particular volume (see Avogadro's Law). So when water molecules (vapor) are added to a given volume of air, the dry air molecules must decrease by the same number, to keep the pressure or temperature from increasing. Hence the mass per unit volume of the gas (its density) decreases."
Therefore, as the air becomes more humid and the air becomes less dense, the bullet will drop less.
The discussion progressed to how humidity affects the lifting force of a wing (or propeller or rotor), the same two diverging thoughts were present - it increases lift and it decreases lift.
Here is the answer:
Density and lift
"Lift is created by deflecting a moving fluid (liquid or gas), and drag is generated on a body in a wide variety of ways. From Newton's second law of motion, the aerodynamic forces on the body (lift and drag) are directly related to the change in momentum of the fluid with time. The fluid momentum is equal to the mass times the velocity of the fluid. Since the fluid is moving, defining the mass gets a little tricky. If the mass of fluid were brought to a halt, it would occupy some volume in space; and we could define its density to be the mass divided by the volume. With a little math which is described on the fluid momentum page, we can show that the aerodynamic forces are directly proportional to the density of the fluid that flows by the airfoil.
Lift and drag depend linearly on the density of the fluid. Halving the density halves the lift, halving the density halves the drag. The fluid density depends on the type of fluid and the depth of the fluid. In the atmosphere, air density decreases as altitude increases. This explains why airplanes have a flight ceiling, an altitude above which it cannot fly. As an airplane ascends, a point is eventually reached where there just isn't enough air mass to generate enough lift to overcome the airplane's weight. The relation between altitude and density is a fairly complex exponential that has been determined by measurements in the atmosphere."
Therefore, when an airfoil (wing, rotor, or propeller) is used to produce lift or propulsion, increasing the humidity decreases its effectiveness and the aircraft performance is negatively affected.
I just LOVE the Old Friends Match and all the talk that goes on before, during, and afterward. If we were Kings of the Planet, we could solve all the problems.