It seemed paradoxical to me the first time I worked it out , but it is a truth that wet air is less dense than dry air. Yes, if you wet air, it gets less heavy. For anyone that has fallen in a pool of water or got drenched in the rain, we are used to wet things being much heavier, but honestly
WET AIR IS LIGHTER!
The reason is pretty obvious. Basic physics means that equal numbers of gas molecules occupy pretty much the same volume. So, the density can be calculated from atomic weight. As most of the air is Nitrogen, the average weight of which is (28) and then you have oxygen (32), CO2 (44), its pretty obvious that H2O is very much the lightweight at (18). When water gets taken into the air, it displaces heavier molecules (i.e. causes it to expand) with the result the air mass is less dense.
That is why clouds form … wet air rises!
But, another intriguing possibility came to mind. What about the “moist air balloon”. Not a hot air balloon where the air is made less dense by heating, but a balloon that rises because the air inside is saturated with water.
At 15C, air of a relative humidity of 50% has a density of 1.2051 KG/m3. If that air is kept at 15C and allowed to saturate with water vapour its density reduces to 1.2013 or 38g/m3.
So, how much air would lift say a 100kg person? 100/0.0038 = 26,000 m3 which is a balloon approximately 30meters across. I make this around 5000m2 of material which at around 60gram/m2 means the balloon material will weigh 300kg. Oops.
But, if we double the size, the air volume goes up by 8x, but the balloon material only goes up by 4x as it is the area of the outside. That means the lift is 800kg, but the weight it 1200kg. So, what if I double it again? The life it now 6400kg and the weight is 4800kg giving us 1600kg of lift.
So … now it’s what? 120m across!!! 120m tall, that’s something like 80,000 m2 of material costing at a rough guess £3million?? …but it would fly! (in theory … and bumble bees can’t!)
Of course the trick is that adding water to air causes it to cool down … or more precisely the liquid is cooled resulting in cold moisture entering the air cooling it down. But as there is no heat loss, in theory (if the balloon material can retain the water), the balloon could fly all day … until it rains and the air becomes 100% saturated! … but it’s more practical than time travel!