Just spent a couple of hours googling. I found some answers.
It seems that a wind at sea level at a given speed (say 21 kts) can have a variation by as much as 10% density/weight.
The odd thing (for me anyway) was that increased humidity decreases the density of air :-? Water vapour is less dense than air by quite a margin apparently.......well I'll be
Not quite true... although water vapor is lighter than the gases found in air, the amount of the
gas, per unit of volume of atmosphere, is reduced when there's a lot of water vapor, and that's what matters to airplanes. Take an empty 1-gallon bucket (full of air, right?) and pour some water into it. There's now less air in that gallon of volume.
And the hotter the air is, the more vapor it can hold. Hotter air, due to the reluctance of the agitated gas particles to stay close together, is less dense as well, so you can see how hot, humid weather can affect density altitude.
The problem for aircraft when it comes to high humidity is not so much with aerodynamics as it is with combustion. Non-turbocharged or supercharged piston engines in particular will have problems, because the engine needs a certain amount of oxygen per second to create the desired horsepower. It's getting the same volume of "air" all the time (for a given throttle setting), but there has to be a certain ratio of gas (specifically oxygen). Water is not useful at all to such an engine, regardless of its density. The fuel will only burn in the presence of oxygen. If the ratio of water vapor to gas starts to skew towards the water side, the problem is obvious. Leaning the fuel/air mixture only helps to a point, as many piston pilots have found when they tried to take off, leaned for as much power as they can, under density altitude conditions that had them starting out at an altitude equivalent to, or exceeding, the service ceiling of the airplane.
...!
People Eating Tasty Animals.
